MX2010009691A - Method for treating subterranean formation with degradable material. - Google Patents

Abstract

A method is for treating a subterranean formation penetrated by a wellbore is given which comprises providing a treatment fluid comprising a viscoelastic surfactant having at least one degradable linkage, a hydrolysable material, and a pH control material, wherein the pH control material has a pH equal or greater than about 9 and comprises a strongly alkaline material and an oxidizing agent; and injecting into the subterranean formation the treatment fluid..

Description

METHOD FOR TREATING UNDERGROUND TRAINING WITH MATERIAL DEGRADABLE FIELD OF THE INVENTION The present invention relates to the technique of treating underground formations and more particularly, to a method for delivering a fluid treatment composition with base mixture and a degradable material in a formation for low temperature application. The invention is particularly applicable to methods for delivering viscoelastic low viscosity surfactant compositions which are capable of transporting large sized holding agents, but which break cleanly without the need for prior rinsing or subsequent rinsing.

BACKGROUND OF THE INVENTION Hydraulic fracturing of underground formations has long been established as an effective means of stimulating the production of hydrocarbon fluids from a well. In hydraulic fracturing, a well stimulation fluid (commonly known as a fracturing fluid) is injected into and through a well and against the surface of an underground formation penetrated by the well at a pressure at least sufficient to create a fracture in the formation.

Usually a "buffer fluid" is injected first to create the fracture and then a fracturing fluid, which often carries granular support agents, is injected at a pressure and rate sufficient to extend the fracture from the well deeper into the formation . If a support agent is employed, the objective is generally to create an area filled with supporting agent from the tip of the fracture to the well. In any case, the hydraulically induced fracture is more permeable than the formation and acts as a way or conduit for the hydrocarbon fluids in the formation to flow into the well and then to the surface where they are collected.

The fluids used as fracturing fluids have also been varied, but many are that most are water-based fluids that have been "viscosified" or thickened by the addition of a natural or synthetic polymer (cross-linked or non-crosslinked) or a viscoelastic surfactant (VES). The carrier fluid is usually water or a brine (for example, diluted aqueous solutions of sodium chloride and / or potassium chloride).

The viscosification polymer is typically a solvatable (or hydratable) polysaccharide, such as a galactomannan gum, a glycomannan gum, or a cellulose derivative. Examples of such polymers include guar, hydroxypropyl guar, carboxymethyl guar, carboxymethylhydroxyethyl guar, hydroxyethyl cellulose, carboxymethyl hydroxyethyl cellulose, hydroxypropyl cellulose, xanthan, polyacrylamides and other synthetic polymers. Among these, guar, hydroxypropyl guar and carboxymethylhydroxypropyl guar are typically preferred due to their commercial availability and cost performance.

In many cases, if not in most, the viscosity polymer is crosslinked with an appropriate crosslinking agent. The crosslinked polymer has an even higher viscosity and is even more effective in transporting support agent in the fractured formation. The borate ion has been used extensively as a crosslinking agent, typically in high pH fluids, for guar, guar derivatives and other galactomannans. Other crosslinking agents include, for example, titanium, chromium, iron, aluminum and zirconium.

Viscoelastic surfactant fluids are usually made by mixing, in a carrier fluid, appropriate amounts of suitable surfactants such as anionic, cationic, nonionic and zwitterionic surfactants. The viscosity of viscoelastic surfactant fluids is attributed to the three-dimensional structure formed by the components in the fluids. When the concentration of the viscoelastic surfactants significantly exceeds a critical concentration, the molecules Surfactants are added in micelles, which can be highly entangled to form a network that exhibits an elastic behavior.

Solutions of viscoelastic surfactants are usually formed by the addition of certain reagents to concentrated solutions of surfactants, which often consist of long chain quaternary ammonium salts such as cetyltrimethylammonium bromide (C ). Common reagents that generate viscoelasticity in the surfactant solutions are salts such as ammonium chloride, potassium chloride, sodium salicylate and sodium isocyanate, and nonionic organic molecules such as chloroform. The electrolyte content of surfactant solutions is also an important control of its viscoelastic behavior.

During fracturing treatments, controlling the growth of the height of a fracture can be an important aspect. In situations where the water level is close to the fracture zone, or where the fracture zones have low stress barriers, where the growth of the fracture height can result in blockages, the control of the fracture height can be critical . A common technique for the control of fracture height is the use of fluids with a lower viscosity, such as VES surfactants. However, low viscosity fluids they do not effectively transport the large supportive agents in the fracture.

One method to solve this problem has been the incorporation of fiber in surfactant fluids. However, the breakage of the fibers and the VES fracturing fluid carried by the fiber may remain a problem especially without previous rinsings or subsequent rinses. Polylactic acid (PLA) fibers have been shown to degrade in soluble materials under temperature and with time. However, all applications are limited to temperatures above 82 ° C based on the rate of degradation. At temperatures below 82 ° C, PLA fibers degrade too slowly to be useful for those applications in the oil field. It would be useful to have a VES fluid that could effectively transport large sized holding agents and still break under low temperature conditions (below 82 ° C, for example 50 ° C or 60 ° C), leaving few or no solid residues in fracture.

Summary of the Invention In one embodiment, the invention provides a method for treating an underground well-penetrated formation comprising providing a treatment fluid comprising a viscoelastic surfactant having at least one ligature degradable, a hydrolysable material, and a pH control material, wherein the pH control material has a pH equal to or greater than about 9 and comprises a strongly alkaline material and an oxidizing agent; and inject the treatment fluid into the underground formation.

In another embodiment, the invention provides a method for treating a subterranean formation penetrated by a well comprising the injection into the underground formation of a treatment fluid made of a viscoelastic surfactant having at least one degradable ligature, a hydrolysable material and a material of pH control, wherein the pH control material is an amine additive.

In some embodiments, the hydrolysable material is a hydrolysable fiber for example selected from the group consisting of polyesters, polyamides, and polylactides. Hydrolysable fiber and viscoelastic surfactants can form non-solid products during hydrolysis.

In some embodiments, the oxidizing agent and / or strongly alkaline material and / or the amine additive can be encapsulated.

In another embodiment, the strongly alkaline material has a pH of at least about 11. The strongly alkaline material can be selected from the group consisting of metal hydroxide, metal oxide, hydroxide calcium, metal carbonates, and metal bicarbonates. The metal hydroxide may be NaOH, Ca (0H) 2, Mg (0H) 2 or KOH and the metal oxide may be CaO, MgO or ZnO. The oxidizing agent can be ammonium persulfate or calcium peroxide. The amine additive can be selected from the group consisting of urea, dimethylolurea, 1,1-diethylurea, 1,1,3,3-tetramethylurea, 1,3-diethylurea, hydroxyurea, 1,3-diallylurea, ethylurea, 1, 1 -dimethylurea, 4-dimethylaminopyridine (D AP) and 1,8-diazabicyl (5.4.0) undec-7-ene (DBU). The amine additive may also have a salt, for example potassium carbonate.

Unless specifically indicated otherwise, all percentages here are percentages by weight.

Brief Description of the Figures Figure 1 is a graph plotting the dissolution of the fiber with time in hours at 75 ° C.

Figure 2 is a graph plotting the degradation of the PLA fiber after 18h 30 min at 50 ° C.

Figure 3 is a graph plotting the degradation of the PLA fiber after 4h at 50 ° C.

Figure 4 is a graph plotting the degradation of PLA fiber with different oxidation agents at 50 ° C.

Figure 5 is a graph plotting the conductivity of Fores 12/18 support agents with fiber and different amount of sodium hydroxide.

Figure 6 is a graph plotting fiber degradation with various urea additives and amine additives at 66 ° C and 100 ° C.

Figure 7 is a graph plotting fiber degradation as a function of the DBU concentration at 50 ° C and 66 ° C.

Figure 8 is a graph plotting the degradation of fibers with DMAP as a function of the DBU concentration at 50 ° C and 66 ° C.

Figure 9 is a graph plotting the degradation of fibers with DMAP at 50 ° C for 58 hours.

Figure 10 is a graph plotting the degradation of fibers with DMAP at 66 ° C for 21 hours.

Figure 11 is a graph plotting the degradation of fibers with DBU at 50 ° C for 68 hours.

Figure 12 is a graph plotting the degradation of fibers with DBU at 66 ° C for 21 hours.

Figure 13 is a graph plotting fiber degradation with 1,1-diethylurea at 50 ° C for 5 hours.

Figure 14 is a graph plotting fiber degradation with 1,1-diethylurea at 66 ° C for 21 hours.

Detailed Description of the Invention From the beginning, it should be noted that in the development of any of such current modality, they should make numerous specific implementation decisions to achieve the specific objectives of the promoter, such as in accordance with the related system and businesses related to the limitations, which will vary from one implementation to another. Therefore, it will be appreciated that such a development effort can be complex and time-consuming but should nevertheless be a guaranteed routine for those of ordinary skill in the art who have the benefit of this description. The description and examples are presented solely for the purpose of illustrating the preferred embodiments of the invention and should not be construed as limiting the scope and applicability of the invention. Although the compositions of the present invention are described herein as comprising certain materials, it should be understood that the composition may optionally comprise two or more chemically different materials. In addition, the composition may also comprise some different components in those already mentioned.

In the summary of the invention and this description, each numerical value should be read once as modified by the term "around" (unless already expressly modified), and then read again as not to be modified unless indicated otherwise in the context. Also, in the summary of the invention and this detailed description, it should be understood that a concentration range listed or described as being useful, adequate, or the like, it is intended that any and every concentration within the range, including the end points, is to be considered as having been established. For example, "a range of from 1 to 10" is to be read as indicated in each and every possible number along the continuum between about 1 and about 10. In this way, even if the specific data points within the range, or even without data points within the range, are explicitly identified or referred to for only a few specific data points, it is to be understood that the inventors appreciate and understand that any and all data points within the range are for considered to have been specified, and that the inventors have described and allowed the complete interval and all points within the range.

A first embodiment is a petroleum field treatment method which includes providing a viscosified fluid with a viscoelastic surfactant, including a degradable material and a base mixture for hydrolysis of PLA at low temperature.

According to some embodiments, the degradable material is a degradable fiber or degradable particle.

For example, degradable fibers or particles made of degradable polymers are used. The molecular structures differ from the degradable materials which are suitable for the present embodiments given in a wide range of possibility with respect to the regulation of the rate of degradation of the degradable material. The degradability of a polymer depends at least in part on its backing structure. One of the most common structural characteristics is the presence of hydrolysable and / or oxidizable bonds in the structure. The degradation rates of, for example, polyesters, are dependent on the type of repeating unit, composition, sequential, length, molecular geometry, molecular weight, morphology (e.g., crystallinity, spherulite size, and orientation), hydrophilicity, area of surface, and additives. Also, the environment to which the polymer is subjected can affect how the polymer degrades, for example, temperature, presence of moisture, oxygen, microorganisms, enzymes, pH, and the like. One of ordinary skill in the art, with the benefit of this description, will be able to determine that the optimum polymer should be for a given application considering the characteristics of the polymer used and the environment to which it will be subjected.

Suitable examples of polymers that can be used according to the appended embodiments include, but are not limited to, homopolymers, random aliphatic polyester copolymers, aliphatic blocking polyester copolymers, star aliphatic polyester copolymers, or hyper-branched aliphatic polyester copolymers. Such suitable polymers can be prepared by polycondensation reactions, ring opening polymerizations, free radical polymerizations, anionic polymerizations, carbocationic polymerizations, coordinated ring opening polymerization for, such as, lactones, and any other suitable process. Specific examples of suitable polymers include polysaccharides such as dextran or cellulose; chitins; chitosans; proteins; aliphatic polyesters; poly (lactides); poly (glycolides); poly (e-caprolactones); poly (hydroxy ester ethers); poly (hydroxybutyrates); polyanhydrides; polycarbonates; poly (orthoesters); poly (acetals); poly (acrylates); poly (alkyl acrylates); poly (amino acids); poly (ethylene oxide); poly ether esters; polyester amides; polyamides; polyphosphazenes; and copolymers or mixtures thereof. Other degradable polymers that undergo hydrolytic degradation may also be suitable. Of these suitable polymers, aliphatic polyesters are preferred. Of the appropriate aliphatic polyesters, the polyesters of hydroxy a or β acids are preferred. Poly (lactide) is the most preferred. The cop. { lactide) is synthesized either from lactic acid by a condensation reaction or more commonly by ring-opening polymerization of the cyclic lactic acid monomer. Lactide monomer generally exists in three different forms: two stereoisomers of lactide L and D; and lactide D, L (meso-lactide). The chirality of the lactide units provides a means to adjust, inter alia, rates of degradation, as well as the physical and mechanical properties after the lactide is polymerized. Poly (lactide L), for example, is a semi-crystalline polymer with a relatively slow hydrolysis rate. This may be desirable in applications where slow degradation of the material degradable material is desired. Poly (lactide D, L) is an amorphous polymer with a hydrolysis rate with a much faster rate of hydrolysis. This may be suitable for other applications. The stereoisomers of lactic acid may be used individually or combined for use in the compositions and methods of the present embodiments. Additionally, they can be copolymerized with, for example, glycolide or other monomers such as e-caprolactone, 1,5-dioxepan-2-one, trimethylene carbonate, or other suitable monomers to obtain polymers with different properties or times of degradation. Additionally, the stereoisomers of lactic acid can be modified by mixing polylactide of high and low molecular weight or by mixing polylactide with other aliphatic polyesters. For example, the rate of degradation of polylactic acid can be affected by mixing, for example, molecular weight polylactides. high and low; mixtures of polylactide and lactide monomer; or by mixing the polylactide with other aliphatic polyesters.

A guideline for choosing which composite particles are used in a particular application is that the degradation products will result. Another guideline is the conditions that surround a particular application. When choosing the appropriate degradable material, someone should consider the degradation products that will result. For example, some may form an acid during degradation, and the presence of the acid may be undesirable; others may form degradation products that must be insoluble, and these may be undesirable. Therefore, these degradation products should not adversely affect other operations or components.

The physical properties of degradable polymers may depend on various factors such as the composition of the repeating units, chain flexibility, presence of polar groups, molecular mass, degree of branching, crystallinity, orientation, etc. For example, short chain branches reduce the degree of crystallinity of polymers although the long chain branches lower the melt viscosity and impart, inter alia, extensional viscosity with tension stretching behavior. The properties of the material used can be tailored in addition to mixing, and copolymerized with another polymer, or by a change in the macromolecular architecture (eg, hyper-branched polymers, star-shaped polymers, or dendrimers, etc.). The properties of any such suitable degradable polymers (such as hydrophilicity, rate of biodegradation, etc.) can be tailored by introducing functional groups along the polymer chains. One of ordinary skill in the art, with the benefit of this disclosure, will be able to determine the appropriate functional groups to introduce to the polymer chains to achieve the desired effect.

In one embodiment, the method employs degradable fiber when exposed to high pH conditions for a period of time. Examples of such fibers include, but are not limited to, polyesters, polyamides, polylactides, and the like.

In one embodiment, the method employs polylactic acid, which is subjected to hydrolysis to form a liquid when exposed to a high pH environment as shown in the following reaction scheme: Diagram e Reaction i: Reaction to polylactic iodine nictroiysis.

In order to provide an environment of pH suitable for hydrolysis of the fiber to occur at low temperature (as low as 40 ° C and up to 85 ° C), a base mixture is used. The base mixture can be a pH control agent.

Useful pH control agents will vary with the specific degradable fiber selected for use, but may generally include those agents that are strongly alkaline materials that can provide a high pH environment. Generally, the control agents of pH that have a pH of 9 or more are considered to be strongly alkaline materials. Examples of such strongly alkaline materials include, but are not limited to, metal hydroxides, metal oxides, calcium hydroxide, metal carbonates or bicarbonates, and the like. For example, the strong alkaline material can be CaO, Ca (OH) 2, MgO as well as liquid additives such as NaOH and KOH.

The pH control agent may also contain oxidizing agents such as (? 4) 232? 4 and Ca02. Oxidizing agents were found to increase the rate of fiber degradation when used in conjunction with the metal oxide.

The pH control agent may also contain additives based on amines such as urea and its derivatives, as well as nucleophilic amines such as 4-dimethylaminopyridine (DMAP) and 1,8-Diazabicyclo [5.4.0] undec-7-ene ( DBU). In one embodiment, the pH control agent may also contain a combination of amines with potassium carbonate (K2CO3).

In a first embodiment, the pH control agent is made of a strongly alkaline material and an oxidizing agent. In a second embodiment, the pH control agent is made of an amine additive. The amine additive may be an amine base and / or a nucleophilic amine. In one embodiment, the amine additive can also be an amine and a salt In a third embodiment, the pH control agent becomes a strongly alkaline material, an oxidizing agent and an amine additive.

The amount of pH control agent that is required to provide hydrolysis at low temperature will vary with the particular control agent selected and with the system, but in general, the pH control agent may comprise about 0.5 weight percent to about 15 weight percent of the treatment fluid. In one embodiment, the fluid may contain from about 1 weight percent to about 10 weight percent. In another embodiment, the fluid may comprise from about 3 weight percent to about 10 weight percent. In yet another embodiment, the fluid may contain from about 3 percent to about 7 percent by weight.

When the fluids are viscosified by the addition of viscoelastic surfactant systems, it is believed that the increase in viscosity is due to the formation of micelles, for example worm-type micelles, which entangle to provide structure to the fluid which leads to viscosity . In addition to the viscosity itself, an important aspect of the fluid properties is the degree of viscosity recovery or re-cure when the fluid is subjected to high shear and the cut is then reduced.

For VES fluids, the cut may disturb the micelle structure, after which the structure is formed again. The control of the degree of re-assembly (re-cure) is necessary to optimize the performance of the surfactant system for different applications. For example, in hydraulic fracturing it is critical that the fluid recover viscosity as soon as possible after leaving the high cut region in the tubulars and entering the low cut environment in the hydraulic fracture. On the other hand, it is beneficial in helical pipe cleanings to provide a slight delay in full recovery of the viscosity in order to "push" the solids more efficiently from the bottom of the well in the annulus. Once in the anulus, the recovered viscosity ensures that the solids are effectively transported to the surface. Therefore, it is desirable to control the recovery of the viscosity and the time required for said recovery.

Many viscoelastic surfactants can be used in this application. Surfactants with a degradable link in the molecule will hydrolyze to separate the hydrophilic head and the hydrophobic tail. While we do not wish to be bound by any theory, it is believed that such separation will degrade the micelles formed by the VES surfactant.

Exemplary cationic viscoelastic surfactants include amine salts and quaternary amine salts disclosed in the Patents of E.U.A. Nos. 5,979,557, and 6,435,277 having a common Attorney as the present application and which are incorporated herein by reference.

In one embodiment, the viscoelastic surfactant has an amide bond in the head group, in accordance with the reaction scheme Examples XX of suitable cationic viscoelastic surfactants include cationic surfactants having the structure: iN + (R2) (R3) (4) X " wherein Ri has from about 14 to about 26 carbon atoms and can be branched or straight chain, aromatic, saturated or unsaturated and can contain a carbonyl, an amide, a retroamide, an imide, or an amine; R2, R3, and R are each independently hydrogen or an Ci aliphatic group at about C6 which may be the same or different, branched or straight chain, saturated or unsaturated and one or more of which may be substituted with a group which makes the R2, R3 and R groups more hydrophilic, the R2, R3 and R4 groups can be incorporated in a 5- or 6-membered heterocyclic ring structure including the nitrogen atom; the groups R2, R3 and R may be the same or different; and X "is an anion Mixtures of such compounds are also suitable As a further example, Ri has from about 18 to about 22 carbon atoms and may contain a carboxyl, an amide, or an amine and R2, R3 and R4 they are the same among them and contain from 1 to about 3 carbon atoms Cationic surfactants having the structure RiN + (R2) (R3) (R4) X ~ can optionally contain amines having the structure RiN (R2) (R3) It is well known that commercially available cationic quaternary amine surfactants frequently contain the corresponding amines (wherein Ri, R2 and R3 in the cationic surfactant and in the amine have the same structure) The concentrated formulations of commercially available VES surfactant received, for example cationic VES surfactant formulations, may also optionally contain one or more members of the group consisting of solvents, mutual solvents, organic acids, salts of organic acids, inorganic salts, and oligomers, polymers, co-polymers and mixtures of these members. They may also contain mej orators of performance, such as viscosity improvers, for example polysulfonates, for example polysulphonic acids in accordance with that described in U.S. Patent Application Publication No. 2003-0134751 having a common Proxy as the present application and incorporated herein. in the present as a reference.

Another suitable cationic VES is bis (2-hydroxyethyl) methyl ammonium erucyl chloride ("EMHAC"), which is also known as (Z) -13 docosenyl-N-N-bis (2-hydroxyethyl) methyl ammonium chloride. It is commonly obtained from manufacturers in the form of a mixture containing about 60 weight percent surfactant in a mixture of iso-propanol, ethylene glycol and water. In this patent, when we refer to "EMHAC", we refer to this solution. Other suitable amine salts and salts of quaternary amines include (either alone or in combination), erucyl trimethyl ammonium chloride; N-methyl-N, -bis (2-hydroxyethyl) rape ammonium chloride; oleyl methyl bis (hydroxyethyl) ammonium chloride; erucylaminopropyltrimethylamine chloride, octadecyl methyl bis (hydroxyethyl) ammonium bromide; octadecyl tris (hydroxyethyl) ammonium bromide; octadecyl dimethyl hydroxyethyl ammonium bromide; cetyl dimethyl hydroxyethyl ammonium bromide; cetyl methyl bis (hydroxyethyl) ammonium salicylate; 3, 4-dichlorobenzoate of cetyl methyl bis (hydroxyethyl) ammonium; cetyl tris (hydroxyethyl) ammonium iodide; cosyl dimethyl hydroxyethyl ammonium bromide, cosyl methyl bis (hydroxyethyl) ammonium chloride; tris (hydroxyethyl) ammonium bromide; dicosyl dimethyl hydroxyethyl ammonium bromide; dicosyl methyl bis (hydroxyethyl) ammonium chloride; dicosyl tris (hydroxyethyl) ammonium bromide; hexadecyl ethyl bis (hydroxyethyl ammonium chloride; hexadecyl isopropyl bis (hydroxyethyl) ammonium iodide; and cetylamino chloride, N-octadecyl pyridinium.

Zwitterionic viscoelastic surfactants are also suitable. Exemplary zwitterionic viscoelastic surfactants include those described in U.S. Pat. No. 6,703,352 which has a common Attorney as the present application and which is incorporated herein by reference. The zwitterionic surfactants of example have the structure: wherein Ri is a hydrocarbon group which may be branched or straight chain, aromatic, aliphatic or olefinic and contains from about 14 to about 26 carbon atoms and may include an amine; R2 is a hydrogen or an alkyl group having 1 to about 4 carbon atoms; R3 is a hydrocarbyl group having from 1 to about 5 carbon atoms; and Y is a group that removes electrons. More particularly, the zwitterionic surfactant may have the betaine structure: wherein R is a hydrocarbyl group which may be branched or straight chain, aromatic, aliphatic or olefinic and has from about 14 to about 26 carbon atoms and may contain an amine; n = approximately 2 to approximately 4; and p = 1 to about 5. Mixtures of these compounds can also be used.

Two examples of suitable betaines are, respectively, BET-O-30 and BET-E-40. The VES surfactant in BET-O-30 is oleylamidopropyl betaine. It is designated BET-O-30 here, because as it is obtained from the supplier (Rhodia, Inc. Cranbury, New Jersey, United States of America) it is known as Mirataine BET-O-30; contains an oleic acid amide group (including an alkene tail group C17H33) and is supplied as an active surfactant approximately 30%; the balance is substantially water, sodium chloride, glycerol and propane-1,2-diol. A suitable analogous material, BET-E-40, was used in the experiments described below; A chemical name is erucilaminopropyl betaine. BET-E-40 is also available from Rhodia; it contains an erucic acid amide group (including an alkene tail group C21H41) and is supplied as approximately 40% active ingredient, with the balance being substantially water, sodium chloride, and isopropanol. BET surfactants and other suitable surfactants are described in the Patent of E.Ü.A. No. 6,703,352.

Certain co-surfactants may be useful for extending brine tolerance, for increasing gel strength, for reducing re-cure time after cutting, and / or for reducing the cutting sensitivity of zwitterionic VES fluid systems, such as for example VES betaine fluids. An example provided in the U.S. Patent. No. 6,703,352 is sodium dodecylbenzene sulfonate (SDBS). Another example is polinaphthalene sulfonate. The zwitterionic VES surfactants can be used with or without this type of co-surfactant, for example, those having an SDBS type structure having a saturated or unsaturated C6 to Ci6 chain, branched chain or straight chain; the additional examples of this type of co-surfactants are those that have a chain Cs to Ci6 saturated or unsaturated, branched or straight. Other suitable examples of this type of co-surfactant, especially for BET-O-30 are certain chelating agents such as trihydroxyethylethylenediamine trisodium triacetate. Many suitable additives are known in the sense that they improve the performance of VES surfactant systems in gel form; any of these systems can be employed in the present invention; they must be tested to determine their compatibility with the compositions and methods of the present modalities before use; Simple laboratory experiments for these tests are well known.

The viscoelastic systems of zwitterionic surfactants typically contain one or more members of the group consisting of organic acids, salts of organic acids, inorganic salts, and oligomers, polymers, copolymers and mixtures of these members. This member is typically present only in a minor amount and does not have to be present. The organic acid is typically a sulfonic acid or a carboxylic acid and the anion counter ion of the organic acid salts are typically sulfonates or carboxylates. The representative examples of these Organic molecules include various sulfonates and aromatic carboxylates such as p-toluene sulfonate, naphthalene sulfonate, chlorobenzoic acid, salicylic acid, phthalic acid and the like, wherein such counter-ions are soluble in water. More preferred are salicylate, phthalate, p-toluene sulfonate, hydroxynaphthalene carboxylates, for example 5-hydroxy-l-naphthoic acid, 6-hydroxy-l-naphthoic acid, 7-hydroxy-l-naphthoic acid, l-hydroxy acid -2-naphthoic, preferably 3-hydroxy-2-naphthoic acid, 5-hydroxy-2-naphthoic acid, 7-hydroxy-2-naphthoic acid, and 1,3-hydroxy-2-naphthoic acid and 3,4-dichlorobenzoate . The organic acid or salt thereof typically helps in the development of an increased viscosity which is characteristic of the preferred fluids. The organic acid or its salt is typically present in the zwitterionic viscoelastic fluid (after sufficient concentration of the viscoelastic surfactant to viscosify the fluid) in a concentration by weight of from about 0.1% to about 10%, more typically from about 0.1% to about 7%, and still more typically from about 0.1% to about 6%.

Inorganic salts that are particularly suitable for use in the zwitterionic viscoelastic fluid include water soluble potassium, sodium and ammonium salts such as potassium chloride and ammonium chloride. In addition, salts of calcium chloride, calcium bromide and zinc halide can also be used. The inorganic salts can help to develop an increased viscosity which is characteristic of preferred fluids. In addition, the inorganic salt can help maintain the stability of a geological formation to which the fluid is exposed. The stability of the formation and in particular the stability of clay (by inhibiting the hydration of the clay) are achieved at a level of concentration of some per hundred by weight. The inorganic salt is typically present in the zwitterionic viscoelastic fluid (after sufficient concentration of the viscoelastic surfactant to viscosify the fluid) in a concentration by weight of from about 0.1% to about 30%, more typically from about 0.1% to about 10%, still more typically from about 0.1% to about 8%. The organic salts, for example trimethylammonium chlorohydrate and tetramethylammonium chloride, can also be used in addition to the inorganic salts or as a replacement thereof. Optionally, these systems can be formed in dense brines, including brines that contain polyvalent cations.

As an alternative to organic salts and inorganic salts, or as a partial substitute for them, a medium to long chain alcohol can be used (preferably an alkanol), preferably having from five to ten carbon atoms, or an alcohol ethoxylate (preferably an alkanol ethoxylate), preferably of an alcohol of 12 to 16 carbon atoms and having 1 to 6 oxyethylene units, preferably 1 to 4 oxyethylene units.

Amphoteric viscoelastic surfactants are also suitable. Example amphoteric viscoelastic surfactants include those described in the U.S. Patent. No. 6,703,352, for example, amine oxides. A useful amine oxide surfactant has the formula: wherein R 1, R 2 and R 3 are independently selected from alkyl, alkenyl, arylalkyl or hydroxyalkyl groups wherein each of said alkyl groups contains from about 8 to about 24 carbon atoms and can be branched or straight chain, saturated or unsaturated.

Mixtures of zwitterionic surfactants and Amphoteric surfactants are also suitable. An example, which is known herein as BET-E-40 / AO, is a mixture of about 13% isopropanol, about 5% 1-butanol, about 15% ethylene glycol monobutyl ether, about 4% sodium chloride , approximately 30% water, approximately 30% cocamidopropyl betaine and approximately 2% cocamidopropylamine oxide.

The fluid can be used, for example, in oil field treatments. As examples, the fluid can be used as a buffer fluid and as a carrier fluid in hydraulic fracturing, as a carrier fluid for lost circulation control agents, and as a carrier fluid for gravel packaging.

The optimum concentration of a given rheology enhancing additive for a given choice of VES surfactant fluid system at a given concentration and temperature, and with other given materials present, can be determined through simple experiments. The total concentration of viscoelastic surfactant must be sufficient to form a viscoelastic gel under conditions in which the surfactants have a tendency to aggregate. The appropriate amounts of surfactant and rheology enhancer are those necessary to reach the viscosity and recovery time after the desired cut as determined by experiment. In general, the amount of surfactant (as active ingredient) is from about 1 to about 10%. Concentrates of commercially available surfactants may comprise certain materials that can be used as rheology enhancers, for example, to depress the concentrate freezing point, but usually the amount of this material is not sufficient in the final fluid when the concentrate is diluted . The amount of rheology enhancer that is used, in addition to what may already be present in the surfactant concentrate as received, is from about 0.1 to about 6%, for example, from about 0.25 to about 3.5%, more particularly from about 0.25 to about 1.75%. Mixtures of surfactants and / or mixtures of rheology enhancers can be used.

EXAMPLES The present embodiments can be further understood from the following examples: Figure 1 shows the results of fiber degradation PLA (3.6kg / m3) in polymer solution with a different amount of added NaOH, at a temperature of 75 ° C. Table 1 shows the results of fiber d-egradation in fluids at 60 ° C prepared with 3.6kg / m3 fiber PLA and different amounts of CaO and Ca (OH) 2. As can be seen, the higher the concentration of CaO and Ca (OH) 2 results in a more rapid degradation of PLA fibers.

Chemical Concentration, g / L Degradation time, days CaO 1 > 10 days CaO 3 7 CaO 5 2 Ca (OH) 2 1 > 10 days Ca (OH) 2 3 7 Ca (OH) 2 5 2 Table 1 Additives such as CaO or Ca (OH) 2 can be made either of a large mesh size or encapsulated to prevent rapid dissolution in fracture fluids during pumping and feedback (constant dissolution for Ca (OH) 2 is 6xl0" 6 mol3 / L3) Figure 2 shows degradation of PLA fiber at low temperature (bottles heated from room temperature to 50 ° C) in the presence of Ca (OH) 2 (fine powder and encapsulated in paraffin (ratio hydroxide: paraffin is 1: 1)). Also, degradation is shown in the presence of Ca (OH) 2 encapsulated with hexane (oil mimic).

The use of the pre-heated sample at 50 ° Cf the thick mixture Ca (OH) 2 shows even higher degradation rates (Figure 3). Also, positive influences of peroxide-type oxidants on the rate of degradation were also observed.

Figure 4 shows that MgO does not have a major impact on PLA degradation at 50 ° C. However, when mixed with oxidizing agents such as ammonium persulfate and calcium peroxide, MgO significantly increases the degradation rate of PLA. Interestingly, the addition of sodium bromate that does not generate oxidant to MgO may not have an impact on the degradation of PLA at the same temperature. It is expected that the amount of degraded fiber will have a significant impact on the fracture conductivity as shown in Figure 5.

A series of amine derivatives were also evaluated in order to increase the rate of degradation of the polymer. The nucleophilic bases were chosen and in particular amine bases. Among these, urea and its derivatives were evaluated. In fact, urea and its derivatives self-decompose and release ammonia that reacts rapidly with ester bonds, leading to oligomers ending in amide. The nucleophilic attack of the amine together with a High pH environment accelerates the rate of degradation of the fibers as shown in Figure 6. It was used in all experiments leading to fibers of lkg / m3 of fluid.

Some derivatives such as 4-dimethylaminopyridine (DMAP) and 1,8-Diazabicyclo [5. .0] undec-7-ene (DBU) significantly accelerates the rate of degradation at 66 ° C. Further experiments were then carried out to further study the action of DMAP and DBU on the degradation of fibers at low temperatures. Figure 7 shows the influence of DBU (with various concentrations) on fiber degradation at 50 ° C and 66 ° C. It seems that the concentration of 0.5mol / L gives a complete degradation at 66 ° C after 21h. Much higher concentrations are required to achieve complete degradation at 50 ° C.

The results obtained with the addition of DMAP are presented in Figure 8. Compared with DBU, DMAP more significantly accelerates degradation at low temperatures. A concentration of 0.2mol / L up to 0.5mol / L of DMAP gives a significant degradation of the fibers. The combination of these derivatives with the presence of a base such as K2C03 was also investigated and the results are shown in Figures 9 and 10.

Figure 9 and 10 show the influence of the presence of various amounts of K2C03 together with DMAP (at 0.01M). The experiments were performed at 66 ° C for 21h and 50 ° C for 68h. The Results show that there is an increase in fiber degradation when the combination of the two products is used. K2C03 is a base that helps maintain a high pH environment. Since the degradation of the polymer occurs, lactic acid is generated which reduces the overall pH of the solution. In an acidic environment, the protonation of the amine may occur which will then limit its activity in the degradation process. Therefore, maintaining alkaline pH is a much better option to ensure faster degradation.

Similar results were obtained with DBU and 1,1-diethyl urea as shown in Figures 11, 12, 13 and 14.

The disclosure and description of the invention is illustrative and of explanation thereof and it can be readily appreciated by those in the art that various changes in size, shape and materials can be made, as well as in the details of the illustrated construction or combinations of the elements described herein without departing from the spirit of the invention.

Claims (21)

1. A method for treating an underground formation penetrated by a well, characterized in that it comprises: providing a treatment fluid comprising a viscoelastic surfactant having at least one degradable ligature, a hydrolysable material, and a pH control material, wherein the pH control material has a pH equal to or greater than about 9 and that it comprises a strongly alkaline material and an oxidizing agent; and Inject the treatment fluid into the underground formation.

2. The method according to claim 1, characterized in that the hydrolysable material is a hydrolysable fiber.

3. The method according to claim 2, characterized in that the hydrolysable fiber and the visoelastic surfactant form non-solid products during hydrolysis.

4. The method according to claim 2, characterized in that the fiber is selected from the group consisting of polyesters, polyamides, and polylactides.

5. The method according to claim 1, characterized in that the oxidizing agent and / or strongly alkaline material is encapsulated.

6. The method according to claim 1, characterized in that the pH control material has a pH of at least about 11.

7. The method according to claim 1, characterized in that the strongly alkaline material is selected from the group consisting of metal hydroxide, metal oxide, calcium hydroxide, metal carbonates, and metal bicarbonates.

8. The method according to claim 7, characterized in that metal hydroxide is NaOH, Ca (OH) 2, Mg (OH) 2 or KOH.

9. The method according to claim 7, characterized in that the metal oxide is CaO, gO or ZnO.

10. The method according to claim 1, characterized in that the pH control material comprises at least two strongly alkaline materials.

11. The method according to claim 10, characterized in that the pH control material comprises a metal hydroxide, a metal oxide and the oxidizing agent.

12. The method according to claim 1, characterized in that the oxidizing agent is ammonium persulfate or calcium peroxide.

13. The method according to claim 1, characterized in that the pH control material further comprises an amine additive.

14. The method according to claim 13, characterized in that the amine additive is selected from the group consisting of urea, dimethylolurea, ethylurea, 1,1-dimethylurea, 4-dimethylaminopyridine (D AP) and 1/8-diazabicyl (5.4. 0) undec-7-ene (DBU).

15. The method according to claim 13, characterized in that the amine additive further comprises a salt.

16. The method according to claim 15, characterized in that the salt is potassium carbonate.

17. The method according to claim 1, characterized in that the viscoelastic surfactant contains an amide ligature.

18. The method according to claim 17, characterized in that the viscoelastic surfactant is represented by the formula:

19. The method according to claim 1, characterized in that the fluid further comprises a support agent.

20. The method according to claim 1, characterized in that the fluid also comprises an additive selected from the group consisting of corrosion inhibitors, fluid loss additives, and mixtures thereof.

21. The method according to claim 1, characterized in that the fluid further comprises a gas component to provide a foam or energizing fluid wherein the gas component comprises a gas selected from the group consisting of nitrogen, air, and carbon dioxide.