[0001] Mesoporous Silica Nanoparticles as Fluorescent Tracers for Reservoir Characterization

CROSS-REFERENCE TO RELATED APPLICATION

[0002] This application claims the benefit of U.S. Provisional Patent Application No.

62/291,862, filed February 5, 2016, entitled "Mesoporous Silica Nanoparticles as Fluorescent Tracers for Reservoir Characterization," which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

[0003] The present invention generally relates to the characterization and synthesis of stable fluorescent silica nanoparticles, and their use in estimating properties of subterranean formations. BACKGROUND OF THE INVENTION

[0004] Various methods have been developed to synthesize fluorescent silica nanoparticles, and various methods have been developed to generate statistics of subterranean formations.

[0005] Silica nanoparticles with incorporated fluorescent dyes are used in many applications ranging from biomedical sciences to downhold oil and gas technologies. US Pat. No. 8,298,677 describes a method of incorporating organic dyes inside of a silica matrix wherein the organic dyes do not undergo quenching activity, as is often the case during syntheses derived from the work of Stober et al. However, in all derivatives of this technology, there have been documented difficulties with stability of the particles in a variety of conditions, including ranges of temperature, pressure, ionic content, and in the presence of certain chemicals. Difficulties with stability can include aggregative qualities of particles based on the Stober and derived syntheses, a loss in fluorescence intensity as a result of dye aggregation within the particle given external conditions, and a loss of structure based on forces or stresses caused by externalities.

[0006] There is a need for development of coatings or additional chemical additives in silica nanoparticles for long term stability applications.

[0007] In the same manner, various methods have been developed to estimate the properties of subterranean formations. US Pat. No. 8,959,991 describes a method of estimating properties of a subterranean formation penetrated by a wellbore by introducing tracer agents and measuring their size, concentration, and type distribution upon tracer recovery. Similar technologies have been commercialized in the past to characterize formations. However, all technologies that have been developed as derivatives of injectable tracer agents have been developed using intermittent sampling techniques. Conclusions based on infrequent sampling of tracer agents and their concentration or conformational changes are insufficient, and the development of technologies with high frequency sampling is necessary in the production of subterranean formation properties.

BRIEF SUMMARY OF THE INVENTION [0008] In an embodiment a nanoparticle comprises a silica structure, one or more fluorescent agents embedded within the silica structure, and a plurality of zwitterionic functional groups adhered to the silica structure. In an embodiment a nanoparticle comprises a mesoporous silica structure, which may comprise pores having, for example, an average pore size of from 0.05 to 5 nanometers. In some embodiments a nanoparticle has at least one dimension less than 300 nm in length.

[0009] In some embodiments where a nanoparticle comprises one or more fluorescent agents within the silica structure, the fluorescent agent may be selected from the group consisting of acridine dyes, benzopyrone dyes, cyanine dyes, fluorone dyes, oxazine dyes, phenanthridine dyes, rhodamine dyes, or a combination thereof. In some embodiments the fluorescent agent has an absorption spectra and an emission spectra lying within a range comprising the visible light, near- infrared and infrared spectra, (e.g. 400 to 1500 nm). In some embodiments the fluorescent agent has an absorpotion spectrum that is distinct from, or has minimal overlap with, the emission spectra of other fluorescent agents. A fluorescent agent may be encapsulated within the silica structure in some embodiments, or covalently bonded to the silica structure in other embodiments. In some

embodiments a nanoparticle has a surface and the fluorescent agent is functionalized on the surface of the nanoparticle.

[0010] In some embodiments where a nanoparticle comprises a plurality of zwitterionic functional groups adhered to the silica structure, the zwitterionic functional groups are selected from the group consisting of: sulfobetaines, phosphatidylcholines, ammonium betaines, carboxybetaines, imidazoliumn salts, pyridinium salts, piperidinium salts, phosphonate salts, or a combination thereof. In some embodiments the zwitterionic functional group is polymeric. In some embodiments the zwitterionic functional group is amphoteric over a variable pH range of 2 through 11. In some embodiments a nanoparticle further comprising a zwitterion silane derived from the zwitterionic functional group being adhered to the silica structure through hydrolysis with a silane precursor. For example, the zwitterion silane may be 3-(dimethyl(3-(trimethoxysilyl)propyl)ammonio)propane-l- sulfonate. In some embodiments the zwitterionic functional groups are adhered to the silica nanoparticle surface via covalent bonding. [0011] According to some embodiments, a nanoparticle is stable in a pH range of from 2 through 11 for a time period taken from the group consisting of 1 day, 1 week, 1 month and 1 year. According to some embodiments, a nanoparticle is stable in high saline conditions of at least 100,000 parts per million for a time period taken from the group consisting of 1 day, 1 week, 1 month, and 1 year. According to some embodiments, a nanoparticle is stable in temperatures of at least 200 °C and/or at pressures of at least 12,000 psi.

[0012] According to some embodiments, a nanoparticle is capable of adsorbing in a quantifiable amount to a subterranean formation. According to some embodiments, a fluorescence intensity of a nanoparticle is not affected by one or more of temperature, pressure, variable pH, presence of carbonate groups, presence of shale rock, presence of surfactants, and presence of friction reducers. In particular, according to some embodiments, a fluorescence intensity of a nanoparticle is not affected by one or more of high temperature, high pressure, variable pH, presence of carbonate groups, presence of shale rock, presence of surfactants, and presence of friction reducers. According to some embodiments, a nanoparticle is capable of being dispersed in a hydrophilic solvent, while in other embodiments a nanoparticle is capable of being dispersed in a hydrophobic solvent. In some embodiments a nanoparticle is capable of being initially dispersed in a hydrophobic solvent and then partitioning into a hydrophilic solvent or being initially dispersed in a hydrophilic solvent and then partitioning into a hydrophobic solvent.

[0013] In some embodiments, a method of synthesizing a nanoparticle comprises creating an emulsion of one or more silica precursors, one or more fluorescent agents, one or more surfactants, and one or more hydrophobic catalysts and/or structure - directing agents; condensing the precursors into a silica structure having small channels or pores; breaking the emulsion and releasing newly formed nanoparticles; removing the hydrophobic catalyst, surfactant, and unreacted silica precursor, hereafter referred to as the nanoparticle template; stabilizing the newly formed nanoparticles in aqueous solution, and functionalizing the newly formed nanoparticles with one or more zwitterionic functional groups.

[0014] In some embodiments, a method of synthesizing a nanoparticle comprises forming a first mixture comprising one or more fluorescent agents, one or more surfactants, one or more hydrophobic catalysts and/or structure - directing agents, and water; forming a silica nanoparticle by adding one or more silica precursors to the first mixture, wherein when more than one silica precursors are added, the more than one silica precursors are added sequentially; forming a functionalized silica nanoparticle by functionalizing the structure of the silica particle with one or more polymeric zwitterionic functional groups; and stabilizing the functionalized silica nanoparticles in solution.

[0015] In some embodiments the one or more silica precursors are independently selected from the group consisting of: tri- and tetra-alkoxysilanes such as tetramethoxysilane (TMOS),

tetraethoxysilane (MTMS), methyltriethoxysilane (MTES), ethyltriethoxysilane (ETES),

octyltriethoxysilane (OTES), octyltrimethoxysilane (OTMS), hexadecyltrimethoxisilane (HDTMS) and hexadecyltriethoxisilane (HDTES), octadecyltrimethoxysilane (ODTMS),

octadecyltriethoxyisilane (ODTES) as well as methyl polysilicate (MPS), ethyl polysilicate (EPS), polydiethoxysilane (PDES), hexamethyl disilicate, hexaethyl disilicate or functional tnalkoxysilanes (methacryloyloxypropyltrimethoxysilane, phenyltriethoxysilane (PTES), phenyltrimethoxysilane (PTMS), glycidoxypropoxyltrimethoxysilane (GLYMO), glycidoxypropyltriethoxysilane (GLYEO), mercaptopropyltriethoxysilane (MPTES), mercaptopropyltrimethoxysilane (MPTMS),

aminopropyltrimethoxysilane (APTMS), aminopropyltriethoxysilane (APTES), 3-(2- aminoethylamino)propyltrimethoxysilane (DATMS), 3-[2-(2- aminoethylamino)ethylamino]propyltrimethoxysilane (TATMS), [2- (cyclohexenyl)ethyl]triethoxysilane (CHEETES), vinyltrimethoxysilane (VTMS),

vinyltriethoxysilane (VTES), and a combination thereof. In some embodiments the forming a silica nanoparticle step is through a co-condensation process, while in other embodiments the forming a silica nanoparticle step is through a sequential grafting process.

[0016] In some embodiments a method of synthesizing a nanoparticle further comprises conjugating one or more fluorescent dyes to a silica precursor prior to forming the first mixture.

[0017] In some embodiments, a method of synthesizing a nanoparticle including forming a first mixture comprising one or more surfactants, the one or more surfactants are independently selected from the group consisting of anionic surfactants (comprising carboxylates, sulfonates, petroleum sulfonates, alkylbenzenesulfonates, naphthalenesulfonates, olefin sulfonates, alkyl sulfates, sodium lauryl sulfate, sodium dodecyl sulfate, sulfates, sulfated natural oils & fats, sulfated esters, sulfated alkanolamides, alkylphenols, ethoxylated & sulfated), cationic surfactants (comprising quaternary ammonium salts - including but not limited to alkyltrimethylammonium salts: cetyl

trimethylammonium bromide (CTAB), cetyl trimethylammonium chloride (CTAC), cetylpyridinium chloride (CPC), benzalkonium chloride (BAC), benzethonium chloride (BZT), 5-Bromo-5-nitro-l,3- dioxane, dimethyldioctadecylammonium chloride, cetrimonium bromide,

dioctadecyldimethylammonium bromide (DODAB), amines with amide linkages, polyoxyethylene alkyl & alicyclic amines, Ν,Ν,Ν',Ν' tetrakis substituted ethylenediamines, 2- alkyl 1- hydroxethyl 2- imidazolines), nonionic surfactants (comprising ethoxylated aliphatic alcohol, polyoxyethylene surfactant, carboxylic esters, polyethylene glycol esters, anhydrosorbitol ester & associated ethoxylated derivatives, glycol esters of fatty acids, carboxylic amides, monoalkanolamine condensates, polyoxyethylene fatty acid amides), amphoteric surfactants and a combination thereof.

[0018] In some embodiments the forming a first mixture further comprises forming a microemulsion comprising the one or more surfactants. The first mixture may further include water, and in embodiments where the forming a first mixture further comprises forming a microemulsion, the microemulsion may be a water-in-oil microemulsion. In some embodiments of a method of synthesis of a nanoparticle that includes forming a microemulsion, the method may further comprise breaking the emulsion is via dilution, while in other embodiments the method may further comprise breaking the emulsion is via changing the pH.

[0019] In some embodiments, a method of synthesizing a nanoparticle including forming a first mixture comprising one or more hydrophobic catalysts and/or structure-directing agents, the hydrophobic catalysts and/or structure-directing agents are selected from the group consisting of alkanolamines such as ethanolamine, diethanolamine, triethanolamine, tetramethylammonium hydroxide, ammonium hydroxide, N-methylglucamine, polyalkylene oxide homo- and copolymers, fatty alcohols, silanes, metal alkoxides and a combination thereof.

[0020] According to some embodiments of the invention, a process of collecting data concerning one or more subterranean regions includes injecting one or more fluorescently distinct nanoparticles to one or more discrete subterranean regions, wherein the one or more fluorescently distinct nanoparticles are each in accordance with an embodiment of the invention (e.g., comprises a silica structure, one or more fluorescent agents embedded within the silica structure, and a plurality of polymeric zwitterionic functional groups adhered to the silica structure); detecting the fluorescence data of the one or more fluorescently distinct nanoparticles in a produced fluid, wherein the produced fluid comprises the injected fluid and a formation fluid from one or more subterranean regions; in certain of such embodiments the detection of the one or more fluorescently distinct nanoparticles in a produced fluid is performed using an in-flow device. According to some embodiments, more than one fluorescently distinct nanoparticles are injected in a single

subterranean reservoir comprising a plurality of regions, and wherein each fluorescently distinct nanoparticle is injected in a different region of the subterranean reservoir; in certain of such embodiments each fluorescently distinct nanoparticle comprises a different fluorescent agent or a different combination of fluorescent agents. In some embodiments comprising more than one fluorescently distinct nanoparticles, each fluorescently distinct nanoparticle comprises a different fluorescent agent or a different combination of fluorescent agents.

[0021] According to some embodiments of the invention, detection of the one or more fluorescently distinct nanoparticles is carried out using an in-flow detector (e.g. an in-flow fluorescent measurement technique), which, in some embodiments, is installed on the wellsite at the surface of the wellsite. In some embodiments, detection of the nanoparticles occurs at a rate of no less than one sample every 30 seconds.

[0022] According to some embodiments of the invention, a process of collecting data concerning one or more subterranean regions further comprises forming or providing an aqueous dispersion of nanoparticles. According to some embodiments of the invention, a process of collecting data concerning one or more subterranean regions further comprises dispersing an aqueous suspension of nanoparticles in a drilling fluid, a fracturing fluid, or an injection fluid prior to the injecting step. In some embodiments an aqueous suspension of nanoparticles is dispersed using high pressure homogenizer, polymer addition and / or addition of a nonionic, anionic, or cationic surfactant or a combination thereof.

[0023] According to some embodiments, the nanoparticles are conservative to an aqueous phase and remain suspended in the produced fluid.

[0024] According to some embodiments, detecting is optically detecting. In some embodiments optically detecting nanoparticles is at a wellbore in the produced fluid using a fluorescent measurement technique. In some embodiments optically detecting is performed via photometer, fluorometer, spectrofluorometer, Raman spectrometer or a combination thereof. In some

embodiments optically detecting nanoparticles is performed downhole inside the wellbore, while in other embodiments optically detecting nanoparticles is performed on a surface near the wellbore.

[0025] According to some embodiments of the invention, a process of collecting data concerning one or more subterranean regions further comprises detecting fluorescence data in realtime from the optical detection device as it reads fluorescence measurements. In some embodiments fluorescence data are detected at a rate of at least once every 30 seconds.

[0026] According to some embodiments of the invention, a process of collecting data concerning one or more subterranean regions further comprises converting the fluorescence data into nanoparticle concentration measurements. According to some embodiments of the invention, a process of collecting data concerning one or more subterranean regions further comprises calculating stage-specific flow from the nanoparticle concentration measurements. According to some embodiments of the invention, a process of collecting data concerning one or more subterranean regions comprises calculating subterranean characteristics from the fluorescence data.

[0027] According to some embodiments, the nanoparticles serve as partitioning nanoparticle tracers that can selectively phase from an aqueous to an oil phase, or from an oil phase to an aqueous phase. In some embodiments adhered functional groups confer higher oil-phase solubility to the nanoparticle tracers relative to conservative nanoparticle tracers, for example nanoparticle tracers without adhered functional groups.

[0028] According to some embodiments of the invention, a process of collecting data concerning one or more subterranean regions further comprises injecting a set of partitioning nanoparticle tracers with conservative nanoparticle tracers and determining residual oil saturation in one or multiple subterranean regions.

[0029] In some embodiments the nanoparticles are capable of responding to different stimuli, selected from changes in temperature, changes in pressure, changes in salinity, changes in proppant type and concentration, changes in fracture geometry and surface area, changes in reservoir rock type, changes in oil-to-water ratios in the produced fluid or a combination thereof.

[0030] According to some embodiments of the invention, a process of collecting data concerning one or more subterranean regions according to an embodiment of the invention comprises concurrently injecting nanoparticles having different physical properties in one or many subterranean regions. For example, according to an embodiment of the invention a process comprises concurrently injecting nanoparticles comprising a fluorescent dye elected from the group consisting of acridine dyes, benzopyrone dyes, cyanine dyes, fluorone dyes, oxazine dyes, phenanthridine dyes, rhodamine dyes, or a combination thereof and nanoparticles that are stable in high salinity in one or many subterranean regions.

[0031] In some embodiments the one or more fluorescently distinct nanoparticles are adhered to proppant molecules, including sand and other silicates, resins, surfactants, ceramics, and other particles used during stimulation, completion, and production.

[0032] In some embodiments, a process for injecting one or more fluorescently distinct nanoparticles is carried out during stimulation, completion, and/or production in one or multiple subterranean regions during hydraulic fracturing. In other embodiments, a process for injecting one or multiple fluorescently distinct nanoparticles is carried out during enhanced oil recovery or other unconventional recovery operations. In still other embodiments, a process for injecting one or multiple fluorescently distinct nanoparticles is carried out during analysis of enhanced geothermal systems (EGS) to analyze water flow. [0033] According to some embodiments, a process for injecting one or more fluorescently distinct nanoparticles includes a detecting step, wherein the detecting step provides data that allows for quantification of breakthrough, quantification of stage-specific production, other subterranean statistics or a combination thereof.

[0034] In some embodiments the nanoparticle is used in a binary manner to verify breakthrough from one or a few stages, including but not limited to the toe stage of a horizontal well.

[0035] According to some embodiments, a nanoparticle is produced by a method described herein.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS [0036] The foregoing summary, as well as the following detailed description of embodiments of the fluorescent nanoparticles and their use will be better understood when read in conjunction with the appended drawings of an exemplary embodiment. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

[0037] In the drawings:

[0038] Fig. 1 is a schematic of a silica nanoparticle featuring zwitterionic functional groups in accordance with an exemplary embodiment of the present invention;

[0039] Fig. 2 is a schematic of nanoparticle detection scheme in accordance with an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION [0040] Silica nanoparticles have well-characterized chemical and physical properties that confer stability in difficult environments, such as oil and gas reservoirs. Silica nanoparticles can remain stable in adverse environments where concerns may include high temperature, high salinity, high or low pH, and high pressure. Particularly, silica particles are inert and unreactive, and depending on their size and surface properties, they can be versatile as either single-phase particles or intra-phase particles. With the incorporation of fluorescent agents, the silica nanoparticles can take on the same photoluminescent (absorption and emission spectra) properties of the incorporated fluorescent agents.

[0041] Embodiments of the present invention relate to the synthesis and methods of synthesis of fluorescent nanoparticles, and their use in subterranean formation data collection. In one

embodiment, the nanoparticles include a silica nanoparticle structure and one or more embedded fluorescent agents to enable optical detection. In some embodiments nanoparticles further include adhered zwitterionic functional groups, which may confer nanoparticle stability.

[0042] The nanoparticle may include, or in some embodiments, consist essentially of a core and an external coating. The core may include, or in some embodiments may consist essentially of, one or more fluorescent agents. In one embodiment, the core of the nanoparticle includes a mesoporous silica matrix. The silica matrix may comprise the self-assembled form of a silicon dioxide lattice structure. The term mesoporous generally refers to the matrix's pores, which can range in size from 0.05 nm to 5 nm. In other embodiments, the pore sizes may be between 0.1 nm to 1 nm, 0.1 nm to 2.5 nm, 1 nm to 5 nm, or 2.5 nm to 5 nm. The external coating is the external surface of the mesoporous silica matrix, or core. The external coating can be chemically modified on the surface to add or remove functional groups and thereby change the particle's stability in a variety of conditions, for example the ability of the particle to remain colloidally dispersed with minimal aggregation in its local conditions.

[0043] Nanoparticles in accordance with embodiments of the invention can subsequently vary over a large size range. In one embodiment, the nanoparticle's longest dimension is less than 300 nm in length. In one embodiment the nanoparticle's longest dimension is less than about 600 nm in length, less than about 500 nm in length, less than about 400 nm in length, less than about 300 nm in length, less than about 250 nm, in length, or less than about 200 nm in length. In some embodiments the nanoparticle has a longest dimension between about 100 nm and about 500 nm, between about 200 nm and about 400 nm, or between about 250 nm and about 250 nm. The overall shape

(spherical, rod, cylinder, etc.) as well as overall size (in any dimension) can be tuned via method of synthesis.

[0044] In certain embodiments, one of more of the fluorescent agents that may be embedded in the silica core of the nanoparticle is a dye. The fluorescent dyes can be independently selected from the group consisting of acridine dyes, benzopyrone dyes, cyanine dyes, fluorone dyes, oxazine dyes, phenanthridine dyes, rhodamine dyes, or a combination thereof. In some embodiments the fluorescent dye is one or more of Nile Blue A, Nile Red, Texas Red, Eosin Y, Sodium Fluorescein, fluoroscein 5-isothiocyanate (FITC), TRITC, Rhodamine 6G, Rhodamine B, Rhodamine WT, Alexa Fluor647, Alexa Fluor660, Alexa Fluor680, Alexa Fluor700, Alexa Fluor780, Cy5, Cy5.5, Cy7, indocyanine green (ICG), Cypate, ITCC, NIR820, NIR2, IRDye680, IRDye700, IRDye800, DiD, DiR, Cresy Vllet, Oxazine 750, Rhodamine800, AlexaFluor350, Alexa Fluor488, Alexa Fluor532, Alexa Fluor568, Alexa Fluor633, Alexa Fluor647, Cy2, Cy3, Cy3.5, Dansyl Chloride (DNS-C1), 5- (iodoacetamida)fluoroscein (5-IAF), Eu-TDPA [Tris(dibenzoylmethane) mono(5-amino phenanthroline)europium], 6-acryloyl-2-dimethylaminonaphthalene (acrylodan), 7-nitrobenzo-2- oxa-l,3,-diazol-4-yl chloride ( BD-C1), ethidium bromide, Lucifer Yellow, BODIPY™, 1- anilinonaphthalene-8-sulfonic acid (ANS), 6-(p-toluidinyl)naphthalen-e-2-sulfonic acid (TNS), Anthroyl fatty acid, DPH, Parinaric acid, TMA-DPH, Fluorenyl fatty acid, Fluorescein- phosphatidylethanolamine, Texas red-phosphatidylethanolamine, Pyrenyl-phophatidylcholine, Fluorenyl-phosphotidylcholine, Merocyanine 540, l-(3-sulfonatopropyl)-4-[-.beta.-[2[(di-n- butylamino)-6 naphthyl]vinyl]pyridinium betaine (Naphtyl Styryl), 3,3'dipropylthiadicarbocyanine (diS-C3-(5)), 4-(p-dipentyl aminostyryl)-l-methylpyridinium (di-5-ASP), IR-125, Thiazole Orange, Azure B, Al Phthalocyanine, Oxaxine 1.4, 6-diamidino-2-phenylindole (DAPI), Hoechst 33342, TOTO, Acridine Orange, Ethidium Homodimer, N(ethoxycarbonylmethyl)-6-methoxyquinolinium (MQAE), Fura-2, Calcium Green, Carboxy SNARF-6, BAPTA, coumarin, phytofluors, and Coronene. The dyes may be embedded using covalent bonding or other encapsulation techniques. In this embodiment, covalent bonding refers to chemical linkage between a dye or dyes and the silica matrix core. The nanoparticles may exhibit increased fluorescence intensity compared to free fluorescent dye as a result of the covalently-bonded dyes, resulting in a higher per-dye-molecule fluorescence signal from the particles. The increase in fluorescence intensity may be up to but not limited to being 500x or greater than the intensity of free fluorescent dye. In some embodiments the fluorescence intensity of a nanoparticle comprising a fluorescent dye covalently bonded to a silica matrix core may be increased by greater than lOOx, 200x, 300x, 400x, or 500x compared to free fluorescent dye. Encapsulation techniques may include other non-covalent linkages that encapsulate one or more dyes within the core of the matrix through confinement, or growth of the silica matrix core around contained volumes of the dye. Additionally, the fluorescent dye can be adhered to the external coating of the nanoparticle via covalent or non-covalent linkage.

[0045] One or multiple dyes can be encapsulated or embedded within a nanoparticle, conferring the ability to have one or multiple distinct absorption and emission spectra per nanoparticle type. Fluorescence emission wavelengths of the silica particles in some embodiments will be no lower than 300nm. In some embodiments, the fluorescence emission wavelengths will range from 300nm to 2000nm. In some embodiments, the fluorescence emission wavelengths encompass the visible, near-infrared, and infrared spectra. The dyes used in the silica nanoparticles exhibit fluorescence in aforementioned range of 300nm to 2000nm, and excitation of the particles will occur in this range. Below 300nm, hydrocarbons and other possible compounds in oil and gas reservoirs confound the fluorescence signal. Fluorescence emission and intensity can be measured using a spectrofluorometer, fluorometer, Raman spectrometer and / or portable fluorometer, or other means known to a person skilled in the art.

[0046] In embodiments where two or more fluorescent agents are included in a nanoparticle, the fluorescent agents used in the nanoparticles preferably have emission spectra that are distinct enough from one another to enable statistical resolution between different nanoparticle types that may have multiple fluorescent agents. In this example, a single emissions measurement will not cause any confounding of the optical signal. The distinctiveness of the label is measured by ensuring that the area under the emissions spectra of one fluorescent label will have minimal overlap

(generally less than 10%) with the area under the emissions spectra of other fluorescent labels; in the event that there is significant overlap of emission peaks, there would be confounding of the optical signal. Therefore, in other embodiments, the labels are chosen to reduce the possibility of confounding an optical signal. For example, nanoparticles made with Rhodamine WT (Maximum Emission Peak: 589 nm) will not experience crosstalk with particles using Fluorescein

Isothiocyanate (FITC) (Maximum Emission Peak: 525 nm) as their emission spectra are staggered significantly enough as to ensure minimal overlap; therefore, both are suitable nanoparticles that are differentially detectible in solution. Distances between the maximum value of the emissions peaks of the dyes can range between 30 nm and 200 nm, and the necessary distance between the peaks can be minimized if the range of emissions for the dye does not span a significant number of wavelengths. The fluorescent emissions, or labels, are distinctly detectable without the presence of crosstalk - overlap of fluorescent emission spectra leading to signal attenuation - contributing in a detrimental manner for detection. In this embodiment, multiple dyes can be encapsulated or covalently attached to the core of the particle such that the emission spectra and fluorescent intensity of each dye does not interfere with the emission spectra or fluorescent intensity of any of the other dyes. For example, FITC and Indocyanine Green may both be embedded in one embodiment of the nanoparticle. This nanoparticle would be able to have distinct fluorescent emission spectra for both dyes. Additionally, based on methods of synthesis, it may be possible to control the relative amount of each dye embedded per particle, thereby modifying the fluorescent intensity of each dye per nanoparticle and the relative fluorescent intensity between the dyes per nanoparticle.

[0047] In certain embodiments, the functional groups added to the external coating of the nanoparticle can be one or more zwitterionic compounds. In some embodiments zwitterionic compounds are amphoteric over a variable pH range including but not limited to 2 through 11.

[0048] The zwitterionic functional groups may be selected from the group consisting of:

sulfobetaines, phosphatidylcholines, ammonium betaines, carboxybetaines, imidazolium salts, pyridinium salts, piperidinium salts, phosphonate salts, or a combination thereof. Without being bound by theory, zwitterionic nanoparticles can confer particle stability by having net neutral electrostatic charge, by minimizing the charge shielding effect, and by experiencing minimal ion exchange with other cations and anions in their local conditions. In certain embodiments, a nanoparticle comprising one or more zwitterionic functional groups can maintain stability in salient concentrations up to but not limited to 100,000 ppm, 200,000 ppm, or 300,000 ppm, whereby the saline concentrations can include monovalent and divalent ions, for example Ca ²⁺ , Mg ²⁺ , S0 ₄ ^2" , K ⁺ , CI ^" and Na ⁺ . Additionally, the inclusion of zwitterionic functional groups may maintain nanoparticle stability in temperatures up to but not limited to, 200 °C, 300 °C, or 400 °C, and pressures up to but not limited to, 12,000 psi, 15,000 psi, or 20,000 psi. In this embodiment, stability again refers to colloidal stability as well as mechanical stability, or the ability of the nanoparticle to maintain the conformation of its core and external coating in the presence of extreme environmental conditions. In this example, stability can be measured by determining the fluorescent emission spectra and quantifying the fluorescent intensity both before and after exposure to environmental conditions - emission spectra with minimal shift and decrease in intensity (generally less than 10%) are viewed as being stable after exposure to the appropriate environmental condition. In certain embodiments, the zwitterionic functional group is adhered to the external coating of the nanoparticle through hydrolysis with a silane precursor, forming a zwitterion silane. In one embodiment, the zwitterion silane can be 3 -(dimethyl(3-(trimethoxysilyl)propyl)ammonio)propane-l -sulfonate.

[0049] Fig. 1 provides a schematic of a nanoparticle in accordance with an embodiment of the invention. Referring to Fig. 1, the core of the nanoparticle comprises a silica structure 01 and one or more fluorescent agents 02. The fluorescent agents 02 can be incorporated into the silica structure 01 during synthesis through, for example, encapsulation or covalent bonding. Polymeric zwitterions 03 are adhered to the surface of the silica nanoparticle.

[0050] In certain embodiments, the nanoparticles have quantifiable adsorption to a subterranean formation or reservoir. Based on the size distribution of the nanoparticles and coupled with data regarding pore and fluid channel size of the reservoir, it is possible to determine any potential adsorption of the nanoparticle to the subterranean formation. Adsorption can be quantified by calculating a mass balance of nanoparticle concentration both before and after addition to the rock formation. Additionally, in certain embodiments, the intensity of the nanoparticle fluorescence is minimally reduced (generally less than 10%) by exposure of the nanoparticles to high temperature, high pressure, variable pH, carbonate groups, shale rock, surfactants, friction reducers, slickwater formulations, acids for use in acid fracturing, brine solution, produced water, emulsifiers, or a combination thereof.

[0051] NANOP ARTICLE SYNTHESIS

[0052] In one embodiment, the nanoparticles are synthesized via a micro-emulsion method that forms an emulsion with one or more silica precursors, one or more fluorescent agents, one or more surfactants, and one or more hydrophobic catalysts / structure - directing agents. In this

embodiment, the surfactant and hydrophobic catalyst help to create the emulsion to encapsulate and / or covalently embed the fluorescent agents, whereby the silica precursors can then be condensed to form the silica core encompassing these fluorescent agents with small channels or pores.

Afterwards, the emulsion can be broken or disrupted, enabling the separation of the newly formed nanoparticles, and the nanoparticle template including the hydrophobic catalyst, surfactant, and unreacted silica precursor can be removed. The remaining nanoparticles can be stabilized in aqueous solution by the external coating of silane groups that form on the outside of the silica core as part of the silica precursor condensation. The silane groups, forming the external coating of the

nanoparticle, can then be chemically modified by attaching new chemical functional groups, such as zwitterionic functional groups, on the external coating of the nanoparticle.

[0053] In one embodiment of the synthesis, one or more silica precursors can be independently selected from the group consisting of: tri- and tetra-alkoxysilanes such as tetramethoxysilane (TMOS), tetraethoxysilane (MTMS), methyltriethoxysilane (MTES), ethyltriethoxysilane (ETES), octyltriethoxysilane (OTES), octyltrimethoxysilane (OTMS), hexadecyltrimethoxysilane (HDTMS) and hexadecyltriethoxysilane (HDTES), octadecyltrimethoxysilane (ODTMS),

octadecyltriethoxyisilane (ODTES) as well as methyl polysilicate (MPS), ethyl polysilicate (EPS), polydiethoxysilane (PDES), hexamethyl disilicate, hexaethyl disilicate or functional trialkoxysilanes (methacryloyloxypropyltrimethoxysilane, phenyltriethoxysilane (PTES), phenyltrimethoxysilane (PTMS), glycidoxypropoxyltrimethoxysilane (GLYMO), glycidoxypropyltriethoxysilane (GLYEO), mercaptopropyltriethoxysilane (MPTES), mercaptopropyltrimethoxysilane (MPTMS),

aminopropyltrimethoxy silane (APTMS), aminopropyltriethoxy silane (APTES), 3-(2- aminoethylamino)propyltrimethoxysilane (DATMS), 3-[2-(2- aminoethylamino)ethylamino]propyltrimethoxysilane (TATMS), [2- (cyclohexenyl)ethyl]triethoxy silane (CHEETES), vinyltrimethoxy silane (VTMS),

vinyltriethoxysilane (VTES), or a combination thereof.

[0054] In certain embodiments of the synthesis, the silica precursors that form the core of the nanoparticle can be added through a co-condensation process (see example 2). In a co-condensation process, two or more silica precursors can be added to the reaction mix in varying molar ratios in order to more closely control the chemical formation of the silica matrix and enhance nanoparticle stability. Additionally, in other embodiments, the silica precursors of the nanoparticle can be added sequentially with varying time increments to the reaction mix in the process of sequential grafting (see example 3). Like co-condensation, sequential grafting may allow for close control of the silica core formation as well as impart different structural, chemical, and physical properties of the nanoparticle (e.g. size, shape, surface chemistry, hydrophilicity, hydrophobicity, electrical charge, or mechanical stability)

[0055] In some embodiments, prior to creation of the water-in-oil emulsion, one or more fluorescent agents may be conjugated to the silica precursor(s). Thereafter, the water-in-oil emulsion can be facilitated through the addition of one or more surfactants, which can be independently selected from the group consisting of: anionic surfactants (comprising carboxylates, sulfonates, petroleum sulfonates, alkylbenzenesulfonates, naphthalene sulfonates, olefin sulfonates, alkyl sulfates, sodium lauryl sulfate, sodium dodecyl sulfate, sulfates, sulfated natural oils & fats, sulfated esters, sulfated alkanolamides, alkylphenols, ethoxylated & sulfated), cationic surfactants

(comprising quaternary ammonium salts - including but not limited to alkyltrimethylammonium salts: cetyl trimethylammonium bromide (CTAB), cetyl trimethylammonium chloride (CTAC), cetylpyridinium chloride (CPC), benzalkonium chloride (BAC), benzethonium chloride (BZT), 5- Bromo-5-nitro-l,3-dioxane, dimethyldioctadecylammonium chloride, cetrimonium bromide, dioctadecyldimethylammonium bromide (DODAB), amines with amide linkages, polyoxyethylene alkyl & alicyclic amines, η,η,η',η' tetrakis substituted ethylenediamines, 2- alkyl 1- hydroxethyl 2- imidazolines), nonionic surfactants (comprising ethoxylated aliphatic alcohol, polyoxyethylene surfactant, carboxylic esters, polyethylene glycol esters, anhydrosorbitol ester & associated ethoxylated derivatives, glycol esters of fatty acids, carboxylic amides, monoalkanolamine condensates, polyoxyethylene fatty acid amides), amphoteric surfactants or a combination thereof.

[0056] In certain embodiments, one or more hydrophobic catalysts or structure-directing agents are added to chemically stabilize the emulsion. These hydrophobic compounds can be selected from a group consisting of alkanolamines such as ethanolamine, diethanolamine, triethanolamine, tetramethylammonium hydroxide, ammonium hydroxide, N-methylglucamine, polyalkylene oxide homo- and copolymers, fatty alcohols, metal alkoxides or a combination thereof. After creation of the water-in-oil emulsion, there are multiple methods to break the emulsion. In one embodiment, the emulsion can be broken via dilution with either excess hydrophilic or hydrophobic solvent to remove the stability of the emulsion, while in another embodiment, the emulsion can be broken via variation of the reaction pH to change the protonation of individual chemical compounds in the reaction mix (i.e. surfactant & hydrophobic catalysts) which contain varying pKa values. Disruption of the emulsion is preferably performed after the silica precursors have condensed to form the core matrix and external coating of the nanoparticle.

[0057] In certain embodiments, the nanoparticle template materials (unreacted silica precursor, hydrophobic catalyst, and surfactant) can be removed via dialysis, e.g. with a 10,000 Da membrane, or aforementioned dilution. In one embodiment, nanoparticle template extraction can be performed by adding a mixture of acid or an acidic salt with an alcohol solvent (such as ethanol or methanol). The extraction mixture can be added to the reaction mixture and washed and centrifuged (e.g. at 10,000 rpm for 3 rounds) to ensure appropriate removal of nanoparticle template materials. The nanoparticle external coating, initially including silane groups from the previously mentioned embodiment of the synthesis, can be modified to adhere zwitterionic functional groups to the external coating via covalent linkage between the silane group and a zwitterionic functional group, (see example 4).

[0058] In one embodiment, the nanoparticles may remain suspended in one phase (either hydrophobic or hydrophilic, depending on functional groups on external coating). In other embodiments, the functional groups of the external coating can be selected to provide the nanoparticle with greater solubility in the opposite phase of the nanoparticle before addition of zwitterionic functional groups, (e.g. a hydrophobic nanoparticle can become a relatively more hydrophilic particle and vice versa). In some embodiments this may be achieved through the addition of functional groups comprising benzyl alcohols, fatty acyl chains, and long-chain polymers. In this embodiment, the nanoparticle with increased solubility for the opposing phase can serve as a partitioning tracer that does not remain fully suspended in its original phase and can switch to the other phase under certain environmental conditions. These conditions can include high temperature, high pressure, changes in local concentrations of brine solution, and more.

[0059] In certain embodiments, the nanoparticle can be dispersed in any hydrophilic or hydrophobic solvents. For example, the previously mentioned zwitterionic groups, which can be selected from the group of: sulfobetaines, phosphatidylcholines, ammonium betaines,

carboxybetaines, imidazolium salts, pyridinium salts, piperidinium salts, phosphonate salts, or a combination thereof, would confer nanoparticle stability in hydrophilic solvents. In other embodiments, the external coating of the nanoparticle may be functionalized with hydrophobic functional groups that would confer nanoparticle stability in any hydrophobic solvents. [0060] NANOP ARTICLE SYNTHESIS EXAMPLES

[0061] Certain aspects of the present invention may be better understood as illustrated by the following examples, which are meant by way of illustration and are not meant to be limiting.

[0062] Example 1 : Silica Nanoparticle (SNP) Micro-emulsion

[0063] In embodiments of this invention, the silica nanoparticles can range in size from tens of nanometers to tens of microns. The size of the nanoparticle can be regulated by the use of a hydrophobic catalyst / structure - directing agent and correspondingly, the size of the water-in-oil emulsions generated by the solution containing a surfactant. One nanoparticle formation process includes the following steps: providing an emulsion comprising droplets of a hydrophilic material such as water dispersed in a continuous hydrophobic phase such as cyclohexane, wherein the droplets are solubilized, forming reverse micelles in a micro-emulsion; introducing a silica precursor to the stirring emulsion for growth; and breaking the emulsion after the reaction. One possible overall molar ratio composition may be: 8 triethanolamine (TEA) : 130 H ₂ 0 : 0.25 CTAC/CTAB: 0.01- 0.04 fluorescent dye: 1 silanes. In some embodiments, the molar ratio may be varied from 1 - 10 for triethanolamine (or the hydrophobic catalyst) : 50 through 500 for H ₂ 0 : 0.01 through 5 for CTAC/CTAB or any other surfactant used : 0.0001 through 1 for the fluorescent dye : 0.05 through 5 for silanes.

[0064] In one embodiment of the nanoparticle synthesis, the synthesis may include two initial reaction mixtures: (1) silica precursor, or silane, and hydrophobic catalyst mix and (2) surfactant and dye mix in aqueous solution. Reaction mixture 1 may include the addition of a hydrophobic catalyst like triethanolamine (TEA) added to silica precursor, e.g. tetraethylorthosilane (TEOS), in a 1 : 10 molar ratio. This molar ratio may range from 1 : 1 to 1 : 1000. In one embodiment, the silane may be added during the mixture formation. In another embodiment, the silane may be added after mixture formation, instead added during the combination of both reaction mixtures for micro-emulsion formation. Reaction mixture 1 may then be heated (e.g. to about 90 °C) and stirred (e.g. for aboutl hour). The temperature for the heated reaction mixture 1 may range from 40 °C to 140 °C and the stirring time can range from 30 minutes to 24 hours.

[0065] Reaction mixture 2 may include surfactant, dye, and water in following molar ratio - 0.2 cetyltrimethylammonium chloride (CTAC) : 0.03 dye : 140 H ₂ 0. In this example, the molar ratio may be varied from 0.01 to 10 for CTAC, varied from 0.001 to 5 for the dye and 50 to 500 for H ₂ 0. Reaction mixture 2 may then be heated (e.g. to 60°C) and stirred (e.g. for one hour). The

temperature for the heated reaction mixture 2 may range from 40 °C to 140 °C and the stirring time can range from 30 minutes to 24 hours. [0066] Reaction mixtures 1 and 2 may then be combined and stirred vigorously (e.g. for 5 hours) to create a water-in-oil micro-emulsion and induce reverse micelle formation, leading to the growth of the nanoparticles. The time of stirring may range from 1 hour to 48 hours. During this time, one or more silanes may be added to the micro-emulsion mix, for example via the processes of co- condensation or sequential grafting (see examples 2 and 3).

[0067] Example 2: S P Co-Condensation

[0068] Additional silanes, if any, can co-condensate with the TEOS, or pertinent silica precursor, if any, in the micro-emulsion via addition to the micro-emulsion at the beginning (time t = 0) of micro-emulsion formation (or mixing of the two reaction mixtures mentioned in Example 1) to modify the silica condensation process during formation of silica nanoparticles. In one example, three additional silane precursors could be added to the reaction mixture with the following possible molar ratio: 2: Tetramethylorthosilane (TMOS), 1 : Methyltrimethoxysilane (MTMS), 1 :

Phenyltriethoxysilane (PTES). In this example, the cumulative sum of the three additional silane precursors can have a 4:6 molar ratio with respect to the original silane, TEOS. The relative percentages of any silane precursor can range from 0% (not included in the mix) to 100% (only one included in the mix). In this example, TEOS was 60%, TMOS was 20%, MTMS was 10% and PTES was 10%.

[0069] Example 3 : SNP Sequential Grafting

[0070] Additional silanes, if any, can be sequentially grafted with the TEOS, or pertinent silica precursor, if any, in the micro-emulsion via addition to the micro-emulsion at varying times after micro-emulsion formation and initial silica condensation begins. In one example, three additional silane precursors could be added to the reaction mixture with the following possible molar ratio: 2: Tetramethylorthosilane (TMOS), 1 : Methyltrimethoxysilane (MTMS), 1 : Phenyltriethoxysilane (PTES). Additionally, the MTMS and PTES may be added to the mixture immediately; after another 30 minutes of stirring, the TMOS may be added to the mixture. The time between sequential addition of precursors may range from 0 minutes to 6 hours. Furthermore, any combination of the chosen silane precursors may be added at the beginning, middle, or end of the sequential addition of precursors. The relative ratios of each precursor can range from 0% (not included in the mix) to 100% (only one included in the mix). In this example, TEOS was 60%, TMOS was 20%, MTMS was 10% and PTES was 10%.

[0071] Example 4: Zwitterion Coating

[0072] In an embodiment, 1,3-propane sultone is conjugated with APTES or APTMS under oxygen free conditions. In this example, the ratio of 1,3-propane sultone to APTMS is 0.072: 1. The resulting monomers are then dissolved in water and are heated in the presence of silica nanoparticles, after which they are conjugated to the external coating of the nanoparticles. The zwitterion polymer coats the silica nanoparticles according to the concentration of the polymer in solution, the ratio of silane precursor to zwitterion, and the concentration of silica nanoparticles in the final synthesis solution. In this example, the ratio of silane precursor to zwitterion is 1 : 0.05. This ratio can range from 1 : 5 through 1 : 0.001, depending on the reactivity of the chemical functional group with the silane functional group on the external coating of the nanoparticle.

[0073] NANOPARTICLE USES

[0074] One embodiment relates to the process of using one or multiple sets of nanoparticles, also referred to as nanoparticle tracers, in the following arrangement: firstly, injecting one or more nanoparticles underground to one or more discrete and unique subterranean regions; secondly, detecting the nanoparticles as they flow back out of the subterranean regions in produced fluids. In some embodiments the nanoparticles may be detected by detecting a fluorescence emitted by the nanoparticles. In some embodiments a process may include deriving a nanoparticle concentration from an intensity of the detected fluorescence. In some embodiments a process may further include measuring a time elapsed between injecting the nanoparticles and detecting the nanoparticles.

[0075] During the recovery of hydrocarbons with treatments including hydraulic fracturing and enhanced oil recovery, in addition to other secondary and tertiary recovery methods, underground regions can be differentiated from each other due to the type of treatment. For example, hydraulic fracturing occurs in stages, each with its own perforations and high pressure fluid treatment. Similar formation separations are present in enhanced oil recovery and other secondary and tertiary recovery methods, where regions are separated due to geological features or the presence of an injector and / or producer well, for example. In some embodiments of this invention, one or more different nanoparticles can be injected in one or more discrete subterranean regions during these hydrocarbon recovery processes. In some embodiments a first nanoparticle including a first fluorescent agent is injected at a first discrete subterranean region and a second nanoparticle including a second fluorescent agent is injected at a second discrete subterranean region, such that the first and second nanoparticles have differentiated fluorescence emission spectra. Any number of nanoparticles with differing fluorescence emission spectra can be injected into any number of subterranean regions. The nanoparticles can be detected in produced water, which is made of both the injected fluids used during hydrocarbon recovery and the natural formation fluid, i.e. the fluid that was already in the subterranean reservoir that is released as a result of the treatment. [0076] In most embodiments, multiple different varieties of tracer nanoparticles are optically distinct enough from each other as a result of the inclusion of different fluorescent agents in the nanoparticle structure, which has an effect on the nanoparticle' s fluorescent emissions.

Nanoparticles comprising multiple fluorescent agents can also result in even more optically distinct nanoparticles. For example, a first nanoparticle with two fluorescent agents has the ability to carry an optical signature that is distinct from two separate nanoparticles, each with one of the two fluorescent agents in the first nanoparticle.

[0077] Fig. 2 provides a schematic of a method of collecting data on one or more subterranean regions in accordance with an embodiment of the invention. After the nanoparticles 11, 12, and 13 are injected into the wellbore 15 during a hydrocarbon recovery process, the nanoparticles 11, 12, and 13 flow through the wellbore 15 to the fractures in the subterranean formation 14. The surface of a wellbore 15 features a fluorescence detection mechanism 10, which outputs fluorescence emissions data based on the flow of fluorescently distinct nanoparticles 11, 12, and 13. The fluorescence detection mechanism 10 converts fluorescence data to concentration of nanoparticles in the produced fluid.

[0078] In some embodiments of the invention, injecting the nanoparticles into a wellbore includes the placement of the nanoparticles into an aqueous suspension. This aqueous suspension can comprise a solution of nanoparticles in an alcohol, e.g. ethanol, a solution of nanoparticles in water, or a solution of nanoparticles in any polar solvent. In some embodiments, when the nanoparticles are delivered to the wellbore for injection into the subterranean formation, they are mixed with drilling fluids at the wellsite. Such drilling fluids can include but are not limited to: slickwater formulations, proppant formulations, acid fracturing formulations, or emulsification formulations. The nanoparticle can be added to the one or more drilling fluids with a volume ratio of nanoparticles to total drilling fluid of 1 : 1, or 1 :2, or 1 :4, or 1 : 10, or 1 :20, or 1 : 100, or 1 : 1000, or 1 : 100,000, or 1 : 1,000,000, or 1 : 10,000,000. In some embodiments, prior to injection into the subterranean formation, the nanoparticles can be dispersed in additional fluids, also referred to herein as dispersal solutions, which can include but are not limited to: polymer-stabilized solutions, solutions containing nonionic surfactants, solutions containing anionic surfactants, or solutions containing cationic surfactants. The nanoparticle can be added to the dispersal solutions with a volume ratio of nanoparticles to dispersal solution of 1 : 1, or 1 :2, or 1 :4, or 1 : 10, or 1 :20, or 1 : 100, or 1 : 1000, or 1 : 100,000, or 1 : 1,000,000, or 1 : 10,000,000. The dispersal solutions enable the nanoparticles to be homogeneously mixed prior to addition with drilling fluid and subsequent injection during hydrocarbon recovery techniques. For example, the nanoparticles can be dispersed evenly throughout fluids used in hydraulic fracturing for a specific stage to ensure that the tracer is evenly distributed throughout the subterranean formation.

[0079] In some embodiments, the nanoparticles are synthesized to act as passive, also known as conservative, tracers, that remain in the water phase and do not partition into the oil phase. In one embodiment, chemical group functionalization is used to modify the zeta potential of the particles in aqueous solution. Specifically, the surface of the tracer particles can be functionalized with one or multiple charge-stabilizing polymers, hydrophilic ligands, and / or zwitterionic compounds that confer colloidal stability to the tracer particles within the injected fluid. The use of these stabilizing polymers enables the particles to stay in the water phase, even in the presence of differing environmental changes such as but not limited to fluctuations in temperature, pressure, pH, and salinity. In some embodiments, these passive tracers can be used to specifically track water or aqueous fluid flow throughout one or many subterranean regions, without undergoing any conformational change, chemical change, or phase change throughout the lifetime of the

hydrocarbon recovery treatment.

[0080] In most embodiments, the presence (including concentration) or absence of the nanoparticles in produced fluid is determined through fluorescence measurements, including fluorescent spectral measurements and single emission wavelength measurements. The fluorescence measurements can be taken in the range between 300 nm and 2000 nm, corresponding to the range from blue visible light to near-infrared and infrared electromagnetic spectra. The detection of the nanoparticles can occur in the produced fluid through stimulation of the produced fluid with excitation and corresponding detection of fluorescence emissions.

[0081] Optical detection of the particles can occur either or both downhole and on the surface near the wellbore using devices that read fluorescence measurements, through the mechanism of excitation and emission spectral measurements. For downhole measurements, downhole fluorescent spectroscopy tools can be used and are currently in development by several oilfield service companies. For surface-level detection, portable fluorescent spectrometers, modified Raman spectrometers, or photometers, for example, could be utilized. In this arrangement, there are many possibilities for unique identification of the particles. One method would be to excite the sample of injection fluid with nanoparticle tracer at a characteristic wavelength and detect specific emission wavelengths via a photometer. Another method would be to multiplex and excite the sample over two or more distinct excitation wavelengths to similarly detect emission wavelengths of the sample. Additionally, another arrangement would be to excite the sample at a characteristic wavelength and record emission wavelengths over an entire range over the spectrum. In most embodiments of fluorescent detection, the fluorescent devices can take produced fluid - which can include a mixture of injected and formation fluid - and divert the flow in real time for continuous sampling and detection.

[0082] In some embodiments of the invention, the data that is generated is data regarding the absence, presence, and concentration of nanoparticle tracers in produced solution. This data is compiled using measurements of the fluorescence intensity of the nanoparticle in the produced fluid and correlating the fluorescence measurements with calculated statistical models that convert the intensity of fluorescence to concentration of the nanoparticle. Concentration models can be calibrated using stock solutions of known concentrations of nanoparticles in fluid, but the mechanism of creating the models correlating fluorescence to concentration are not limited to this method of calibration. By combining this data with data regarding time at which nanoparticle tracer fluorescence is detected, multimodal, useful data for some or all possible tracer applications can be achieved. Data can be collected in real-time using the fluorescence measurement devices that are present either in the wellbore or on the surface at the wellsite.

[0083] In some embodiments of the invention, the detection limit of the fluorescence devices and their correlated nanoparticle concentration in produced fluid is less than 1 part per trillion, or less than 1 part per billion, or less than 1 part per million, or less than 1 part per thousand. In one embodiment of the invention, the data points of fluorescence intensity are collected at a rate of 1 point per time, where the sampling time can range from 0.1 and 30 seconds. This rate of detection represents a near-continuous data collection mechanism, enabling real-time concentration calculations for the nanoparticle tracers.

[0084] In one embodiment of the invention, the nanoparticles can be used during hydraulic fracturing operations of an oil or natural gas reservoir. In this application, a plurality of nanoparticle tracers may be delivered to a single fracture site or a plurality of nanoparticle tracers may be delivered to different fracture sites stemming from a single wellbore. A possibility within this invention is to deliver one or multiple nanoparticle tracers at any of the pre-treatment, treatment, and post-treatment stages. To elaborate, there are many opportunities to use a nanoparticle tracer in the course of hydraulic fracturing operations: the particles can be used before a fracturing treatment and can mix with the formation fluid; the particles can be used during the fracturing treatment and will mix with injected fluid as well as the formation fluid; the particles can be used after fracturing has occurred.

[0085] In some embodiments of the invention, the fluorescent agents, which may be embedded or covalently attached to the core of the nanoparticles, can feature a single peak wavelength of emission, or can feature more than one wavelength emissions peak if two or more varied fluorescent agents are used. In some embodiments of this invention, there are additional combinations of fluorescent agents that can result in differentiated fluorescence emissions based on the relative concentration of each fluorescent agent in the particle. For example, in a nanoparticle containing two or more fluorescent agents, the ratio of the amount of each fluorescent agent can result in a differentiated particle that can be distinguished from one or multiple nanoparticles containing simply one of the fluorescent agents used in the nanoparticle containing two or more fluorescent agents. These particles and the configuration of multiple fluorescent agents allow for an increase in the number of discrete nanoparticles, allowing for larger operations involving increased numbers of subterranean regions that are treated with nanoparticle tracers.

[0086] In one embodiment, a nanoparticle can be used to identify a particular attribute of the reservoir formation through the calculation of the concentration of the nanoparticle in the produced fluid. In embodiments where two or more types of nanoparticles are used, each nanoparticle may be optically distinct from the other. For example, in some embodiments of the invention, nanoparticle tracers can be utilized to obtain statistics informing on production including binary data informing on breakthrough (i.e. efficacy of the fracturing treatment in one singular stage or subterranean region of the reservoir). This data can be non stage-specific data, with tracers placed in singular select stages of the overall horizontal well; due to the mixing of fluids between the stages, this data may not maintain stage specificity and the tracer concentration would potentially be less important than simply the presence or absence of the tracer in the produced fluid.

[0087] In some embodiments of the invention, the particles can provide data and statistics in a stage-specific manner as a result of the injection of two or more fluorescently distinct tracers in different stages. These embodiments utilize two or more of these optically distinct particles for determination of additional reservoir attributes, and the data on the particle concentration may be analog data. This data can generate statistics including but not limited to breakthrough, subterranean permeability, fracture treatment efficacy, and projected hydrocarbon production. Statistics on breakthrough are representative of the distance in which the fractures permeate the formation, and tracer concentrations calculated over time are very representative of the distance of these propagations relative to breakthrough from other stages. Permeability values may be obtained through the combination of the tracer data with known pressure data and subterranean formation characteristics, including but not limited to porosity, rock type and stress qualities, and other rock qualities. Fracture treatment efficacy may be calculated via tracer concentration and the flowback time of the tracer in the produced fluid; the quicker the tracer flowback relative to other tracers, the less effective the treatment in the stage featuring the fast-flowing tracer. Projected hydrocarbon production may be a longer-term study of the tracers as they elute from the reservoir in a steady- state fashion. Concentration of the tracer as it flows out of the formation throughout the fracturing process, and concentration of the tracer as it flows out of the formation after the fracturing process, can be used to determine projected production values over the course of 1 week, or 1 month, or 2 months, or 3 months, or 6 months, or 1 year.

[0088] In another embodiment of this invention, nanoparticles can be injected into different wells across a pad to characterize lateral breakthrough. In this application, nanoparticles can be injected into offset wells as well as adjoining wells during zipper fractures. The real-time absence or presence of tracer recovery can inform operations for lateral well placement and production.

Statistics generated from mass tracer recovery and timescale recovery can be used to characterize lateral well connectivity and estimated production lifetime.

[0089] In another embodiment of this invention, nanoparticles can be sampled up to 1 week, or 1 month, or 2 months, or 3 months, or 6 months, or 1 year, or 2 years, after initial well production and characterization. Mass recovery of tracer coupled with timescale from this longer-term testing can inform on absence or presence of breakthrough and if applicable, breakthrough rates of the subterranean formation.

[0090] In another embodiment of this invention, nanoparticles can be injected up to 1 week, or 1 month, or 2 months, or 3 months, or 6 months, or 1 year, or 2 years, after initial well production and characterization. Mass recovery of tracer coupled with timescale from this longer-term testing can inform on well breakthrough rates, and well and formation integrity.

[0091] In some embodiments of the invention, the polymer functional groups on the silica structure can be cross-linked with chemical functional groups, molecular additives, or other polymeric groups, rendering the particles with a new surface coating. This surface coating can alter the fluid flow characteristics of the nanoparticles in hydrophilic or hydrophobic media. These embodiments utilize two or more of these optically distinct particles for characterization of chemicals or molecular additives used during hydrocarbon production. Differential mass recovery concentrations of the tracers as well as the timescales of tracer recovery can be used to determine efficacy of chemicals or molecular additives used during production operations.

[0092] In some embodiments of the invention, the size of the silica nanoparticle can be increased to match the diameter of proppants used during fracturing operations. These embodiments can include nanoparticles with sizes ranging up to 500 nm in diameter. At these increased sizes, nanoparticles of these embodiments can have fluid flow characteristics that match the fluid flow characteristics of select proppant additives. These embodiments utilize two or more optically distinct particles for characterization of proppant proliferation during fracturing operations. Differential mass recovery concentrations of the tracers as well as the timescales of tracer recovery can be used to determine proppant proliferation between relative sub-stages of fracturing operations and characterize proppant mobility, proppant flowback from discrete stages, and proppant stability.

[0093] In some embodiments of the invention, two or more optically different nanoparticles can be used to evaluate the efficacy of tools used during fracturing operations. In this embodiment, differential tracer recovery and timescale would enable generation of statistics to evaluate relative sub-stage / stage-specific production associated with specific tools. These tools can include diverters, frac plugs, reamers, and cutters. In one embodiment, two or more optically different nanoparticles can be used to evaluate the efficacy of a diverter used during fracturing operations. Increased or decreased tracer recovery from one stage relative to another can inform on tracer proliferation through the subterranean formation and may indicate whether the diverter was effectively deployed during an operation. In another embodiment of the invention, two or more optically different nanoparticles can be used to evaluate the efficacy of a firac plug used during fracturing operations. The binary absence or presence of recovery from one stage outfitted with a frac plug - compared both before and after tracer injection - may indicate whether a firac plug was effectively deployed during an operation. Information indicating frac plug efficacy can be used to inform on cleanout operations.

[0094] Additional statistics that can be generated from the nanoparticles include but are not limited to: overall formation permeability, calculated distance of perforations prior to the fracture treatment, extent of fracturing into the formation, fracture diameters for discrete stages, differential pressure and differential temperature between subterranean regions and intra-subterranean regions, and fluid wetting properties.

[0095] The data obtained from the absence, presence and / or concentration of tracer

nanoparticles, in conjunction with other data such as chemical kinetics of the nanoparticles, diffusion of the nanoparticles into the reservoir, sorption of the nanoparticles to the subterranean formation, seismic measurements of the formation, environmental pressure, and environmental temperature of the subterranean formation can be used to determine fracture geometry as well as other statistics such as near-wellbore fracture-surface area, inter-well fracture-surface area, and fracture volume.

[0096] In addition to aforementioned passive, or single-phase, tracers that remain colloidal in the water phase, modification of chemical group functionalization to the external coating of the nanoparticles can allow for nanoparticles to serve as partitioning tracers, meaning they will selectively phase from the aqueous to the oil phase. In one embodiment of the invention, these particles make use of a varied functional group or multiple varied functional groups that can be more hydrophobic than the functional groups used in passive tracers.

[0097] In one embodiment, the partitioning ability of the aforementioned nanoparticle tracers confers higher oil-phase solubility relative to single-phase nanoparticle tracers which enables their selective emulsion into oil. In this embodiment, the nanoparticles may be stable and colloidal in an aqueous solution given an absence of oil, but upon a change in the water to oil ratio in the fluid to a specific amount of oil in water, the nanoparticle may phase into the oil phase and remain stable in that phase.

[0098] Additionally, the nanoparticles can function as active tracers that can undergo a conformational change under including but not limited to the following conditions: a specific environmental temperature, or a specific environmental pressure, a specific concentration of ions and salinity, or a specific oil-to-water ratio in the formation. In this arrangement, the tracer particles can be functionalized with chemical groups that confer particle miscibility in both phases and increase retention rates of tracer nanoparticles in the reservoir matrix. These functional groups can be adhered to the external coating of the nanoparticle using the same mechanism as the original zwitterionic polymer groups, but have a different function when the particles undergo environmental changes. Examples of environmental changes causing a conformational change in the active tracer can include but are not limited to: variations in the proppant type and concentration of the proppant, variations in formation characteristics such as but not limited to changes in the fracture geometry and pore size of the subterranean rock, presence, absence, or concentration of fluid wetting agents, and variations in the surface area of the formation that comes into contact with the nanoparticles.

[0099] In one embodiment of this invention, passive tracer nanoparticles can be mixed with a stimulating fluid and injected at different fracture sites, or subterranean regions, during plug-and- perf operations. After completion and after the removal of firac plugs, the concentration of the nanoparticles can be determined in the produced water, giving statistics in a stage-by-stage manner regarding the success, failure, or varied degree of success of the plug and perf operation.

Determining the success of the operation can include but is not limited to determining the extent and distance of the perforations, the extent and estimated distance of the fracturing channels, and determination of the amount of proppant remaining in a specific stage.

[00100] In another possible example of this application, passive and partitioning tracers previously discussed can be combined into the same stage, which allows for the generation of different statistics through the calculation of their concentration in the flowback fluid. These statistics can include the determination of residual oil saturation in one or multiple subterranean regions. This is achieved by differentially detecting the concentration of the two tracers as they flow in the produced fluid as well as the calculation of the time delay between their arrival at the detection device. The relative flow rates, breakthrough volumes, differentiated concentrations, and time delay dilation will enable the generation of statistics on different residual oil saturations of the specific stage that was tested.

[00101] In another possible example of this application, passive and active tracers previously discussed can be combined into the same stage, which allows for alternate statistics to be calculated using a similar concentration and time delay calculation. The statistics generated by this type of test can include but are not limited to the differential pressure in the wellbore versus the fracturing channels, the stability of the proppant in the formation, differential temperatures that may result in differences in hydrocarbon chain length in the produced hydrocarbons, and calculated fluid wetting properties.

[00102] In another embodiment of the invention, the nanoparticles can also be used in two or more separate horizontal wells located in a similar geographical region to test or confirm

breakthrough between them.

[00103] In another embodiment of the invention, the nanoparticles can be used during other stimulation techniques, in addition to hydraulic fracturing, for oil & gas reservoir production including but not limited to water-flooding, gasification, and matrix acidization. During enhanced oil recovery operations, techniques such as waterflooding and gasification make use of additional stimulants to increase fluid movement in the subterranean region or regions of choice, in an effort to increase hydrocarbon production. Nanoparticle tracers can be used to test and determine the efficacy of these treatments. For example, nanoparticles injected in aqueous solution, during a waterflooding operation in an oilfield, can be detected at the producer wellbore after the nanoparticles have moved throughout the reservoir.

[00104] In one embodiment, the use of passive nanoparticle tracers in an interwell tracer test (IWTT) to flow precisely with the water front during a waterflooding enhanced oil recovery technique can produce statistics including but not limited to: the time between water injection and water breakthrough in the production well, and fluid velocities in differential areas in a subterranean reservoir. In another embodiment, the use of passive nanoparticle tracers in a single well tracer test (SWTT) can produce statistics regarding the movement of fluid in a wellbore. In another embodiment, the use of partitioning tracers in an IWTT can produce statistics regarding residual oil saturation in differentiated areas in an underground oil reservoir. As the partitioning tracer flows from an injection well to the reservoir containing hydrophobic groups including oil, the partitioning tracer can experience phase changes. When the particle concentrations are detected at the wellbore, the ratio of particles in oil to particles in the aqueous phase can determine statistics regarding the ongoing operation of the enhanced oil recovery technique.

[00105] In some embodiments, the tracer nanoparticles can adhere to the surface of a proppant particle, including but not limited to: sands, coated sands and other silicates, ceramics, other structurally-ordered particles, and other injected particulates. These proppant particles can range in size from 200nm to 20mm, or from 200nm to 2mm. In this embodiment, the nanoparticle tracers can be used to determine the effectiveness of a proppant in one or multiple stages during hydraulic fracturing or enhanced oil recovery. Additionally, the nanoparticle tracers can be used to determine the flow of specific proppants in one or multiple subterranean regions. In one embodiment, the tracer nanoparticles are covalently bonded to the surface of the proppant particle. In another embodiment, the tracer nanoparticles can adhere to the surface of proppant particle but are effectively released from the surface of the proppant particles in the presence of an environmental change, including but not limited to: changes in temperature, changes in pressure, changes in pH, changes in salinity, changes as a result of the increase of concentration of a particulate such as an ion, and changes in fluid velocity.

[00106] In another embodiment, the nanoparticles can be used to trace water or brine reservoirs used in power generation in enhanced geothermal systems (EGS) plants. EGS operations rely on the flow of fluid deep underground around formations, including hot dry rocks (HDR), that will heat the fluid. As the fluid moves from an injector well and towards a producer well, it heats up, and power is generated through the thermodynamic difference in the injected and produced fluids. The tracer nanoparticles can be used to track the flow of fluid from the injector well to the producer well, which can generate statistics including but not limited to: fluid flow rates, percentage of fluid recovered, efficacy of producer well placement, estimated fluid recovery over time, fluid channel pore size, and subterranean formation permeability.

[00107] It will be appreciated by those skilled in the art that changes could be made to the exemplary embodiments shown and described above without departing from the broad inventive concepts thereof. It is understood, therefore, that this invention is not limited to the exemplary embodiments shown and described, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the claims. For example, specific features of the exemplary embodiments may or may not be part of the claimed invention and various features of the disclosed embodiments may be combined. Unless specifically set forth herein, the terms "a", "an" and "the" are not limited to one element but instead should be read as meaning "at least one".

[00108] It is to be understood that at least some of the figures and descriptions of the invention have been simplified to focus on elements that are relevant for a clear understanding of the invention, while eliminating, for purposes of clarity, other elements that those of ordinary skill in the art will appreciate may also comprise a portion of the invention. However, because such elements are well known in the art, and because they do not necessarily facilitate a better understanding of the invention, a description of such elements is not provided herein.

[00109] Further, to the extent that the methods of the present invention do not rely on the particular order of steps set forth herein, the particular order of the steps should not be construed as limitation on the claims. Any claims directed to the methods of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the steps may be varied and still remain within the spirit and scope of the present invention.