Crosslinkable cellulose derivatives

Abstract: A subterranean formation is treated by injecting a treating fluid through the well bore, the treating fluid optionally containing a particulate agent and comprising an aqueous liquid, a crosslinking agent (such as titanium or zirconium ions) and a cellulose ether derivative (eg hydroxyalkylcellulose) which has been chemically modified by reaction with a vinyl phosphonic acid in the presence of the reaction product of hydrogen peroxide and a ferrous salt.

Description: This invention relates to a method of preparing a treating fluid composition for injection into a subterranean formation, and to the use of a composition so made. Petroleum recovery operations, such as well stimulation and gravel packing, often require the use of fluid compositions capable of suspending particles. In gravel packing operations, a pack of gravel is placed on the exterior of a perforated or slotted liner or screen which is positioned across an unconsolidated formation. The resulting structure presents a barrier to migrating sand from the formation while still permitting fluid flow. The gravel is carried to the formation in the form of a slurry by mixing gravel with a viscosified fluid. Once the gravel is placed in the wellbore, the viscosified carrier fluid is degraded and returned to the surface. Treating fluids are used similarly in stimulating subterranean formations. The viscosified fluid carries a propping agent through the wellbore and into both natural fractures and fractures in the formation caused by hydraulic pressure. Once the desired fracture occurs, the fluid is degraded and returned to the surface leaving the proppant in the formation to provide a conductive channel through which formation fluids can flow. In both stimulation and gravel packing operations, the most desirable treating fluid reduces friction pressure as the fluid is pumped through the tubular goods and transports the propping agent or gravel to the formation without particle settling in the wellbore during placement. Treating fluids with these properties are generally comprised of a hydratable polysaccharide, including but not limited to guar, guar derivatives, and cellulose derivatives. These polymers viscosify aqueous liquids to form solutions which inhibit particle settling to a limited extent by virtue of viscosity. However, these polymer solutions can approach near zero particle settling rates upon crosslinking with multivalent metal cations to form highly viscoelastic gels. The utility of these gels is well known in the art of petroleum recovery operations. Cellulose derivatives are the preferred viscosifying polymers for certain petroleum recovery operations because they degrade, i.e. lose viscosity, without generating water insoluble particles or residue. The water insoluble particles are believed to remain in the formation and may cause formation plugging or impair the permeability of sand or gravel packs. However, cellulose derivatives have had limited use in many petroleum applications because most derivatives are salt sensitive or not crosslinkable. Non-ionic derivatives of cellulose are generally not crosslinkable because the polymer lacks a site for attachment of a multivalent metal cation. Examples of this type include hydroxyalkyl cellulose ethers, methyl cellulose, ethyl cellulose, and hydroxyalkyl methyl cellulose. A crosslinkable non-ionic cellulose derivative has been prepared and described in U.S. patent specification nos. 4,523,010 and 4,552,215. In these specifications, dihydroxypropyl hydroxyalkyl cellulose is prepared by a condensation reaction of glycidol with hydroxyethyl cellulose under alkaline conditions. The glycidol addition along the HEC polymer chain provides a site of attachment for multivalent metal cations. Anionic cellulose derivatives are normally substituted with carboxyl groups along the polymer chain. The carboxyl groups complex with polyvalent metal cations, such as aluminium. Gels formed with this chemistry tend to have limited structural stability at formation temperatures of about 250°F (121°C). In addition, carboxylate substituents render the polymer salt sensitive, i.e. the viscosity of the polymer in a salt solution is less than the viscosity in water. Salt sensitivity is not a desirable property because the aqueous liquids used in recovery operations must generally contain chloride salts to inhibit the swelling of formation clays. In our European patent specification no. 387002A, we described a method of treating a subterranean formation by injecting a treating fluid composition into the formation through a well bore, wherein the composition comprises an aqueous liquid, a cellulose ether that has been chemically modified by reaction under acidic conditions with a ceric salt, and a crosslinking agent. We have now found that, surprisingly, similarly useful compositions can be made by using instead of the ceric salt in acid, the product of reaction of hydrogen peroxide and a ferrous compound. The latter is easier to use than the acidic ceric salt and produces a better product in different yield. According to the present invention, there is provided a method of preparing a treating fluid composition for injection into a subterranean formation, in which method a cellulose ether derivative comprising a hydroxyalkyl cellulose, a carboxymethyl cellulose or a carboxymethylhydroxyalkyl cellulose or a mixture thereof, is chemically modified by reaction with a vinyl phosphonic acid, and mixed with a crosslinking agent, characterised in that said reaction is effected in the presence of the reaction product of hydrogen peroxide and a ferrous salt, and wherein water is included in the composition to partially hydrate the composition and increase the viscosity of said composition. The composition may optionally also include a particulate agent. A cellulose ether derivative is chemically modified by reaction thereof with a vinyl phosphonic acid in the presence of a crosslinking agent. In the present invention, the cellulose derivative forms a graft copolymer with the vinyl phosphonic acid. These crosslinkable graft copolymers of cellulose derivatives are generally non-ionic in character. Methods of grafting monomers on polyhydroxy containing compounds are well known in the art. The process is described for example, in U.S. patent specification no. 2,922,768, in which a vinylidene monomer is polymerized with an organic reducing agent, such as cellulose, in the presence of a ceric salt in aqueous medium under acidic conditions. In accordance with the present invention, the crosslinkable cellulose derivatives are prepared by grafting a vinyl phosphonic acid monomer onto the cellulose derivative, so that the resulting copolymer is non-ionic and crosslinkes readily with polyvalent metal cations to form stable viscoelastic gels, thus overcoming the deficiencies of non-ionic and anionic cellulose derivatives currently used in petroleum recovery operations. The present invention provides a method of preparing a composition for treating subterranean formations. The treating fluid composition comprises an aqueous liquid, a crosslinkable copolymer of HEC and a selected crosslinking agent. The fluid composition can be used in petroleum recovery operations, such as stimulation, gravel packing and other well completion operations. In these operations, the treating fluid performs a variety of functions, for example, (1) reducing friction pressure as the fluid is pumped through tubular goods and down the wellbore and (2) to carry propping agents or gravel packing materials to the formation without settling. A highly viscoelastic fluid is often times required to transport propping agents or gravel packing materials to the formation without settling. In addition, the treating fluid must have stable viscosity at formation temperatures. The present invention provides such a fluid. The term "aqueous liquid" is used hereafter to mean any liquid containing sufficient water to at least partially hydrate the copolymer and result in an increase in the viscosity of the fluid. Aqueous liquids used in petroleum recovery operations normally contain sodium chloride, potassium chloride, or other salts to inhibit the swelling of clays generally found in subterranean formations. The pH of the aqueous liquid must be compatible with the selected crosslinking agent and must not adversely affect the hydration of the copolymer. The cellulose derivative used in this invention is preferably a hydroxyalkyl cellulose having a hydroxyalkyl molar substitution from 1.5 to 3.0. Molar substitution is defined as the average number of moles of a substituent group present per anhydroglucose unit of the cellulose material. The alkyl group is preferably ethyl or propyl (mixtures can be used). The preferred hydroxyalkyl cellulose is hydroxyethyl cellulose (HEC) having a molar substitution in the range of 1.8 to 2.5. Preferably in this invention, the hydroxyalkyl cellulose is preformed in a separate reaction. Hydroxyethyl cellulose is usually formed by reacting ethylene oxide with cellulose under extreme alkaline conditions and is available commercially. The copolymers used in the present invention are crosslinkable by virtue of the vinyl phosphonic acid which has been reacted therewith. It has the reactive CH₂=C- moiety which enables it to attach to a hydroxyl group of the cellulose derivative. It also provides a crosslinkable substituent, i.e. a phosphonate group, which enables the copolymer to crosslink upon solvation. The crosslinkable copolymers are prepared by reacting vinyl phosphonic acid with a cellulose derivative using a redox system comprising the reaction product of hydrogen peroxide with a ferrous salt. The generalized redox reaction is believed to be represented by the formula: H₂0₂ + Fe⁺² → HO· + HO⁻ + Fe⁺³ and the generalized reaction is believed to be represented by the general formula: RCH₂OH + HO· → RCH ˙OH + H₂O An advantage of this initiator is that radical production occurs at a reasonable rate over a wide temperature range whereby reactions can be carried out at room temperature, if desired. The free radical produced on the cellulose derivative initiates polymerization with the vinyl group of the monomer to produce the graft copolymer. Typically, the copolymerization is carried out in aqueous media wherein the polymer is dissolved or dispersed. Copolymers were prepared in acetone/water mixtures containing from about 55 to about 90% acetone. Reactions were carried out in a 1 liter kettle with a stirrer or a 1 liter jar at a temperature of from about 20 to about 60°C. The ratio of cellulose derivative to aqueous medium ranges from about 1 gram per 100 ml. to about 1 gram per 4 ml. The preferred ratio is from about 1 gram per 9 to 30 ml. The ratio of cellulose derivative to vinyl phosphonic acid ranges from 6 to 40 grams per 1 gram of monomer. The preferred ratio is from 8 to 30. It is to be understood that the ranges set forth above are merely exemplary and that other temperatures, concentrations and the like maybe utilized to prepare the reaction product. The polymerization reaction is chemically initiated by a redox system comprising the reaction product of hydrogen peroxide with a ferrous salt. Ferrous ions may be provided, for example, by salts such as ferrous ammonium sulfate, ferrous chloride, ferrous sulfate, ferrous acetate, ferrous oxalate, ferrous acetylacetonate and the like. A preferred source of ferrous ions is ferrous ammonium sulfate. The initiator may be added slowly to the reaction material over a time period of about 30 to 90 seconds or longer. Reaction times vary from about 15 min to about 4 hours depending upon the reaction conditions. Grafting reaction efficiency (% of monomer grafted) is generally less than 50%. After the reaction is complete, the polymerization product is washed with acetone, filtered and dried. The modified cellulose ether derivative used in the present invention readily solubilizes in aqueous liquids and substantially increases the viscosity of aqueous liquids. The viscosity of the solution is further increased by the addition of a crosslinking agent. Preferred crosslinking agents in accordance with this invention comprise polyvalent metal cations such as aluminium, titanium, zirconium, chromium, antimony, and the like. Most preferred crosslinking agents provide titanium (IV) or zirconium (IV) ions in the range of 0.005 weight percent to 0.05 weight percent in the aqueous liquid. Examples of such crosslinking agent include triethanolamine titanate, titanium acetylacetonate, zirconium acetate, zirconium lactate and other organic complexes of titanium and zirconium capable of providing Ti (IV) and Zr (IV) ions. The modified cellulose ether derivative of the present invention can be used with non-modified such derivatives, for example, using about 20% of the modified cellulose ether with 80% non-modified HEC, for example, provides a cost-effective means for obtaining substantially increased gel stability. The viscosity stability of the polymer blends exceeds the stability of CMHEC alone when crosslinked with zirconium and evaluated at 250°F (121°C). The following Examples are given by way of illustration only. Example 1 To a 1 liter jar, the following reagents were added: 26 g HEC (about 2.2 Ms), 384 ml acetone, 94 ml deionized water and 0.12 ml of a 60% active solution of 1-hydroxyethylidene-1,1-diphosphonic acid and 2 ml of FAS solution. The FAS solution comprised 2 grams of ferrous ammonium sulfate (reagent grade) dissolved in 20 ml deonized water. The jar contents are mixed using a magnetic stirrer at moderate speeds while purging with nitrogen for 30 minutes. Thereafter, 2 ml 30% H₂0₂ were added to the reaction mixture while purging and the reaction was permitted to proceed at room temperature for one hour. The reaction product then is recovered by washing with acetone, filtration and drying under vacuum until a constant weight was obtained. The reaction product at 0.72% by weight was hydrated in 2% potassium chloride solution. The viscosity of the polymer solution measured 62 cps (P.s) at 511 Sec⁻¹ after 1 hour and 90 cps (P.s) at 24 hours using a Fann Model 35® viscometer. Example 2 The VPA/HEC copolymer of Example 1 is utilized to perform crosslinking tests at 180°F (82°C) in a water bath. The reaction product at 0.48% by weight was hydrated in 2% potassium chloride solution. The crosslinking agent was admixed with the sample in an amount of 1 vol per 1000 vol of gelled fluid in a Waring Blendor and transferred to a sample jar that was inserted into the water bath. The samples were inspected after 30 minutes for crosslinking, then the samples were permitted to cool to room temperature and the pH was measured. The results are set forth in the table, below. Crosslinker % Active Ingredients Rigid Crosslink formed pH triethanolamine titanate 80 yes 8.0 zirconium oxychloride 20 yes 4.0 aluminum acetate 20 yes 6.0 zirconium N-peroxide hydroxyethyl tris-hydroxyethylenediamine 52 yes 7.6 zirconium lactate 25 yes 6.4 CL - 24™, a zirconate complex - yes 8.1

Claims: A method of preparing a treating fluid composition for injection into a subterranean formation, in which method a cellulose ether derivative comprising a hydroxyalkyl cellulose, a carboxymethyl cellulose or a carboxymethylhydroxyalkyl cellulose or a mixture thereof, is chemically modified by reaction with a vinyl phosphonic acid, and mixed with a crosslinking agent, characterised in that said reaction is effected in the presence of the reaction product of hydrogen peroxide and a ferrous salt, and wherein water is included in the composition to partially hydrate the composition and increase the viscosity of said composition, A method according to claim 1 in which a particulate agent is also included in the composition. A method according to claim 1 or 2, wherein the cellulose ether derivative is hydroxyethyl cellulose. A method according to claim 1, 2 or 3, wherein said ferrous salt is ferrous chloride, ferrous ammonium sulfate, ferrous sulfate, ferrous acetate, ferrous oxalate or ferrous acetylacetonate. A method according to claim 1, 2, 3 or 4, wherein the crosslinking agent is a polyvalent metal cation providing titanium (IV) or zirconium (IV) ions. A method according to any one of the preceding claims, wherein a non modified cellulose ether derivative is further included in the composition. A method according to claim 6, wherein said non modified cellulose ether derivative is carboxymethyl hydroxyethyl cellulose. A method according to claim 1 or 2, wherein the modified cellulose ether derivative is the reaction product of a hydroxyalkyl cellulose having a molar substitution of from 1.5 to 3.0 moles per anhydroglucose unit of the hydroxyalkyl cellulose, the alkyl group being ethyl or propyl, and a vinyl phosphonic acid, at a temperature of from 20 to 60°C. A method according to claim 8, wherein the hydroxyalkyl cellulose is hydroxyethyl cellulose having a molar substitution of 1.8 to 2.5 moles per anhydroglucose unit of the hydroxyalkyl cellulose.

Inventors: HOLTMYER MARLIN D, HUNT CHARLES V, HOLTMYER, MARLIN D., HUNT, CHARLES V.
Applicants: HALLIBURTON CO, HALLIBURTON COMPANY
Application year: 1991
Main classification: E21B43
IPC codes: C09K8