WO2013138156A1

WO2013138156A1 - Synthesis and application of high pressure high temperature fluid loss additive and rheology stabilizer

Info

Publication number: WO2013138156A1
Application number: PCT/US2013/029610
Authority: WO
Inventors: Jianzhao Janice WANG; Jun Zheng; David Farrar; Feng SONG
Original assignee: Isp Investments Inc.
Priority date: 2012-03-13
Filing date: 2013-03-07
Publication date: 2013-09-19

Abstract

A high pressure, high temperature (HPHT) fluid loss additive and, in particular, a fluid loss additive for oil-field drilling applications is described. In accordance with one aspect, the fluid loss additive comprises a terpolymer of acrylamide (AM), 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and a cationic monomer such as acrylamidopropyl-trimethyl ammonium chloride (APT AC) and/or methacrylamidopropyltrimethyl ammonium chloride (MAPTAC).

Description

SYNTHESIS AND APPLICATION OF HIGH PRESSURE HIGH TEMPERATURE FLUID LOSS ADDITIVE AND RHEOLOGY STABILIZER

[0001] The present application is directed to a high pressure, high temperature (HPHT) fluid loss additive and, in particular, to a fluid loss additive for oil-field drilling applications. In accordance with one aspect of the invention, the fluid loss additive comprises a terpolymer of acrylamide (AM), 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and a cationic monomer such as acrylamidopropyl-trimethyl ammonium chloride (APTAC) and/or methacrylamidopropyltrimethyl ammonium chloride (M APT AC).

[0002] In accordance with particularly useful embodiments, the polymer composition comprises from about 30 -70 wt. % acrylamide, 30 -70 wt. % AMPS, and 2 -50 wt. % APTAC and/or MAPTAC. AMPS can be in either acid or neutralized form used in the polymerization process.

BACKGROUND

[0003] Fluid loss additives (FLAs) are widely used in drilling fluids and cementing slurries to control the loss of fluid to the formation through filtration. Drilling Fluids or cementing slurries that lose water can also lose or degrade their design properties. Fluid loss additives help operators retain the key characteristics of their drilling fluids or cementing slurries, including viscosity, thickening time, rheology and comprehensive strength-development. Moreover, FLA's minimize the high risk of permeability damage.

[0004] Natural biopolymers such as cellulosic polymers, starches, modified starches, CMC/polysaccharide have been used as FLAs. However the thermal stability of the starch and cellulose derivatives is in the range of 120-150°C, which is not suitable for challenging wellbore drilling operations such as high pressure, high temperature (HPHT). Therefore, synthetic polymers are typically used as FLAs in the severe drilling and cementing conditions.

[0005] Polyacrylamide and its copolymers with other monomers (e.g., 2-acrylamido-2- methyl-propanesulfonic acid (AMPS), vinylpyrrolidone (NVP), N-vinylacetamide, alkylacrylamide, etc) have been reported to have fluid loss control effects. Solution polymerization and other polymerization techniques are typically used to manufacture synthetic fluid loss additives.

[0006] As more and more challenging conditions are encountered in oilfield drilling operations, there is a need for improved high-performance fluid loss additives and rheology modifiers. The enhanced performance of the drilling fluids especially the High Pressure, High Temperature (HPHT) compatibility will allow faster and safer drilling.

SUMMARY

[0007] The present application is directed to thermally stable amphoteric polymers with high fluid loss control and excellent retention of rheological properties under HPHT conditions. In accordance with certain aspects, the present application is directed to a dispersion polymerization process and chemistry modification to make thermally stable amphoteric polymers. The polymers disclosed herein are effective fluid loss additives and, in certain cases, rheology modifiers. Moreover, the fluid loss additive or rheology modifier disclosed herein can be delivered to the field as a water dispersion or a dry powder to facilitate handling and processing.

[0008] The fluid loss additive described herein exhibits rheological and thermal stability properties that are particularly useful in high-pressure/high temperature drilling operations. In accordance with one aspect, the fluid loss additive comprises an amphoteric terpolymer of poly(NaAMPS/AM/MAPTAC) or poly(NaAMPS/AM/APTAC). The polymers may be made via water dispersion polymerization. This polymer not only can be used as HPHT FLAs, but imparts excellent retention of drilling mud rheological properties. Moreover, the FLA described herein exhibits interesting synergistic effects with other ingredients (e.g., rheology modifiers) commonly used in water-based drilling fluids, further enhancing thermal stability. The polymer can be used as a fluid loss control additive and secondary viscosifier and stabilizer of rheological property in HPHT water-based drilling mud application. The amphoteric polymers taught in this application can also be useful in oilfield cement slurries as high performance fluid loss control additives and secondary viscosifiers.

[0009] In accordance with one aspect of the present invention, the FLA comprises a terpolymer comprising about 20 -80 wt. % acrylamide, 20 -80 wt. % AMPS, and 2 -50 wt. % of a cationic monomer such as APT AC and/or MAPTAC. In accordance with particularly useful aspects of the present invention, the rheology modifier comprises a terpolymer comprising about 40-60 wt. % acrylamide, 40 -60 wt. % AMPS, and 5 -30 wt. % cationic monomer. In accordance with another embodiment of the present invention, the polymer comprises approximately equal parts by weight acrylamide and AMPS. In certain embodiments, the cationic monomer such as APT AC and/or MAPTAC may be present in an amount of about 8 - 25 wt . The weight percentages provided herein are based on the total weight (100%) of AM and AMPS and/or salts monomers. In some of the working examples provided below weight percentages are provided based on total weight of all monomers. The context in each case clearly indicates which weight percentage calculation is being used.

[0010] In accordance with certain aspects of the present invention, the fluid loss additive exhibits improved salt-tolerance, temperature-stability, and fluid loss properties as compared to conventional fluid loss additives. The fluid loss additive may be provided in water dispersion or powder form to facilitate processing and the use of the additive in water-based drilling muds as a temperature- stable fluid loss additive. In accordance with one aspect, the present application provides a method of preventing fluid loss during oilfield drilling operations, wherein the method includes drilling a wellbore and circulating a fluid containing an effective amount of the fluid loss additive described herein.

[0011] The fluid loss additives described herein typically have a weight average molecular weight (Mw) over 3,000 daltons, more particularly over 10,000 daltons, more particularly from about 100,000 to 10,000,000 daltons, and in certain cases from about 1,000,000 to 5,000,000 daltons as determined by GPC. Weight average molecular weight (Mw) is defined in the following equation

[0012] Moreover, the compositions can be produced at relatively high polymer solids (e.g. 20 - 30% in water dispersion form) while still providing acceptable bulk viscosity for processing the water dispersion (e.g., spray drying).

DETAILED DESCRIPTION

[0013] The present application is directed to a thermally- stable fluid loss additive and, in particular, to a polymer suitable for use under HPHT conditions. HPHT refers generally to wells that are hotter or at higher pressure than most wells. In accordance with some aspects, HPHT may refer to a well having an undisturbed bottomhole temperature of greater than 300°F [149°C] and a pore pressure of at least 0.8 psi/ft (-15.3 lbm/gal). The present application describes an HPHT filtration test (i.e., HPHT fluid loss test) wherein the test is conducted at conditions that provide an indication as to how the composition would perform under HPHT conditions. In accordance with this test, static filtration behavior of water mud or oil mud is measured at elevated temperature, up to about 380°F [193°C] maximum (450°F [227°C] maximum if a special cell is used), usually according to the specifications of API with the exception of temperature and pressure. The standard API test is conducted at room temperature and a differential pressure of lOOpsi. Although the HPHT test method described herein can simulate downhole temperature conditions, it does not simulate downhole pressure. Total pressure in a cell should not exceed 700 psi [4900 kPa], and the differential pressure across the filter medium is specified as 500 psi [3500 kPa]. Therefore, in the examples described herein, the HPHT fluid loss test is conducted at temperatures of at least 200°F or more and at differential pressures of about 500 psi.

[0014] In accordance with one aspect of the present application, a thermally- stable fluid loss additive is provided. The additive comprises a polymer of an acrylamide, a sulfonic acid or salt thereof and a cationic monomer such as APTAC and/or MAPTAC. In accordance with one aspect, the polymer composition comprises from about 20 -80 wt. % of an acrylamide, 20 -80 wt. % of a sulfonic acid or salt thereof, and 5 -30 wt. % of a cationic monomer such as APTAC and/or MAPTAC. In particular, the present application is directed to a fluid loss additive for oil-field drilling applications. In accordance with one aspect of the invention, the fluid loss additive comprises a terpolymer of acrylamide (AM), 2-acrylamido- 2-methyl-propanesulfonic acid (AMPS) and acrylamidopropyl-trimethyl ammonium chloride (APTAC) and/or methacrylamidopropyltrimethyl ammonium chloride (MAPTAC).

[0015] The fluid loss additive described herein is particularly useful in oil-field drilling applications. The polymer described herein may also find use in other oil well applications. For example, it may be used in applications including, but not limited to, rheology modifier/ thickener for drilling fluids and cementing, friction reducer (lime, freshwater, salt water muds), shale swell inhibitor/clay stabilizer, viscosifier (fresh water, seawater, saline muds), filtration control, cementing retarder, oil well fracturing (e.g., friction reducer), oil well stimulation (viscosifier for acidizing), drilling aids (oil, water, geological drillings), completion fluids and workover fluids, and polymer flooding (enhanced oil recovery).

[0016] The polymer can also find use in HI&I (household, industrial and institutional products) applications including, but not limited to, thickener of bleach (e.g., disinfectants, bleaching material, sterilization, washing concentrates, etc), alkaline environments (>KOH) gels (e.g., battery applications), thickener for hydrogen peroxide (e.g., antiseptics, disinfectants, sterilization agents, cleaners), thickener for acidic hard surface cleaners, air fresheners gel applications (thickener, fragrance delivery), controlled release of actives (antiseptics, biocides, fragrances), formation of clear gels for handwash and hair styling products.

[0017] The polymers described herein can also be used in adhesives, coatings and textiles. Examples of particular applications include, but are not limited to, latex adhesives and paints, water based resins (thickener), adhesive hardeners and catalysts (thickener, where extreme pH conditions are common). Additional applications include lubricants for the batch dyeing of textiles and thickeners for adhesives and defoamers.

[0018] The described polymers can also be used in applications relating to solid-liquid separation (flocculation). Specific applications include, but are not limited to, flocculation of municipal and industrial effluents, particularly at low or high pH, clarification of acidic and alkaline mining and mineral slurries, separation of oily waters, dewatering of paper slurries, and thickener for clay slurries and tailings.

[0019] Although the present application is primarily described with respect to polymer compositions comprising acrylamide, 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and APTAC and/or MAPTAC, it is believed that the present invention can be practiced utilizing other ethylenically unsaturated monomelic acrylamides and other monomeric sulfonic acids, phosphonic acids and/or salts thereof as well as other cationic monomers. In accordance with certain embodiments, these other monomers can be used to produce a polymer capable of providing the attributes discussed in more detail below.

[0020] Examples of ethylenically unsaturated acrylamides include acrylamide,

alkylacrylamide such as methacrylamide, and furmaramide. Other monomers believed to be useful include without limitation N-alkyl Substituted or Ν,Ν' -alkyl disubstituted amides such as: N-methyacrylamide, Ν,Ν'-dimethylacrylamide, N- isopropylacrylamide, Ν,Ν'- diethylacrylamide. N-(dimethylaminomethyl)acrylamide, N-

(trimethylammoniummethyl)acrylamide, N-(trimethylammoniumpropyl)methacrylamide chloride, p-acrylamidomethylbenzyl bromide, p-acrylamidomethylbenzyl chloride, p- acrylamidomethylphenethyl chloride, p-acrylamidomethylphenethyl bromide, 3- acrylamidomethyl-p-xylyl chloride, 3-acrylamidomethyl-p-xylyl bromide, 3- methacrylamidomethyl-p-xylyl chloride, 3-methacrylamidomethyl-p-xylyl bromide, p- acrylamidomethyl-0-(2-bromethyl)-phenol, p-acrylamidomethyl-0-(2-chloroethyl)-phenol may also be useful.

[0021] Examples of sulfonic acids or salts can be summarized as shown in the following structure:

CH₂

S0₃ ^"M⁺ where Rl and R2 are independently hydrogen or methyl group (CH₃), n is a number from 0 to 18. M is H or metal salt Na, K, NH₄ etc. For example, when n=0, Ri is H, R₂ is methyl, the structure represents acrylamidopropanesulfonic acid (AMPS) and when n=2, Ri is H, R₂ is methyl, the structure represents acrylamidobutanesulfonic acid, etc.

[0022] Examples of other sulfonic acids that may be used include those represented by the following structure: where R is hydrogen or lower alkyl (C1-C5) group; X is a direct bond or a functionalized or unfunctionalized, branched or linear, alkylene, cycloalkylene, alkenylene, or arylene group, typically having from 1 to 6 carbon atoms, wherein any of the before mentioned groups may be with or without heteroatoms groups. Salts of the foregoing acids may also be used.

Specific examples of these monomers include, but are not limited to, sodium vinyl sulfonic acid/salts (SVS), sodium sulfonic acid/salts (SSS), styrenesulfonic acid/salts (SSA) and 1- Allyloxy 2-Hydroxy Propyl Sulfonic Acid and salts thereof (AHPS). AHPS is a particularly useful monomer that can be used instead of AMPS in any of the compositions disclosed herein. APHS is thermally and hydrolytically stable at high pH, saturated salt and elevated temperature conditions.

[0023] Examples of sulfonate monomers also include: 2-chloroethylene sulfonic acid, ethylenesulfonic acid, ethylenedisulfonic acid, 1 -nitriloethylenesulfonic acid, 2- formylethylenesulfonic acid, 1 -carboxyethylenesulfonic acid, 1-propene-l -sulfonic acid, 1- propene-2-sulfonic acid, 2-formyl-l -methylethylene sulfonic acid, l-carboxy-2- methylethylene sulfonic acid, 2-methyl-l,3-propenedisulfonic acid, 1-butene-l -sulfonic acid, 1 -carboxy-2,2-dimethyl-ethylene sulfonic acid, 1-pentene-l- sulfonic acid, 1-hexene-l- sulfonic acid, 2-(p-nitrophenyl)ethylene sulfonic acid, 2-phenylethylene sulfonic acid, 2-(p- hydroxyphenyl)ethylene sulfonic acid, 2-(2-aminophenyl)ethylene sulfonic acid, l-methyl-2- phenylethylene sulfonic acid, 2-(p-methoxyphenyl)ethylene sulfonic acid, 4-phenyl-l ,3- butadiene sulfonic acid, 2-(p-acetamidophenyl)ethylene sulfonic acid, 3-chloroallyl sulfonic acid, allyl sulfonic acid, 1-hydroxyallyl sulfonic acid, 2-cynoallyl sulfonic acid, 3- chloromethallyl sulfonic acid, 1-carboxyallyl sulfonic acid, 3-carboxyallyl sulfonic acid, methallyl sulfonic acid, 2-methylene-4,4-dimethyl-l ,3-disulfo-pentene, 4-methylene-4,4- dimethyl pentene sulfonic acid, l-hydroxy-3-phenylallyl sulfonic acid, 3-phenylallyl sulfonic acid, 2-benzylallyl sulfonic acid, 2-(p-methylphenoxy)allyl-sulfonic acid, 3- phenoxymethallyl sulfonic acid, 2-sulfoethyl acrylate, 2-sulfoethyl maleate, 3-sulfopropyl acrylate, 2-sulfonyl methacrylate, 3-sulfopropyl acrylate, 2-sulfo-l-(sulfomethyl)ethyl methacrylate, 3-sulfopropyl maleate, 4-sulfobutyl methacrylate, 2-(acyloxymethyl)-c- sulfuran, bis-2-sulfoethyl fumarate, 3-sulfopropyl itaconate, p-sulfophenyl acrylate, 2-(2- methylacryloxymethyl)-sulfofuran, bis(2-sulfoethyl)itaconate, p-sulfophenyl methacrylate, bis(3-sulfopropyl)maleate, bis(3-sulfopropyl)fumarate, bis(2-sulfopropyl)maleate, bis(2- sulfopropyl)fumarate, 5-methyl-2-(methallyloxy)benzene sulfonic acid, bis(2- sulfopropyl)itaconate, ar-(2-acryloyloxyethoxy)-2-naphthalene sulfonic acid, ar-(2- methacryloyloxyethoxy)-naphthalene sulfonic acid, dodecyl-4-sulfopropyl itaconate, dodecyl-4-sulfobutyl itaconate, n-acryloyl taurine, allylthioethyl sulfonic acid, alloxy propene sulfonic acid, n-allyl-n-methylaminoethane-sulfonic acid, n-(methacrylamidomethyl)- sulfoacetamide, vinyloxybenzene sulfonic acid, n-(p-sulfophenyl)methacrylamide, p-[(2- vinylsulfonyl)ethoxy] -benzene sulfonic acid, n-methyl-n-(2-vinylsulfonyl-ethyl)-p- (sodiumsulfo) benzyl amine, dichlorostyrene sulfonic acid, 2-chlorostyrene sulfonic acid, p- styrene sulfonic acid, p-sulfonic acid, vinyltoluene sulfonic acid, 2-methyl styrene sulfonic acid, the potassium, sodium and ammonium salts of each of the foregoing compounds, 4- methylene-2,2,6,6-tetramethyl-3,5-disulfoheptene, allyloxyethyl sulfonic acid, allyl oxybenzene sulfonic acid, and styrene sulfonic acid.

[0024] Phosphonic acid and salts may also be useful in accordance with certain aspects. Examples of phosphonic acids or salts can be represented by following structure:

H₂)nCH₃

OM OM where Ri and R₂ are independently hydrogen or methyl group (CH₃), n is a number from 0 to 18. M is H or a metal salt such as Na, K, etc.

[0025] Examples of phosphonic acid and phosphonate monomers include without limitation: vinylidene diphosphonic acid, vinylphosphonic acid (VP A), styrenephosphonic acid (SPA), 4-vinylbenzylphoshonic acid (VBPA), or a-phenylvinylphosphonic acid (PVPA).

[0026] Cationic monomers that may be used include diallydimethyl ammonium chloride (DADMAC) and N-methyl 2-vinyl pyridinium methyl sulfate. [0027] Also suitable are quaternized (meth)acrylate or (meth)acrylamide monomers including those described by the following structure:

wherein:Ri is hydrogen or methyl, X is O or NH,

Q is selected from a functionalized and unfunctionalized alkylene, cycloalkylene, alkenylene, or arylene group, wherein any of the before mentioned groups may be with or without heteroatoms (more particularly, C1-C6 alkylene or cycloalkylene groups),

R2, R3, and R4 are independently selected from the group consisting of functionalized or unfunctionalized alkyl groups (more particularly, independently selected C1-C8 alkyl groups), and

M is independently selected from the group consisting of alkali metal ions, alkaline earth metal ions, and the ammonium ion, and combinations thereof.

[0028] To maintain the polymer' s water solubility, the groups Q, R2, R3, and R₄ can be selected to modulate the polymer' s overall hydrophilic/hydrophobic balance.

[0029] Examples of quaternized (meth)acrylates and (meth)acrylamides include, but are not limited to: (meth)acrylamidopropyltrimethyl ammonium chloride , dimethylaminoethyl methacrylate (DMAEMA), 3-methacryloyloxy-2-hydroxypropyl trimethyl ammonium chloride, 3-acrylamido-3-methylbutyl trimethyl ammonium chloride, N-propyl acrylamido trimethyl ammonium chloride, and 2-methacryloyloxy-ethyl trimethyl ammonium methosulfate.

[0030] The polymers according to the invention may be readily synthesized by procedures known by those skilled in the art, and include free radical polymerization, solution polymerization, emulsion polymerization, and inverse emulsion polymerization (including Liquid dispersion polymerization (LDP), and "dispersion polymerization" (water-in water)). Dispersion polymerization is a particularly useful method for producing the polymers described herein. "Dispersion polymer" means a water-soluble polymer dispersed in an aqueous continuous phase containing one or more inorganic salts. In the process of dispersion polymerization, the monomer and the initiator are both soluble in the polymerization medium, but the medium is a poor solvent for the resulting polymer. Accordingly, the reaction mixture is homogeneous at the onset, and polymerization is initiated in a homogeneous solution. Depending on the solvency of the medium for the resulting oligomers or macroradicals and macromolecules, phase separation occurs at an early stage. This leads to nucleation and the formation of primary particles called "precursors" and the precursors are colloidally stabilized by adsorption of stabilizers. The particles are believed to be swollen by the polymerization medium and/or the monomer, leading to the formation of spherical particles. Typically, the particles range from about 0.1 - 500 microns, more particularly from about 1 - 200 microns.

[0031] In dispersion polymerization, the variables that are usually controlled are the concentrations of the stabilizer, the monomer and the initiator, solvency of the dispersion medium, and the reaction temperature and choice of initiator. It has been found that these variables can have a significant effect on the particle size, the molecular weight of the final polymer particles, and the kinetics of the polymerization process.

[0032] Particles produced by dispersion polymerization in the absence of any stabilizer are not sufficiently stable and may coagulate after their formation. Addition of a small percentage of a suitable stabilizer to the polymerization mixture produces stable dispersion particles. Particle stabilization in dispersion polymerization is usually referred to as "steric

stabilization". Good stabilizers for dispersion polymerization are polymer or oligomer compounds with low solubility in the polymerization medium and moderate affinity for the polymer particles.

[0033] Typically, as the stabilizer concentration is increased, the particle size decreases, which implies that the number of nuclei formed increases with increasing stabilizer concentration.

[0034] As the solvency of the dispersion medium increases, (a) the oligomers will grow to a larger MW before they become a precursor nuclei, (b) the anchoring of the stabilizer moiety will probably be reduced and (c) the particle size increases. As the initiator concentration is increased, it has been observed that the final particle size increases. As for the kinetics, it is reported that when the dispersion medium is a non-solvent for the polymer being formed, then the locus of polymerization is largely within the growing particles and the system follows the bulk polymerization kinetics, n (the kinetic chain length) = Rp/Rt, where Rp is the propagation rate and Rt is the termination rate. As the solvency of the dispersion medium for the growing polymer particle is increased, polymer growth proceeds in solution. The polymeric radicals that are formed in solution are then captured by growing particles.

Consequently, the locus of the particle polymerization process changes and there is a concomitant change in the kinetics of polymerization.

[0035] Stabilizers as used herein include anionically charged water soluble polymers having a molecular weight of from about 100,000 to about 5,000,000 and preferably from about 1,000,000 to about 3,000,000. The stabilizer polymer should be soluble or slightly soluble in the salt solution, and should be soluble in water.

[0036] Particularly useful stabilizers include polyacrylic acid, poly (meth) acrylic acid, poly (2- acrylamido-2-methyl-l-propanesulfonic acid) and copolymers of 2-acrylamido-2- methyl- 1-propanesulfonic acid and an anionic comonomer selected from acrylic acid and methacrylic acid.

[0037] The stabilizer polymers may be prepared using conventional solution

polymerization techniques, are prepared in water-in-oil emulsion form or are prepared in accordance with the dispersion polymerization techniques described herein. The choice of a particular stabilizer polymer will be based upon the particular polymer being produced, the particular salts contains in the salt solution, and the other reaction conditions to which the dispersion is subjected during the formation of the polymer.

[0038] Preferably from about 0.1 to about 20 percent by weight, more preferably from about 0.5 to about 15 percent and still more preferably, from about 2 to about 10 percent by weight of stabilizer, based on the weight of the total dispersion polymer solids is utilized.

[0039] The remainder of the dispersion comprises an aqueous solution containing from about 10 to about 40 weight percent based on the total weight of the dispersion of a water soluble salt selected from the group consisting of ammonium, alkali metal and alkaline earth metal halides, sulfates, and phosphates.

[0040] The salt is important in that the polymer produced in such aqueous media will be rendered insoluble on formation, and polymerization will accordingly produce particles of water soluble polymer when suitable agitation is provided. The selection of the particular salt to be utilized is dependent upon the particular polymer to be produced, and the stabilizer to be employed. The selection of salt, and the amount of salt present should be made such that the polymer being produced will be insoluble in the salt solution.

[0041] Particularly useful salts include a mixture of ammonium sulfate and sodium sulfate in such quantity to saturate the aqueous solution. Salts containing di-or trivalent anions are particularly useful because of their reduced solubility in water as compared to for example alkali, alkaline earth, or ammonium halide salts, although monovalent anion salts may be employed in certain circumstances. The use of salts containing di-or trivalent anions generally results in polymer dispersions having lower percentages of salt materials as compared to salts containing monovalent anions.

[0042] The particular salt to be utilized is determined by preparing a saturated solution of the salt or salts, and determining the solubility of the desired stabilizer and the desired polymer. Preferably from about 5 to about 30, more preferably from about 5 to about 25 and still more preferably from about 8 to about 20 weight percent based on the weight of the dispersion of the salt is utilized. When using higher quantities of monomer less salt will be required.

[0043] In addition to the above, other ingredients may be employed in making the polymer dispersions of the present invention. These additional ingredients may include chelating agents designed to remove metallic impurities from interfering with the activity of the free radical catalyst employed, chain transfer agents to regulate molecular weight, nucleating agents, and co-dispersant materials. Nucleating agents when utilized generally encompass a small amount of the same polymer to be produced. Thus if a polymer containing 70 mole percent AMPS acid (or its water soluble salts) and 30 percent acrylamide were to be produced, a nucleating agent or "seed" of the same or similar polymer composition may be utilized. Generally up to about 10 weight percent, preferably about 0.1 to about 5, more preferably from about 0.5 to about 4 and still more preferably from about 0.75 to about 2 weight percent of a nucleating agent is used based on the polymer contains in the dispersion is utilized.

[0044] Co-dispersant materials that may be utilized include dispersants from the classes consisting of water soluble sugars, polyethylene glycols having a molecular weight of from about 2000 to about 50,000, and other polyhydric alcohol type materials. Amines and polyamines having from 2-12 carbon atoms are often times also useful as co-dispersant materials, but, must be used with caution because they may also act as chain transfer agents during polymerization. The function of a co-dispersant is to act as a colloidal stabilizer during the early stages of polymerization. The use of co-dispersant materials is optional, and not required to obtain the polymer dispersions of the invention. When utilized, the co-dispersant may be present at a level of up to about 10, preferably from about 0.1- 4 and more preferably from about 0.2-2 weight percent based on the dispersion.

[0045] The total amount of water soluble polymer prepared from the anionic and the nonionic water soluble monomers in the dispersion may vary from about 5 to about 50 percent by weight of the total weight of the dispersion, and preferably from about 10 to about 40 percent by weight of the dispersion. Most preferably the dispersion contains from about 15 to about 30 percent by weight of the polymer prepared from the nonionic and anionic water soluble monomers.

[0046] Polymerization reactions described herein may be initiated by any means which results in generation of a suitable free -radical. Thermally derived radicals, in which the radical species results from thermal, homolytic dissociation of an azo, peroxide,

hydroperoxide and perester compound are preferred. Especially preferred initiators are azo compounds including 2,2'-azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis [2- (2- imidazolin-2- yl) propane] dihydrochloride, 2,2'-azobis (isobutyronitrile) (AIBN), 2,2'-azobis (2,4- dimethylvaleronitrile) (AIVN), and the like.

[0047] The monomers may be mixed together with the water, salt and stabilizer prior to polymerization, or alternatively, one or both monomers may be added stepwise during polymerization in order to obtain proper incorporation of the monomers into the resultant dispersion polymer. Polymerizations of this invention may be run at temperatures ranging from -10°C to as high as the boiling point of the monomers employed. Preferably, the dispersion polymerization is conducted at from -10°C to about 80°C. More preferably, polymerization is conducted at from about 20°C to about 60°C.

[0048] The dispersion polymers of this invention are prepared at a pH greater than 5, preferably at a pH of about 6 - 8. After polymerization the pH of the dispersion may be adjusted to any desired value as long as the polymer remains insoluble to maintain the dispersed nature. Preferably, polymerization is conducted under inert atmosphere with sufficient agitation to maintain the dispersion.

[0049] The polymer dispersions made through the "dispersion polymerization" process of the instant invention typically have apparent solution viscosities of less than about 50,000 cps at 25 °C (Brookfield), more preferably less than 30,000 cps and still more preferably less than about 20,000 cps and in certain embodiments from about 300-3000 cps. At these viscosities, the polymer dispersions are easily handled in conventional polymerization equipment and are suitable for subsequent manufacturing processing (e.g., drum and spray drying).

[0050] A powdered polymer product can be made through drying (e.g., vacuum drying, spray drying, belt drying, drum drying, etc.) the above mentioned polymer dispersions. Powdered polymers can also be manufactured through drying the solutions, emulsions, inverse emulsions or suspensions described in the following text.

[0051] In another aspect, this invention is directed to a method of preparing a moderate molecular weight dispersion polymer having a bulk Brookfield viscosity of from about 200 to about 8000 cps at 25°C comprising a) adding an initiator to an aqueous mixture comprising: i) from about 20 -30 weight percent of a mixture comprising 5-55, more particularly 10-43, mole percent of Na AMPS, 30-95, more particularly 43 -88, mole percent of acrylamide and 0.5-30, more particularly 1 -25, mole percent of MAPTAC and/or APTAC; ii) from about 2 - 20 of weight percent based on the total weight of the dispersion of a stabilizer; and iii) from about 10 -40 of weight percent based on the weight of the dispersion of a water soluble salt selected from the group consisting of ammonium, alkali metal and alkaline earth metal halides, sulfates, and phosphates; and b) polymerizing the monomers.

[0052] Free radical polymerization is another useful polymerization method, especially when using water-dispersible and/or water-soluble reaction solvent(s), and is described in "Decomposition Rate of Organic Free Radical Polymerization" by K.W. Dixon (section II in Polymer Handbook, volume 1, 4th edition, Wiley-Interscience, 1999), which is incorporated by reference.

[0053] Compounds capable of initiating the free-radical polymerization include those materials known to function in the prescribed manner, and include the peroxo and azo classes of materials. Exemplary peroxo and azo compounds include, but are not limited to: acetyl peroxide; azo bis-(2-amidinopropane) dihydrochloride; azo bis-isobutyronitrile; 2,2'-azo bis- (2-methylbutyronitrile); benzoyl peroxide; di-tert-amyl peroxide; di-tert-butyl diperphthalate; butyl peroctoate; tert-butyl dicumyl peroxide; tert-butyl hydroperoxide; tert-butyl perbenzoate; tert-butyl permaleate; tert-butyl perisobuty Irate; tert-butyl peracetate; tert-butyl perpivalate; para-chlorobenzoyl peroxide; cumene hydroperoxide; diacetyl peroxide;

dibenzoyl peroxide; dicumyl peroxide; didecanoyl peroxide; dilauroyl peroxide; diisopropyl peroxodicarbamate; dioctanoyl peroxide; lauroyl peroxide; octanoyl peroxide; succinyl peroxide; and bis-(ortho-toluoyl) peroxide.

[0054] Also suitable to initiate the free-radical polymerization are initiator mixtures or redox initiator systems, including: ascorbic acid/iron (II) sulfate/sodium peroxodisulfate, tert-butyl hydroperoxide/sodium disulfite, and tert-butyl hydroperoxide/sodium

hydroxymethanesulfinate.

[0055] Alternatively, monomer units may be polymerized concurrently together, using an appropriate initiator and optional solvent(s). Alternatively, the polymerization reaction may be initiated with one or more of the monomers, the reaction temporarily slowed or stopped, and then reinitiated upon the addition of more or different monomers and initiator.

[0056] The initiators are often called "free radical initiators." Various decomposition methods of the initiators are discussed first, followed by a description of the emulsion, solution, and suspension polymerization methods. The initiator can be decomposed homolytically to form free radicals. Homolytic decomposition of the initiator can be induced by using heat energy (thermolysis), using light energy (photolysis), or using appropriate catalysts. Light energy can be supplied by means of visible or ultraviolet sources, including low intensity fluorescent black light lamps, medium pressure mercury arc lamps, and germicidal mercury lamps. [0057] Catalyst induced homolytic decomposition of the initiator typically involves an electron transfer mechanism resulting in a reduction-oxidation (redox) reaction. This redox method of initiation is described in Elias, Chapter 20 (detailed below). Initiators such as persulfates, peroxides, and hydroperoxides are more susceptible to this type of

decomposition. Useful catalysts include, but are not limited to, (1) amines, (2) metal ions used in combination with peroxide or hydroperoxide initiators, and (3) bisulfite or mercapto- based compounds used in combination with persulfate initiators.

[0058] Presently, preferred methods of initiation comprise thermolysis or catalysis.

Thermolysis has an additional advantage in that it provides ease of control of the reaction rate and exotherm.

[0059] Useful initiators are described in Chapters 20 & 21 Macromolecules, Vol. 2, 2nd Ed., H. G. Elias, Plenum Press, 1984, New York. Useful thermal initiators include, but are not limited to, the following: (1) azo compounds such as 2,2-azo-bis-(isobutyronitrile), dimethyl 2,2'-azo-bis-isobutyrate, azo-bis-(diphenyl methane), 4-4'-azo-bis-(4- cyanopentanoic acid); (2) peroxides such as benzoyl peroxide, cumyl peroxide, tert-butyl peroxide, cyclohexanone peroxide, glutaric acid peroxide, lauroyl peroxide, methyl ethyl ketone peroxide; (3) hydrogen peroxide and hydroperoxides such as tert-butyl hydroperoxide and cumene hydroperoxide; (4) peracids such as peracetic acid and perbenzoic acid;

potassium persulfate; ammonium persulfate; and (5) peresters such as diisopropyl percarbonate.

[0060] Useful photochemical initiators include but are not limited to benzoin ethers such as diethoxyacetophenone, oximino-ketones, acylphosphine oxides, diaryl ketones such as benzophenone and 2-isopropyl thioxanthone, benzyl and quinone derivatives, and 3- ketocoumarins as described by S. P. Pappas, J. Rad. Cur., July 1987, p.6.

[0061] Various other polymerization methods are summarized below that may be employed to synthesize the polymer.

[0062] Solution polymerization and optional inversion

[0063] The polymers of the present invention can be made by solution polymerization followed by an optional inversion step. In one illustrative solution polymerization method, the monomers and suitable inert solvents are charged into a reaction vessel. The monomers and the resultant copolymers are soluble in the solvent. After the monomers are charged, an initiator, preferably a thermal free radical initiator is added. The vessel is purged with nitrogen to create an inert atmosphere. The reaction is allowed to proceed, typically using elevated temperatures, to achieve a desired conversion of the monomers to the copolymer. In solution polymerization, preferably the initiator used comprises a thermally decomposed azo or peroxide compound for reasons of solubility and control of the reaction rate.

[0064] Suitable solvents for solution polymerizations include but are not limited to (1) esters such as ethyl acetate and butyl acetate; (2) ketones such as methyl ethyl ketone and acetone; (3) alcohols such as methanol and ethanol; (4) aliphatic and aromatic hydrocarbons; and mixtures of one or more of these. The solvent, however, may be any substance which is liquid in a temperature range of about -10°C to 50°C, does not interfere with the energy source or catalyst used to dissociate the initiator to form free radicals, is inert to the reactants and product, and will not otherwise adversely affect the reaction. The amount of solvent, when used, is generally about 30% to 80% (w/w) based on the total weight of the reactants and solvent. Preferably, the amount of solvent ranges from about 40% to 65% (w/w), based upon the total weight of the reactants and solvent, to yield fast reaction times.

[0065] Polymers prepared by solution polymerization optionally can be inverted to yield dispersions of small average particle size, typically less than about one micrometer, preferably less than about 0.5 micrometer.

[0066] In accordance with certain aspects, the polymer may be prepared in a water-miscible solvent which has a boiling point below 100°C such as ethylene glycol. Alternatively, a non- water-miscible polymerization solvent such as ethyl acetate may be used. The non-water- miscible polymerization solvent may be removed from the polymer by using a rotary evaporator. The resulting polymer can then be dissolved in a water-miscible solvent such as those described above or mixtures including isopropanol, methanol, ethanol, and

tetrahydrofuran.

[0067] The resulting solutions are added with stirring to an aqueous solution of a base, (in the case of polymers containing acidic functionality), or an acid (in the case of polymers containing basic functionality). Alternatively, the base or acid can be added to the polymer solution prior to adding water or adding to water. Suitable bases include (1) ammonia and organic amines, such as aminomethyl propanol, triethyl amine, triethanol amine, methyl amine, morpholine, and (2) metal hydroxides, oxides, and carbonates, etc. Suitable acids include (1) carboxylic acids such as acetic acid, and (2) mineral acids, such as HC1. In the case of a volatile weak base (e.g., ammonia) or acid (e.g., acetic acid), the ionic group formed (an ammonium carboxylate) is non-permanent in nature. For example, for an acrylic acid containing polymer neutralized with aqueous ammonia, the polymer remains as the ammonium acrylate derivative when dispersed in water, but is thought to revert to its original free acid state as the coating dries on the surface. This is because there is an equilibrium between the neutralized and free acid which is shifted towards the free acid as the ammonia is driven off on drying. Acid or base at less than an equivalent is preferably used, more preferably at slightly less than an equivalent, to ensure near neutral pH and thus providing the lowest potential for skin irritation.

[0068] Suspension Polymerization

[0069] The polymers of the present invention can be made by a suspension polymerization method in the absence of surfactants. Instead, colloidal silica in combination with a promoter is used as the stabilizer. Using this process, surfactant-free copolymers can be obtained with a relatively narrow particle size distribution. The preferred method involves making a monomer premix comprising the first, second, and third monomer. The premix is combined with a water phase, preferably deionized water, containing colloidal silica, and a promoter. Amphiphilic polymers represent one class of useful promoters.

[0070] The pH of the mixture is adjusted so as to be in the range of 3 to 11 , preferably in the range of 4 to 6, without coagulation of the particles. For certain monomers, the initial pH of the mixture can be as low as about 2.5. This pH is low enough for the colloidal silica to stabilize the monomer droplet, but the final product may contain a small amount of coagulum. Similar observation can be made at very high pH. It has been observed that when the mixture is treated with ammonia or hydrochloric acid to about pH 4 to 6, the reaction is more stable and the final product is basically free of coagulum.

[0071] The mixture is exposed to high shear, such as that capable in a WaringTM blender, to break the monomer droplets down to a diameter size of 1 micrometer or less. The shearing action is then reduced to a lower agitation (or temporarily stopped) to allow for the partial coalescence of the small droplets and formation of a suspension. Initiator is added. The silica-promoter mixture stabilizes the droplets and limits their coalescence yielding very uniform, and sometimes nearly monodisperse particles. The suspension polymerization is completed under moderate agitation and a stable, aqueous dispersion is obtained.

[0072] The above described suspension polymerization has several advantages. For example, the method yields a polymer with a narrow distribution of mean particle size and limited coalescence. When coalescence is present, the particles tend to migrate towards one another and can form large masses. Coalescence hampers the handling and transportation of the particles and thus is undesirable. The particles are sterically stabilized by the colloidal silica.

[0073] Emulsion polymerization

[0074] The polymers of the present invention can be made by emulsion polymerization. In general, it is a process where the monomers are dispersed in a continuous phase (typically water) with the aid of an emulsifier and polymerized with the free-radical initiators described above. Other components that are often used in this process include stabilizers (e.g., copolymerizable surfactants), chain transfer agents for minimizing and/or controlling the polymer molecular weight, and catalysts. The product of this type of polymerization is typically a colloidal dispersion of the polymer particles, often referred to as "latex." In one preferred emulsion polymerization process, a redox chemistry catalyst, such as sodium metabisulfite, used in combination with potassium persulfate initiator and ferrous sulfate heptahydrate, is used to start the polymerization at or near room temperature. Typically, the copolymer particle size is less than one micrometer, preferably less than 0.5 micrometer.

[0075] Emulsion polymerization can be carried out in several different processes. For example, in a batch process the components are charged into the reactor at or near the beginning. In a semi-continuous process, a portion of the monomer composition is initially polymerized to form a "seed" and the remaining monomer composition is metered in and reacted over an extended time. In one exemplary multistage process, a seed polymer of one monomer composition (or one molecular weight distribution) is used to nucleate the polymerization of a second monomer composition (or the same composition with a different molecular weight distribution) forming a heterogeneous polymer particle. These emulsion polymerization techniques are well known by those skilled in the art and are widely used in industry.

[0076] Inverse Emulsion Polymerization

[0077] The polymers described herein can also be made through an Inverse Emulsion Polymerization process. Inverse emulsion polymerizations may be conducted by dissolving the desired monomers (e.g., Acrylamide and AMPS) in an aqueous phase, dissolving an emulsifying agent(s) (e.g., surfactant) in the oil phase (e.g., low boiling point solvent or mineral or vegetable oils), emulsifying the water phase in the oil phase to prepare a water-in- oil emulsion, in some cases, homogenizing the water-in-oil emulsion, polymerizing the monomers dissolved in the water phase of the water-in-oil emulsion to obtain the polymer as a water-in-oil emulsion.

[0078] In oilfield applications any convenient concentration of fluid loss additive or rheology modifier can be used, so long as it is effective in its purpose. Generally, the fluid loss additive or rheology modifier is used in an amount of about 0.05% to about 5%, more particularly about 0.1% to about 3% and in certain cases about 0.5% to about 2% by weight based on total formulation. The compositions also may include (without limitation) one or more viscosifiers and protective agents to achieve and maintain proper rheology of the fluids. Particularly useful rheology modifiers include terpolymers of acrylamide, 2-acrylamido-2- methyl-propanesulfonic acid (AMPS) and a long-chain alkyl acrylate having a chain length for the alkyl group of from 12 - 25 as described in provisional application No. 61/406,402 filed October 25, 2010, the contents of which are hereby incorporated by reference. Techwax Aqua AA-350 from ISP/Ashland is a particularly useful rheology modifier. It is a terpolymer of AM/ AMPS/ n-stearyl acrylate.

[0079] It is contemplated that higher concentrations may be preferred in some applications. Nonetheless, it is understood that the actual concentration will vary, depending upon many parameters. A suitable concentration for a particular application, however, can be determined by those skilled in the art by taking into account the rheology modifier' s performance under such application. [0080] Aspects of the present application will be described in more detail by reference to the following non-limiting examples.

[0081] Example 1 (Comparative): To make anionic poly(AMPS) as dispersant.

[0082] To a 1 L 4-neck round bottom flask, equipped with water bath, thermal couple, mechanic stirrer and nitrogen inlet and outlet, was added 80 g 2-acrylamido-2-methyl propanesulfonic acid (AMPS acid), and 700 g deionized water, the mixture as purged with N₂ at 40°C for 60 min, 0.10-0.20 g ammonium persulfate dissolved in 20 g deionized water was added in batches over 1 hour. The reaction was maintained for 10 hrs at 40°C to obtain viscous poly (AMPS). A 10% by weight actives solution of polyacrylamidomethylpropane sulfonic acid [poly)AMPS)] was recovered with a bulk viscosity of 2,000 - 10,000 cps. The polymer had a Mw of about 1,000,000 as measured by GPC.

[0083] Example 2: To make AM/AMPS/MAPTAC terpolymer by dispersion

polymerization:

To a 1L resin reactor, equipped with water bath, thermocouple, mechanical stirrer and nitrogen inlet and outlet, was added 58g of 10% poly(AMPS) made in a manner similar to example 1 with a bulk viscosity of 2,000-10,000 cps. 54.27 g AMPS acid and 290g water were added into the reactor and the temperature was maintained below 30~40°C. After AMPS acid and poly(AMPS) were completely dissolved, approximately 24g of 50% NaOH was dropwise added to the above solution to adjust the pH value to 7. 130 g ammonium sulfate, 60g acrylamide, and 12g of 50% 3-trimethylammonium propyl methacrylamide chloride (MAPTAC) were added in the mixture. The above mixture was heated to 60°C and purged with nitrogen for 30 minutes. 7.5 g 2% ammonium persulfate and 7.5g 2% potassium metabisulfite were added into the reactor over 30 minutes. Polymerization began after 20-30 minutes after initiators and the solution become a milky dispersion after another 30 min. The reaction was kept for an additional 1 hour at 60°C, then the temperature was raised to 72°C and 0.05g VA-044 dissolved in 2g DIW was added into the reactor. The reaction was kept for an additional 2 hours. The resulting polymer dispersion with a bulky viscosity in the range of 200-1500cps was poured into a glass jar. [0084] The polymerization reaction can be summarized as set forth in Scheme 1 wherein:

Where: AM mole range is 30-95%

NaAMPS mole% range is 5-55%

APTAC mole% range is 0.5-30%

Scheme 1. Synthesis of terpolymer of AM, NaAMPS, and APTAC

[0085] A ~12.5ppg drilling fluid formulation as described in Table 1 was made on a 600g scale containing weighting agents, rheology modifier (AA 350), and fluid loss control additives (PAC-L ) as shown in following table. Sufficient mixing was required to facilitate dissolving of the polymer and avoid local viscosified agglomerates (fish eyes). The drilling fluids were allowed to agitate for 5-15 minutes between the addition of each component and with 60 minutes total for complete and homogenous mixing. Rheological properties were then measured on Fann 35 before and after hot rolling (BHR and AHR) aging tests.

Table 1. For comparison: mud formulation using AA 350 alone without FLA

PAC-L: Polyanionic Cellulose, a high specification carboxymethylcellulose

[0086] The drilling fluid muds were prepared from the formulation provided in Table 1 and sealed in OFITE stainless cells under N₂ pressure of 150psi for 350°F for 16 hours aging. HPHT fluid loss tests on drilling fluid formulations were conducted in accordance with the procedures detailed in API RP 13B-1. BHR and AHR rheology results and HPHT Fluid loss control properties are provided in Table 2. The Table 2 results show the good retention of rheological profiles of 1% AA350 without FLA. However, HPHT Fluid loss control properties of the muds are poor after 350°F/16hr aging in the absence of the FLA polymer described herein.

Table 2. For comparison: Mud Rheology before and after hot rolling at 350F containing 1 % or 0.5% AA 350 +PAC, without FLA.

*Fluid Loss tests were conducted at high pressure (ΔΡ=500 psi) and high temperature (260°F)

[0087] The measured specifications of the fluid are outlined herein. Plastic viscosity (PV), Yield Point (YP) and Gel Strength are measured on an oilfield type rotational viscometer Fann 35. PV is a measure of the high-shear-rate viscosity of the fluid and is calculated from the measurements at 600 and 300 rpm rotational speeds and is equal to PV=9₆oo- Θ300 cps. YP is a measure of the yield stress of the fluid and is calculated from

θ₆οο lb/100 ft². The unit lb/100ft² is an oilfield unit, which is equivalent to 0.48 Pa.

[0088] Gel strength is the ability of fluid to suspend mud while mud is in static condition. Before testing gel strength, mud must be agitated for a while in order to prevent precipitation and then let mud is in static condition for a certain limited time (10 seconds, 10 minutes) and then slowly turn the gel knob counter wise and read the maximum reading value. The measured 10-second or 10-min gel strength of a fluid is the maximum reading (deflection) taken from a direct-reading viscometer after the fluid has been quiescent for 10 seconds or 10 minutes. The reading is reported in lb/ 100 ft². [0089] Gel strength should be just high enough to suspend weighing agents and drilling cuttings when circulation is stopped. Higher gel strengths are undesirable because they retard the separation of cuttings and of entrained gas at the surface, and also because they raise the pressure required to re-establish circulation after changing bits.

[0090] OFITE Aging Cells are patented pressure vessels that enable samples to be subjected to temperatures higher than the boiling point of water and still be maintained in a liquid state. The cells may be used for static temperature exposure or in a dynamic mode in a roller oven with a normal minimum aging time of 16 hours. The mud formulations described herein were aged in 500 ml OFITE 303 grade stainless cells sealed with Teflon liner and O- rings in a OFITE roller oven.

[0091] After mud formulations were aged at certain temperature, and their PV, YP, gel strength were still maintained > 40% of their original values, they are considered as passing the aging tests. In accordance with certain embodiments, these values may be maintained at values that are at least 60%, 80% or even 90% of the original values for one or more of the specified tests. HPHT fluid losses are measured in accordance with the procedures detailed in API RP 13B-1. HPHT fluid loss value should be less than 50 ml/30 min, more particularly less than 30 ml/30 min and in some cases less than 20 ml/30 min.

[0092] The effects of FLA polymers described herein were examined in a water based drilling fluid formulation as shown in Table 3. The drilling fluid muds were prepared and sealed in OFITE stainless cells under N₂ pressure of 150psi for 350°F, 200psi for 375°F, 250psi for 400°F, for 16 hours aging. HPHT fluid loss tests on drilling fluid formulations were conducted in accordance with the procedures detailed in API RP 13 B-l. Their BHR and AHR rheology results and HPHT Fluid loss measurements are provided in Tables 4-6.

Table 3 Mud formulation containing 0.3% AA 350 and 0.7-1% FLA for fluid loss measurements

Components Weight (g) Dosage in lb/barrel (ppb)

Fresh water 310 271.25

Soda Ash 0.6 0.53

KC1 55 48.13

AA 350 HPHT viscosifier, -20% 8 1.4

PAC-L 0.5 0.44

FLA , -20% 20g (-0.7%) or 3.5

25g (0.8%), or 30g or 4.38

(1%) or 5.25

NaOH, 50% (pH value 10-10.5) Variable --

Baroid 41 (ground barium sulfate 190 166.25 weighting agent)

Hymod Prima Clay 15 13.13

Total volume (mL) 400ml

Table 4. Mud properties and fluid losses containing 0.3% AA 350 and 0.7-0.8% FLA+0.08 PAC before and after 350F/16hr aging.

*: Fluid Loss tests were conducted at high pressure (ΔΡ=500 psi) and high temperature (260°F)

FLA compositions: NaAMPS/AM/MAPTAC (45.5/45.5/9.0 by weight ratio)

[0093] Table 4 listed HPHT fluid loss control of muds made from Table 3 after 350°F/16hr aging. The muds containing a combination of 0.3% AA-350 and 0.7-0.8% FLA with or without PAC exhibited excellent HPHT fluid loss control after aging (<20mL/30min). The aged muds also showed excellent retention of rheological properties (-100%). These results can be compared with those in Tables 2 in which muds containing 1% AA-350 and PAC only gave poor HPHT fluid loss control in the absence of FLA (350 16 hr aging).

[0094] Table 5 shows HPHT fluid loss control of muds made from Table 3 after 375°F/16hr aging. The muds containing combination of 0.3% AA-350 and 0.7-0.8% FLA and 0.08% PAC also imparted excellent HPHT fluid loss control (<20mL/30min) after aging. The aged muds also retained excellent rheological properties with 80-100% of their original values. These results can be compared with those in Tables 2 in which muds containing 1% AA-350 and PAC gave poor HPHT fluid loss control in the absence of FLA (350°F/16hr aging). Furthermore, mud containing AA 350 and FLA only without PAC generated HPHT fluid loss of 116ml/30min, indicating that even a small amount of PAC may play an important role in fluid loss control at temperatures of about 375°F or higher.

Table 5. Mud properties and fluid losses containing 0.3% AA 350 and Q.7-0.8%%

FLA+0.08%PAC before and after 375°F/16hr aging. (Synergistic combination of AA 350 and FLA and PAC)

FLA polymers are terpolymers of NaAMPS/AM/MAP AC (weight ratio) as described herein. *: Fluid Loss tests were conducted at high pressure (ΔΡ: =500 psi) and high temperature (250°F)

[0095] Table 6 summarizes HPHT fluid loss control of muds made from Table 3 after 400°F/16hr aging. The muds containing a combination of 0.3% AA-350 and 0.8-1% FLA and 0.08% PAC showed excellent HPHT fluid loss control attributes (<20mL/30min), as well as good rheological retention with 60-80% of original values after aging. The mud containing FLA polymer and PAC only without AA 350 generated HPHT fluid loss of 66ml/30min, indicating that AA 350 may also play an important role in conjunction with FLA polymer. These results can be compared with those in Tables 2, in which muds containing 1% AA-350 and PAC only gave poor HPHT fluid loss control in the absence of FLA at 350°F aging.

Table 6. Mud properties and fluid losses containing 0.3% AA 350 and 0.8-1% FLA and 0.08% PAC before and after 400°F/16hr aging, (synergistic combination of AA 350 and FLA and PAC)

FLAs are terpolymer of NaAMPS/AM/APTAC (weight ratio) as described herein.

*: Fluid Loss tests were conducted at high pressure (ΔΡ=500 psi) and high temperature

(250°F)

[0096] FLA polymers perform very well in conjunction with AA 350 in drilling fluids to generate excellent HPHT fluid loss control properties after 350-400°F/16hr aging.

[0097] Table 7 summarizes HPHT fluid loss control of muds after 350°F/16hr aging.

Samples contained lower dosage of AA 350 and FLA polymer. The results indicate that FLA polymer works well with AA 350 by providing good FL control and excellent rheology retention. FLA polymer also works well with Xanthan at 350°F even it didn't provide rheology retention to xanthan, it gives good FL control. PAC-L alone cannot provide FL control at 350°F to AA 350 in the mud. PAC and Xanthan was unacceptable after aging.

Table 7. Mud properties and fluid losses containing Xanthan/PAC, Xanthan/FLA, AA 350/PAC, AA

FLAs are terpolymer of NaAMPS/AM/APTAC (weight ratio) as described herein.

*: Fluid loss tests were conducted at high pressure (AP=500psi) and high temperature (250°F)

[0098] Table 8 summarizes results for various compositions in accordance with some aspects of the present invention:

[0099] Table 8. FLA compositions vs. Fluic losses.

FLA Composition wt ratio *HPHT FL HPHT FL HPHT FL HPHT FL ACM/NaAMPS/ APTAC(or After 350F After 375F After 400F aging After 425F MAPTAC) aging aging (ml/30min) aging

(the ratios in parenthesis are (ml/30min) (ml/30min) (ml/30min) normalized wt ratio)

30/70/7.5 Unacceptable, non-homogeneous dispersion

( 27.9 /65.1/7.0)

30/70/5 80 in lOmin -- — -- (28.5/66.6/4.8) (0.75%), fail

50/50/2 - 84 82 —

1. *: HPHT: 250°F, 500psi

2. **: the % in parenthesis is FLA dosage level

3. If FLAs perform well at lower aging temperature (e.g. 350°F), they will be aged further at higher aging temperature (e.g., 375°-425°F) and tested.

4. If FLAs perform well at higher aging temperature (> 400°F), they most likely will impart give good FL performance at low temperature (e.g., 250°-375°F). Therefore FLA aged at 350°-375°F are omitted in those cases.

5. FL >=50ml in 30min is considered as failing the FL test, <50ml is considered as passing the test.

[00100] The inclusion of some monomers may affect the basic characteristics of the polymer and typically should be avoided. For example the following composition resulted in an unacceptable reaction mixture:

NaAMPS/ACM/MAPTAC/DMACM DMACM is insoluble in the reaction mixture. Gel (60/40/6/16) particles formed.

[00101] What is claimed is:

Claims

1. A polymer polymerized from monomers comprising (A) 20-80 wt. % acrylamide, (B)

20-80 wt. % 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and/or salt(s) thereof, and (C) 2-50 wt. % of a cationic monomer selected from the group consisting of quaternized (meth)acrylate monomers, quaternized (meth)acrylamide monomers, diallydimethyl ammonium chloride (DADMAC), dimethylaminoethyl methacrylate (DMAEMA), 3- methacryloyloxy-2-hydroxypropyl trimethyl ammonium chloride, 3-acrylamido-3- methylbutyl trimethyl ammonium chloride, N-methyl 2-vinyl pyridinium methyl sulfate, N- propyl acrylamido trimethyl ammonium chloride, 2-methacryloyloxy-ethyl trimethyl ammonium methosulfate and combinations thereof, wherein said weight percentages are based on the total weight of (A) and (B).

2. The polymer in accordance with claim 1 wherein monomer (C) comprises a quaternized (meth)acrylate or (meth)acrylamide monomer represented by the following structure:

wherein: Ri is hydrogen or methyl, X is O or NH,

Q is selected from a functionalized and unfunctionalized alkylene, cycloalkylene, alkenylene, or arylene group, wherein any of the before mentioned groups may be with or without heteroatoms,

R2, R3, and R4 are independently selected from the group consisting of functionalized or unfunctionalized alkyl groups, and

3. The polymer in accordance with claim 2 wherein monomer (C) comprises acrylamidopropyl trimethyl ammonium chloride (APTAC) and/or methacrylamidopropyl trimethylammonium chloride (MAPTAC).

4. The polymer in accordance with claim 3 wherein said polymer comprises from about 40-60 wt. % acrylamide, 60-40 wt. % AMPS, and 5 - 30 wt. % APTAC and/or MAPTAC.

5. The polymer in accordance with claim 1 having a weight-average molecular weight of at least 3,000 Da.

6. The polymer in accordance with claim 4 having a weight-average molecular weight from 100,000 Da to 10,000,000 Da.

7. The polymer in accordance with claim 1 wherein said monomer (C) is MAPTAC.

8. The polymer in accordance with claim 1 wherein said monomer (C) is APTAC.

9. The polymer in accordance with claim 1 wherein said polymer is stable under High Pressure, High Temperature (HPHT) drilling conditions.

10. A method of preparing a dispersion polymer having a bulk Brookfield viscosity of from about 200 to about 8,000 cps at 25°C comprising: a) adding an initiator to an aqueous mixture comprising: i. from about 20 to about 30 weight percent of a mixture comprising 5-55 mole percent of Na AMPS, 30-95 mole percent of acrylamide and 0.5-30 mole percent of MAPTAC and/or APTAC; ii. from about 2 -20 weight percent based on the total weight of the dispersion of a stabilizer, and iii. from about 10 -40 weight percent based on the weight of the dispersion of a water soluble salt selected from the group consisting of ammonium, alkali metal and alkaline earth metal halides, sulfates, and phosphates; and b) polymerizing the monomers.

11. The method of claim 10 wherein the stabilizer comprises

polyacrylamidomethylpropane sulfonic acid.

12. A method of preventing fluid loss during oilfield drilling operations, said method comprising: drilling a wellbore; and circulating a fluid containing an effective amount of a fluid loss additive wherein said fluid loss additive comprises a polymer polymerized from monomers consisting essentially of (A) 20-80 wt. % acrylamide, (B) 20-80 wt. % 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and/or salt(s) thereof, and (C) 2-50 wt. % of a cationic monomer selected from the group consisting of quaternized (meth)acrylate monomers, quaternized (meth)acrylamide monomers, diallydimethyl ammonium chloride (DADMAC), dimethylaminoethyl methacrylate (DMAEMA), 3-methacryloyloxy-2-hydroxypropyl trimethyl ammonium chloride, 3-acrylamido-3-methylbutyl trimethyl ammonium chloride, N-methyl 2-vinyl pyridinium methyl sulfate, N-propyl acrylamido trimethyl ammonium chloride, 2- methacryloyloxy-ethyl trimethyl ammonium methosulfate and combinations thereof, wherein said weight percentages are based on the total weight of (A) and (B).

13. The method of claim 12 wherein monomer (C) comprises a quaternized

(meth)acrylate or (meth)acrylamide monomer represented by the following structure:

wherein: Ri is hydrogen or methyl, X is O or NH,

14. The method of claim 12 wherein monomer (C) comprises acrylamidopropyl trimethyl ammonium chloride (APT AC) and/or methacrylamidopropyl trimethylammonium chloride (MAPTAC).

15. The method of claim 12 wherein said polymer comprises from about 40-60 wt. % acrylamide, 60-40 wt. % AMPS, and 5 - 30 wt. % APTAC and/or MAPTAC.

16. The method of claim 12 wherein said polymer has a weight- average molecular weight of at least 3,000 Da.

17. The method of claim 12 wherein said polymer has a weight- average molecular weight from 100,000 Da to 10,000,000 Da.

18. The method of claim 12 wherein said monomer (C) is MAPTAC.

19. The method of claim 12 wherein said monomer (C) is APTAC.

20. A method of preventing fluid loss during cementing operations, said method comprising: drilling a wellbore; and circulating a cementing fluid containing an effective amount of a fluid loss additive wherein said fluid loss additive comprises a polymer polymerized from monomers consisting essentially of (A) 20-80 wt. % acrylamide, (B) 20-80 wt. % 2-acrylamido-2-methyl- propanesulfonic acid (AMPS) and/or salt(s) thereof, and (C) 2-50 wt. % of a cationic monomer selected from the group consisting of quaternized (meth)acrylate monomers, quaternized (meth)acrylamide monomers, diallydimethyl ammonium chloride (DADMAC), dimethylaminoethyl methacrylate (DMAEMA), 3-methacryloyloxy-2-hydroxypropyl trimethyl ammonium chloride, 3-acrylamido-3-methylbutyl trimethyl ammonium chloride, N-methyl 2- vinyl pyridinium methyl sulfate, N-propyl acrylamido trimethyl ammonium chloride, 2-methacryloyloxy-ethyl trimethyl ammonium methosulfate and combinations thereof, wherein said weight percentages are based on the total weight of (A) and (B).