POLYVINYL ALCOHOL BASED LOST CIRCULATION MATERIALS

Field of the Inventio

[0001] This present invention relates to particulate, polyvinyl alcohol-based loss circulation control compositions for minimizing or eliminating loss of circulation of fluids during the process of well drilling, workover, completion and cementing. In particular, the present invention relates to particulate, water-soluble loss control polymer manufactured by compacting specified polyvinyl alcohol copolymers with one or more certain specified fillers, optionally with other additives and other polyvinyl alcohols.

[0002] During drilling operations for oil production from subterranean formations, drilling fluids are pumped down through the drill pipe, through openings in the drill bit, and then upward to ground level, where the fluid is screened of the accumulated cuttings and is recirculated through the system

[0003] The functions of the drilling fluids include, for example, to lubricate the drilling tool and drill pipe, to provide a medium for removing formation cuttings from the well to the surface, to counterbalance formation pressure in order to prevent the inflow to the well bore of gas, oil, and/or water from permeable or porous formations which may be encountered at various levels as drilling progresses, to maintain hole stability prior to setting the casing, to minimize formation damage, and to hold the drill cuttings in suspension.

[0004] It is necessary for the drilling fluid to circulate in the wellbore (down the drill pipe and back up the annulus) in order to perform ail of the desired functions.

[0005] A problem which sometimes occurs in the oil field is the loss of the drilling fluids into the permeable zones of the wellbore, which can dramatically increase the costs of the drilling operation.

[0006] In order to minimize the loss of the circulation fluids, it is desirable to plug the flow passages responsible for the fluid losses.

[0007] Drilling fluids are designed to seal porous formations while drilling; this occurs as the result of suction of the fluid onto the permeable surface (pressure greater in the well than in the formation) and the creation of a mud cake to seal a porous formation during drilling and for the purpose of wellbore stabilization.

[0008] The loss of fluids to the formation can reach an extent such that no mud cake can be created to secure the surface and create an effective barrier. In extreme situations, when the borehole penetrates a fracture in the formation through which most of the drilling fluid may be lost, the rate of loss may exceed the rate of replacement. Drilling operations may have to be stopped until the lost circulation zone is sealed and fluid loss to the fracture is reduced to an acceptable level. In the worst case, the consequences of this problem can be loss of the well.

[0009] Several techniques have been developed to cure or to reduce lost circulation of mud to the wellbore. Curing lost circulation while drilling is the subject of many publications and patents.

[0010] For example, certain drilling fluid additives can form a thin, low permeability filter cake that can seal openings in formations to reduce the unwanted influx of fluids or the loss of drilling fluids to permeable formations.

[0011] Lost circulation materials (LCMs) capable of bridging or blocking seepage into the formation can also be added to the drilling fluid. Many different types of materials have been added to drilling fluids as LCMs including, for example, chopped up corn stalks; sugar cane, beet pulp; conttonseed hulls; rice hulls; sawdust; wood shavings; flake cellophane; shredded paper; mica flakes; materials made from whole ripe flax straw (US2610149); processed and shaped wet pulp residue with solid and fiber content with particle sizes between 200-1000 micron with up to 70% inorganic filler (kaolinite clays and calcium carbonate) and at least 30% cellulous fiber (W093/1811 1A1); whole corncobs or the woody ring portion of corncobs (US4247403); sized coca bean shell material with a particle size distribution of 2 to 100 mesh (US4474665); a thermoplastic polymer (such as polypropylene, polyethylene and polyvinyl chloride) in a flexible, elongated laminar form (US4579668); ground oat hulls, optionally in combination with one or more of ground corn cobs, cotton, citrus pulp, and ground cotton burrs (US5004553 and US507I575); ground cotton burrs, optionally in combination with one or more of ground oat hulls, ground corn cobs, cotton, ground citrus pulp ground peanut shells, ground rice hulls, and groundnut shells (US5076944); and ground tannin-containing organic waste product including grape pumice, tomato pumice, yellow pine bark, yellow pine, wood bark and the like (US6399545).

7 [0012] Polyvinyl alcohol optionally in combination with other materials has been described as a fluid loss additive for use in cement for cementing oil and gas well bores. See, for example,

US5105885, US5207831, US2006/0041060A1 and EP0587383A1.

Summary of the Invention

[0013] In accordance with the present invention, it has been found that certain specified polyvinyl alcohol copolymer resins and blends of the copolymers with homopolymers compacted in combination with one or more specified additives are highly effective as loss circulation control materials for use in subterranean treatments.

[0014] More specifically, the present invention provides a particulate loss circulation control composition comprising particles of a compacted mixture of a (1) poly vinyl alcohol component, characterized in that the mixture further comprises (2) an acid-soluble weighting agent, and the polyvinyl alcohol component comprises a hydrolyzed copolymer of vinyl acetate and one or more unsaturated acids as comonomers, wherein wherein

[0015] (a) the unsaturated acid is selected from the group consisting of

[0016] (i) a monocarboxylic un saturated acid,

[0017] (ii) a dicarboxylic unsaturated acid,

[0018] (iii) an alkyl ester of (i),

[0019] (iv) an alkyl ester of (ii),

[0020] (v) an alkali metal salt of (i),

[0021] (vi) an alkali metal salt of (ii),

[0022] (vii) an alkaline earth metal salt of (i),

[0023] (viii) an alkaline earth metal salt of (ii), and

[0024] (ix) an anhydride of (i) and (x) an anhydride of (ii), and

[0025] (b) the copolymer has

[0026] (i) an unsaturated acid content of from about 0.1 mol% to about 15 mol% based on the total moles of monomers,

[0027] (ii) a viscosity-average degree of polymerization of from about 300 to about 3000,

[0028] (iii) a degree of hydrolysis of from about 70 mol% to 100 mol%, and [0029] (iv) is substantially soluble in water and brine at a temperature of 195°F or higher;

[0030] (c) the composition has a bulk density greater than about 0.9 g/cc; and

[0031] (d) the composition has a D(10) particle size of 4 mesh (U S Sieve Series).

[0032] In one embodiment, the particulate loss circulation control composition further comprises at least one additional additive selected from the group consisting of a starch, a plasticizer and a filler (other than (2)).

[0033] In another embodiment, the composition has a D(90) particle size of 1 inch.

[0034] In another embodiment, the polyvinyl alcohol component comprises of a blend of an acid-functional polyvinyl alcohol copolymer with at least one other polyvinyl alcohol. The second polymer can be a fully hydrolyzed high molecular weight polyvinyl alcohol homopolymer which has a water solubility significantly lower than the acid-functional polyvinyl alcohol copolymer.

[0035] In another embodiment, the other polyvinyl alcohol is partially-hydrolyzed polyvinyl alcohol homopoiymer.

[0036] In yet another embodiment, the polyvinyl alcohol resin component is a transition product as explained below.

[0037] The particulate loss circulation control compositions of the present invention are prepared by placing the mixture (polyvinyl alcohol resin component and additive(s)) under extreme pressure. As the resin adheres to itself in the compaction process, no additional binder is needed to agglomerate the mixture. In other words, the specified polyvinyl alcohol component functions as the binder for the agglomerate. Additives such as fillers, starches, and plasticizers are added to the resin as necessary. The resin compaction can be carried out using conventional compaction methods and equipment, such as a double roll compactor. The compacted mixture can be crushed and screened to appropriate particle size. Pelletization using conventional methods may also be utilized to the extent that sufficient density can be achieved

[0038] The particulate loss circulation control compositions of the present invention are particularly suitable for use in subterranean formations where formation temperatures are typically about 200°F or lower. In some cases, however, the particulate loss circulation control compositions can have suitable stability for suffi cient time periods at temperatures of up to about 250°F. [0039] An additional advantage of the polyvinyl alcohol-based particulate loss circulation control compositions of the present invention is that they are environmentally friendly as they are temporary, and the specified polyvinyl alcohols are considered non-toxic and biodegradable.

[0040] These and other embodiments, features and advantages of the present invention will be more readily understood by those of ordinary skill in the art. from a reading of the following detailed description.

[0041] The present invention relates to particulate polyvinyl alcohol-based loss circulation control compositions for use in subterranean treatments. In particular, the present invention relates to particulate loss circulation control compositions manufactured by compacting specified polyvinyl alcohol copolymers with certain additives and optionally other fully and partially hydrolyzed polyvinyl alcohols. Further details are provided below.

[0042] In the context of the present description, all publications, patent applications, patents and other references mentioned herein, if not otherwise indicated, are explicitly incorporated by reference herein in their entirety for all purposes as if fully set forth

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary' skill in the art to which this disclosure belongs. In case of conflict, the present specification, including definitions, will control.

[0044] Except where expressly noted, trademarks are shown in upper case.

[0045] Unless stated otherwise, all percentages, parts, ratios, etc., are by weight.

[0046] Unless stated otherwise, pressures expressed in psi units are gauge, and pressures expressed in kPa units are absolute. Pressure differences, however, are expressed as absolute (for example, pressure 1 is 25 psi higher than pressure 2).

[0047] When an amount, concentration, or other value or parameter is given as a range, or a list of upper and lower values, this is to be understood as specifically disclosing all ranges formed front any pair of any upper and lower range limits, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the present disclosure be limited to the specific values recited when defining a range. [0048] When the term "about" is used in describing a value or an end-point of a range, the disclosure should be understood to include the specific value or end-point referred to.

[0049] As used herein, the terms "comprises,” "comprising," "includes," "including," "has," "having" or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0050] The transitional phrase "consisting of excludes any element, step, or ingredient not specified in the claim, closing the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith. When the phrase "consists of appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole

[0051] The transitional phrase "consisting essentially of limits the scope of a claim to the specified materials or steps and those that do not materially affect the basic and novel charaeteristie(s) of the claimed invention. A“consisting essentially of’ claim occupies a middle ground between closed claims that are written in a“consisting of’ format and fully open claims that are drafted in a“comprising” format. Optional additives as defined herein, at a level that is appropriate for such additives, and minor impurities are not excluded from a composition by the term“consisting essentially of.

[0052] Further, unless expressly stated to the contrary', "or" and“and/or” refers to an inclusive and not to an exclusive. For example, a condition A or B, or A and/or B, is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

[0053] The use of "a" or "an" to describe the various elements and components herein is merely for convenience and to give a general sense of the disclosure. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.

[0054] The term“predominant portion”, as used herein, unless otherwise defined herein, means that greater than 50% of the referenced material. If not specified, the percent is on a molar basis when reference is made to a molecule (such as hydrogen, methane, carbon dioxide, carbon monoxide and hydrogen sulfide), and otherwise is on a weight basis (such as for carbon content). [0055] The term “substantial portion” or“substantially”, as used herein, unless otherwise defined, means ail or almost all or the vast majority, as would be understood by a person of ordinary skill in the relevant art in the context used. It is intended to take into account some reasonable variance from 100% that would ordinarily occur in industrial-scale or commercial- scale situations.

[0056] The term“depleted” or“reduced” is synonymous with reduced from originally present. For example, removing a substantial portion of a material from a stream would produce a material-depleted stream that is substantially depleted of that material. Conversely, the term “enriched” or“increased” is synonymous with greater than originally present.

[0057] As used herein, the term“copolymer” refers to polymers comprising copolymerized units resulting from copolymerization of two or more comonomers. In this connection, a copolymer may be described herein with reference to its constituent comonomers or to the amounts of its constituent comonomers, for example“a copolymer comprising vinyl acetate and 15 mol% of a comonomer”, or a similar description. Such a description may be considered informal in that it does not refer to the comonomers as copolymerized units; in that it does not include a conventional nomenclature for the copolymer, for example International Union of Pure and Applied Chemistry (IUPAC) nomenclature; in that it does not use product-by-process terminology; or for another reason. As used herein, however, a description of a copolymer with reference to its constituent comonomers or to the amounts of its constituent comonomers means that the copolymer contains copolymerized units (in the specified amounts when specified) of the specified comonomers. It follows as a corollary that a copolymer is not the product of a reaction mixture containing given comonomers in given amounts, unless expressly stated in limited circumstances to be such.

[0058] The term“unit” refers to a unit operation. When more than one“unit” is described as being present, those units are operated in a parallel fashion unless otherwise stated A single “unit”, however, may comprise more than one of the units in series, or in parallel, depending on the context. For example, a thermal treating unit may comprise a first cooling unit followed in series by a second cooling unit.

[0059] The term“free-flowing” particles (or agglomerates) as used herein means that the particles do not materially further agglomerate (for example, do not materially further aggregate, cake or clump), as is well understood by those of ordinary skill in the relevant art. Free-flowing particles need not be“dry” but, desirably, the moisture content of the particles is substantially internally contained so that there is minimal (or no) surface moisture.

[0060] The term“D(X) particle size” means the diameter at which X% of the sample's mass is comprised of particles with a diameter less than this value. For example,“D(10) particle size” means the diameter at which 10% of the sample's mass is comprised of particles with a diameter less than this value, and“D(90) particle size” means the diameter at which 90% of the sample's mass is comprised of particles with a diameter less than this value.

[0061] The term“substantially soluble in water” and“soluble in water” means substantially completely (or completely) soluble in deionized water under the stated conditions.

[0062] The term“substantially soluble in brine” and“soluble in brine” means substantially completely (or completely) soluble in“brine” under the stated conditions. For the purposes of the present invention,“brine” generally means a water solution with NaCi concentration of up to 2.9 wt%.

[0063] The term“acid-soluble weighting agent” means a material that is soluble in an acidic medium, or reacts in acidic medium to result in a product that is soluble in water. For example, calcium carbonate reacts in an acidic medium to generate calcium salt that is soluble in water.

[0064] For convenience, many elements of the present invention are discussed separately, lists of options may be provided and numerical values may be in ranges; however, for the purposes of the present disclosure, that should not be considered as a limitation on the scope of the disclosure or support of the present disclosure for any claim of any combination of any such separate components, list items or ranges. Unless stated otherwise, each and every combination possible with the present disclosure should be considered as explicitly disclosed for all purposes.

[0065] Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described herein. The materials, methods, and examples herein are thus illustrative only and, except as specifically stated, are not intended to be limiting.

Polyvinyl Alcohol Copolymers

[0066] The resins used in the manufacture of the loss circulation control compositions of the present invention are polyvinyl alcohol based. Polyvinyl alcohol homopolymers and copolymers are in a general sense well-known polymers and are generally commercially available in many forms for a variety' of end-uses.

[0067] Polyvinyl alcohol is produced on a commercial scale by polymerizing vinyl acetate (with optional comonomers) to generate polyvinyl acetate, after which the acetate groups are hydrolyzed to hydroxyl groups in varying degrees. Several different hydrolysis methods are well-known and can be used for this purpose.

[0068] Polyvinyl alcohol copolymers for use in the present invention are hydrolyzed“acid- functional” polyvinyl acetate copolymers.

[0069] The polyvinyl acetate copolymer starting material is typically produced by the free radical polymerization of the vinyl acetate monomer with one or more “acid-functional” comonomers in the presence of a polymerization catalyst. The solvent commonly used in the commercial polymerization of vinyl acetate is methanol. The polymerization is typically conducted in the temperature range of from about 10°C to about 80°C. The lower end of the polymerization range is known to give products with improved properties. The percent conversion of vinyl acetate to polyvinyl acetate can vary over a wide range. Though conversions ranging fro about 20% to 100% have been found satisfactory _' , commercially at least about 30% conversion is preferable.

[0070] The“acid-functional” comonomer is one or more of (i) a monocarboxylic unsaturated acid, (ii) a dicarboxylic un saturated acid, (iii) an alkyl ester of (i), (iv) an al yl ester of (ii), (v) an alkali metal salt of (i), (vi) an alkali metal salt of (ii), (vii) an alkaline earth metal salt of (i), (viii) an al aline earth metal salt of (ii), (ix) an anhydride of (i), and (x) an anhydride of (ii).

[0071] Some of the examples of such comonomers include methacrylic acid, methyl methacrylate, 2-hydroxyethyl acrylate, hydroxyl methacrylate, ethyl methacrylate, n-butyl methacrylate, maleic acid, monomethyl maleate, dimethyl maleate, maleic anhydride, itaconic acid, monomethyl itaconate, dimethyl itaconate, and itaconic anhydride.

[0072] Preferred are lower alkyl (C1 -C8, or C1 -C4) acrylates and methacryies. Non-limiting examples of such comonomers include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methyacrylate, i -propyl acrylate, i-propyl methacrylate, n-propyl acrylate, n-propyl methacrylate, i-butyl acrylate, i -butyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t- butyl acrylate, t-butyl methacrylate, ethylhexyl acrylate, ethyl hexyl methacrylate and others. Preferred comonomers include methyl acrylate, methyl methacylate and mixtures thereof, and particularly methyl acrylate.

[0073] The comonomer content of polyvinyl acetate copolymer (and resulting polyvinyl alcohol copolymer (2)) ranges from about 0.1 mol%, or from about 0.5 mol%, or from about 1 mol%, to about 15 mo!%, or to about 10 mol%, or to about 8 mol%. In the case of methyl acrylate, the amount is typically about 10 mol% or less, based on the total moles of monomer. In the case of methyl methacrylate, the amount is typically about 5 mol% or less, based on the total moles of monomer.

[0074] The viscosity-average degree of polymerization of polyvinyl acetate copolymer (and resulting polyvinyl alcohol copolymer (2)) varies anywhere from about 300, or from about 500, or from about 700, to about 3000, or to about 2000. The viscosity-average degree of polymerization of a polyvinyl alcohol copolymer is a value measured in accordance with JIS K6726 (1994), as described above.

[0075] The polyvinyl acetate is converted to polyvinyl alcohol via hydrolysis or alcoholysis processes generally known to those of ordinary skill in the relevant art. In such processes, the polyvinyl acetate is contacted with an alkali catalyst such as sodium hydroxide or sodium methylate. The major products of this reaction are polyvinyl alcohol and methyl acetate.

[0076] Examples of commercially available acid -functional polyvinyl alcohols include KURARAY POVAL™ K-series grades such as 32-97KL, 25-88KL, 6-77KL and 30-94KL (Kuraray Co., Ltd., Tokyo Japan).

[0077] One such process is a slurry alcoholysis process in which polyvinyl alcohol is obtained from polyvinyl acetate and recovered as a slurry in a methanol and methyl acetate solvent system. Such process is desirably continuous. The slurry alcoholysis process is in a general sense well known to those of ordinary skill in the relevant art, such as disclosed in US2734048, and the resulting polyvinyl alcohols are agglomerated particles of a“popcorn” morphology as described below A suitable acid-functional polyvinyl alcohol copolymer produced by slurry alcoholysis is commercially available under the trade designation ELVANOL™ 80-18 (Kuraray America, Inc., Houston, Texas USA).

[0078] Regardless of the hydrolysis process, the resulting polyvinyl alcohols, of course, will have substantially the same monomer makeup and degree of polymerization as the starting polyvinyl acetates. [0079] The polyvinyl alcohol copolymer should have a degree of hydrolysis of from about 70 moi%, or from about 75 mol%, or from about 85 mol%, or from about 93 mol%, or from about 95 mol%, or from about 98 mol%, or from about 99 mol%, to 100 mol% (maximum). The degree of hydrolysis can be measured in accordance with JIS K6726 (1994).

[0080] One method of varying the rate of the dissolution of the particulate loss circulation control compositions is by varying the crystallinity of the polyvinyl alcohol resin.

[0081] For example, for polyvinyl alcohol copolymers with increased water solubility, the slurry alcoholysis processes as described in US2017/0260309A1 and US2019/0055326A1, the disclosure of which is incorporated by reference herein for all purposes as if fully set forth) may used for hydrolyzing the polyvinyl acetate copolymer to polyvinyl alcohol copolymer

[0082] As described in the above-incorporated disclosures, suitable polyvinyl alcohol copolymers can be produced by a slurry alcoholysis process in which polyvinyl alcohol is obtained from polyvinyl acetate and recovered as a slurry in a methanol and methyl acetate solvent system.

[0083] In one embodiment, a first solution of typically about 30 wt% to about 60 wt% polyvinyl acetate copolymer in methanol, and a second solution of dilute sodium methylate alcoholysis catalyst in methanol, are continuously fed to an alcoholy sis unit wherein the reaction proceeds to produce a first slurry of the alcoholyzed polyvinyl acetate (polyvinyl alcohol) and methyl acetate.

[0084] Catalyst amount typically ranges from about 0.2 wt% to about 0.5 wt% based on the weight of the reaction mixture

[0085] The temperature of the alcoholysis reaction in the alcoholysis unit is typically from about 58°C, or fro about 64°C, to about 70°C, or to about 68°C. The pressure within the alcoholysis unit ranges from slightly below atmospheric pressure to slightly above atmospheric pressure, but is typically slightly above atmospheric pressure.

[0086] The alcoholysis unit contains an agitation means so that the alcoholysis is at least partially conducted under agitation conditions. Such agitation means are well known to those of ordinary skill in the relevant art.

[0087] When the alcoholysis reaches about 40-50%, the polymer partially precipitates. The insoluble material takes the form of a gel of polymer molecules solvated with methanol. As the solubility decreases by further alcoholysis, the gel becomes tougher and begins to reject the associated solvent molecules. When the alcoholysis is completed, the polymer and solvent are mutually insoluble. If this gel is allowed to stand undisturbed, alcoholysis proceeds and the product is obtained in a massive, unworkable form. However, if the gel is worked mechanically (agitated) during this range above about 40% alcoholysis, the polymer will break down to finely- divided solids insoluble in the alcohol. The collapsing gel traps and sticks together with the fine particles from the previous alcoholysis cycle producing polyvinyl alcohol of a desired“popcorn ball” morphology.

[0088] In one embodiment, the alcoholysis unit is made up of a primary alcoholysis vessel where the reaction proceeds to produce a slurry of partially alcoholyzed polyvinyl acetate. The slurry from the primary alcoholysis vessel overflows to an agitated hold vessel which provides additional residence time for completing the alcoholysis reaction. The slurry from the agitated hold vessel is then pumped through one or more finisher units to react short-circuited polyvinyl acetate, thus ensuring that the conversion is raised to 99.5% or higher of desired completion.

[0089] Preferred conversion is a degree of hydrolysis as set forth above.

[0090] The resulting first polyvinyl alcohol slurry may then optionally be fed to a neutralizing unit along with an acid to fully or partially neutralize remaining alcoholysis catalyst. In one embodiment, the catalyst is substantially neutralized. In another embodiment, less than a predominant portion (less than 50 equivalent %), or less than 25 equivalent %, or less than 10 equivalent %, or less than 5 equivalent %, of any excess alkali catalyst, can be neutralized. Typically, the acid employed is acetic acid. The temperature entering neutralizing unit is slightly lower than in the alcoholysis unit, generally in the range of from about 53°C to about 60°C, and typically in the range from about 55°C to about 58°C. Pressure conditions in the neutralizing unit are typically similar to those in the alcoholysis unit.

[0091] A second slurry is generated from the neutralizing unit. If present, the neutralizing unit can also be used to control the pH of the resulting second slurry.

[0092] In one embodiment, the neutralizing unit is not present (or is bypassed if present, or is present with substantially no acid feed, or is present with no acid feed), and the excess alkali catalyst is substantially not neutralized (or not neutralized) and remains in first slurry.

[0093] In one embodiment, the second slurry' resulting from the neutralization unit, if present, or if not present the first slurry, is then fed to an optional thermal treating unit. The temperature of the first slurry, or the second slurry' if present, may be elevated or reduced in the thermal treating unit depending on the desired solubility of the resulting polyvinyl alcohol copolymer. If higher solubility is desired, the temperature can be reduced to less than 50°C, or to less than 40°C, or to less than 35°C, or to less than 30°C, or to less than 25°C, or to less than ambient conditions, with the lower temperatures resulting in higher amorphous and less crystalline content

[0094] The thermal treatment unit can be a holding tank with mild heating, or no heating or even active cooling so that the temperature of the slurry is increased or reduced between entry and exit.

[0095] In one embodiment, the thermal treating unit is not present

[0096] In one embodiment, the thermally treated slurry', or the second slurry' if the thermal treatment unit is not present or not utilized, or the first slurry if the thermal treatment unit and the neutralizing unit are not present or not utilized, is fed to a solids-liquid separation unit where polyvinyl alcohol is separated from the slurry to generate a polyvinyl alcohol wet cake and separated liquids. The solids-liquid separation unit can be a centrifuge and/or filtration device or other conventional solids-liquid separation device.

[0097] In an alternate embodiment, the thermal treatment unit and solids-liquid separation unit can be combined in a single unit operation where the residence time of the slurry and solids is sufficient to reduce the temperature of the second slurry to the desired level.

[0098] In one embodiment, the process further comprises the step of washing the polyvinyl alcohol wet cake to produce a purified polyvinyl alcohol wet cake, which is then subject to the drying step. The resulting polyvinyl alcohol wet cake can optionally be purified by feeding the wet cake into a washing unit where it is contacted typically with a fresh or recycled methanol stream to remove ash components and other contaminates to generate a purified polyvinyl alcohol wet cake.

[0099] In order to generate the final particulate agglomerated polyvinyl alcohol particles, the purified polyvinyl alcohol wet cake after centrifugation, or the wet cake if the washing unit is not present or not utilized, is fed to a drying unit where it is dried via conventional means to remove sufficient remaining liquid content so that the resulting particulate agglomerated polyvinyl alcohol copolymer particles can be recovered, preferably as a free-flowing powder.

[00100] D(10) particles sizes of the polyvinyl alcohol copolymer agglomerated particles produced by the above slurry alcohol process are about 1 pm, or about 10 pm, with D(90) particle sizes being about 1000 pm, or about 400 pm. [00101] Bulk density of the polyvinyl alcohol copolymer agglomerated particles produced by the above slurry alcohol process is generally about 0.55 g/cm3 or less, or about about 0.50 g/cm ³ or less.

[00102] Additional process details can be had by reference to previously incorporated U S2017/0260309 A 1 and US2019/0055326A1, as well as US2734048, US3497487, US3654247 and general knowledge of those of ordinary' skill in the relevant art.

Blends with Other High Molecular Weight Homopolymer Polyvinyl Alcohols

[00103] In addition to crystallinity modification of the polyvinyl alcohol copolymer, another method for controlling the dissolution time of the particulate loss circulation control compositions of the present invention is by blending the acid-functional polyvinyl alcohol copolymer described above with one or more other fully- or partially-hydrolyzed polyvinyl alcohols of the types disclosed in some of the above incorporated references, or that are otherwise commercially available or generally known to those of ordinary' skill in the art.

[00104] Such other polyvinyl alcohols may be chosen to be more soluble than the acid- functional polyvinyl alcohol copolymer, but typically they are chosen to be less soluble and thus extend the dissolution rate of the lost circulation polymers comprising the combination.

[00105] In one embodiment, the polyvinyl alcohol component comprises a blend, wherein the acid-functional polyvinyl alcohol copolymer is present in the blend in an amount of from about 10 wt%, or from about 20 wt%, or from about 25 wt%, or from about 33 wt%, or from about 40 wt%, to about 90 wt%, or to about 80 wt%, or to about 77 wt%, or to about 67 wt%, or to about 60 wt%, based on the total weight of the polyvinyl al cohol component.

[00106] In one embodiment, the other polyvinyl alcohol is one or more partially- or fully- hydrolyzed polyvinyl alcohol homopolymers. Such polyvinyl alcohol homopolymers are generally commercially available, for example, under the brands KURARAY POVAL™ and ELVANOL™ from Kuraray Co., Ltd. (Tokyo, Japan) and its affiliates.

Transition Grade Blends (“Transition Products”)

[00107] In one embodiment, the polyvinyl alcohol component is a transition product produced in a continuous hydrolysis process. Such transition product is in essence an intimate reactor blend of multiple polyvinyl alcohol grades as would be recognized by one of ordinary skill in the relevant art.

[00108] For example, in many commercial continuous polyvinyl alcohol hydrolysis processes, instead of completely stopping the process and cleaning the equipment, the polyvinyl acetate feed is transitioned and/or the reaction conditions are transitioned from grade to grade. At some point, the process starts producing one grade of specified properties then transitions over time to a second grade of specified properties. This interim production is referred to as a transition grade.

[00109] In one embodiment, this transition grade is produced by transitioning production of the acid-functional polyvinyl alcohol copolymer to production of a polyvinyl alcohol homopolymer (or vice versa). In this case, the polyvinyl alcohol homopoiymer is less soluble than the acid- functional polyvinyl alcohol copolymer so that the dissolution rate of the particulate lost circulation can be modified.

[00110] In another embodiment, the transition grade is produced by altering the hydrolysis conditions, for example, thermal treatment step and/or level of excess catalyst neutralization, which can result in different solubility polyvinyl alcohols from the same starting polyvinyl acetate.

[00111] In another embodiment, the transition grade is produced by transitioning both the starting polyvinyl acetate and the hydrolysis conditions (for example, thermal treatment step and/or level of excess catalyst neutralization).

[00112] While the exact composition of the transition grade varies as a function of time, when different polyvinyl acetate starting materials are used the average composition should fall within the blend proportions as described above.

Additives

[00113] The particulate loss circulation control compositions of the present invention comprise an acid-soluble weighting agent as an additive, and may also optionally include one or more otheradditives. Such additives include, for example, fillers other than the acid-soluble weighting agents, plasticizers and starches. [00114] For example, fillers may be blended with the resin component to enhance mechanical properties and regulate the solubility curves of the loss circulation control compositions of the present invention.

[00115] The total amount of filler added (including acid-soluble weighting agent) can vary widely depending on the desired property modification, for example, up to about 50 wt%, or up to about 30 wt%, or up to about 5 wt%, based on the total weight of the loss circulation control compositions.

[00116] In many instances it is desirable to have the specific gravity of the loss circulation control compositions to be close to that of carrier fluid in order to allow for pumping and satisfactory placement of the loss circulation control compositions using the selected carrier fluid.

[00117] An acid-soluble weighting agent filler is blended with the resin prior to compaction. Weighting agent generally refers to any additive used to increase the density of the resin component.

[00118] Acid-soluble weighting agents generally include substances such as natural minerals and inorganic and organic salts. For example, the weighting agent can comprise a metal ion selected from the group consisting of calcium, magnesium, silica, barium, copper, zinc, manganese and mixtures thereof, and a counterion is selected from the group consisting of fluoride, chloride, bromide, carbonate, hydroxide, formate, acetate, nitrate, sulfate, phosphate and mixtures thereof.

[00119] Specific examples of fillers include minerals such as CaC0 ₃ , CaCl ₂ and ZnO.

[00120] One skilled in the art will recognize that plasticizers may be included in manufacturing of the loss circulation control compositions of the present invention to improve the flow' characteristics of the polyvinyl alcohol.

[00121] In order to obtain a uniform plasticizer coating it is preferred to utilize a spray mechanism to coat the polyvinyl alcohol.

[00122] A secondary' effect of such plasticizers is to reduce any dusting issues with the polyvinyl alcohol materials and ultimate particulate loss circulation control compositions.

[00123] Materials commonly used as plasticizers for polyvinyl alcohols are generally known to those of ori dinary skill in the relevant art, and are generally commercially available. Suitable plasticizers include, for example, compounds such as water, glycerol, polyglycerol, ethylene glycol, polyethylene glycols, ethanol acetamide, ethanol formamide, and acetates of triethanolamine, glycerin, trimethylolpropane and neopenty glycol, and mixtures of one or more of the above.

[00124] Plasticizers which are solid or crystalline at ambient temperatures, such as trimethylolpropane, may be dissolved in water or another liquid plasticizer medium for use as a sprayabie plasticizer.

[00125] Typically the level of the plasticizer can vary up to about 40 wt%, or up to about 30 wt%, or up to about 20 wt%, based on the weight of the polyvinyl alcohol component.

[00126] In one embodiment, a loss circulation control composition which yields a combination of good solubility properties and density comprises: (a) from about 60 wt% to about 94 wt% polyvinyl alcohol resin component: (b) from about 5 wt% to about 40 wt% acid-soluble weighting agent; and (c) from about 1 wt% to about 15 wt% plasticizer, based on the combined weight of (a), (b) and (c ).

[00127] In yet another embodiment, the present invention provides a loss circulation control compositions comprising of a blend of the polyvinyl alcohol resin component and acid-soluble weighting agent with a starch. Such blend can typically comprise from about 10 to about 90 parts by weight of the polyvinyl alcohol resin component and from about 90 to about 10 parts by weight of a starch, based on 100 parts by weight of the combination of polyvinyl alcohol resin component and starch. Preferably, however, there should be at least about 30 parts by weight polyvinyl alcohol resin component in any starch blend.

[00128] Suitable starches for use in the present invention include natural starches, synthetic starches, physically modified starches, chemically modified starches and mixtures thereof.

[00129] One or more other additives can be incorporated to the loss circulation control compositions as necessary. The additives include but are not limited to chelators, pH-adjusting agents, oxidizing agents, other lost circulation materials (such as described in the previously incorporated references), scale inhibitors, corrosion inhibitors, clay control additives, iron control additives, reducers, oxygen scavengers and the like. Use of such other additives in subsurface well operations is generally known to those of ordinary skill in the relevant art., as exemplified by many of the previously incorporated references.

Preparation of Loss Circulation Control Compositions [00130] In one embodiment, the particulate loss circulation control compositions of the present invention are prepared by compacting the mixtures described above under pressure.

[00131] The mixture compaction can be carried out using conventional compaction methods and equipment, such as a double roll compactor.

[00132] In a double roll compactor, the mixture is fed between two counter-rotating roll presses. The rolls apply extreme pressure to press the mixture into a sheet-like form. Desirably, the pressure applied during compaction is at least 5 T, or at least 10 T. After a certain pressure point, the compaction reaches an effective maximum where there is very little increase in density per unit of additional pressure. In one embodiment of the present invention, this effective maximum is about 30 T of pressure.“T” refers to ton (US)/sq. inch.

[00133] Compaction should be sufficient to achieve the desired or necessary' bulk density of the resulting loss circulation control composition as described below.

[00134] This sheet of material is then fed through a granulator, where it is broken up into sized granules that are random in shape but are desirably reasonably uniform in size. A screener sorts the agglomerated particles according to size. Particle that fall outside the desired size range are recycled fro the screener back to the compactor.

[00135] Desirably such compaction and granulation is a dry process that does not require an additional drying step.

[00136] Suitable particles sizes for the loss circulation control compositions of the present invention are as set forth in the previously incorporated references.

[00137] In general, the particle size of the loss circulation control compositions may be graded from 3 mesh, or from 4 mesh, to 200 mesh, or to 170 mesh (U.S. Sieve Series). A typical parti cle size of the loss circulation control compositions in accordance with the present inventi on is from 2 mesh, or from 3 mesh or from 4 mesh. The“mesh” size refers to US standard mesh.

[00138] Particle size distribution can vary widely depending on the permeability of the substrate, carrier fluid, subsurface temperature profile, composition of the loss circulation control compositions and other factors recognized by those of ordinary skill in the relevant art.

[00139] In accordance with the present invention, the composition has a D(10) particle size of 4 mesh (U.S. Sieve Series). In one embodiment, the composition has a D(90) particle size of 1 inch. [00140] In an embodiment with both plasticizer and filler, the plasticizer is preferably first added to the polyvinyl alcohol resin component, which is then preferably uniformly blended with one or more of the fillers. The blend is then compacted as described above

[00141] In an embodiment with both plasticizer and starch, the plasticizer is again preferably first added to the polyvinyl alcohol resin component, which is then preferably uniformly blended with the starch. The blend is then compacted as described above.

[00142] The particulate loss circulation control compositions of the present invention have an average density from about 0.9 g/mL or greater, or about 1 g/niL or greater, or about 1.1 g/mL or greater, or about 1.2 g/mL or greater, about 1.3 g/mL or greater, or about 1.4 g/mL or greater, or about 1.5 g/mL or greater. Bulk density is measured according to ASTM 1895C-17.

[00143] The shape of the loss circulation control compositions of this invention can change in roundness, area, length, width, aspect ratio, and roughness depending on the compacti on/granulation/peiletizati on methods.

Use of Loss Circulation Control Compositions

[00144] The loss circulation control compositions of the present invention can be used in fluid injection operations for subsurface wells by processes as generally known to those of ordinary skill in the art, and as exemplified in many of the previously incorporated references.

[00145] As indicated previously, the loss circulation control compositions of the present invention are particularly suitable for use in subterranean formations where formation temperatures are typically about 200°F or lower, although in some cases the loss circulation control compositions can have suitable stability for sufficient time peri ods at temperatures of up to about 250°F.

Examples

[00146] The following examples will facilitate a more complete understanding of the present invention but it is understood that the invention is not limited to the specific embodiments incorporated therein.

[00147] Solubility Test: 30 grams of a loss circulation control composition and 470 grams of deionized water were added into a vessel equipped with an agitator. The vessel was then placed in a water bath. The water bath heat controller was set at the desired temperature (122°F or 149°F). The agitator speed inside the vessel was set at 20 RPM. The timer was started as soon as the temperature inside of the vessel reached the desired temperature (122°F or 149°F). 10 mL of sample in the vessel was then collected in a centrifuge tube at the following times: 15, 30, 60, 120, 180, 240, 300, 360 and 420 minutes. The 10 mL sample was placed in centrifuge for 10 minutes at 1,500 RPM. The supernatant liquid was filtered through a 200 mesh sieve screen and placed into a pre- weighed aluminum pan. The sample together with the aluminum pan was then placed in an oven set to 105°C and left overnight to dry. The pan and contents were weighed and the % solubles was calculated using the following equation:

[00148] % Water Solubles (Weight of Residue plus pan - Weight of pan) * 100/Weight of sample

[00149] The solubility in salt w'ater (brine) was determined using the above procedure except salt water (5.84 grams of sodium chloride added to 994.16 grams of deionized water) was used instead of DI water to dissolve the loss circulation control composition.

[00150] Examples 1 and 2 below will present a more complete understanding of the present invention by describing the composition and the solubility curves of a relatively high-solubility loss circulation control composition (LCM-l) and a relatively low-solubility loss circulation control compositions (LCM-2A and LCM-2B). It should be understood that the invention is not limited to the specific embodiments incorporated in the examples.

[00151] Example 1 : The high-solubility loss circulation control compositions (LCM-l) was produced from the resin C-l described below.

[00152] The resin C-2 was a substantially fully-hydrolyzed acid-functional polyvinyl alcohol copolymer commercially available under the trade designation ELVANOL™ 80-18 (Kuraray America, Inc., Houston, Texas USA). This product is produced by utilizing the commercial process described above with neutralization, heat treatment by adding heat to raise temperature from about 50°C to about 110-1 15°C, and methanol washing.

[00153] The resin C-l on the other hand was produced by bypassing the neutralization unit in the production process for ELVANOL™ 80-18 resin, and as a result the excess alkali catalyst was not neutralized and remained in first slurry. The resulting slurry from the neutralization unit was then fed to a thermal treating unit where the slurry temperature w'as reduced to less than 50°C, after which it was fed to a solids-liquid separation unit where polyvinyl alcohol was separated from the shiny _' to generate a polyvinyl alcohol wet cake and separated liquids. The resulting wet cake after centrifugation was fed to a drying unit where it was dried via conventional means to remove sufficient remaining liquid content so that the resulting particulate agglomerated polyvinyl alcohol copolymer particles were recovered as a free-flowing powder [00154] The C-l resin was sprayed with 1.5 parts of a polyethylene glycol plasticizer (commercially available under the trade designation CARBOWAX™ Polyethylene Glycol 200, The Dow Chemical Company, Freeport, TX, USA). The plasticized polymer was then uniformly blended with 5 wt % of CaC0 ₃ . The uniform blend composed of the resin, plasticizer and filler was then compacted by placing it between two counter-rotating a double roll compactor. The rolls applied 20 T of pressure to press the mixture into a sheet-like form. This sheet of material was then fed through a granulator, where it was broken up into sized granules that are random in shape but desirably reasonably uniform in size. A screener sorted the agglomerated particles according to size. Particles that fell outside the desired size range were recycled from the screener back to the compactor.

[00155] The solubility of LCM-1 was determined in deionized water and brine at 122°F and at 149°F. Results are shown in Table 1 and Table 2. The results show ^? that LCM-1 is substantially soluble in both DI and brine water.

Table 1

Solubility of LCM-1 in Deionized Water

Table 2

Solubility of LCM-1 in Salt Water

[00156] Example 2: The relatively low solubility lost circulation polymers (LCM-2A and LCM-2B) were produced using the resin C-l, C-2 and C-3. As discussed above, C-2 was a substantially fully-hydrolyzed acid-functional polyvinyl alcohol copolymer commercially available under the trade designation ELVANOL™ 80-18 (Kuraray America, Inc., Houston, Texas USA), while C-3 was a fully-hydrolyzed polyvinyl alcohol homopolymer commercially available under the trade designation ELVANOL™ 71-30 (Kuraray America, Inc., Houston, Texas USA).

[00157] The dissolution time of the C-l resin (described in Example 1 above) was substantially decreased by blending it with C-2 and C-3.

[00158] C-l, C-2 and C-3 wore first plasticized by the spraying mechanism with 1.5 parts of a polyethylene glycol plasticizer (commercially available under the trade designation CARBOWAX™ Polyethylene Glycol 200, The Dow Chemical Company, Freeport, TX, USA). The lost circulation control compositions LCM-2A and LCM-2B were then prepared from a blend composed of a 50/50 wt% of C-l resin with the C-2 resin, and the C-l resin with the C-3 resin respectively, both uniformly blended with 2.5 wt % of CaC0 _3. The solubility of the loss control polymers LCM-2A and LCM-2B was determined in deionized water at 68°F and at I94°F. Results are shown in Table 3 and Table 4. The results clearly show that the solubility of the C-l resin is significantly reduced by bending it with a fully hydrolyzed polyvinyl alcohol homopolymer (such as ELVANOL™ 71 -30) and a fully neutralized acid copolymer (such as ELVANOL™ 80-18).

Table 3

Table 4