THE SAME

BACKGROUND

[0001] The present invention relates to additives used in treatment fluids in subterranean operations. More specifically, the present invention relates to modified particulate weighting agents used in treatment fluids such as drilling and cementing fluids.

[0002] Treatment fluid roles include, for example, stabilizing the well bore and controlling the flow of gas, oil or water from the formation to prevent the flow of formation fluids or prevent the collapse of pressured earth formations. The column of a treatment fluid exerts a hydrostatic pressure proportional to the depth of the hole and the density of the fluid. For example, high-pressure formations may require a fluid with a specific gravity as high as 3.0.

[0003] A variety of materials are presently used to increase the density of treatment fluids, including the use of dissolved salts such as sodium chloride, calcium chloride and calcium bromide. Alternatively, powdered minerals such as barite, calcite and hematite may be added to a fluid to form a suspension of increased density. The use of finely divided metal, such as iron, as a weight material in a drilling fluid has also been described. The use of finely powdered calcium or iron carbonate has also been indicated; however, the plastic viscosity of such fluids rapidly increases as the particle size decreases, thus limiting the utility of these materials.

[0004] Another demand on a typical treatment fluid additive is that it should form a stable suspension that does not readily settle out. Secondarily, the suspension may beneficially exhibit a low viscosity to facilitate pumping and minimize the generation of high pressures. Ideally, the treatment fluid slurry should also exhibit low fluid loss. Conventional weighting agents such as powdered barite may require the addition of a gellant such as bentonite for water-based fluids, or organically modified bentonite for oil-based fluids. A soluble polymer viscosifier may be also added to slow the rate of the sedimentation of the weighting agent. However, as more gellant is added to increase the suspension stability, the fluid viscosity (plastic viscosity and/or yield point) increases undesirably.

[0005] Submicron particles have also been employed as weighting agents using surfactant-based coatings to help disperse the particles in the base fluid. However, in such applications the surfactants are only weakly linked to the surface of the particles and the adherence of the surfactant to the particle competes with other phenomenon such as the formation of emulsion droplets and/or the interaction of the surfactant with other solids that may have a higher affinity for the surfactant than the weighting particle.

SUMMARY OF THE INVENTION

[0006] The present invention relates to additives used in treatment fluids in subterranean operations. More specifically, the present invention relates to modified particulate weighting agents used in treatment fluids such as drilling and cementing fluids.

[0007] In some embodiments, the present invention provides a method comprising the steps of providing a treatment fluid for use in a subterranean formation comprising a weighting agent, the weighting agent comprising a particulate metal oxide and a polymer linked to the particulate metal oxide, and introducing the treatment fluid into the subterranean formation.

[0008] In other embodiments, the present invention provides a method comprising providing a treatment fluid for use in a subterranean formation comprising a weighting agent, the weighting agent comprising a particulate metal oxide and a polymer covalently linked to the particulate metal oxide, and introducing the treatment fluid into the subterranean formation.

[0009] The features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the preferred embodiments that follows.

DETAILED DESCRIPTION

[0010] The present invention relates to additives used in treatment fluids in subterranean operations. More specifically, the present invention relates to modified particulate weighting agents used in treatment fluids such as drilling and cementing fluids.

[0011] Of the many advantages, the present invention provides methods employing polymer-modified particulate weighting agents with anti- sagging properties that are readily dispersible in any base fluid of a treatment fluid. By judicious choice of polymer modification, the micronized particles may be rendered compatible with water, oil, or mixed water-oil base fluids. Thus, in some embodiments, the polymer characteristics may be substantially hydrophilic for use in aqueous-based treatment fluids. In other embodiments, the polymer characteristics may be substantially hydrophobic for used in oil-based treatment fluids. In some such embodiments, hydrophobic polymers maybe covalently linked to the particulate weighting agent to provide superhydrophobic particles which may be useful in, for example, the continuous oil phase of inverse emulsions. In yet further embodiments, the polymer characteristics may render the modified particulates useful in other mixed oil-water treatment fluids by employing copolymers having both hydrophobic portions and hydrophilic portions.

[0012] Weighting agents employed in methods of the invention may also be designed to enhance repulsive forces via the linked polymer, which may assist in dispersion of the micronized particles, and reduce and/or eliminate agglomeration, which may help prevent settling. The linked polymer structures are believed to minimize Van der Waals forces, which may cause particle aggregation and subsequently result in solids settling as typically observed with certain weighting materials, especially in oil-based inverse emulsions.

[0013] Moreover, weighting agents employed in methods of the invention may display low viscosity and gel strength properties, even when using micronized particulates on 1-500 nanometer scale. Without being bound by theory, the steric bulkiness of the linked polymer may reduce particle-particle interactions and thus reduce the shear radius.

[0014] The modified particulate weighting agents employed in methods of the invention may further exert low impact on the subterranean location resulting in little to no damage to the formation. First, the small particle size and minimized aggregation may allow the modified particulate weighting agents to be readily flushed out of the formation. In the case of hydrophobically modified particulate weighting agents, for example, such agents may be taken up into the oil phase of the reservoir pore matrix structure, with minimal interaction with potential water layers attached to pore linings in water-wet reservoir rocks. Thus, in some embodiments, the combination of size and surface properties may aid in removal of the weighting agent inside the formation. In some embodiments, the modified particulate weighting agents may be solubilized in acids providing advantages where acid stimulating treatments are employed. In some such embodiments, the removal of the weighting agent may be facilitated by the dissolution of the polymer, the core metal oxide particulate, or both.

[0015] Finally, modified particulate weighting agents employed in methods of the invention may incorporate stimuli responsive smart polymers to alter the properties of the weighting agent as desired. For example, a superhydrophobic surface may be altered to a hydrophilic surface after exposing the linked polymer to changes in pH or other stimuli, such as temperature. Given the guidance provided herein, other advantages will be apparent to the skilled artisan.

[0016] In some embodiments, the present invention provides methods comprising the steps of providing treatment fluids for use in subterranean formations, the treatment fluids comprising modified particulate weighting agents, the weighting agents comprising particulate metal oxides and polymers linked to the particulate metal oxides, and the methods further comprising introducing the treatment fluids into the subterranean formation.

[0017] As used herein, the term "treatment fluid" includes any fluid used in drilling, cementing, stimulation, or other operations conducted in a subterranean location. The term "treatment" does not imply any particular action by the fluid relative to the subterranean formation. Treatment fluids may include a base fluid comprising a hydrocarbon, water, or mixtures thereof (e.g., emulsions, invert emulsions, foamed fluids, etc.). In addition to the modified particulate weighting agents disclosed herein, treatment fluids may include other additives such as viscosifiers, emulsifiers, proppants, pH modifying agents, cementing compositions, lost circulation materials, corrosion inhibitors, other subterranean treatment fluid additives, and the like, depending on the function of the treatment fluid.

[0018] As used herein, the term "weighting agent" refers to particulates used to modulate the density of the treatment fluid. In particular, weighting agents employed in methods of the invention may be used to increase the density of the treatment fluids.

[0019] As used herein, the term "modified" means the particulate weighting agent and polymer are linked by way of chemical modification. Such chemical modification may include covalent bonding, ionic boding, metal coordination chemistry, and other chemisorptive (i.e. chemical adsorption) processes. "Modified," as used herein, is distinguished from mere physical adsorption (physisorption). Such physical adsorptive processes include, for example, the physical adsorption of polymers or surfactants to a weighting agent particulate surface through weak induced dipole moments. Such physical adsorption may be highly reversible and lead to undesired separation of the polymer/surfactant and the weighting agent particulate.

[0020] "Modified," as used herein, is also distinguished from encapsulation without bonding or physical/shear grinding together of polymer and weighting agent components. Both of these motifs lack chemical bonding between a polymer and particulate weighting agent. In the case of mere encapsulation, changes in the polymer morphology upon exposure to solvents or other conditions, such as pH or temperature, may lead to a "loose" weighting agent particle within a polymer cage and may even result in the loss of the weighting agent particle in the case of sufficiently porous polymer structures. In addition to the robust connectivity between polymer and particulate weighting agent, the modified particulates disclosed herein are readily tailored in a bottom- up approach allowing modification at the particulate weighting agent surface with a first polymer and subsequent modification with a second polymer or other chemical agent via further chemical reaction. Such bottom-up tailoring of the particulate weighting agent surface may not be accessible using intimate mixing/shear mixing/dry blending processes which may result in exposed surfaces lacking homogeneity. That is the surface may comprise a random array of exposed polymer and exposed particulate weighting agent. Moreover, the use of chemical modification may allow for superior control of coating thickness of the polymer layer relative to physical mixing/coating processes. [0021] As used herein, the term "particulate" refers to particles having dimensions ranging from about 1 nm to about 10 microns. In some embodiments, the particulate weighting agents may be nanoparticles ranging in size from about 1 nm to about 100 nm, including any value inbetween or fractions thereof. In some embodiments, the particulate weighting agents may range in size from about 1 nm to about 500 nm, including any value inbetween or fractions thereof. In some embodiments, the particulate weighting agents may range in size from about 0.5 microns to about 1 microns, including any fractional value inbetween. In some such embodiments, the particulates may be referred to as sub-micron particles. Sub-micron particles may be distinguished from nanoparticles based on bulk matter behavior of sub-micron particles versus quantum behavior of nanoparticles. In some embodiments, particulates may range in size from about 1 micron to about 10 microns, including any value inbetween or fractions thereof. In some embodiments, particulates may range in size from about 2 microns to about 5 microns, including any value inbetween or fractions thereof.

[0022] Any of the aforementioned ranges of sized particulates may be accessed via micronization techniques as known in the art. As used herein, the term "micronized" refers to particulates that have been processed to provide particle sizes on micron scale or less. For example, micronized particles may have an effective diameter from between about 1 micron to about 10 microns in some embodiments, and from about 1 micron to about 5 microns in other embodiments, including any value inbetween or fractions thereof. The effective diameter refers to an average particle diameter based on an idealized spherical geometry, with the understanding that the particles may exhibit imperfections that cause the particle to deviate from perfect spherical shape. The term "micronized" also encompasses submicron sized particles including particles less than about 1 micron. Submicron particles also include nanometer scale particulates ranging in size from about 1 nanometer to about 1000 nanometers, the distinction between bulk and quantum behavior notwithstanding. Thus, where quantum behavior may be evident, the particulates may more appropriately be referred to as nanoparticles.

[0023] Micronized particulates are accessed via any methods known in the art. Such methods include milling, bashing, grinding, and various methods employing supercritical fluids such as the RESS process (Rapid Expansion of Supercritical Solutions), the SAS method (Supercritical Anti-Solvent) and the PGSS method (Particles from Gas Saturated Solutions).

[0024] As used herein, the term "metal oxide" refers to any oxidized form of a metal, metalloid, semimetal, or semiconducting element, including any alkaline earth metals (Group II), transition metals (d-block), lanthanides or actinides (f-block), and the like. Exemplary metal oxides include, without limitation, oxides of iron, silicon, aluminum, manganese, barium, calcium, magnesium, zinc, titanium, and the like.

[0025] As used herein, the term "linked," when used in reference to the relationship between the polymer and the particulate weighting agent, means chemically bonded. Bonding motifs include, for example, covalent bonding and ionic bonding. In some embodiments, bonding encompasses metal-ligand coordination chemistry. In some embodiments, the chemical bonding provided may be substantially irreversible, meaning that forcing conditions may sever the bonding between the weighting agent and the polymer. In some embodiments, the chemical bonding provided may be moderately reversible. In some such embodiments, reversible attachment may include cleavage of the polymer from the weighting agent under special reaction conditions such as base labile detachment, acid labile detachment, photolabile detachment, oxidative or reductive detachment, and the like. "Linked" also encompasses the use of smaller organic fragments, such as linkers, to indirectly connect the polymer and the weighting agent. Linkers may be of any type commonly employed in the art of solid phase synthesis. Linkers may include oligomers, such as peptides, polyethylene glycols, propylene glycols, and the like. Examples of linkers may be any found in, for example, "Linker Strategies in Solid-Phase Organic Synthesis," Peter H. Scott editor, John Wiley & Sons, Inc., Somerset, NJ, December 2009.

[0026] In some embodiments, providing the treatment fluid may include providing a fluid intended for use as a drilling fluid. The methods of the invention may include the use of drilling fluids to control formation pressure. In some such embodiments, the drilling fluid may include the weighting agents disclosed herein along with viscosifiers, other densifying additives such as brines, and other agents depending on the nature of the of the formation being drilled. Drilling fluids may be formulated to be thixotropic to aid in the removal of drill cuttings from the wellbore. Drilling fluids may further include bridging agents, lost circulation materials, and other agents to provide zonal isolation in porous formations. Drilling fluids may include other additives to minimize formation damage, provide lubrication during drilling and provide cooling to the drill bit.

[0027] In some embodiments, the drilling fluid may be a water-based drilling mud. In some embodiments, such a mud may include bentonite clay as a gellant, with weighting agents disclosed herein. Various thickeners may be employed to modulate the viscosity of the fluid. Exemplary thickeners may include, without limitation, xanthan gum, guar gum, glycol, carboxymethylcellulose (polyanionic cellulose, PAC or CMC), scleroglucan gum, synthetic hectorite, hydroxyethyl cellulose (HEC), diutan gum or starch, or any combination thereof. In some embodiments, a drilling fluid according to the present invention may include, deflocculants to reduce viscosity when employing clay-based muds; anionic polyelectrolytes, such as acrylates, polyphosphates, lignosulfonates or tannic acid derivates such as Quebracho. Other additives may include lubricants, shale inhibitors, fluid loss control additives to control loss of drilling fluids into permeable formations, anti-foaming agents, pH-modulating additives, antimicrobial agents, H ₂ S/C0 ₂ and/or oxygen scavengers, corrosion inhibition agents.

[0028] In some embodiments, a drilling fluid according to the present invention may be an oil-based drilling mud. As used herein, "oil-based drilling mud" includes invert-emulsion oil muds. Oil-based mud may include a petroleum product such as diesel fuel as a base fluid. Oil-based muds maybe used to provide increased lubricity, enhanced shale inhibition, and greater cleaning abilities with lower viscosity. Oil-based muds also withstand greater heat without breaking down. Any of the additives described herein above maybe included in the oil based mud in conjunction with the weighting agents disclosed herein.

[0029] In some embodiments, the drilling fluid is a synthetic-based mud (SBM). SBMs may include systems based on commercially available formulations such as the ENCORE® fluid (on the world-wide web at halliburton.com/hpht, Halliburton, Houston, TX). Any such commercial formulation maybe modified by inclusion of weighting agents disclosed herein.

[0030] The base fluid, or carrier fluid, suitable for use in the drilling fluids of the present invention may include any of a variety of fluids suitable for use in a drilling fluid. Examples of suitable carrier fluids include, but are not limited to, aqueous-based fluids (e.g., water, oil-in- water emulsions), oleaginous-based fluids (e.g., invert emulsions). In certain embodiments, the aqueous fluid may be foamed, for example, containing a foaming agent and entrained gas. In certain embodiments, the aqueous-based fluid comprises an aqueous liquid. Examples of suitable oleaginous fluids that may be included in the oleaginous-based fluids include, but are not limited to, alpha-olefins, internal olefins, alkanes, aromatic solvents, cycloalkanes, liquefied petroleum gas, kerosene, diesel oils, crude oils, gas oils, fuel oils, paraffin oils, mineral oils, low- toxicity mineral oils, olefins, esters, amides, synthetic oils (e.g., polyolefins), polydiorganosiloxanes, siloxanes, organosiloxanes, ethers, acetals, dialkylcarbonates, hydrocarbons, and combinations thereof; in certain embodiments, the oleaginous fluid may comprise an oleaginous liquid.

[0031] Generally, according to the present invention, the carrier fluid may be present in a treatment fluid in an amount sufficient to form a pumpable fluid. By way of example, the carrier fluid may be present in a drilling fluid according to the present invention in an amount in the range of from about 20% to about 99.99% by volume of the drilling fluid, including any value inbetween and fractions thereof. One of ordinary skill in the art with the benefit of this disclosure will recognize the appropriate amount of carrier fluid to include within the drilling fluids of the present invention in order to provide a drilling fluid for a particular application.

[0032] In addition to the carrier fluid, the weighting agent may be present in the drilling fluid in an amount sufficient for a particular application. For example, the modified particulate weighting agent may be included in a drilling fluid to provide a particular density. In certain embodiments, the modified particulate weighting agent may be present in the drilling fluid in an amount up to about 60% by volume of the drilling fluid (v%) (e.g., about 5%, about 15%, about 20%, about 25%, about 30%, about 35%, about 40%, about 45%, about 50%, about 55%, and about 60%, including all values inbetween and fractions thereof). In certain embodiments, the weighting agent may be present in the drilling fluid in an amount in a range from about 10 v% to about 60 v%.

[0033] In some embodiments of the present invention, providing the treatment fluid entails providing a cementing fluid comprising the weighting agents disclosed herein. In some embodiments, some such methods of the invention further include allowing the cementing fluid to set in an area in the subterranean formation.

[0034] Cementing fluids include any cement composition comprising a cementitious particulate. Cementing fluids may include any hydraulic or non- hydraulic cement composition, such as a Portland or Sorel cement, respectively. Suitable examples of hydraulic cements that may be used include, but are not limited to, those that comprise calcium, aluminum, silicon, oxygen, and/or sulfur, which set and harden by reaction with water. Examples include, but are not limited to, Portland cements, pozzolanic cements, gypsum cements, calcium phosphate cements, high alumina content cements, silica cements, high alkalinity cements, and mixtures thereof. Cementing fluids may include any composition used in the formation of set cement sheath in a wellbore. Cementing fluids may include cementing kiln dust (CKD), fly ash, and other additives as recognized by one skilled in the art.

[0035] Cementing fluids according to the present invention may include lost circulation materials, defoaming agents, foaming agents, plastic fibers, carbon fibers or glass fibers to adjust a ratio of the compressive strength to tensile strength (CTR), elastomers, and rubber, accelerator or retarders to modulate the setting time, and the like, any of which may be used in any combination. In some embodiments, weighting agents disclosed herein are used in conjunction with spacer fluids ahead of cementing fluids. In some such embodiments, the spacer fluid may employ weighting agents disclosed herein, while the cementing fluid does not require a weighting agent.

[0036] One skilled in the art will appreciate that while drilling and cementing fluids are described herein above, other subterranean treatment fluids may employ weighting agents as disclosed herein, for example, those that may benefit from the additional weight provided by the weighting agents of the present invention or any of the advantages disclosed herein. Any such treatment fluid maybe oil-based, water-based, or a water-oil mixture and/or emulsions.

[0037] In some embodiments, the treatment fluid may be introduced into a subterranean formation or a particular zone in a subterranean formation. While the most common methods for introducing fluids into a formation comprise pumping the fluid into the formation via the casing string, other treatment fluids may be delivered in the annulus between the casing string and the wall of the formation. In some embodiments, a treatment fluid maybe delivered via the casing string and then into targeted fractures within the formation. In some embodiments, the treatment fluid comprising weighting agents disclosed herein are introduced into fractures created by a perforation gun. In some such embodiments, the weighting agent is part of a fracturing fluid.

[0038] In some embodiments, treatment fluids employing weighting agents disclosed herein may be useful during 1) drilling, 2) cementing, 3) completion (including perforation), 4) well intervention or work-over, 5) hydraulic fracturing or acidification and 6) as packer fluid (fluid left between surface casing and production tubing, above reservoir isolating packer). The skilled artisan will recognize the utility of treatment fluids incorporating weighting agents disclosed herein in other applications.

[0039] In some embodiments, methods of the invention employ a particulate weighting agent may be a metal oxide particle less than about 5 microns. For example, the metal oxide particle maybe about 1 micron, about 2 microns, about 3 microns, about 4, microns or about 5 microns, including fractions thereof. In some embodiments, the metal oxide particle may be less than about 1 micron. Sub-micron metal oxide particles may have a particle size distribution such that at least 90% of the particles have a diameter ("d ₉₀ ") below about 1 micron. In certain embodiments, the sub-micron metal oxide particles may have a particle size distribution such that at least 10% of the particles have a diameter ("di ₀ ") below about 0.2 microns, 50% of the particles have a diameter ("d ₅₀ ") below about 0.3 microns and 90% of the particles have a diameter (d ₉₀ ) below about 0.5 micron.

[0040] In some embodiments, the metal oxide particles have at least one dimension that is about 500 nm or less. In some embodiments, the metal oxide maybe about 500 nm, about 400 nm, about 300 nm, about 200 nm, about 100 nm, about 50 nm, about 10 nm, including any value inbetween and fractions thereof. Advantageously, in some embodiments, where the particle is smaller than about 500 nm, the treatment fluid need not include any suspending agent to maintain suspension of the modified particulate weighting agent. Thus, in some embodiments, the modified particulate weighting agent is capable of self- suspending without the aid of a suspending agent. In some such embodiments, the treatment fluid may exclude viscosifying agents, although, this will depend on the actual function of the treatment fluid. For example, a viscosifying agent may still be needed in a drilling fluid to aid in removing drilling cuttings. The use of smaller particle sizes may also help prevent sagging when used with or without suspending agents.

[0041] In some embodiments, the metal oxide particle may comprise a standard size weighting agent particle size including a d ₅₀ of about 20 microns and a d ₉₀ of about 70 microns. In some such embodiments, the treatment fluid may include suspending agents to aid in preventing the settling of the weighting agent.

[0042] As described above, methods of the invention may utilize weighting agents comprising a metal oxide particle comprising any number of metals, metalloids, or semi-conducting metals. In some embodiments, the metal oxide comprises a metal selected from the group consisting of manganese, iron, titanium, silicon, zinc, and any combination thereof. While the oxide form of a metal is chosen for its ability to provide a point of attachment for chemically bonding a polymer or linker/polymer combination, the skilled artisan will recognize that metal forms other than oxides may serve this purpose. For example, in some embodiments, the metal may comprise a zero-valent metal- or metal ion-polymer pairing in which at least a portion of the polymer is capable of linking to the zero-valent metal or metal ion via ligand coordination chemistry. As used herein, zero-valent means a metal having no formal charge associated with higher oxidations states. When engaging in ligand coordination, the polymers may contain organic functional groups for this purpose including, without limitation, alcohols, carboxylates, amines, thiols (mercaptans), or other heteroatom function groups serving as a ligand donor to the zero-valent metal or metal ion.

[0043] In some embodiments, the metal oxide particle comprises manganese tetraoxide (Mn ₃ 0 ₄ ). In some such embodiments, the particle is provided as a nanoparticle. Manganese tetraoxide is particularly useful in the present invention due to the ability to degrade the weighting agent by dissolution of the manganese tetraoxide upon treatment with an acid source.

[0044] In some embodiments, the polymer maybe hydrophobic. Hydrophobic polymers may include any degree of crosslinking, but generally lack the presence of substantial numbers of heteroatoms that confer polar character to the polymer. The term "hydrophobic polymer" is used herein to mean any polymer resistant to wetting, or not readily wet, by water, that is, having a lack of affinity for water. Examples of hydrophobic polymers may include, without limitation, polyolefins, such as polyethylene, poly(isobutene), poly(isoprene), poly(4-methyl- l-pentene), polypropylene, ethylene-propylene copolymers, ethylene-propylene-hexadiene copolymers, and ethylene-vinyl acetate copolymers; metallocene polyolefins, such as ethylene-butene copolymers and ethylene-octene copolymers; styrene polymers, such as poly(styrene), poly(2- methylstyrene), and styrene-acrylonitrile copolymers having less than about 20 mole-percent acrylonitrile; vinyl polymers, such as poly(vinyl butyrate), poly(vinyl decanoate), poly(vinyl dodecanoate), poly(vinyl hexadecanoate), poly(vinyl hexanoate), poly(vinyl octanoate), and poly(methacrylonitrile); acrylic polymers, such as poly(n-butyl acetate), and poly(ethyl acrylate); methacrylic polymers, such as poly(benzyl methacrylate), poly(n-butyl methacrylate), poly(isobutyl methacrylate), poly(t-butyl methacrylate), poly(t-butylaminoethyl methacrylate), poly(do-decyl methacrylate), poly(ethyl methacrylate), poly(2- ethylhexyl methacrylate), poly(n-hexyl methacrylate), poly(phenyl methacrylate), poly(n-propyl methacrylate), and poly(octadecyl methacrylate); polyesters, such a poly(ethylene terephthalate) and poly(butylene terephthalate); and polyalkenes and polyalkynes, such as polybutylene and polyacetylene.

[0045] The term "polyolefin" is used herein to mean a polymer prepared by the addition polymerization of one or more unsaturated monomers which contain only carbon and hydrogen atoms. Examples of such polyolefins may include, without limitation, polyethylene, polypropylene, poly(l-butene), poly(2- butene), poly(l-pentene), poly(2-pentene), poly(3-methyl-l-pentene), poly(4- methyl- l-pentene), and the like. In addition, such term is meant to include blends of two or more polyolefins and random and block copolymers prepared from two or more different unsaturated monomers.

[0046] In some embodiments, methods of the invention employ hydrophobic polymers to provide modified particulate weighting agents that are superhydrophobic. In some such embodiments, the hydrophobic polymer may include fluorinated polyolefins and other perfluoroalkyl polymers and perfluoropolyethers. In some such embodiments, the weighting agents constructed form such polymers may be particularly well suited for oil-based treatment fluids, including oil-based drilling muds. [0047] In some embodiments, the polymer is hydrophilic, while in other embodiments the polymer is an amphiphilic copolymer comprising at least one hydrophobic portion and at least one hydrophilic portion. Hydrophilic polymers may include any array of heteroatoms that confer polarity to the polymer. Moreover, some such polymers may contain organic functional groups capable of supporting a formal charge, such as carboxylates, amines/ammonium groups, including mono alkyl ammonium, dialkyl ammonium, trialkylammonium, and tetraalkyl ammonium salts, sulfonates or alkyl sulfonates, phosphates or alkyl phosphates, or other charged functional groups. Examples of hydrophilic polymers may include, without limitation, polyethylene glycol (PEG), poly(vinyl alcohol), polyvinylpyrrolidone, chitosan, starch, sodium carboxymethylcellulose, cellulose, hydroxyethyl cellulose, sodium alginate, guar, scleroglucan, diutan, welan, gellan, xanthan, and carrageenan.

[0048] Other suitable hydrophilic polymers may include homopolymers, copolymers, or terpolymers including, without limitation, polyacrylamides, polyvinylamines, poly(vinylamines/vinyl alcohols), alkyl acrylate polymers, and combinations thereof. Additional examples of alkyl acrylate polymers may include polydimethylaminoethyl methacrylate, polydimethylaminopropyl methacrylamide, poly(acrylamide-dimethylaminoethyl methacrylate), poly(methacrylic acid-dimethylaminoethyl methacrylate), poly(2-acrylamido-2- methyl propane sulfonic acid/dimethylaminoethyl methacrylate), poly(acrylamide-dimethylaminopropyl methacrylamide), poly (acrylic acid/dimethylaminopropyl methacrylamide), poly(methacrylic acid- dimethylaminopropyl methacrylamide), and combinations thereof. In certain embodiments, the hydrophilic polymers may comprise a polymer backbone and reactive amino groups in the polymer backbone or as pendant groups, the reactive amino groups capable of engaging a zero-valent metal or metal ion ligand coordination sphere. In some embodiments, the hydrophilic polymers may comprise dialkyl amino pendant groups. In some embodiments, the hydrophilic polymers may comprise a dimethyl amino pendant group and a monomer comprising dimethylaminoethyl methacrylate or dimethylaminopropyl methacrylamide. In certain embodiments, the hydrophilic polymers may comprise a polymer backbone that comprises polar heteroatoms, wherein the polar heteroatoms present within the polymer backbone of the hydrophilic polymers include oxygen, nitrogen, sulfur, or phosphorous. Suitable hydrophilic polymers that comprise polar heteroatoms within the polymer backbone include, without limitation, homopolymer, copolymer, or terpolymers, such as, but not limited to, celluloses, chitosans, polyamides, polyetheramines, polyethyleneimines, polyhydroxyetheramines, polylysines, polysulfones, gums, starches, and combinations thereof. In some embodiments, the starch maybe a cationic starch. A suitable cationic starch maybe formed by reacting a starch, such as corn, maize, waxy maize, potato, tapioca, or the like, with the reaction product of epichlorohydrin and trialkylamine.

[0049] In some embodiments, the polymer employed in methods of the invention maybe a synthetic polymer or a naturally occurring polymer. In some embodiments, the polymer may be based on amino acids and maybe a protein. In some embodiments, the polymer maybe based on polysaccharides or glycoproteins. In some embodiments, the polymer may be a PEG-based polymer. In some embodiments, the polymer may be selected to swell in polar solvent such as water. In some embodiments, the polymer may be selected to swell in a nonpolar solvent, such as a hydrocarbon based solvent like diesel. In some embodiments, the polymer may be selected to resist swelling regardless of what solvent is employed.

[0050] In some embodiments, smart polymers may be employed to allow a change in the polymers character, including, without limitation, polarity molecular weight, and degree of crosslinking. In some embodiments, the polymer may comprise a block copolymer. In some such embodiments, the block copolymer may be a diblock, triblock, tetrablock, or other multiblock copolymer. In some embodiments, the polymer may comprise a graft copolymer. In some embodiments, the polymer may be a periodic copolymer. In some embodiments, the polymer may be an alternating copolymer. In some embodiments, the polymer may be an interpolymer.

[0051] In some embodiments, the linked polymer may be selected to be degradable. Suitable examples of degradable polymers that may be used in accordance with the present invention include, but are not limited to, those described in the publication of Advances in Polymer Science, Vol. 157 entitled "Degradable Aliphatic Polyesters," edited by A.C. Albertsson, pages 1- 138. Specific examples include homopolymers, random, block, graft, and star- and hyper-branched aliphatic polyesters. Such suitable polymers may be prepared by polycondensation reactions, ring-opening polymerizations, free radical polymerizations, anionic polymerizations, carbocationic polymerizations, coordinative ring-opening polymerizations, as well as by any other suitable process.

[0052] Examples of suitable degradable polymers that may be used in conjunction with the methods of this invention include, but are not lim ited to, aliphatic polyesters; poly(lactides) ; poly(glycolides); poly(s-caprolactones) ; poly(hydroxy ester ethers); poly(hydroxybutyrates) ; poly(anhydrides) ; polycarbonates; poly(orthoesters); poly(amino acids); poly(ethylene oxides) ; poly(phosphazenes) ; polyether esters, polyester am ides, polyamides, and copolymers or blends of any of these degradable polymers, and derivatives of these degradable polymers. The term "copolymer" as used herein is not lim ited to the combination of two polymers, but includes any com bination of polymers, e.g . , terpolymers and the like.

[0053] As referred to herein, the term "derivative" is defined herein to include any compound that is made from one of the listed compounds, for exam ple, by replacing one atom in the base compound with another atom or group of atoms. Of these suitable polymers, aliphatic polyesters such as poly(lactic acid), poly(anhydrides), poly(orthoesters), and poly(lactide)-co- poly(glycolide) copolymers maybe beneficially em ployed, especially poly(lactic acid) and poly(orthoesters) . Other degradable polymers that are subject to hydrolytic degradation also may be suitable. One's choice may depend on the particula r application or use and the conditions involved . Other guidelines to consider include the degradation products that result, the time for required for the requisite degree of degradation, and the desired result of the degradation, such as removal of the weighting agent.

[0054] Suitable aliphatic polyesters have the general formula of repeating units shown belo

Formula I

where n is an integer between 75 and 10,000 and R is selected from the group consisting of hydrogen, alkyl, aryl, alkylaryl, acetyl, heteroatoms, and mixtures thereof. In certain em bodiments of the present invention wherein an aliphatic polyester is used, the aliphatic polyester may be poly(lactide) . Poly(lactide) is synthesized either from lactic acid by a condensation reaction or, more commonly, by ring-opening polymerization of cyclic lactide monomer. Since both lactic acid and lactide may achieve the same repeating unit, the general term poly(lactic acid) as used herein is included in Formula I without any lim itation as to how the polymer was made (e.g. , from lactides, lactic acid, or oligomers), and without reference to the degree of polymerization or level of plasticization.

[0055] The lactide monomer exists generally in three different forms: two stereoisomers (L- and D-lactide) and racemic D,L-lactide (/meso-lactide) . The oligomers of lactic acid and the oligomers of lactide are defined by the formula :

where m is an integer in the range of from greater than or equal to about 2 to less than or equal to about 75. In certain embodiments, m may be an integer in the range of from greater than or equal to about 2 to less than or equal to about 10. These limits may correspond to num ber average molecular weights below about 5,400 and below about 720, respectively.

[0056] The chirality of the lactide units provides a means to adjust, inter alia, degradation rates, as well as physical and mechanical properties. Poly(L-lactide), for instance, is a semicrystalline polymer with a relatively slow hydrolysis rate. This could be desirable in applications or uses of the present invention in which a slower degradation of the degradable material is desired . Poly(D, L-lactide) may be a more amorphous polymer with a resultant faster hydrolysis rate. This may be suitable for other applications or uses in which a more rapid degradation may be appropriate. The stereoisomers of lactic acid may be used individually, or may be combined in accordance with the present invention. Additionally, they may be copolymerized with, for example, glycolide or other monomers like ε-caprolactone, l,5-dioxepan-2-one, trimethylene carbonate, or other suitable monomers to obtain polymers with different properties or degradation times. Additionally, the lactic acid stereoisomers maybe modified by blending high and low molecular weight polylactide or by blending polylactide with other polyesters, in embodiments wherein polylactide is used as the degradable material, certain preferred embodiments employ a mixture of the D and L stereoisomers, designed so as to provide a desired degradation time and/or rate. Examples of suitable sources of degradable material are poly(lactic acids) that are commercially available from NatureWorks ^® of Minnetonka, MN, under the trade names "300 ID" and "4060D. "

[0057] Aliphatic polyesters useful in the present invention may be prepared by substantially any of the conventionally known manufacturing methods such as those described in U.S. Patent Nos. 6,323,307; 5,216,050; 4,387,769; 3,912,692; and 2,703,316, the relevant disclosures of which are incorporated herein by reference.

[0058] Polyanhydrides are another type of degradable polymer that may be suitable for use in the present invention. Examples of suitable polyanhydrides include poly(adipic anhydride), poly(suberic anhydride), poly(sebacic anhydride), and poly(dodecanedioic anhydride). Other suitable examples include, but are not limited to, poly(maleic anhydride) and poly(benzoic anhydride).

[0059] The physical properties of degradable polymers may depend on several factors including, but not limited to, the composition of the repeat units, flexibility of the chain, presence of polar groups, molecular mass, degree of branching, crystallinity, and orientation. For example, short chain branches may reduce the degree of crystallinity of polymers while long chain branches may lower the melt viscosity and may impart, inter alia, extensional viscosity with tension-stiffening behavior. The properties of the material utilized further may be tailored by blending, and copolymerizing it with another polymer, or by a change in the macromolecular architecture (e.g., hyper-branched polymers, star-shaped, or dendrimers, and the like). The properties of any such suitable degradable polymers (e.g., hydrophobicity, hydrophilicity, rate of degradation, and the like) maybe tailored by introducing select functional groups along the polymer chains. For example, poly(phenyllactide) will degrade at about one-fifth of the rate of racemic poly(lactide) at a pH of 7.4 at 55 °C. One of ordinary skill in the art, with the benefit of this disclosure, will be able to determine the appropriate functional groups to introduce to the polymer chains to achieve the desired physical properties of the degradable polymers. [0060] In some embodiments, methods of the invention include a weighting agent in which the polymer is covalently linked to the metal oxide particle. In some embodiments, the polymer is linked via ionic bonding. In some embodiments, the polymer is linked to any metal center, not just metal oxides, via ligand coordination chemistry. As described herein above, the nature of the chemical bonding maybe configured to be substantially irreversible or moderately reversible. In some embodiments, the polymer is linked to the metal oxide via a linker molecule as described above.

[0061] Polymers employed in the present invention may vary in molecular weight and degree of cross-linking suitable for compatibility with the intended application of the weighting agent. For example, the molecular weight of the polymer and its degree of cross-linking may be chosen for any number of physical properties such as swellability, stiffness, strength, and toughness.

[0062] In some embodiments, the present invention provides a method comprising providing a treatment fluid for use in a subterranean formation comprising a weighting agent, the weighting agent comprising a micronized metal oxide particle and a polymer covalently linked to the metal oxide particle, the method further including introducing the treatment fluid into the subterranean formation.

[0063] In some such embodiments, the fluid is a drilling fluid or a cementing fluid as described herein. In some such embodiments, the metal oxide particle is a nanoparticle. In some such embodiments, the polymer comprises one selected from the group consisting of a hydrophobic polymer, a hydrophilic polymer, and a copolymer comprising at least one hydrophobic portion and at least one hydrophilic portion. In some such embodiments, the weighting agent is capable of self-suspending without the aid of a suspending agent.

[0064] In some embodiments, the weighting agent disclosed herein are used to increase the treatment fluid density to provide at least one function selected from the group consisting of controlling formation pressure, maintaining borehole stability, and preventing the introduction of formation fluids into a borehole. Although the weighting agents are described herein are described in the context of treatment fluids for subterranean operations, other uses will be recognized by the skilled artisan. [0065] Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the present invention. The invention illustratively disclosed herein suitably may be practiced in the absence of any element that is not specifically disclosed herein and/or any optional element disclosed herein. While compositions and methods are described in terms of "comprising," "containing," or "including" various components or steps, the compositions and methods may also "consist essentially of" or "consist of" the various components and steps. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, every range of values (of the form, "from about a to about b," or, equivalently, "from approximately a to b," or, equivalently, "from approximately a-b") disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles "a" or "an," as used in the claims, are defined herein to mean one or more than one of the element that it introduces. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted.