POLYVINYL ALCOHOL BASED LOST CIRCULATION MATERIALS

Abstract

Provided are particulate polyvinyl alcohol-based loss circulation control compositions for use in subterranean treatments, which are prepared by compacting a specified polyvinyl alcohol copolymer with an acid-soluble weighting agent, optionally with other specified additives and other polyvinyl alcohols.

Claims

1. A particulate loss circulation control composition comprising particles of a compacted mixture comprising (1) a polyvinyl alcohol component comprising a hydrolyzed copolymer of vinyl acetate and one or more unsaturated acids as comonomers (acid-functional polyvinyl alcohol copolymer), and (2) an acid-soluble weighting agent, wherein (a) the unsaturated acid is selected from the group consisting of (i) a monocarboxylic unsaturated acid, (ii) a dicarboxylic unsaturated acid, (iii) an alkyl ester of (i), (iv) an alkyl 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 alkaline earth metal salt of (ii), and (ix) an anhydride of (i) and (x) an anhydride of (ii), and (b) the copolymer has (i) an unsaturated acid content of from about 0.1 mol % to about 15 mol % based on the total moles of monomers, (ii) a viscosity-average degree of polymerization of from about 300 to about 3000, (iii) a degree of hydrolysis of from about 70 mol % to 100 mol %, and (iv) is substantially soluble in water and brine at a temperature of 195 F. or higher; (c) the composition has a bulk density greater than about 0.9 g/cc: and (d) the composition has a D(10) particle size of 4 mesh (U.S. Sieve Series).

2. The particulate loss circulation control composition of claim 1, wherein the acid-soluble weighting agent comprises 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.

3. The particulate loss circulation control composition of claim 2, wherein the acid-soluble weighting agent comprises at least one of CaCO.sub.3, CaCl.sub.2 and ZnO.

4. The particulate loss circulation control composition of claim 1, further comprising at least one additional additive selected from the group consisting of a starch, a plasticizer and a filler (other than (2)).

5. The particulate loss circulation control composition of claim 4, further comprising a plasticizer.

6. The particulate loss circulation control composition of claim 4, further comprising a starch.

7. The particulate loss circulation control composition of claim 1, wherein the polyvinyl alcohol component comprises a mixture of the hydrolyzed copolymer with at least one other polyvinyl alcohol.

8. The particulate loss circulation control composition of claim 7, wherein the at least one other polyvinyl alcohol has a water solubility lower than the hydrolyzed copolymer.

9. The particulate loss circulation control composition of claim 7, wherein the at least one other polyvinyl alcohol is a fully- or partially-hydrolyzed polyvinyl alcohol homopolymer.

10. The particulate loss circulation control composition of claim 8, wherein the at least one other polyvinyl alcohol is a fully- or partially-hydrolyzed polyvinyl alcohol homopolymer.

11. The particulate loss circulation control composition of claim 1, wherein the polyvinyl alcohol resin component is a transition product.

12. The particulate loss circulation control composition of claim 1, having a D(90) particle size of 1 inch.

13. The particulate loss circulation control composition of claim 7, having a D(90) particle size of 1 inch.

14. The particulate loss circulation control composition of claim 11, having a D(90) particle size of 1 inch.

15. The particulate loss circulation control composition of claim 1, having a bulk density of about 0.95 g/mL or greater.

16. The particulate loss circulation control composition of claim 7, having a bulk density of about 0.95 g/mL or greater.

17. The particulate loss circulation control composition of claim 11, having a bulk density of about 0.95 g/mL or greater.

Description

DETAILED DESCRIPTION

[0042] 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.

[0043] 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.

[0044] 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.

[0045] Except where expressly noted, trademarks are shown in upper case.

[0046] Unless stated otherwise, all percentages, parts, ratios, etc., are by weight.

[0047] 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).

[0048] 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 from 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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 characteristic(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.

[0053] 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).

[0054] 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.

[0055] 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).

[0056] The term substantial portion or substantially, as used herein, unless otherwise defined, means all 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.

[0057] 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.

[0058] 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 comonomers, 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] The term substantially soluble in water and soluble in water means substantially completely (or completely) soluble in deionized water under the stated conditions.

[0063] 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 NaCl concentration of up to 2.9 wt %.

[0064] 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.

[0065] 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.

[0066] 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

[0067] 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.

[0068] 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.

[0069] Polyvinyl alcohol copolymers for use in the present invention are hydrolyzed acid-functional polyvinyl acetate copolymers.

[0070] 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 from about 20% to 100% have been found satisfactory, commercially at least about 30% conversion is preferable.

[0071] The acid-functional comonomer is one or more of (i) a monocarboxylic unsaturated acid, (ii) a dicarboxylic unsaturated acid, (iii) an alkyl ester of (i), (iv) an alkyl 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 alkaline earth metal salt of (ii), (ix) an anhydride of (i), and (x) an anhydride of (ii).

[0072] 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.

[0073] Preferred are lower alkyl (C1-C8, or C1-C4) acrylates and methacryles. 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, ethylhexyl methacrylate and others. Preferred comonomers include methyl acrylate, methyl methacylate and mixtures thereof, and particularly methyl acrylate.

[0074] 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 mol %, 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.

[0075] 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.

[0076] 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.

[0077] 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).

[0078] 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 U.S. Pat. No. 2,734,048, 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, Tex. USA).

[0079] 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.

[0080] The polyvinyl alcohol copolymer should have a degree of hydrolysis of from about 70 mol %, 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 HS K6726 (1994).

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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 alcoholysis unit wherein the reaction proceeds to produce a first slurry of the alcoholyzed polyvinyl acetate (polyvinyl alcohol) and methyl acetate.

[0085] Catalyst amount typically ranges from about 0.2 wt % to about 0.5 wt % based on the weight of the reaction mixture.

[0086] The temperature of the alcoholysis reaction in the alcoholysis unit is typically from about 58 C., or from 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] Preferred conversion is a degree of hydrolysis as set forth above.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] In one embodiment, the thermal treating unit is not present.

[0097] 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.

[0098] 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 shiny and solids is sufficient to reduce the temperature of the second slurry to the desired level.

[0099] 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.

[0100] 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.

[0101] D(10) particles sizes of the polyvinyl alcohol copolymer agglomerated particles produced by the above slurry alcohol process are about 1 m, or about 10 m, with D(90) particle sizes being about 1000 m, or about 400 m.

[0102] 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.sup.3 or less.

[0103] Additional process details can be had by reference to previously incorporated US2017/0260309A1 and US2019/0055326A1, as well as U.S. Pat. No. 2,734,048, U.S. Pat. No. 3,497,487, U.S. Pat. No. 3,654,247 and general knowledge of those of ordinary skill in the relevant art.

Blends with Other High Molecular Weight Homopolymer Polyvinyl Alcohols

[0104] 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.

[0105] 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.

[0106] 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 alcohol component.

[0107] 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)

[0108] 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.

[0109] 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.

[0110] 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 homopolymer is less soluble than the acid-functional polyvinyl alcohol copolymer so that the dissolution rate of the particulate lost circulation can be modified.

[0111] 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.

[0112] 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).

[0113] 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

[0114] 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 other additives. Such additives include, for example, fillers other than the acid-soluble weighting agents, plasticizers and starches.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] Specific examples of fillers include minerals such as CaCO.sub.3, CaCl.sub.2 and ZnO.

[0121] 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.

[0122] In order to obtain a uniform plasticizer coating it is preferred to utilize a spray mechanism to coat the polyvinyl alcohol.

[0123] A secondary effect of such plasticizers is to reduce any dusting issues with the polyvinyl alcohol materials and ultimate particulate loss circulation control compositions.

[0124] Materials commonly used as plasticizers for polyvinyl alcohols are generally known to those of ordinary 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 neopentyl glycol, and mixtures of one or more of the above.

[0125] 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 sprayable plasticizer.

[0126] 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.

[0127] 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).

[0128] 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.

[0129] Suitable starches for use in the present invention include natural starches, synthetic starches, physically modified starches, chemically modified starches and mixtures thereof.

[0130] 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

[0131] In one embodiment, the particulate loss circulation control compositions of the present invention are prepared by compacting the mixtures described above under pressure.

[0132] The mixture compaction can be carried out using conventional compaction methods and equipment, such as a double roll compactor.

[0133] 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.

[0134] Compaction should be sufficient to achieve the desired or necessary bulk density of the resulting loss circulation control composition as described below.

[0135] 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 from the screener back to the compactor.

[0136] Desirably such compaction and granulation is a dry process that does not require an additional drying step.

[0137] Suitable particles sizes for the loss circulation control compositions of the present invention are as set forth in the previously incorporated references.

[0138] 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 particle size of the loss circulation control compositions in accordance with the present invention is from 2 mesh, or from 3 mesh or from 4 mesh. The mesh size refers to US standard mesh.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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.

[0143] 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/mL 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.

[0144] 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 compaction/granulation/pelletization methods.

Use of Loss Circulation Control Compositions

[0145] 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.

[0146] 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 periods at temperatures of up to about 250 F.

EXAMPLES

[0147] 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.

[0148] 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:

% Water Solubles=(Weight of Residue plus panWeight of pan)*100/Weight of sample

[0149] The solubility in salt water (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.

[0150] 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-1) 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.

[0151] Example 1: The high-solubility loss circulation control compositions (LCM-1) was produced from the resin C-1 described below.

[0152] 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, Tex. 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 110-115 C., and methanol washing.

[0153] The resin C-1 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 was 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.

[0154] The C-1 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, Tex., USA). The plasticized polymer was then uniformly blended with 5 wt % of CaCO.sub.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.

[0155] 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-US-00001 TABLE 1 Solubility of LCM-1 in Deionized Water Loss Dissolution Loss Dissolution Control Time (wt %) @ Control Time (wt %) @ Polymer (min) 122 F. Polymer (min) 149 F. LCM-1 0 0.00 LMC-1 0 0 15 52.33 15 38.1 30 61.56 30 68.1 60 69.64 60 78.0 120 78.97 120 87.8 180 89.25 180 96.7 240 94.63 240 96.7 300 96.80 300 98.2 360 98.22 360 98.7 420 98.98 420 38.1

TABLE-US-00002 TABLE 2 Solubility of LCM-1 in Salt Water Loss Dissolution Loss Dissolution Control Time (wt %) @ Control Time (wt %) @ Polymer (min) 122 F. Polymer (min) 149 F. LCM-1 0 0.00 LCM-1 0 0 15 51.17 15 47.2 30 56.58 30 53.5 60 62.83 60 67.7 120 76.92 120 80.0 180 81.86 180 91.8 240 85.84 240 92.3 300 88.82 300 92.8 360 90.51 360 93.1 420 92.00 420 94.1

[0156] Example 2: The relatively low solubility lost circulation polymers (LCM-2A and LCM-2B) were produced using the resin C-1, 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, Tex. USA), while C-3 was a fully-hydrolyzed polyvinyl alcohol homopolymer commercially available under the trade designation ELVANOL 71-30 (Kuraray America, Inc., Houston, Tex. USA).

[0157] The dissolution time of the C-1 resin (described in Example 1 above) was substantially decreased by blending it with C-2 and C-3.

[0158] C-1, C-2 and C-3 were 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, Tex., 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-1 resin with the C-2 resin, and the C-1 resin with the C-3 resin respectively, both uniformly blended with 2.5 wt % of CaCO.sub.3. The solubility of the loss control polymers LCM-2A and LCM-2B was determined in deionized water at 68 F. and at 194 F. Results are shown in Table 3 and Table 4. The results clearly show that the solubility of the C-1 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-US-00003 TABLE 3 Concentration of dissolved polymer at time, in minutes Temp. (4% solution in deionized Water) Resin F. 0 1 10 20 30 40 50 60 C-1 68 0 58.9 64.4 66.4 67.7 69.5 70.8 72.6 C-2 68 0 4.6 5.0 5.6 5.7 5.8 6.0 6.4 C-3 68 0 6.8 6.8 7.2 7.4 7.5 7.6 7.7 LCM- 68 0 43.1 46.2 47.4 47.7 47.7 47.8 48.1 2A LCM- 68 0 37.0 45.8 46.1 46.9 47.1 47.3 47.3 2B

TABLE-US-00004 TABLE 4 Concentration of dissolved polymer at time, Temp. in minutes (4% solution in deionized Water) Resin F. 0 1 10 20 30 40 50 60 C-1 194 0 92.1 92.1 92.3 92.7 93.0 94.0 94.1 C-2 194 0 97.5 97.5 98 98.5 98.8 98.8 98.9 C-3 194 0 98.1 98.3 98.4 98.7 99 99 99.7 LCM- 194 0 92.9 94.4 94.9 94.9 95.1 95.4 95.8 2A LCM- 194 0 89.9 91.0 92.2 2.2 93.1 93.2 93.2 2B