Field of the Invention

The present disclosure relates to tracer-containing materials for use in flow monitoring in subterranean reservoirs. In particular, the present invention relates to materials providing a controlled and/or sustained release of a tracer material, especially a gas tracer material, within a subterranean reservoir.

Background to the Invention

Tracer technology is used extensively in oil and gas exploration and recovery, and both radioactive and non-radioactive tracers that can be measured at low concentrations are applied. Tracers may be injected as pulses in well-to-well studies for measuring flow paths and velocities.

Tracer sources may also be placed in oil or gas production wells to monitor in-flow from the surrounding formation. For in-flow monitoring studies the tracers may be encapsulated in a polymer which is placed along the external surface of the production tube at different locations before completion of the well as described in US 6,645,769. The tracers can be attached to or encapsulated in polymers or in different types of particles, and the release of the tracers can be made dependent on type of fluid passing (oil or water), of chemical properties of the fluid (for instance pH or salinity) or for example of temperature.

Hydraulic fracturing is frequently used for stimulating gas or oil production from petroleum reservoirs, and is commonly applied in connection with hydrocarbon production from tight formations. Hydraulic Fractures radiate outwardly from the wellbore (typically from hundreds to just a few meters) and extend a surface area from which hydrocarbons can be produced efficiently. This process calls for a slurry (fluid that may or not contain propping agents) pumped at high pressure into a defined section of the well. As the slurry is pumped into the formation, a pressure differential between the wellbore and the reservoir pressure is generated. Eventually, the pressure differential will reach a point at which this will exceed the natural stresses in the formation generating a tensile failure, breaking the rock apart and creating a fracture. The propping agents (usually ceramic or sand particles) from the slurry will prevent the fracture faces to close on each other, leaving a permeable path for the reservoir fluids to flow towards the wellbore. If the treatment has been designed correctly, this will lead to an increase in production.

Fracturing slurry can be accompanied by particles including tracers, whose release shall provide information about the fractured formation, as described in WO2010/140033. Oilfield chemicals, for example tracers, are distributed in the form of "particles" - preferably resulting from the solidification of an emulsion - within a relatively large carrier matrix particle through which tracer diffuses.

WO2010/140033 relates to tracers for rapid release in 5-10 days or less and does not consider the tough mechanical conditions to which these particles are submitted. Furthermore, the tracer particles will easily be broken into smaller particles, or be limited to very small particles. The largest median size cited is 800 microns. As tracer diffuses through the whole surface of the particle, it will be difficult to obtain controlled tracer release or flow measurements over a long period, as tracer will release quickly and after a short time will drop to very low concentrations. In addition, as there is no control provided on the size of particles and tracer release will depend critically upon the surface area and diffusion distances, tracer concentration will be far from reflecting the flow for given physical characteristics of the environment, composition of the production fluid and temperature in particular.

In view of the above, it would evidently be an advantage to provide a tracer release material that could provide tracer release for an extended duration. . It would be a further advantage to provide a tracer release material with a predictable rate of release and/or change of release rate over time. It would also be an advantage to provide a tracer release material that is physically robust enough to maintain its release characteristics when delivered to an underground reservoir. This applies particularly to tracer release materials that may be applied to the sites of hydraulic fracturing, for example by injection with the proppant materials. It would be a further advantage if the tracer release material allowed for convenient and reproducible production methods.

Brief Description of the Invention

The present inventors have now established that by provision of a tracer release material having a resilient, low permeability outer layer combined with a more permeable inner matrix and a controlled degree of encapsulation of the matrix by the outer layer, a tracer release material may be provided which addresses one of more of the issues indicated above and allows for use in a variety of configurations.

In a first aspect, the invention therefore provides a tracer-release material in the form of particles comprising: a) a partial outer coating comprising at least one resilient material; b) a matrix within said outer coating, said matrix comprising at least one polymeric material; c) at least one porous material encapsulated within said matrix; d) at least one tracer contained within said porous material.

Typically the tracer-release material may be in the form of a tube having at least one open end. In such cases the tube will be formed of the resilient material and will contain the matrix, porous material and tracer components.

The tracer-release materials of all aspects and embodiments of the invention provide release of a tracer material over a sustained period. This tracer material may be detected at low levels in the fluid produced from a subterranean reservoir and may thereby give information on the movement of fluids within the subterranean reservoir. In a further aspect, the invention thus provides a method for the assessment of flow of fluid(s) within a subterranean reservoir, said method comprising applying a first tracer release material as described herein comprising a first tracer to at least one position within said reservoir and detecting the presence, absence, concentration and/or rate of change of concentration of said tracer in at least one fluid produced from at least one producing site on said reservoir. Such detections may be made under normal well operating conditions or, in an alternative embodiment, may be made under conditions chosen to elucidate certain properties of the reservoir and/or flow of fluids therethrough. For example, the particles of the present invention may be used in "Multizone flow estimation" methods based on Tracer transient analysis. In such a method, particles comprising different tracers are positioned in different production zones. After a well shut-in or at least throttling, production is resumed, and the different tracers are analyzed downstream, for example topside. They will generally present a peak of concentration

(corresponding to the build-up of tracer released during the period of restricted flow), and the analysis of the time interval between the peaks of the respective tracers will provide information as to the flow produced at the level of each zone marked with a tracer (and possibly also from other zones). Depending on the characteristics of the tracer release design (release constant with time, release proportional to the flow), additional information may also be drawn from the quantity of tracer flowing up. In such a method, it is typically the various tracer concentrations and the rate of change of those concentrations that will provide information on the relative flow rates.

The methods of the invention may be carried out using one or more tracer materials. In such an embodiment, each tracer material will typically contain a different and distinguishable tracer. Thus, for example, the above method may additionally comprise applying a second tracer material comprising a second tracer to a second position within the reservoir and detecting the presence, absence, concentration and/or rate of change of concentration of said first tracer and said second tracer, wherein said second tracer is different from said first tracer. This can provide the additional advantage of determining or helping to determine regions of production within the mentioned reservoirs.

In more general terms, the methods may employ a plurality (e.g. 2 to 30, preferably 2 to 10) of tracer materials as described herein. Generally each tracer material will comprise a different tracer from each other material used in the same reservoir. In such an aspect the invention provides a method of assessment of flow of at least one fluid within a subterranean reservoir, said method comprising applying a plurality of tracer release materials as claimed in claim 1 each comprising a different tracer at a plurality of positions within said reservoir and detecting the presence, absence, concentration and/or rate of change of concentration of each of said tracers in at least one fluid produced from at least one producing site on said reservoir, and relating the detected presence, absence,

concentration and/or rate of change of concentration of each of said tracers and the position of application of each tracer release material to a flow of fluid within said reservoir. In a similar embodiment, different tracer types can be placed in a single fracture stage. This can provide information on the hydraulic fracture, such as its effective dimensions, such as the extent of fractures of various dimensions, and on fluid movement and drainage patterns at the fracture site.

The materials of the present invention are highly effective in analyzing and monitoring the flow of at least one fluid within a subterranean reservoir. The robust nature and/or controlled release properties provided by the materials of the invention allow for highly advantageous uses both for individual materials over prolonged periods and for multiple materials (typically comprising different tracers) distributed at one or multiple sites. In a further aspect, the invention therefore provides the use of at least one tracer release material as described herein for monitoring the flow of at least one fluid in a subterranean reservoir. Preferred embodiments of such a use include use in monitoring the relative production of hydrocarbons from a plurality of sites at different layers within a subterranean reservoir. An example of this would be monitoring he relative production of gas from various sites in a shale gas reservoir.

The materials of the present invention are typically formed under controlled conditions so as to ensure effective encapsulation and release of the tracer in the subterranean reservoir environment. In a still further aspect, the present invention therefore provides a method of formation of a material as described herein comprising: i) entrapping at least one tracer within particles of at least one porous material whereby to provide an entrapped tracer; ii) mixing said entrapped tracer with at least one matrix precursor wherein to provide a tracer-containing matrix precursor; iii) introducing said tracer-matrix precursor into at least one void within at least one particle coating to provide a coated tracer-matrix precursor; iv) curing the matrix precursor component of said tracer-matrix precursor whereby to provide at least one tracer-release particle or a tracer-release particle assembly. v) optionally dividing said tracer-release particle assembly whereby to provide a plurality of tracer-release particles.

Generally the methods of the invention will be carried out at a temperature below the glass transition temperature of the cured matrix component. The methods may also be carried out at or below the boiling point of the tracer. Curing of the matrix precursor will typically be by non-thermal methods. Such methods include exposure to electromagnetic radiation, such as visible or preferably UV light. Where the production process involves elevated temperatures, such as for curing of the matrix material, this will generally be a temperature below the boiling point of the tracer under the conditions within the particle. Thus, in one embodiment, the temperature of the process may be above the boiling point of the tracer at atmospheric pressure but below the boiling point of the tracer at the pressure of the process. Such a process may be carried out at, for example, 1.2 to 10 bar pressure.

The particles of the present invention may be formed as a single size and/or optional division step v) may generate a single size and/or shape of particle. Alternatively, if more than one size and/or shape of particle is formed by the production methods of the invention then the particles may be sorted to generate a desirable size and optionally size distribution of particles (as discussed herein). The sizes and/or size distributions may be controlled by selection of appropriately sized particle coating(s), by division of a particle assembly in optional step v) and/or by sorting of the particles. The division method including optional step v) has the advantage of allowing strict control of particle size and ability to re-configure particle size by controlling the division process. Sorting of particles into desired sizes and/or size distributions constitutes and optional additional step vi). Detailed Description of the Invention

The tracer-release materials of the present invention comprise three key components. Firstly, an outer layer of resilient material with at least one defined opening which is insoluble under field conditions. The primary purpose of this resilient material is to give mechanical strength and/or to define a tracer/chemical release surface (surface area) by providing a partial encapsulation.

Secondly, a matrix component (which could be a polymer) that fills the volume inside the outer resilient layer and serves to retard and/or control the release rate of tracer or other chemicals from the open capsule. The matrix also gives structure to the open part of the capsule volume and provides the area through which the tracer can diffuse into the surrounding fluid. Thirdly, within the matrix component are distributed particles of a porous material (e.g. Silica gel or any of the porous materials described herein) that contain (within their porous volume) tracers or other oil field chemicals.

In one aspect the invention provides controlled release of at least one tracer and/or oil field chemical. The tracer or oil field chemical is released by diffusion through the matrix and out of the defined opening(s). The release rate is generally retarded in comparison with un-encapsulated tracer or oil field chemicals. The release rate may be predictable and/or reproducible due to the defined capsule opening and diffusion path.

The tracer-release material of the invention may be used by placement in the formation during hydraulic fracturing or in defined positions in a subterranean reservoir, such as in a gravel packed section of the well annulus or at a downhole screen (such as a sand screen) or Inlet Control Device (ICD) completion site. Many suitable sites for tracer utilization are widely known in the oil industry and may utilize the compositions of the present invention.

Compounds and compositions

The compositions of the present invention (which are applicable to all aspects of the invention) are "controlled release" in that the tracers or other oil-field chemicals included in the compositions are not released into the fluid instantaneously, but rather are released over a period of time. This release may be dependent solely or primarily upon the period of contact between the tracer-release material and the relevant fluid in the formation (e.g. oil or gas). Alternatively the release may be dependent upon the flow and/or conditions of the reservoir. Generally, 50% of the tracer or other chemical may be released over a period of 2 days to 180 days. That is to say the release of tracer or other chemical may have a half-life (preferably under the conditions of the subterranean reservoir) of 2 to 180 day. If the half-life is 180 days, one quarter of the initial release rate will be maintained after one year under stable reservoir conditions. The tracers are typically detectable at

concentrations down to O.lppt and will, depending on the amount of tracer placed in a fracture initially and the total fluid flow in which the tracer is dispersed in the well, be detectable effectively for many half-lives of release. Thus the tracer-release compositions of the invention may remain effective in providing detectable tracer for at least 1 year, preferably at least 3 years following introduction to the environment of the reservoir, preferably 5 to 120 days, or 10 to 60 days (e.g. 5 to 60 days). At a constant temperature and pressure the diffusion release rate may be predictable or substantially predictable and may be dependent on a small number of factors, such as the matrix composition, the tracer/oil field chemical boiling point, the dimensions of the particles and surface area of the opening. This allows prediction of the expected release rate under constant conditions, thus allowing changes in those conditions (e.g. changes in flow or temperature) to be detected where the observed concentrations differ from what is expected. Release may therefore be modelled and/or compared against one or more experimentally derived standards in order to predict the rate of release at varying times. From this information, predictions may be made as to the concentration of tracer expected in produced fluids under various conditions and/or conditions in the reservoir may be derived by fitting the modelled release rate to the observed concentration of tracer in produced fluid. Changes in tracer concentration in the produced fluid over time may serve to verify that conditions in the reservoir are stable (e.g. if the drop in concentration follows the predicted rate for stable conditions), or may indicate a change in flow or temperature (e.g. if the change deviates from that expected for stable conditions). In embodiments described herein one tracer or a plurality of tracers may be used to indicate absolute and/or relative conditions at sites within a reservoir.

Although potentially advantageous to derive quantitative data from the tracer concentration(s) in the produced fluid(s), this is not essential in order to derive valuable information from the tracer release and detection. Tracer measurements may thus be qualitative (e.g. presence or absence at detectable levels) semi-quantitative (e.g. concentration above or below a threshold), or quantitative as either an absolute concentration or a relative concentration of one tracer in comparison with at least one other.

Tracer-release material/ Oil Field Chemical

The controlled release particles of the present invention may be used in the release of many "Oil Field Chemicals". Such chemicals include all suitable chemicals having utility in a subterranean hydrocarbon reservoir, such as scale inhibitors, corrosion inhibitors, wax inhibitors, hydrate inhibitors, surfactants, antifoaming agents, biocides, demulsifiers, hydrogen sulfide or oxygen scavengers, and/or asphaltene inhibitors. A key embodiment of the present invention is the use of tracers as this "oil-field chemical". Many embodiments of the present invention are thus described and illustrated with respect to tracers. The particles of the present invention are, however, effective for the release of a variety of chemicals having down-hole utility and these chemicals may be substituted for the "tracers" indicated herein wherever context allows. Some properties described of tracers, such as detectability, will be specific to those chemicals. Other properties, however, such as boiling point, concentration etc. can be applied to any suitable down-hole chemical and should be applied to those other chemicals equally. The skilled worker will be aware of those few properties which are specific to tracers. In all other situations, "tracer" may be read as "chemical" herein throughout.

A key aspect of the present invention relates to a "tracer-release material". As used herein, this is a composite material which serves to encapsulate a tracer or other chemical useful in a petroleum reservoir and to release that over a period as described herein. The first component of that composite material is a "resilient material" which serves to form at least one outer layer(s), partially encapsulating the other components and providing physical and release characteristics. The "resilient" material used in the present invention will typically be in the form of a tube (e.g. a hollow cylindrical or other hollow prismatic shaped tube). The outer layer of resilient material serves to define the size of the particles of the tracer-release material and, in that final material, will thus have the size and distribution indicated herein for the particles of the tracer-release material. In one embodiment of the invention, the tracer-release material may be formed by introduction of the remaining components into a tube of resilient material, followed by curing and dividing (cutting). In such an embodiment, the resilient material will initially have one or two dimensions equivalent to that of the final product but will be longer in at least one dimension before being filled, cured and divided.

Where a tube of resilient material is used in the present invention, this will typically be of outside diameter 0.1 to 50mm, preferably 0.2 to 10mm and most preferably 0.5 to 5mm. The inside diameter will evidently be less than the outside diameter and may be, 0.005 to 45 mm, preferably 0.01 to 9.5mm, more preferably 0.025 to 4.5mm. The inside diameter will typically be between 10% and 90% of the outside diameter.

The resilient material for use in the present invention provides at least a part of the mechanical strength of the tracer-release material and thus should be physically robust. For example, the resilient material when cut to the dimensions of the tracer-release particles may exhibit crush resistance under closures stress conditions of at least 100 psi, (e.g. at least 500 or at least lOOOpsi), preferably at least 5000 psi, and more preferably at least 10,000 psi (e.g. at least 12,000 psi, up to 500,000 psi).. The resilient material may additionally or alternatively have a compressive strength at yield (according to ASTM D695) of at least 5,000 psi, preferably at least 8,000 psi, more preferably at least 10,000 psi (e.g. at least 50,000 psi, such as 5,000 to 200,000 psi). Furthermore, the resilient material, may withstand temperatures of at least 140°C, preferably at least 160°C, more preferably at least 180°C or 200°C (e.g. up to 250 or 300°C). A typical resilient material may have a heat deflection temperature at 264psi (according to ASTM648) of at least 140°C, preferably at least 160°C and most preferably at least 200°C. Certain advantageous resilient materials may have a heat deflection temperature at 264psi (according to ASTM 648) of at least 220°C, preferably at least 250°C.

Typical materials suitable for use as the resilient material of the particles of the present invention include polymers and metals. Suitable polymers include cross-linked and thermo-setting polymers, especially those having the physical characteristics described herein. Some preferred examples of suitable polymers include Polyphenylsulfone (Radel R), Polyetheretherketone (PEEK), polyetherimide, and Polyketone.

In one embodiment the resilient material is transparent in at least one region of the electromagnetic spectrum, preferably transparent or partially transparent (such as absorbing no more than 70% of the incident radiation) to visible and/or UV radiation, most preferably UV. This has the advantage of allowing curing of the matrix material by exposure to radiation, such as UV irradiation (see below).

An alternative material suitable for use as the resilient material of the particles of the present invention is a metal. Metal materials are advantageous in having high strength and rigidity but are not transparent to electromagnetic radiation of frequencies useful for curing the matrix material. Suitable metals include steel, stainless steel and other iron alloys, aluminium and aluminium alloys, nickel and nickel alloys, brass and titanium. A further advantage of metal is that it has a

comparatively high density and so can be used to increase the overall density of the tracer-release material. This may allow the density of the tracer-release material to more closely match that of the proppant particles, for example in embodiments where the tracer-release material is injected with the proppant slurry during a hydraulic fracturing process.

In a further embodiment of the invention, the resilient material may comprise more than one layer, such as two, three or four layers. In such a multi-layer resilient outer material, the layers may be of differing materials, such a two-layer material having a metal layer outermost with a polymer lining. Such multi-layer outer materials have advantages over single-layer coatings. For example, a fused metal oxide material such as fused silica may be used as one layer of the resilient material for its excellent barrier properties and low reactivity. However, such a layer would require an outer coating of another material such as a polymer to provide sufficient toughness. Similarly a metal resilient material may be coated with a polymer layer to reduce the risk of damage to pumping equipment and to enhance filling of the fissures when used in combination with a proppant in hydraulic fracturing methods. Furthermore, if the outer polymer coating allows for greater diffusion of the tracer then the polymer coating may encapsulate both the metal resilient material and the opening(s) through which the tracer diffuses to help maintain the integrity of the matrix. The reverse layer ordering having a polymer layer coated within a metal resilient material may serve to allow control over the internal diameter of the resilient layer and may also aid in controlling the release rate.

The outer layer(s) of resilient material (such as those layers outside of the matrix material) additionally serves to define at least one release area of the tracer-release particles of the present invention. This is enhanced by the resilient material having a significantly lower permeability to the tracer than the permeability of the matrix material. In this way, the tracer material is transported out of the particle primarily by contact between the matrix material and the reservoir environment in those regions where the outer layer of resilient material is not present. In one embodiment the permeability of the outer resilient material to the tracer is no more than 10% (e.g. 0.001 to 10%) of the permeability of the matrix material to that tracer. This may preferably be no more than 5% and more preferably no more than 1% of the permeability of the matrix material to the tracer. In the case of a metal resilient material, or of a multi-layer resilient outer material comprising at least one layer of metal, the permeability of the resilient material to the tracer may be close to zero.

Where the resilient material is a polymer, it will be desirable that the glass transition point of this polymer is higher than the temperature of the reservoir environment. This will help to ensure that the permeability of the resilient material to the tracer remains low. Thus, in one embodiment, the resilient material will be a polymer with a glass transition temperature (GTT) above 60°C, such as above 100°C, preferably above 140°C (e.g. above 160°C, such as 160°C to 250°C or even up to 300°C) and optionally above 180°C or 200°C. In a further embodiment, the resilient material has a glass transition temperature higher than that of the matrix component. The GTT of the resilient material may thus be higher than that of the matrix component. The GTT of the resilient material may thus be at least 50°C (e.g. 50-150°C) higher than that of the matrix component. This is preferably at least 70°C higher and more preferably at least 80°C higher. Evidently, where the resilient material is a metal then this will not have a corresponding glass transition temperature but will have a melting point higher than the GTT of the matrix material. In one embodiment, the resilient material is not soluble in oil, or water. In a further embodiment the resilient material is not soluble in any fluid typically encountered in a subterranean reservoir under the conditions of such a reservoir.

In the tracer-release particles of the present invention, within the outer casing of resilient material is held a matrix material. The matrix material is typically a polymer and may be a cross-linked polymer. The matrix material serves to entrap the porous material within the outer resilient material and helps to control the rate of release of tracer to the reservoir environment.

In order to achieve a comparatively high permeability of the matrix material to the tracer, it is preferable that the matrix material has a glass transition point below the temperature of the reservoir. It may also be advantageous for the matrix material to have a GTT above normal handling temperatures (e.g. above room temperature) in order to limit the permeability of the matrix to the tracer prior to introduction to the reservoir. The matrix material may therefore have a glass transition point of 30 to 200°C(e.g. 30°Cto 180°C), preferably 50 to 145°C, more preferably 60 to 135°C.

Matrix materials will typically be polymers, especially cross-linked polymers. Some suitable polymers include chemically and/or ultra-violet (UV) curing polymers. Thermally curing polymers are generally less preferred since the application of heat to cure the polymer matrix may cause significant release of the tracer/chemical component from the tracer/chemical release material. Specific examples of suitable matrix polymers include polyethylene terephthalate (PET), polyvinyl chloride (PVC), polyvinyl alcohol (PVA), polystyrene, polymethyl methacrylate, acrylonitrile butadiene styrene (ABS), polytetrafluoroethylene (PTFE), polycarbonate, epoxy resins (chemically and UV-curing). Most preferred matrix materials include UV curing epoxy resins such as UV-15-DC-80 and UV-15TK.

The matrix material will typically be formed or formable from at least one matrix precursor material. This allows the matrix precursor(s) to be introduced into at least one void in the resilient material and subsequently cured to form the matrix material. It is preferable that the matrix is formed or formable from matrix precursors having a viscosity of 1 to 150 000 cP at 25°C. This is preferably 300 to 100 000 cP and more preferably 5000 to 80 000 cP at 25°C. Most suitable matrix precursors will be curable chemically or more preferably by exposure to electromagnetic radiation, such as visible and/or UV light.

In one embodiment, the matrix of the present invention may be formed as a polymer emulsion containing particles of porous material, themselves containing tracer or chemical as described herein. Such an emulsion may then be cured within a shell of resilient material as described herein.

The tracer/chemical-release materials of the present invention comprise a particulate porous material embedded within the matrix material. Suitable porous materials will typically have a pore size of 2 to 50 nm, preferably 3 to 8 nm. The porous material may also have a specific pore volume of 0.2 to 0.95 cm ³ /g, preferably 0.4 to 0.8, more preferably 0.55 to 0.75 cm ³ /g- The porous material may furthermore have a specific surface area of 100 to 1000 m ² /g, preferably 150 to 800 m ² /g, more preferably 200 to 700 m ² /g- Suitable particles sizes for the porous material will be in the range 10 to 1000 mesh, preferably 30 to 650 mesh. The particles of porous material may be of uniform or mono- modal size distribution or alternatively may be of bi-modal or multi-modal size distribution. A bi- modal or multi-modal particle size distribution may have the advantage of allowing more porous material to be incorporated without unduly increasing the viscosity of the matrix precursor.

Metal oxides and/or metal silicates form a preferable group of materials suitable for forming the porous particulate material for use in the tracer/chemical-release materials of the present invention. Particularly suitable metal oxides include oxides of silicon, aluminium, titanium, tin or magnesium, as well as mixed oxides of these. Silicates of any of these (except for silicon) are also suitable, especially magnesium silicate (Florisil). Preferred examples of porous materials include silica, aluminium oxide, Florisil and Celite.

The porous particulate material may also be (or may comprise) Boron carbide. As Boron has a high likelihood of accepting neutrons, neutron activation logging could be used to determine which fractures have received tracer particles of the invention and correspondingly have also received normal proppant during the hydraulic fracturing process. In this method, a logging tool with a neutron emitting source is placed in the fractured well, as the tool move along the well path the surroundings are irradiated with neutrons. The logging tool may also contain a neutron detector or a gamma radiation detector. As the Boron carbide inside the tracer release particles is irradiated, there is a high likelihood of it accepting neutrons and thus a reduction in the neutron signal at the neutron detector will be observed, alternatively after Boron has accepted a neutron it will release gamma radiation at a known wavelength, this may be detected at the gamma radiation detector included in the logging tool. Both methods could be used to determine which fractures have accepted Boron carbide containing tracer/chemical release particles and there-by which fractures have accepted of normal proppants.

The amount of porous material which is incorporated into the tracer-release material of the invention will depend upon the viscosity of the matrix precursor, the size of the particles and similar factors. Typically an amount of 2 to 50% by weight of porous material will be incorporated into the matrix material, preferably 5 to 20% by weight, especially 7 to 15% by weight.

The porous material in and for use in the tracer/chemical-release materials of the invention are capable of containing at least one tracer or other chemical within the pores thereof. In one embodiment the porous material contains or is capable of containing at least one tracer or other chemical to a level of at least 20% (e.g. 10 to 70%) of the combined weight of tracer/chemical and porous material. This is preferably 30% or 40% and more preferably at least 50% tracer by weight of tracer and porous material.

The porous material of the present invention may "contain" the tracer/chemical material in any suitable form and by any effective method. Containment may be, for example, by physical encapsulation, by absorption into the bulk of the porous material, by adsorption at the porous surface of the porous material or in any other way.

The tracer-release materials of the present invention require a tracer component or other chemical for release. Such a tracer may be an oil tracer or a gas tracer (preferably a gas tracer) and may be detectable down to a very low level. Tracers detectable at a level down to at least 1x10-9 vol/vol (e.g. down to 1x10-9 to 1x10-13 by volume) will be preferred, more preferably detectable to 1x10-12 and most preferably detectable down to 1x10-13. Suitable tracers may be those detectable by radioactivity detection (e.g. scintillation), by photometric methods such as colour or fluorescence but will most commonly be detectable by chromatographic methods, such as gas-chromatography mass- spectrometry (GCMS). Tracer materials will also typically be stable to the conditions of the reservoir, and in particular be stable to water and/or hydrocarbons at elevated temperatures, such temperatures up to 160°C, preferably up to 200°C and more preferably to temperatures above 200°C, such as up to 220°C, 250°C or 300°C . These stabilities apply equally to other chemicals suitable for use in a hydrocarbon reservoir, such as any of those indicated herein.

Suitable classes of tracers include halogenated compounds, especially per-halogenated compounds such as per-fluorinated compounds, including per-fluorinated hydrocarbons. Partially fluorinated and perdeuterated aromatic compounds, partially fluorinated alkyl chains and ethers, are also suitable.

In one embodiment, the tracer is a gas tracer. A suitable gas tracer may have a boiling point below the temperature of the reservoir (e.g. gas reservoir). A suitable boiling point may be below 200°C (e.g. 20 to 200°C), preferably below 180°C, more preferably below 140°C (e.g. below 120°C).

Where the reservoir is operated at elevated pressures, the boiling point of the tracer may be selected to be below the temperature of the reservoir at the pressure of that reservoir. In one embodiment, the boiling point of the tracer at the pressure of the reservoir may thus be below the temperature of that reservoir. Tracers may thus be selected according to the conditions of use as appropriate.

Some example of halogenated tracer compounds are shown below with their boiling points. Each of these tracers forms a highly preferred tracer for use in all aspects of the present invention.

Table 1

In a further embodiment, the tracer material may be an oil tracer. Suitable oil tracers may have a boiling point above the temperature of the reservoir. Thus, for example the tracer may be an oil tracer and have a boiling point above 140°C, preferably above 160°C, more preferably above 200°C. In one embodiment, a plurality of tracer-release materials of the present invention may be used where at least one such tracer-release material comprises a gas tracer and at least one such tracer- release material comprises an oil tracer. Such an embodiment allows for the simultaneous monitoring of the flow of both fluids (oil and gas) and allows a relative increase in the production of one fluid in comparison with the other to be detected. In other respects, this embodiment may be carried out according to any of the methods and uses described herein and utilising any of the materials described herein.

The tracer/chemical-release material of the present invention takes the form of composite particles having components as described herein. The outer capsule of resilient material provides the overall shape and size of the particles and defines at least one opening through which the matrix material contacts the surrounding environment (generally the reservoir environment when in use). Each particle may have one or a plurality of openings (e.g. two, three or four openings). Most commonly each particle will have two openings. In one embodiment, the resilient material is a polymer and the particles each have two openings. In an alternative embodiment, the resilient material is a metal and the particles each have one or two openings. Typically the ratio of the combined surface area of all openings to the volume of matrix enclosed by the resilient material will be in the range 0.0001 to 4 m ^"1 , preferably 0.001 to 4 mm ^"1 , more preferably 0.01 to 2 mm ^"1 .

Typical sizes for the tracer/chemical-release particles of the present invention will be in the range 0.1 to 50mm, preferably 0.2 to 10mm and most preferably 0.5 to 5mm. The inside diameter will evidently be less than the outside diameter and may be, 0.005 to 45 mm, preferably 0.01 to 9.5mm, more preferably 0.025 to 4.5mm. Generally the particles may be fabricated from a tube of resilient material and so may be generally prismatic (especially cylindrical) in overall shape. Any shape of particle may be used in the present invention and preferred shapes will depend upon the method of production and the specific use. Spheres, open tubes, and tubes with one closed end are among the preferred options. Various properties of the tracer-release material (e.g. rate of tracer/chemical release, duration, physical strength) may be controlled by varying the ratio of the cross-section of such a prism to its length. For example, a typical ratio of prism average diameter to length may be 2:1 to 1:20, more preferably 1:1 to 1:10 and more preferably 1:2 to 1:8.

Where the tracer-release particles of the invention are for use or suitable for use in hydraulic fracturing methods, it is preferable that the particles are of comparable size to the proppants used so that the proppant pack permeability is not significantly affected. In such an embodiment the particles will generally be no greater than 2mm in their smallest dimension, more preferably no more than 1.2mm and most preferably no more than 1mm in their smallest dimension. Preferably the particles will conform to this sizing in at least two dimensions.

Uses and methods suitable for the particles of the present invention include application at any suitable site in a reservoir where a fluid flow will or might be expected and there may be a desire to monitor that fluid flow. Examples of processes and sites at which the particles of the present invention may be utilized include: High Rate Water-Pack: injecting tracer particles together with the gravel at high pressure to obtain pressure packing will enable monitoring of the performances and the success of the gravel-pack completion.

Hydraulic fracturing without proppants: even when injecting neither gravels nor proppants, a limited quantity of tracer particles not having the function of proppants may be helpful in monitoring the result of a hydraulic fracturing, especially when such fracturing is performed at multiple zones in the well.

Acid fracturing: provided the particle coating and matrix are designed to resist acid, and the particle is produced with small sizes (for example less than 1 mm) tracer particles entering wormholes and pores may help assessing the result of acid fracking. This is of special interest in multi-zone operation. In acid fracturing embodiments, at least the outer resilient material and/or the coating material thereof should be stable to the acid conditions of acid fracking. Where a coating material is used and where this is acid resistant, the coating may extend over the openings in the resilient material so as to protect the matrix. In such a case, however, the coating should be permeable to the tracer to a similar degree to the permeability of the matrix.

Inlet control devices: tracer particles fixed in the inlet zone of inlet control devices (ICDs) will help monitor the flow at the ICD, and thus different tracers will inform on relative production for multiple zones. Tracer particles may be trapped at the level of screens, for example entrapped into material located such that fluid flowing in the ICD entrains leaked tracers. A plurality of particles of the invention, each containing a different tracer, may be used in such a method (e.g. at different ICDs). This may allow generation of an inflow profile by tracer and thus identify relative flows at each ICD.

Flow and scaling monitoring: an alternative is to introduce tracer particles in sand-screens, prepacked or stand-alone, and monitor inflow profiles and their development over time in order to detect scaling. The tracer particles of the invention might be useable in long screens probably with multiple intervals, either pre-packed or standalone to identify inflow profiles and it's development over time (e.g. due to scaling and/or plugging).

Cementation / squeeze job control: by injecting the particles in the zone to be isolated prior to cementation or squeeze job, integrity control of the job can be achieved by tracer release monitoring. The particles of the invention may be used to validate remedial cementation jobs, e.g. to show isolation of zones or completion intervals or to prove integrity. For example, one might shut- off or abandoned a well, a branch or a completion interval/zone and/or perform a completion repair. The particles of the invention may be used over a long period to prove for integrity of barriers or demonstrate failure.

Injector profile control Flood quality / gas breakthroughs: The particles of the invention may be used in control and monitoring when using gas injectors (oil reservoir pressure maintenance or C0 ₂ flooding) and/or injectors for WAG (Water-Alternate-Gas) systems. By using the particles of the invention, information can be gained on inter-reservoir connectivity, flood quality and gas breakthrough mechanism. Particles bearing a different tracer may be used at each site of injection to aid in breakthrough identification. Tracing efficiency of Improved Oil Recovery (IOR) injection processes and enhanced oil recovery (EOR) injection processes : by prior (or possibly simultaneous during water injection) introduction of tracer particles in the reservoir, monitoring of IOR injection processes such as Water-Alternate-Gas (WAG), polymer flooding and characterization of the reservoir may be possible.

Each of the above application evidently serves as a further aspect of the present invention and may be used in combination with any of the tracer particles described herein as appropriate.

Typically the particles will be used in a mono-modal distribution of particle sizes. In a further embodiment, however, a bi-modal distribution of particles sizes may be used. This will be most useful where a mixture is made of two tracer-release materials having different tracers and different particles sizes. This allows information to be provided on the flow of at least one fluid through features of the reservoir having different sizes. For example, a mixture of small particles (e.g. less than 1.5 mm in smallest dimension) and large particles (e.g. greater than 5mm in smallest dimension) having a first and second tracer respectively, may provide information on the flow of fluid within the network of smaller fractures from tracer 1 and information on the flow of fluid through larger fractures of the well from tracer 2.

In one embodiment the tracer-release particles of the present invention may be used in monitoring or investigation in relation to hydraulic fracturing methods. In such an embodiment the tracer- release materials may be introduced to the reservoir with the proppant materials. In one embodiment it is therefore preferable that the tracer-release particles have a density similar to that of the proppant particles so that they do not have an excessive tendency to sediment out of the proppant slurry, nor to float to the top thereof. In one embodiment, the tracer-release particles therefore have an overall density in the range of 1 to 4 g/cm3. This is preferably 1.5 to 3.5 g/cm3, more preferably 1.8 to 3 g/cm3.

The methods of formation of the tracer-release materials of the present invention comprise steps i) to v) as described herein.

It is preferable that the method of formation be carried out at a temperature below the glass transition temperature of the matrix material. It is also preferable that the method of formation be carried out at a temperature below the boiling point of the tracer. Both of these requirements have the advantage of reducing the amount of tracer lost from the particles during handling and preparation and maximize the usefulness of the tracer-release particles. As such, the process should preferably be carried out at below 50°C, preferably below 35°C and more preferably below 30°C.

Due to preferable limitations on the operating temperature of the process, it is preferable that curing step iv) is not a thermal curing step. Curing by electromagnetic irradiation, especially by visible or UV light is preferable. Chemical curing is possible but this should preferably not require heating above the temperatures indicated herein and should not generate excessive heat during curing such that the temperatures remain as indicated herein above. Use of tracer-release material in-flow monitoring along a production tube or within the formation

The tracer-release materials of the invention may be used individually. Alternatively, they may be used as a plurality of materials of the invention, each containing different tracers. These may be used in studies of fluid (especially gas) in-flow in reservoirs, such as hydrocarbon reservoir production wells. In suitable methods of the invention, two or more tracer-release materials of the invention are used.

In one embodiment, the tracer-release materials containing defined tracers are placed at specific locations, either in the reservoir along the production tube enabling identification of zones where fluid in-flow occurs (see Figure 12) or at the reservoir - completion interface. Each different composition is typically applied at a different known location within the reservoir (e.g. along the well, in the gravel pack or screens (e.g. sand screens), at ICDs, behind or in isolation installations and cemented intervals) and produced fluid at one or more location(s) of said reservoir is monitored for the presence of the tracers. These locations can be different layers of the same reservoir, allowing for monitoring of layer separation or inter-layer flow. Alternatively or in combination, the locations may be different zones of the same layer. By analysis of the produced fluid at well-head, information on flow through the different locations in the reservoir can be inferred. This may be by the presence, absence, quantitative estimation, and/or variation of one or more tracers in the produced fluid.

In an alternative embodiment (which may be used in combination with other embodiments described herein), the tracer-release materials containing defined tracers are placed in the formation during hydraulic fracturing enabling identification of fractures in which fluids flows into the well.

The tracer-release materials of the present invention may be applied to a subterranean reservoir by many known methods and at a variety of sites. Some suitable sites for application of the materials of the present invention are shown in Figures 7 to 11. These include placing the material in the gravel pack around a sand-screen at a defined position, placing the tracer-release material in a contact chamber between the sand screen and the production pipe, or by placing the material of the invention in a contact chamber within the production pipe through which material must flow when passing from the in-flow point to the production pipe. Other methods of application, such as use in hydraulic fracturing are described herein.

The various methods and uses of the present invention can employ various degrees of accuracy in assessment of the detected tracers. In one embodiment the tracers may simply be assessed for presence or absence at one or more threshold concentration. Alternatively, an absolute

concentration may be measured to provide information on the flow of fluid(s) in the region on the tracer-release material or a relative concentration may be measured to allow assessment of the relative flow of fluids at two or more sites. Furthermore, since the rate of change of concentration may provide information on the stability or otherwise of fluid flow in the region of the tracer-release material(s) this may also be the subject of measurement. Application of the invention to investigations of fractures

A preferred use of the tracer-release materials of the present invention is in the investigation and/or monitoring of fractures in hydraulic fracturing operations. The present invention provides a process of making observations of fluid movements in fractures in oil or gas wells. A plurality of sets of tracer- release materials containing different tracers are conveyed down the wellbore and allowed to enter fractures at a first location. The first set of tracer-release materials containing a first set of tracers will be gradually released by diffusion in the fluid present at the first location in the reservoir. A second set of tracer-release materials can be conveyed from the surface to a second location and allowed to enter fractures in the second location. The number of sets of tracer-release materials may be greater than two (e.g. three, four, five or more), thus enabling tracer-release materials to enter fractures in more than two locations. The tracers are distinguishable from each other so that detection of a tracer substance in the produced fluid will allow identification of the tracer-release material and the location where they were placed. In one embodiment, the tracers used may all be detected by the same detection method (e.g. GC or GCMS). Preferably all tracers may be detected simultaneously.

General advantages of application of invention

The advantages of applying tracer-release materials of the invention when compared to other techniques are first of all that the release of the tracer or other chemicals can be spread over a longer time period, be more constant and/or predictable throughout the entire lifetime of the source of tracer, and that the release rate is more easy to estimate as the capsules have a defined opening being rather constant over the life of the capsule. Unlike other applications where tracer particles with low physical strength or designed to dissolve as a whole are used, the tracer-release materials of the invention can be applied that are capable of withstanding hard physical stress over long periods of time. Thus the particles of the invention are designed to withstand the forces of injection, either alone or as part of a fracturing process. Furthermore they may withstand the sustained pressures and forces within the reservoir for an extended period (e.g. up to 3 years). The materials of the invention are further not weakened in the process of tracer or chemical diffusion and thus maintain their strength independently of the tracer/chemical diffusion.

Brief Summary of the Figures

Figure 1 shows a schematic representation of the tracer-release particles of the present invention.

Figure 2 shows a schematic representation of the apparatus used in the Example below.

Figure 3 shows a graph showing the determined release concentration against time (From 7 hours to 134 days) of 1,2-perfluoromethylcyclohexane.

Figure 4 shows a graph showing the determined release concentration against time (83 to 134 days) of 1,2-perfluoromethylcyclohexane.

Figure 5 shows the materials of the present invention applied to the gravel pack surrounding the sand screen at the in-flow point. Figure 6 shows the materials of the invention applied in a tracer contact chamber (e.g. in the form of a cartridge) between the sand screen and the production pipe at the in-flow point.

Figure 7 shows the materials of the invention applied in a delay chamber between the sand screen and the production pipe at the in-flow point.

Figure 8 shows the materials of the invention applied as an annular contact chamber within the production pipe at the in-flow point.

Figure 9 shows the materials of the invention applied with the proppant particles at a fracture within a production zone.

Figure 10 shows the use of three tracer materials of the present invention to monitor material flow at three in-flow point along a production pipe.

The following numerals are used to indicate certain features of the various Figures:

1 - resilient material

2 - matrix

3 - porous material (with contained tracer or chemical as appropriate)

4 - shale or similar low-productivity and/or unstimulated layer

5 - production layer

6 - gravel pack

7 - cartridge of tracer particles

8 - tracer particles in delay chamber

9 - tracer particles in annular contact chamber within production pipe

5.1 - production layer 1

5.2 - production layer 2

5.3 - production layer 3 Examples

Fabrication of the capsules

In an experiment performed in the inventors' laboratory, controlled release open capsules containing four gas tracers were produced, an example is shown in Figures 1 and 2. In short, Silica particles (Merck 10181 40A 35/70 mesh) containing 50% by weight of the tracer mixture were mixed into the resins to a concentration of 10% by weight. The liquid resins were transferred to a plastic syringe. The syringe was connected to a 20 cm long piece of polymer tubing and the mixture was injected into the tubing. After the resins had hardened either chemically (polymethylmetacrylate) or using a UV- lamp, the tubing was chopped in pieces of 3 or 5 mm length. The acrylic resin could only be filled into the 1/8" tubing and not into 1/16" tubing because the tubing tended to be clogged by the silica particles. This issue might be addressed by the use of other resins and/or different sizes or proportions of particles. UV-curing resins with silica could be filled into both 1/8" and 1/16" tubing. Gas tracers such as those listed in Table 1, could be added to the silica to give different

distinguishable controlled release open capsules. Other fluorinated or partially fluorinated carbon compounds could be added, as well as perdeuterated aromatic compounds. Partially fluorinated alkyl chains and ethers are also suitable.

Table 2

The release rates, in a natural gas environment, of a range of tracers from a range of different controlled release open capsules (Table 2) were tested using a flow rig at 160°C and 400 bar of pressure. Figure 5 shows the determined release concentration against time (From 7 hours to 134 days) of 1,2-perfluoromethylcyclohexane from line 3 (UV-15DV-80, 1/8"* 3mm) and Figure 6 shows the determined release concentration against time (83 to 134 days) of 1,2- perfluoromethylcyclohexane from line 3 (UV-15DV-80, 1/8"* 3mm). The curves shown in Figures 5 and 6 are typical for each open capsule type but the release type changes depending of capsule dimension and polymer matrix composition. Table 3 shows the amount of tracer produced from each type of open capsule in the first 134 days of the tracer release experiment.

Table 3 Amount of tracer produced from each line/open capsule type in the first 134 days of the experiment