WO2019147515A1 - Anaerobically biodegradable fluids for drilling applications - Google Patents

Abstract

A class of low viscosity carboxylic acid esters is provided herein which are hydrolytically stable yet anaerobically biodegradable to meet the U.S. EPA Gulf of Mexico (GOM) requirements pertaining to anaerobic biodegradability when used in drilling fluids. The carboxylic acid esters generally (i) have an acid moiety comprising a quaternary alpha carbon and an alcohol moiety comprising fewer than 7 carbon atoms or (ii) have an acid moiety comprising a tertiary alpha carbon.

Description

ANAEROBICALLY BIODEGRADABLE FLUIDS FOR DRILLING APPLICATIONS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority from U.S. Provisional Application No 62/623,097, filed 29 January 2018, which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present disclosure generally relates to anaerobically biodegradable fluids for drilling applications, and more specifically relates to esters having sufficient hydrolytic stability and anaerobic biodegradability properties for use in a drilling fluid.

BACKGROUND OF THE INVENTION

[0003] The process of drilling operations for the extraction of a natural resource generally requires a drilling fluid for removing the cuttings from the wellbore, lubricating and cooling the drill bit, controlling formation pressures, and maintaining hole stability.

[0004] Enhanced mineral oils, consisting primarily of aliphatic hydrocarbons, are often used in drilling fluid. Aliphatic hydrocarbons may be readily biodegradable; however, mineral oils often cannot meet marine anaerobic biodegradability requirements for US Gulf of Mexico (“GOM”). Likewise, other regions are considering adopting similar rules. For example, Brazil recently introduced legislation similar to US GOM requirements. Therefore, the oil phase components of new drilling fluids should be designed to meet or exceed the anaerobic biodegradability of a C₁₆ - C₁₈ internal olefin reference fluid as provided in 40 CFR Part 435, Subpart A Appendix 4 - Protocol for the Determination of Degradation of Non-Aqueous Base Fluids in a Marine Closed Bottle Biodegradation Test System: Modified ISO 11734: 1995 (EPA Method 1647).

[0005] Potential replacements for hydrocarbons have included aliphatic materials with functional groups that facilitate biodegradation. For example, as shown in EP-A-374671; 374672; 386636; and 386638, various esters have been proposed for use in drilling fluid. However, esters are prone to hydrolysis due to high temperature and high alkalinity of the drilling fluids, and degrade much faster than olefins in aerobic and anaerobic environments. Therefore, esters are not widely used in drilling fluids since the by-products of hydrolysis (soap scum, carboxylic acid) increase viscosity and corrosion, in addition to reported problems with odor, chemical changes due to temperature increase and interactions with uncured cement or other contaminants, and poor separation of cuttings from the fluid. On the other hand, esters having sufficient anaerobic biodegradability and hydrolytic stability will not form by-products that increase viscosity. Therefore, esters have the potential to be used widely in the oil phase of drilling fluids as a blend or as a primary component.

[0006] As noted above, desired esters for use in drilling fluid exhibit a combination of hydrolytic stability as well as anaerobic biodegradability. The effect of chemical structure on hydrolytic stability in esters has been shown. See Estimation of Hydrolysis Rate Constants of Carboxylic Acid Ester and Phosphate Ester Compounds in Aqueous Systems from Molecular Structure by SPARC, EPA/600/R-06/105 September 2006. Particularly, linear esters can be susceptible to hydrolysis, while branching has been shown to sterically hinder hydrolysis. For example, pivalic acid esters are resistant to hydrolysis because of steric hindrances. Likewise, the effect of chemical structure on aerobic biodegradability of esters has been shown. See Nasiri, M. et al. Synthesis of a Novel Ester-Based Drilling Fluid Applicable to High Temperature Conditions, Journal of Energy Resources Technology March 2009, Vol. 131/013103. Particularly, the ability of microbes to degrade the molecules decreases as branching and carbon chain length of a molecule increases. However, less is known about how structure affects the anaerobic biodegradability of esters.

[0007] A need exists, therefore, for esters having sufficient hydrolytic stability and anaerobic biodegradability properties for use in the base oil of a drilling fluid.

[0008] Additional potential references of interest include EP 0642 561 and U.S. Pat. No. 9,803,127.

SUMMARY OF THE INVENTION

[0009] Provided herein are drilling fluids including a base oil comprising one or more carboxylic acid esters, the carboxylic acid ester (i) having an acid moiety comprising a quaternary alpha carbon and an alcohol moiety comprising fewer than 7 carbon atoms, or (ii) having an acid moiety comprising a tertiary alpha carbon . Often, the total number of carbons in each carboxylic acid ester is between 6 and 24. Alternatively, the total number of carbons in each carboxylic acid ester can be as few as 5. The carboxylic acid ester generally has a hydrolysis rate of less than about 3.0 mole percent per 15 hours. If a mixture of one or more carboxylic acid esters is used, this hydrolysis rate applies to the mixture, and preferably also applies to each of the carboxylic acid esters. The alcohol moiety of each of said one or more esters can have a degree of branching of about 1.5 or less, preferably 1.3 or less, and more preferably about 1.0 or less, such as from about 0.0 to about 1.5 or from about 1.0 to about 1.3.

[0010] In an aspect, the base oil of the drilling fluid has a kinematic viscosity of less than about 3 cSt at 40°C, more preferably less than about 2.5 cSt at 40°C, such as less than about 2.2 cSt at 40°C. The base oil of the drilling fluid typically has a pour point of less than about -6 °C and can advantageously have a pour point of less than about -60°C. In an aspect, the base oil of the drilling fluid comprises a mixture of two or more of the carboxylic acid esters. The base oil of the drilling fluid generally has a biodegradation ratio of 1.0 or less.

[0011] The drilling fluids provided herein can be a water (or brine)-in-oil emulsion. In certain instances, the emulsion can comprise up to about 95 weight percent (“wt%”) carboxylic acid ester and/or up to about 90 wt% carboxylic acid ester. In particular embodiments, the emulsion can comprise between 40 wt% and 95 wt% of carboxylic acid ester, for example from 50 wt% to 90 wt% of carboxylic acid ester. In particular embodiments, the emulsion can comprise between 1 wt% to 50 wt% water, for example between 10 wt% to 40 wt% of water. In particular embodiments, the emulsion can comprise between 1 and 10 wt% of water or brine.

[0012] Further provided herein are blends comprising (i) a carboxylic acid ester having an acid moiety comprising a tertiary or quaternary alpha carbon, and (ii) an olefin, as well as drilling fluids comprising the same. Typically, the olefin can be a linear olefin and/or have an anaerobic biodegradation ratio of 1.0 or less.

[0013] Also provided herein are methods of drilling a wellbore comprising the step of introducing one or more of the drilling fluids described herein into a wellbore.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1A depicts the chemical structure of a low viscosity ester, Example 1 (“Ex 1”).

[0015] Figure IB depicts the chemical structure of a low viscosity ester, Example 2 (“Ex 2”).

[0016] Figure 1C depicts the chemical structure of a low viscosity ester, Example 3 (“Ex 3”).

[0017] Figure ID depicts the chemical structure of a low viscosity ester, Example 4 (“Ex 4”).

[0018] Figure IE depicts the chemical structure of a low viscosity ester, Example 5 (“Ex 5”).

[0019] Figure IF depicts the chemical structure of a low viscosity ester, Example 6 (“Ex 6”).

[0020] Figure 1G depicts the chemical structure of a low viscosity ester, Example 7 (“Ex 7”).

[0021] Figure 1H depicts a chemical structure of a low viscosity ester, Comparative Example 1 (“Comparative Ex 1”). [0022] Figure 2 is a graph showing the results of anaerobic biodegradation of the esters in U.S. EPA Test 1647, ester hydrolysis measuring in mole percent (mol%) alcohol, and theoretical gas production in“percent”.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Various specific embodiments, versions, and examples are described herein, including exemplary embodiments and definitions that are adopted for purposes of understanding the claimed invention. While the following detailed description gives specific preferred embodiments, those skilled in the art will appreciate that these embodiments are exemplary only, and that the invention can be practiced in other ways. For purposes of determining infringement, the scope of the invention will refer to any one or more of the appended claims, including their equivalents, and elements or limitations that are equivalent to those that are recited. Any reference to the "invention" may refer to one or more, but not necessarily all, of the inventions defined by the claims.

Definitions

[0024] As used herein, the term“hydrocarbon” means a class of compounds containing hydrogen bound to carbon, and encompasses (i) saturated hydrocarbon compounds; (ii) unsaturated hydrocarbon compounds; and (iii) mixtures of hydrocarbon compounds (saturated and/or unsaturated), including mixtures of hydrocarbon compounds having different carbon numbers. As used herein, a“carbon number” refers to the number of carbon atoms in a hydrocarbon. Likewise, a“C_x” hydrocarbon is one having x carbon atoms (i.e., carbon number of x), and a“C_{x -} C_y” or“C_{x - y}” hydrocarbon is one having from x to y carbon atoms.

[0025] As used herein, the term“hydrocarbyl” as used herein means that the radical concerned is primarily composed of hydrogen and carbon atoms but does not exclude the presence of other atoms or groups in a proportion insufficient to detract from the substantially hydrocarbon characteristics of the radical concerned.

[0026] As used herein,“OXO-alcohols” are isomeric linear, branched, or mixtures of linear and branched, organic alcohols. OXO-alcohols can be prepared by hydroformylating olefins, followed by hydrogenation to form the alcohols.

[0027] As used herein, the term“base oil” may be used interchangeably with“base fluid.”

[0028] As used herein, the term“brine” is a solution of one or more salts in water, preferably with a total salt concentration of at least 1 wt% (relative to the total solution weight). The one or more salts may be inorganic or organic salts, and are preferably selected from the group consisting of calcium salts (such as calcium chloride and/or calcium bromide), potassium salts (such as potassium chloride), and sodium salts (such as sodium chloride).

[0029] As used herein, the terms“well” and“wellbore” are used interchangeably and can include, without limitation, an oil, gas or water production well, an injection well, or a geothermal well. As used herein, a“well” includes at least one wellbore. A wellbore can include vertical, inclined, and horizontal portions, and it can be straight, curved, or branched. As used herein, the term "wellbore" includes any cased, and any uncased, open-hole portion of the wellbore. A near wellbore region is the subterranean material and rock of the subterranean formation surrounding the wellbore. As used herein, a“well” also includes the near-wellbore region. The near-wellbore region is generally considered to be the region within about 10 feet of the wellbore. As used herein,“into a well” means and includes into any portion of the well, including into the wellbore or into the near- wellbore region via the wellbore.

[0030] As used herein, the term“alpha carbon” refers to a carbon atom directly bonded to a functional group. For example, the alpha carbon in the acid moiety of the carboxylic acid esters described herein refers to the carbon atom of the acid moiety that is directly bonded to the carboxylic acid ester group.

[0031] As used herein, the term“beta carbon” refers to a carbon atom adjacent to an alpha carbon.

[0032] As used herein, the terms“drilling fluid,”“non aqueous drilling fluid,”“non aqueous fluid,”“non aqueous mud,” and“drilling mud” may be used interchangeably, and refer to substances used in boring or making a borehole or tunnel, such as for extracting or removing crude oil, natural gas, bitumen, tar sands, sulfur, other elements, other compounds, other minerals, and the like. Drilling fluids can be in the form of solutions, mixtures, emulsions, slurries, and suspensions. Drilling fluid can also provide a carrier or transport for cuttings and other debris from a drilling process. The fluid can also form a filter cake, such as to prevent excursions into surrounding formations or media. As described herein, drilling fluids can be predominately hydrocarbon (e.g., oil) based or predominantly water based. More specifically, the present drilling fluids can typically be a water (or brine)-in-oil emulsion having base oil as the continuous phase, sometimes referred to as an oil based emulsion. In such aspects, the water or brine contained in the drilling fluid creates finely dispersed droplets and a stabilized emulsion in the oil phase. Oil- based drilling fluids can have finely divided solids suspended therein, and, as described below, can contain one or more of a variety of additives, e.g., emulsifiers, surfactants, pH and alkalinity control agents, water, inorganic or organic salts, corrosion inhibitors, weighting agents, viscosity modifiers, oxygen and sulfur scavengers, particulates for lost circulation control, lubricants, and fluid-loss additives.

[0033] As used herein,“pour point” (PP) refers to the temperature at which a fluid becomes semi-solid and loses its flow characteristics. As used herein, pour point is measured according to ASTM D5950.

[0034] As used herein, kinematic viscosity (KV) is measured using ASTM standard D-445.

Test Methods

EPA Method 1647 Test

[0035] The base oil, alternatively referred to as base fluid, of the drilling fluids provided herein can have a biodegradation ratio of less than or equal to 1.0 as measured by United States Environmental Protection Agency NPDES 2012 (National Pollution Discharge Elimination System) General Permit for New and Existing Sources and New Discharges in the Offshore Subcategory of the Oil and Gas Extraction Category for the Western Portion of the Outer Continental Shelf of the Gulf of Mexico (GMG290000 and TXG330000), protocol for the determination of degradation of Non- Aqueous base fluids in a marine closed bottle biodegradation test system, modified International Organization for Standardization (ISO) 11734: 1995 method, hereinafter referred to as the“EPA Method 1647 Test”.

[0036] As used herein, the reference fluid used to conduct the EPA Method 1647 Test is an internal olefin fluid comprising 65 mole percent of molecules with a carbon number of 16 carbons and 35 mole percent of molecules with a carbon number of 18 carbons.

[0037] As used herein, the term“biodegradation ratio” refers to the percent of theoretical gas production of the reference fluid divided by the percent of theoretical gas production of the stock base fluid being tested for compliance, as defined by Equation 20 of the EPA Method 1647 Test. Hydrolytic Stability Test

[0038] Unless otherwise specified, the hydrolytic stability of the carboxylic esters useful herein is determined using the following procedure: [0039] In an autoclave, a mixture of 80 percent by volume (“vol%”) ester, 20 vol% water, and 0.57 grams lime per 100 mL of the mixture is stirred at 300 rpm for a selected time frame, typically 15 hours, at l50°C. The aqueous fraction and ester fraction are separated. The ester fraction, containing hydrolyzed alcohol, is then dried over magnesium sulfate and filtered. The extent of hydrolysis is then determined by gas chromatography (“GC”) by comparing the amount of alcohol to the amount of ester in the ester fraction.

[0040] As used herein, the hydrolytic stability of carboxylic esters useful herein is generally expressed as the rate of increase in the alcohol content of the ester fraction over the selected time frame, e.g., 3.0 mole percent per 15 hours.

Degree of Branching

[0041] 1H NMR methods or ¹³C NMR methods can be used to determine the degree of branching of the alcohol moiety of the carboxylic acid esters provided herein. According to the present invention, it is preferable to determine the degree of branching with the aid of 1H NMR spectroscopy on a solution of the esters in deuterochloroform (CDCl₃). The spectra are recorded by way of example by dissolving 20 mg of substance in 0.6 ml of CDCI3_, comprising 1% by weight of tetramethylsilane (TMS), and charging the solution to an NMR tube with a diameter of 5 mm. Both the substance to be studied and the CDCI3 used can first be dried over molecular sieve in order to exclude any errors in the values measured due to possible presence of water. In principle, any commercially available NMR equipment can be used for the NMR- spectroscopic studies, such as INOVA 500 equipment from Varian. Using this equipment, the spectra are recorded at a temperature of 300 K using a delay of d1=10 seconds, 64 scans, a pulse length of 9.7 ps and a sweep width of 13,000 Hz, using a 5 mm BBO (broad band observer) probe head. The resonance signals are recorded in comparison with the chemical shifts of tetramethylsilane (TMS=0 ppm) as internal standard. Comparable results may be obtained with other commercially available NMR equipment using the same operating parameters.

[0042] The degree of branching B can therefore be calculated from the measured intensity ratio in accordance with the following formula:

where B is degree of branching, I(CH₃) is the area integral essentially attributed to the methyl hydrogen atoms, and I(OCH₂) is the area integral for the methylene hydrogen atoms adjacent to the oxygen atom. [0043] Drilling of oil and gas wells involves the circulation of a fluid through the drill string and out through nozzles in the drill bit. Drilling fluid is returned through the annular passage formed between the drill string and the bore. Drilling fluid cools and lubricates the drill. Drilling fluid also stabilizes the stresses surrounding the bore, provides a hydrostatic head to counterbalance pressures, and can remove the cuttings from the drill bit. Drilling fluids can be used in rotary drilling applications and the like. Drilling fluids can lubricate downhole equipment, such as a drill string or a drill bit. Drilling fluids are also employed in other applications such as geothermal drilling, drilling for water and scientific drilling. Here, drilling fluids clean and condition a borehole while counterbalancing formation pressure.

[0044] In an aspect, the present disclosure provides using a carboxylic acid ester in the oil phase (i.e., base oil) of a drilling fluid. Also provided herein are drilling fluids including a base oil comprising at least one carboxylic acid ester (sometimes referred to as a“hydrocarbyl carboxylic acid ester”). Also provided herein are blends comprising (a) a carboxylic acid ester having an acid moiety comprising a tertiary or quaternary alpha carbon, and (b) an olefin, as well as drilling fluids including a base oil comprising such blends.

Carboxylic Acid Esters

[0045] In any embodiment, the carboxylic acid ester has a carboxylic acid moiety (sometimes referred to as an“acid moiety”) comprising a tertiary alpha carbon. Alternatively, the ester comprises an acid moiety having a quaternary alpha carbon and an alcohol moiety having fewer than seven carbon atoms. Often, the total number of carbons in each carboxylic acid ester is between 6 and 24, in particular embodiments ranging from 6 to 16. Alternatively, the total number of carbons in each carboxylic acid ester can be as few as 5.

[0046] In any embodiment, the alcohol moiety of the carboxylic acid esters useful herein can be hydrocarbyl, and aliphatic, more specifically saturated aliphatic. The alcohol moiety can be linear to assist in biodegradability under aerobic conditions and maximize lubricity. However, the alcohol moiety can have a branched chain, and/or mixtures of branched and linear alcohols. Mixtures of carboxylic acid esters having alcohol moieties of various chain lengths may also be used in the present drilling fluids. Preferably, suitable alcohol moieties have a degree of branching ranging from about 0.0 to about 1.5, such as from about 0.0 to about 1.3, more preferably about 0.0 to about 1.0, still more preferably about 0.0 to about 0.5. Typically, the average number of carbons in the alcohol moiety is less than 9 (number average), preferably less than 7. Examples of suitable alcohols include methanol, primary and secondary branched or linear C₂to C₉ alcohols, polyhydric alcohols, e.g., neopentyl glycol, and OXO-alcohols.

[0047] In any embodiment, the acid moiety of the carboxylic acid esters useful herein can be a hydrocarbyl group, preferably a branched saturated or unsaturated aliphatic radical, such as branched alkyl or alkenyl radicals. Typically, the number of carbon atoms in the acid moiety is 16 or less, such as 12 or less, or 9 or less, or 7 or less, e.g., 2-methyl-pentanoic acid and 2-ethyl- hexanoic acid. Especially preferred acids are neo acids, e.g., neodecanoic acid and pivalic acid.

[0048] The esters are typically made by reaction between the acid and the alcohol in the presence of a catalyst, or by reaction of the acid chloride with the alcohol. See Kirk-Othmer, Encyclopedia of Chemical Technology, 3^rd Edition, Vol. 4, 863 to 871 and Vol. 9, 291-310, incorporated herein by reference; Houben-Weil, Methoden der Organische Chemie, 4th Edition, Band Vol 3, 862-873, incorporated herein by reference; W.J. Hickinbottom, Reactions of Organic Compounds, 291-4 (1959); Neoacids - Properties, Chemistry and Applications, Exxon Chemical Americas, 1989, incorporated herein by reference.

[0049] Catalysts for direct esterification include inorganic acids, e.g., sulfuric, hydro- chloric, and Lewis acids (usually boron trifluoride); organic acids, e.g., p-toluene sulfonic, and methane sulfonic acids, and cation exchange resins, or organometallic catalysts, e.g., tin and titanium compounds. Using the acid chlorides, esters may be formed with, for example, thionyl chloride, phosphorus tri or pentachloride, or phosgene as catalysts.

Blends

[0050] In any embodiment, blends useful herein generally comprise (a) one or more carboxylic acid esters having an acid moiety comprising a tertiary or quaternary alpha carbon, preferably one or more carboxylic acid esters (i) having an acid moiety comprising a quaternary alpha carbon and an alcohol moiety comprising fewer than 7 carbon atoms, or (ii) having an acid moiety comprising a tertiary alpha carbon and (b) one or more olefins. Such blends may advantageously be used in drilling fluid as a more economical alternative to using a carboxylic acid ester without an olefin blend component. Typically, the olefin(s) have favorable anaerobic biodegradability properties, such as linear alpha olefins (LAO) or linear internal olefins. For example, typically the olefin, or mixture of olefins, has a biodegradation ratio of less than or equal to 1. The olefin(s) may advantageously have a carbon number ranging from C₁₄ to C₂₀, more preferably from C₁₆ to C₁₈, such as a C₁₆/C₁₈ mixture. The blends may comprise the one or more carboxylic acid esters in an amount ranging from about 1 wt% to about 80 wt%, such as from about 10 wt% to about 60 wt%, or from about 30 wt% to about 50 wt%. The blends may comprise the one or more olefins in an amount ranging from about 20 wt% to about 99 wt%, such as from about 40 wt% to about 90 wt%, or from about 50 wt% to about 70 wt%. Typically, the blend of the carboxylic acid ester and olefin components may exhibit improved pour point, temperature properties, and/or lubricity as compared to the neat olefin component(s).

Drilling Fluids

[0051] Drilling fluids useful herein may include a base oil comprising a carboxylic acid ester or blend according to any of the above embodiments. Additionally or alternatively, the drilling fluid may include a base oil comprising two or more carboxylic acid esters. In any embodiment, the base oil of the present drilling fluids generally meet or exceed the anaerobic biodegradability of a C₁₆ - C₁₈ internal olefin reference fluid as provided in 40 CFR Part 435, Subpart A Appendix 4 - Protocol for the Determination of Degradation ofNon-Aqueous Base Fluids in a Marine Closed Bottle Biodegradation Test System: Modified ISO 11734: 1995 (EPA Method 1647). These newly developed base oils for drilling fluids generally also have low viscosity (e.g., KV(40°C) < 3 cSt) and a low pour point (e.g., <-6°C).

[0052] The drilling fluid can be an emulsion comprising at least one carboxylic acid ester, aqueous phase, and optionally one or more additives. Useful additives include, but not limited to, one or more of a pH buffer, a viscosifier, a rheology modifier, an emulsifier, a wetting agent, a weighting agent, a fluid loss additive, and a friction reducer. Particularly preferably, the drilling fluid comprises at least one surfactant or emulsifier, water, a halide of an alkaline earth or alkali metal, or a weighting substance.

[0053] Useful surfactants include cationic surfactants, for example an imidazoline derivative, for example a fatty imidazoline salt of a strong monoprotic acid, for example one as described in U.S. Pat. No. 3,585,051, or a quaternary ammonium salt having at least one long chain alkyl or alkenyl substituent, e.g., one having from 8 to 20 carbon atoms, the remaining substituents being alkyl groups with up to 4 carbon atoms, especially methyl, the anion being, for example chloride, bromide, iodide, phosphate, sulfamate, or acetate. Additional useful surfactants include non-ionic surfactants, for example, polyalkylene ether derivatives of alcohols and alkyl phenols, particularly, those having from 3 to 30 alkylene, ethylene, oxy groups and/or from 8 to 20 carbon atoms in the hydrocarbon chain. Polyethylene oxide derivatives are also useful having from 5 to 20, from 5 to 15, repeat units with a linear or branched primary alcohol from 8 to 18, from 10 to 16, carbon atoms in the chain or with an alkyl phenol with a linear or branched alkyl group with from 6 to 14 carbon atoms. An ethoxylated fatty alcohol can be useful in the present drilling fluids, particularly ethoxylated fatty alcohol having from 5 to 20, 7 to 10, ethoxy groups and 11 carbon atoms in the chain which may be linear or branched.

[0054] Useful emulsifiers include fatty amine alkoxylates, aromatic alkoxylates or an ether amine alkoxylate. The emulsifier can be a quaternary ammonium salt, e.g., a long chain alkyl trimethyl or dialkyl dimethyl ammonium chloride, or the quaternized reaction product of an oxyalkylated polyamine with a fatty acid, for example the product of reaction of a soya or coco fatty acid, or a blend of the two, with diethylene triamine, ethoxylation, and subsequent quatemization with methyl chloride. The emulsifier can be selected from the group consisting of tall oil-based fatty acid derivatives such as amides, amines, amidoamines, and imidazolines made by reactions of fatty acids and various ethanolamine compounds, vegetable oil-based derivatives, and combinations thereof. Commercially available examples of a suitable emulsifier include, but are not limited to, EZ MUL™ NT, INVERMUL™ NT, LE SUPERMUL™, and combinations thereof, marketed by Halliburton Energy Services, Inc., MEGAMUL™, VERSAMUL™, SureMUL™, VERSACOAT™, marketed by M-I Swaco, a Schlumberger Company, OMNI- MUL™, OMNTMUL 2™ by BakerHughes, a GE Company. The emulsifier may be in at least a sufficient concentration such that the oil-based drilling fluid maintains a stable emulsion or invert emulsion. The emulsifier may be in a concentration of at least about 1 lb/bbl (about 2.85 kg/m³) of the oil-based drilling fluid. The emulsifier can also be in a concentration in the range of about 1 to about 20 lb/bbl (about 2.85 - about 57 kg/m³) of the drilling fluid.

[0055] The drilling fluids provided herein can also contain a viscosity modifier, e.g., clay, for example, hectorite, which may be treated, e.g., with peptizing agents or with organic salts, to render them organophilic. Suitable proportions are, for example, about 0.1 to about 20 lb/bbl (about 0.29 kg/m³ to about 57 kg/m³). The drilling fluid may also contain a non-clay viscosifier or a rheology modifier. Suitable viscosifiers may be selected from the group consisting of inorganic viscosifier, fatty acids, including but not limited to dimer and trimer poly carboxylic fatty acids, diamines, polyamines, organophilic clays, and combinations thereof. Commercially available examples of a suitable viscosifier include, but are not limited to, VG-PLUS™, RHEFLAT™, available from M-I Swaco, a Schlumberger Company; RHEMOD L™, TAU-MOD™, RM-63™, and combinations thereof, marketed by Halliburton Energy Services, Inc, and RHEO-CLAY PLUS™, RHEO-LINE HT by Baker Hughes, a GE Company. In an aspect, the viscosifier and rheology modifier is in a concentration of at least about 0.5 lb/bbl (about 1.4 kg/m³) of the drilling fluid. The viscosifier and rheology modifier can also be in a concentration in the range of about 0.5 to about 20 lb/bbl (about 1.4 to about 57 kg/m³), alternatively of about 0.5 to about 10 lb/bbl (about 1.4 to about 28.5 kg/m3), of the drilling fluid.

[0056] The drilling fluids may also contain weighting agents, for example, barium sulfate (barite), to increase the density of the fluid, e.g., up to about 18.5 lb/gal (about 2218 kg/m³). Conveniently, the weighting agent can be selected from the group consisting of barite, hematite, manganese tetroxide, calcium carbonate, and combinations thereof. Commercially available examples of a suitable weighting agent include, but are not limited to, BAROID™, BARACARB™, BARODENSE™, and combinations thereof, marketed by Halliburton Energy Services, Inc and MICROMAX™, MICRODENSE™ marketed by Elkem. In an aspect, the weighting agent can be absent or substantially absent from the drilling fluid. The weighting agent can also be in a concentration in the range of about 10 to about 1000 lb/bbl (about 28.5 kg/m³ to about 2853 kg/m³) of the drilling fluid.

[0057] The oil-based drilling fluids can also contain, in the aqueous phase, a soluble alkali or alkaline earth metal salt, especially calcium chloride, in an amount up to that needed to saturate the aqueous phase. The drilling fluid may also optionally include one or more metal salts, MX_y, where M is a Group 1 or Group 2 metal, X is a halogen, and y is 1 to 2. Exemplary such salts include, NaCl, KC1, CaCl₂, MgCl₂, CaBr₂ etc. The total amount of such salts in the oil-based drilling fluid is typically about 10 - 35 wt% in the aqueous phase. Organic additives that lower the water phase activity may also be used.

[0058] The drilling fluid can further include wetting agents. The wetting agents can be selected from the group consisting of tall oil-based fatty acid derivatives such as amides, amines, amidoamines, and imidazolines made by reactions of fatty acids and various ethanolamine compounds, vegetable oil-based derivatives, and combinations thereof. Commercially available examples of suitable wetting agents include, but are not limited to, DrillTreat™, OMC™, marketed by Halliburton Energy Services, Inc., VersaWet™, Surewet™ marketed by M-I Swaco, a Schlumberger Company. The wetting agent may not be added into the oil-based drilling fluid, although typically can be in at least a sufficient concentration such that the oil-based drilling fluid maintains a stable emulsion or invert emulsion, in a concentration in the range of about 0.05 to about 20 lb/bbl (about 0.14 kg/m³ to about 57 kg/m³), such as about 0.25 to about 20 lb/bbl (about 0.71 kg/m³ to about 57 kg/m³), of the oil-based drilling fluid. Alternatively, the wetting agent is absent from the drilling fluid.

[0059] The drilling fluids may contain a pH buffer selected from the group consisting of sodium hydroxide, magnesium oxide, potassium hydroxide, calcium oxide, and calcium hydroxide. Commercially available examples of a pH buffer include lime (for calcium hydroxide). The pH buffer can be in a concentration in the range of about 0.5 to about 10.0 pounds per barrel lb/bbl (about 1.4 - about 28.5 kg/m³) of the drilling fluid. Alternatively, the pH buffer is absent from the drilling fluid.

[0060] The drilling fluids may further include a lubricant. The lubricant can be a liquid e.g., U1LTRALUBE™ available from Integrity Industries. Alternatively, the lubricant can comprise a particulate material, e.g., graphite such as STEELSEAL™, available from Halliburton.

[0061] The drilling fluids may also contain a fluid-loss additive, for example a hydrophobic lignite, to assist in developing a low permeable film on the bore wall. The fluid loss additive can be selected from the group consisting of oleophilic polymers, including crosslinked oleophilic polymers, particulates. Commercially available examples of a suitable fluid loss additive include, but are not limited to VERSATROL™, available from M-I Swaco; N-DRIL™ HT PLUS, ADAPTA™, marketed by Halliburton Energy Services, Inc. The fluid loss additive can also be in a concentration in the range of about 0.5 to about 10 lb/bbl (about 1.4 - about 28.5 kg/m³) of the oil-based drilling fluid.

[0062] An oil phase of the drilling fluids described herein may comprise one or more carboxylic acid esters alone, or may comprise the carboxylic acid ester(s) together with other oils. In an aspect, the one or more esters constitute at least 40%, preferably at least 60%, by weight, of the oil phase. Other components may include esters of natural or synthetic, saturated or un-saturated fatty acids with mono- or poly-functional alcohols, ethers, optionally alkoxylated amines, ether alcohols, ether acids, ether esters, ether amines, and/or mineral oils.

[0063] When the drilling fluid is a water-in-oil emulsion, the oil phase generally represents from 55 to 95 vol%, preferably from 65 to 95 vol%, and most preferably from 70 to 90 vol%, based on the total liquid (oil and water) volume (in the absence of solids, including weighting agent, drilling solids and salt). In particular embodiments, the drilling fluid is a water-in-oil emulsion, wherein the emulsion comprises up to 95 wt% of carboxylic acid esters, preferably between 40 wt% to 95 wt% carboxylic acid esters, for example between 45 wt% and 70 wt% carboxylic acid esters; based on the total liquid weight (in the absence of solids).

[0064] The technological and ecological demands on the oil phase components of drilling fluids mandate that drilling fluids meet certain requirements. Therefore, the drilling fluids provided herein are hydrophobic and of low polarity to minimize swelling of clays and shales. More specifically, the present base oils for drilling fluids generally have low viscosity (e.g., KV(40°C) < 3 cSt)) and a low pour point (e.g., <-6°C).

[0065] Typically, the base oil of the drilling fluid provided herein has a kinematic viscosity of less than about 20 cSt (20 mm²/s) at 20°C. Additionally or alternatively, the base oils generally have a kinematic viscosity less than or equal to about 3 cSt when measured at 40° C.

[0066] The base oil of the drilling fluid provided herein has a pour point of generally at least about -6°C or lower, at least about -15°C. or lower, at least about -20°C or lower, at least about - 25°C or lower, at least about -30°C or lower, at least about -35°C or lower, at least about -40°C or lower, at least about -45°C or lower, at least about -50°C or lower, at least about -55°C or lower, and at least about -60°C or lower. The base oils can have a pour point below -15°C and be pumpable between about -5°C and about -10°C.

[0067] Similarly, the carboxylic acid ester, or mixture of esters, can have a pour point below about -15°C, preferably below about -20°C and more preferably below about -30°C. The ester, or mixture of esters, generally has a kinematic viscosity at 20°C of less than about 20 cSt, such as from about 2 to about 15, or from about 5 to about 12, preferably about 3 to about 8, cSt (mm²/s).

[0068] The drilling fluids provided herein generally have a flash point above about 100°C as determined using ASTM D 93, in another aspect above about 130°C, or above about 150°C. The drilling fluid base oils generally have a flash point above about 50° C as determined using ASTM D 93, in another aspect, above about 70°C, or above about 100°C.

[0069] The drilling fluids provided herein generally can remain physically and chemically stable at temperatures up to about 250°C, pressures up to about 20000 p.s.i. (about 138 MPa) and at high pH (up to about 10 lb of lime per barrel, about 30 kg of lime per m^3, or about pH 11) and are biodegradable under aerobic and anaerobic conditions. Particularly, the base oil of the drilling fluid typically has a biodegradation ratio of less than or equal to 1.0, such as less than about 0.95, or less than or equal to about 0.9, or less than or equal to about 0.85. The drilling fluids and any degradation products have low toxicity both to mammals and to marine flora and fauna, and little odor. In addition, the drilling fluids provided herein also allow for pressure control and can provide lubricity during drilling operations.

Methods of Drilling

[0070] Drilling fluids described herein are useful in any number of drilling methods. One exemplary method comprises introducing the drilling fluid into a wellbore. Methods may further include one or more steps of advancing a downhole tool in the well bore.

[0071] The following examples illustrate the present invention. Numerous modifications and variations are possible and it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

[0072] The invention can additionally or alternatively include one or more of the following embodiments.

Embodiment 1. A drilling fluid including a base oil comprising one or more carboxylic acid esters, the carboxylic acid ester(s) (i) having an acid moiety comprising a quaternary alpha carbon and an alcohol moiety comprising fewer than 7 carbon atoms, or (ii) having an acid moiety comprising a tertiary alpha carbon.

Embodiment 2. The drilling fluid of embodiment 1, wherein the alcohol moiety of said carboxylic acid ester(s) has a degree of branching of about 1.5 or less.

Embodiment 3. The drilling fluid of embodiment 2, wherein the alcohol moiety of said carboxylic acid ester(s) has a degree of branching of about 1.3 or less.

Embodiment 4. The drilling fluid of any one of embodiments 1 to 3, wherein the total number of carbons in each of said carboxylic acid ester(s) ranges from 6 to 24.

Embodiment 5. The drilling fluid of any one of embodiments 1 to 3, wherein the total number of carbons in each of said one or more carboxylic acid ester(s) is 5.

Embodiment 6. The drilling fluid of any one of embodiments 1 to 5, wherein the one or more carboxylic acid esters have a hydrolysis rate of less than about 3.0 mole percent per 15 hours. Embodiment 7. The drilling fluid of any one of embodiments 1 to 6, wherein the base oil has a kinematic viscosity of less than about 3 cSt at 40°C.

Embodiment 8. The drilling fluid of embodiment 7, wherein the base oil has a kinematic viscosity of less than about 2.5 cSt at 40°C.

Embodiment 9. The drilling fluid of any one of embodiments 1 to 8, wherein the base oil has a pour point of less than about -6°C.

Embodiment 10. The drilling fluid of embodiment 9, wherein the base oil has a pour point of less than about -60°C.

Embodiment 11. The drilling fluid of any one of embodiments 1 to 10, wherein the base oil comprises a mixture of two or more of the carboxylic acid esters.

Embodiment 12. The drilling fluid of any one of embodiments 1 to 11, wherein the base oil has a biodegradation ratio of 1.0 or less.

Embodiment 13. The drilling fluid of any one of embodiments 1 to 12, wherein the drilling fluid is a water-in-oil emulsion.

Embodiment 14. The drilling fluid of embodiment 13, wherein the emulsion comprises up to about 95 wt% of said one or more carboxylic acid esters, based on the weight of the emulsion; preferably between 50 wt% and 95 wt% of the total liquid weight.

Embodiment 15. The drilling fluid of embodiment 13 or 14, wherein the emulsion comprises from 1 wt% to 45 wt% of water, based on the total liquid weight (i.e. without solids).

Embodiment 16. A blend comprising:

(a) one or more carboxylic acid esters having an acid moiety comprising a tertiary or quaternary alpha carbon; and

(b) one or more olefins.

Embodiment 17. The blend of embodiment 16, wherein the carboxylic acid ester(s) (i) have an acid moiety comprising a quaternary alpha carbon and an alcohol moiety comprising fewer than 7 carbon atoms, or (ii) have an acid moiety comprising a tertiary alpha carbon.

Embodiment 18. The blend of embodiment 16 or 17, wherein the one or more olefins comprises at least one linear olefin.

Embodiment 19. The blend of embodiment 18, wherein the one or more olefins comprises at least one linear alpha olefin.

Embodiment 20. The blend of embodiment 18, wherein the one or more olefins comprises at least one linear internal olefin.

Embodiment 21. The blend of any one of embodiments 16 to 20, wherein the one or more olefins has a biodegradation ratio of about 1.0 or less. Embodiment 22. The blend of any one of embodiments 16 to 21, comprising said one or more carboxylic acid esters in an amount ranging from about 10 wt% to about 60 wt% and said one or more olefins in an amount ranging from about 40 wt% to about 90 wt%.

Embodiment 23. The blend of embodiment 22, comprising said one or more carboxylic acid esters in an amount ranging from about 20 wt% to about 50 wt% and said one or more olefins in an amount ranging from about 50 wt% to about 80 wt%.

Embodiment 24. A drilling fluid comprising the blend according to any one of embodiments 16 to 23.

Embodiment 25. The drilling fluid of embodiment 24, comprising 12 wt% to 60 wt% of said blend, 1 wt% to 45 wt% of water, and optionally up to 85 wt% of one or more additives (such as weighting agents and/or salts), wherein the wt% of water is preferably lower than the wt% of said blend.

Embodiment 26. The drilling fluid of embodiment 25, comprising 35 wt% to 60 wt% of said blend, 8 wt% to 45 wt% of water, and optionally up to 30 wt% of additives.

Embodiment 27. The drilling fluid of embodiment 24, comprising 50 wt% to 90 wt% of said blend, and 10 wt% to 50 wt% of water, based on the total liquid weight (i.e. excluding solid additives).

Embodiment 28. The drilling fluid of embodiment 27, comprising 55 wt% to 75 wt% of said blend, and 25 wt% to 45 wt% of water, based on the total liquid weight.

Embodiment 29. A method of drilling a wellbore comprising the step of introducing a drilling fluid according to any one of embodiments 1 to 12 or embodiments 24 to 28 into the wellbore.

EXAMPLE I

[0073] A class of low viscosity esters (KV(40°C) = 2.12 - 3.59 cSt) shown in Figures 1A-1H were evaluated for hydrolytic stability and anaerobic biodegradability in comparison to a fatty acid ester, with the results summarized in Table 1. As shown in Figures 1A to 1H, the chemical structures of the esters that were evaluated are of different sizes (methyl and ethyl) with branching in different locations with respect to alpha or beta to alcohol or acid.

[0074] The hydrolytic stability results of the esters reported in Table 1 were determined using the aforementioned hydrolytic stability testing procedure at a selected time frame of 15 hours and using the following gas chromatography procedure: [0075] Samples were analyzed on an Agilent 6890 Gas Chromatograph equipped with FID detector and an automatic liquid sampler (ALS). The typical injection size was about 1 μl (split ratio of 20: 1). The column used was a Quadrex UAC-1MS (30 m, 0.25 mm id, 0.1 μm film). The GC was operated in constant flow mode with an initial pressure of about 40 psi and column flow of about 126 ml/min using helium as carrier gas. The following oven procedure was used:

Initial temperature of 40°C, hold for 1 minute;

Ramp 1 at 20°C/min to 300°C, hold for 20 minutes;

Ramp 2 at 10°C/min to 360°C, hold for 10 minutes;

Total analysis time of 50 minutes.

[0076] The anaerobic biodegradability results of the esters reported in Table 1 were measured using the EPA Method 1647 Test. The biodegradation results are reported both in units of percent theoretical gas production (% TGP) at the conclusion of the 275 day measurement period, and also as the biodegradation ratio (represent as the ratio of the % TGP of the C₁₆ - C₁₈ internal olefin (IO) reference fluid to that that of the example products). Ratios of less than or equal to 1.0 indicate the example products pass the EPA Method 1647 Test.

Table 1

[0077] As shown from Table 1, the extent of hydrolysis was dependent on the structure of the ester. Particularly, linear structures resulted in poor hydrolytic stability while branching improved stability. For example, an all-linear ester, Ex 7 was the most unstable (11.4 mol% per 15 hours). Furthermore, mono-substitution affected hydrolysis nearly the same. Esters with methyl or ethyl groups alpha or beta to the acid or alcohol functionality hydrolyzed 3.0 - 3.2 mol% per 15 hours (Ex 3, Ex 4, Ex 5, Ex 6, and Comp Ex 1). In contrast, pivalic acid structures provided exceptional hydrolytic stability, as demonstrated by the pivalic acid ester of Ex 2 (exhibiting hydrolysis less than 0.3 mol% per 15 hours). The hydrolytic stability measurements were further confirmed via visual observations. Particularly, it was visually observed that Examples 1 - 6“flowed” more readily following the hydrolysis experiment. Meanwhile, the drilling fluids comprising the ester Ex 7 and Comparative Ex 1 had more solids, presumably because the extent of hydrolysis was greater (for Ex 7) or the byproducts formed insoluble soaps (Comparative Ex 1).

[0078] As also shown from Table 1, esters Ex 1, Ex 2, Ex 3, Ex 4, Ex 6 and Ex 7 all met the anaerobic biodegradability standard of EPA 1647 (i.e., their percent theoretical gas production equaled or exceeded that of the C₁₆ - C₁₈ IO reference fluid).

[0079] Figure 2 graphically depicts the data shown in Table 1. As depicted in Table 1 and Figure 2, while an all linear structure was equivalent to poor hydrolytic stability, the effect of increasing branching to improve hydrolytic stability on anaerobic biodegradability was found to vary depending on whether the branching was located on the alcohol vs the acid moiety. Particularly, molecules having branching on the acid moiety were shown to have a better anaerobic biodegradation than those having branched alcohol moieties.

EXAMPLE II

[0080] As shown in Table 2, blends of ester Ex 2 from Example I with l-hexadecene (i.e., C₁₆ LAO), a commonly used synthetic base fluid, were prepared by adding the components to a container and shaking to form the blend. The prepared blends were evaluated for pour point and kinematic viscosity in order to assess the suitability of the carboxylic acid esters described herein as a blendstock with a known base fluid. Table 2

[0081] As can be seen from Table 2, blending the Ex 2 ester with l-hexadecene at a concentration of 50 wt% was effective in reducing the pour point to -9°C compared to 3°C for neat l-hexadecene.

[0082] All documents described herein are incorporated by reference for purposes of all jurisdictions where such practice is allowed, including any priority documents and/or testing procedures to the extent they are not inconsistent with this text. As is apparent from the foregoing general description and the specific embodiments, while forms of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited thereby. For example, the compositions described herein may be free of any component, or composition not expressly recited or disclosed herein. Any method may lack any step not recited or disclosed herein. Likewise, the term“comprising” is considered synonymous with the term“including.” And whenever a method, composition, element or group of elements is preceded with the transitional phrase“comprising,” it is understood that we also contemplate the same composition or group of elements with transitional phrases “consisting essentially of,” “consisting of,” “selected from the group of consisting of,” or“is” preceding the recitation of the composition, element, or elements and vice versa.

Claims

CLAIMS What is claimed is:

1. A drilling fluid including a base oil comprising one or more carboxylic acid esters, the carboxylic acid ester(s) (i) having an acid moiety comprising a quaternary alpha carbon and an alcohol moiety comprising fewer than 7 carbon atoms, or (ii) having an acid moiety comprising a tertiary alpha carbon.

2. The drilling fluid of claim 1, wherein the alcohol moiety of said carboxylic acid ester(s) has a degree of branching of about 1.5 or less.

3. The drilling fluid of claim 2, wherein the alcohol moiety of said carboxylic acid ester(s) has a degree of branching of about 1.3 or less.

4. The drilling fluid of any one of claims 1 to 3, wherein the total number of carbons in each of said carboxylic acid ester(s) ranges from 6 to 24.

5. The drilling fluid of any one of claims 1 to 3, wherein the total number of carbons in each of said one or more carboxylic acid ester(s) is 5.

6. The drilling fluid of any one of claims 1 to 5, wherein the one or more carboxylic acid esters have a hydrolysis rate of less than about 3.0 mole percent per 15 hours.

7. The drilling fluid of any one of claims 1 to 6, wherein the base oil has a kinematic viscosity of less than about 3 cSt at 40°C.

8. The drilling fluid of claim 7, wherein the base oil has a kinematic viscosity of less than about 2.5 cSt at 40°C.

9. The drilling fluid of any one of claims 1 to 8, wherein the base oil has a pour point of less than about -6°C.

10. The drilling fluid of claim 9, wherein the base oil has a pour point of less than about -60°C.

11. The drilling fluid of any one of claims 1 to 10, wherein the base oil comprises a mixture of two or more of the carboxylic acid esters.

12. The drilling fluid of any one of claims 1 to 11, wherein the base oil has a biodegradation ratio of 1.0 or less.

13. The drilling fluid of claim 1, wherein the drilling fluid is a water (brine)-in-oil emulsion.

14. The drilling fluid of claim 13, wherein the emulsion comprises up to about 95 wt% carboxylic acid ester based on the weight of the emulsion.

15. A blend comprising:

(a) one or more carboxylic acid esters having an acid moiety comprising a tertiary or quaternary alpha carbon; and

(b) one or more olefins.

16. The blend of claim 15, wherein the carboxylic acid ester(s) (i) have an acid moiety comprising a quaternary alpha carbon and an alcohol moiety comprising fewer than 7 carbon atoms, or (ii) have an acid moiety comprising a tertiary alpha carbon .

17. The blend of claim 15 or 16, wherein the one or more olefins comprises at least one linear olefin.

18. The blend of claim 17, wherein the one or more olefins comprises at least one linear alpha olefin.

19. The blend of claim 17, wherein the one or more olefins comprises at least one linear internal olefin.

20. The blend of any one of claims 15 to 19, wherein the one or more olefins has a biodegradation ratio of about 1.0 or less.

21. The blend of any one of claims 15 to 20, comprising said one or more carboxylic acid esters in an amount ranging from about 10 wt% to about 60 wt%; and said one or more olefins in an amount ranging from about 40 wt% to about 90 wt%.

22. A drilling fluid comprising the blend according to any one of claims 15 to 21.

23. The drilling fluid according to claim 22, comprising 50 wt% to 90 wt% of said blend, and

10 wt% to 50 wt% of water, based on the total liquid weight.

24. A method of drilling a wellbore comprising the step of introducing a drilling fluid according to any one of claims 1 to 12 or claims 22 to 23 into the wellbore.