Plugging a Tubing/Casing Annulus in a Wellbore With a Polymer Gel

1. Technical Field

The invention relates to a wellbore treatment process and more particularly to a wellbore treatment process wherein the wellbore penetrates a subterranean hydrocarbon-bearing formation.

2. Description of Related Art

Portland cement is commonly used in oil field applications such as oil well cement jobs. Portland cements can be tailor-made for the specific conditions of each well. A description of the state of the art in oil well cementing technology is given in Basic Cementing, Petroleum Publishing Co., 1977 (reprinted from a series of articles appearing in the Oil and Gas Journal) and Rike, J. L., et al, Squeeze Cementing: State of The Art, Journal of Petroleum Technology, (January 1982), pp. 37-45.

Formulation of the cement in the field is largely a product of trial and error by field personnel to meet irregularities in the cementing composition and the downhole environment. Cement quality control is difficult to achieve under such conditions. As a result, Portland cement can exhibit premature set-up, cracking, or shrinking upon curing. This feature of Portland cement limits its usefulness in wellbore treatments to repair leaks in wellbore casing or tubing by plugging the tubing/casing pair annulus. Use of other available methods to remedy leaking wellbore tubulars, including workovers and redrilling, can be extremely cumbersome and expensive.

A wellbore treatment process is needed for preventing or repairing leaking tubulars which is both economically and operationally attractive. A process is needed which effectively employs a plugging material having a broad range of highly controllable and predictable set-up times for ease of operation and design. A process is needed which employs a plugging material which is not as susceptible as Portland cement to shrinking and cracking when applied to a tubing/casing annulus in a wellbore.

The present invention provides a wellbore treatment process for plugging a tubing/casing annulus in a wellbore in communication with a subterranean hydrocarbon-bearing formation. The process is particularly applicable to injection or production wells in oil fields wherein the tubing and/or casing have developed leaks which enable undesirable flow of fluids into, out of, or vertically through the annulus formed between the tubing/casing pair. Leaks in the tubing and/or casing can be caused inter alia by corrosion, mechanical abrasion or thread leaks.

The leaks can result in serious operational, safety and/or environmental problems, especially where brines leak into and fill the annulus between the tubing/casing pair. The brines cause severe corrosion of the metal materials in the wellbore which ultimately can lead to loss of the well.

The present invention prevents the unwanted vertical flow of fluids in the tubing/casing annulus as well as unwanted flow of fluids into or out of leaking tubing and/or casing in a cost-effective and operationally attractive manner. The process is applicable as a remedial treatment for existing leaking wells and as a preventive treatment for new or non-leaking wells.

The objectives of the present invention are achieved by means of a plugging material comprising a tailor-made crosslinked polymer gel. The gel contains a high molecular weight, water-soluble carboxylate-containing polymer and a chromic carboxylate complex crosslinking agent. The gel is prepared by forming a uniform gelation solution above ground containing the polymer and crosslinking agent and injecting the solution into the annulus between a tubing/casing pair in a wellbore penetrating a hydrocarbon-bearing formation. The gelation solution is gelled to substantial completion in the annulus thereby plugging the annulus.

The gelation solution may be advantageously designed to be at least partially gelled by the time it reaches the annulus to inhibit or prevent its propagation into a less permeable material, which may adjoin the casing where no plugging is desired, such as a formation matrix. The gelation solution sets up in the annulus without requiring the further injection of any additional components. The gel is a continuous single-phase material which substantially plugs the annulus. After the treatment, the well may be returned to normal operation.

The process provides distinct advantages over processes known in the art. The gelation solution, as initially injected into the well-bore, is a uniform nonviscous liquid solution prepared at the surface which is advantageously substantially free of solids. A solid-free solution is more able to penetrate and propagate through microvoids and microannuluses in the wellbore than slurries, such as Portland cement, which contain suspended solids. This property of the solution provides good penetration, avoids bridging, and reduces fluid loss.

The resulting gel forms a tenacious chemical bond with the tubular surfaces and is substantially impermeable to formation fluids. The gel is designed to be non-flowing to the maximum rheological stress of the system, which enables it to substantially resist displacement from the annulus during oil recovery operations. Yet, the gel is not so strong that placement of the gel in the annulus precludes subsequent tube pulling or workover operations in the wellbore. The gel is substantially permanent and resistant to degradation in the subterranean environment. However, if subsequent removal of the gel from the annulus is desired, it can be dissolved by an external solvent, such as solutions of sodium hypochlorite, hydrogen peroxide, or any other suitable peroxo compound.

The cementing gel employed in the present invention possesses a broad range of highly controllable and predictable set-up times and strengths. The process is applicable to a broad range of temperatures, salinities, rock formations, and environments. The practitioner can customize or tailor a gel for specific operational constraints, downhole characteristics and subsequent performance demands. One can predetermine the gelation rate and resultant gel strength and stability which are required of a gel to meet the performance demands in the wellbore. Thereafter, a cementing gel having the required predetermined properties is produced under controlled conditions at the surface by utilizing observed correlations between specific controllable gelation parameters and resultant gel properties.

The present invention is described in the context of specific terms which are defined as follows. A "tubular" in a wellbore is a comprehensive term including tubing string and casing.

"Gel" as used herein is directed to a continuous three-dimensional crosslinked polymeric network having an ultra high molecular weight. The gel contains a liquid medium such as water which is confined within the solid polymeric network. The fusion of a liquid and a solid component into a single-phase system provides the gel with a unique phase behavior. Gels employed by the present invention have sufficient structure so as not to propagate from the confines of a plugged volume into a less permeable region of the formation adjoining the volume when injected into the volume. "Plugging" is a substantial reduction in permeability of a volume in the wellbore sufficient to prevent or inhibit fluid flow therethrough.

"Partially gelled" solutions are also referred to herein. A partially gelled solution is at least somewhat more viscous than an uncrosslinked polymer solution such that it is incapable of entering a less permeable region where no treatment is desired, but sufficiently fluid such that it is capable of displacement into a desired treatment zone. The crosslinking agent of the partially gelled solution has reacted incompletely with the polymer with the result that neither all of the polymer nor all of the crosslinking agent in the gelation solution is totally consumed by the crosslinking reaction. The partially gelled solution is capable of further crosslinking to completion resulting in the desired gel without the addition of more crosslinking agent.

"Crosslinked to completion" means that the gel composition is incapable of further crosslinking because one or both of the required reactants in the initial solution are consumed. Further crosslinking is only possible if either polymer, crosslinking agent, or both are added to the gel composition.

The gel composition utilized in the present invention is comprised of a carboxylate-containing polymer and a crosslinking agent. The carboxylate-containing polymer may be any crosslinkable, high molecular weight, water-soluble, synthetic polymer or biopolymer containing one or more carboxylate species. The average molecular weight of the carboxylate-containing polymer is in the range of about 10,000 to about 50,000,000 and preferably about 100,000 to about 20,000,000, and most preferably about 200,000 to about 15,000,000.

Biopolymers useful in the present invention include polysaccharides and modified polysaccharides. Examplary biopolymers xanthan gum, guar gum, carboxymethylcellulose, o-carboxychitosans, hydroxyethylcellulose, hydroxypropylcellulose, and modified starches. Useful synthetic polymers include inter alia acrylamide polymers, such as polyacrylamide, partially hydrolyzed polyacrylamide and terpolymers containing acrylamide, acrylate, and a third species. As defined herein, polyacrylamide (PA) is an acrylamide polymer having substantially less than 1% of the acrylamide groups in the form of carboxylate groups. Partially hydrolyzed polyacrylamide (PHPA) is an acrylamide polymer having at least 1%, but not 100%, of the acrylamide groups in the form of carboxylate groups. The acrylamide polymer may be prepared according to any conventional method known in the art, but preferably has the specific properties of acrylamide polymer prepared according to the method disclosed by U.S. Pat. No. Re. 32,114 to Argabright et al incorporated herein by reference.

The crosslinking agent is a chromic carboxylate complex. The term "complex" is defined herein as an ion or molecule containing two or more interassociated ionic, radical or molecular species. A complex ion as a whole has a distinct electrical charge while a complex molecule is electrically neutral. The term "chromic carboxylate complex" encompasses a single complex, mixtures of complexes containing the same carboxylate species, and mixtures of complexes containing differing carboxylate species.

The complex of the present invention includes at least one or more electropositive chromium III species and one or more electronegative carboxylate species. The complex may advantageously also contain one or more electronegative hydroxide and/or oxygen species. It is believed that, when two or more chromium III species are present in the complex, the oxygen or hydroxide species may help to bridge the chromium III species. Each complex optionally contains additional species which are not essential to the polymer crosslinking function of the complex. For example, inorganic mono- and/or divalent ions, which function merely to balance the electrical charge of the complex, or one or more water molecules may be associated with each complex. Representative formulae of such complexes include:

Trivalent chromium and chromic ion are equivalent terms encompassed by the term chromium III species as used herein. The carboxylate species are advantageously derived from water-soluble salts of carboxylic acids, especially low molecular weight mono-basic acids. Carboxylate species derived from salts of formic, acetic, propionic, and lactic acid, lower substituted derivatives thereof and mixtures thereof are especially preferred. The carboxylate species include the following water-soluble species: formate, acetate, propionate, lactate, lower substituted derivatives thereof, and mixtures thereof. The optional inorganic ions include sodium, sulfate, nitrate and chloride ions.

A host of complexes of the type described above and their method of preparation are well known in the leather tanning art. These complexes are described in Shuttleworth and Russel, Journal of The Society of Leather Trades' Chemists, "The Kinetics of Chrome Tannage Part I.," United Kingdom, 1965, v. 49, p. 133-154; "Part III.," United Kingdom, 1965, v. 49, p. 251-260; "Part IV.," United Kingdom, 1965, v. 49 p. 261-268; and Von Erdman, Das Leder, "Condensation of Mononuclear Chromium (III) Salts to Polynuclear Compounds," Eduard Roether Verlag, Darmstadt, Germany, 1963, v. 14, p. 249; and are incorporated herein by reference. Udy, Marvin J., Chromium, Volume 1: Chemistry of Chromium and its Compounds, Reinhold Publishing Corp., N.Y., 1956, pp. 229-233; and Cotton and Wilkinson, Advanced Inorganic Chemistry 3rd Ed., John Wiley & Sons, Inc., N.Y., 1972, pp. 836-839, further describe typical complexes which may be within the scope of the present invention and are incorporated herein by reference. The present invention is not limited to the specific complexes and mixtures thereof described in the references, but may include others satisfying the above-stated definition.

The gel is formed by admixing a carboxylate-containing polymer and crosslinking agent at the surface to form an injectable gelation solution. Surface admixing broadly encompasses inter alia mixing the solution in bulk at the surface prior to injection or simultaneously mixing the solution at or near the wellhead by in-line mixing means while injecting it. Admixing is accomplished for example by dissolving the starting materials for the crosslinking agent in an appropriate aqueous solvent. Exemplary starting materials include solid CrAc.sub.3.H.sub.2 O, solid Cr.sub.3 Ac.sub.7 (OH).sub.2 or a solution labeled "Chromic Acetate 50% Solution" commercially available, for example, from McGean Chemical Co., Inc., 1250 Terminal Tower, Cleveland, Ohio 44113, U.S.A. The crosslinking agent solution is then mixed with an aqueous polymer solution to produce the gelation solution. Among other alternatives, the starting materials for the crosslinking agent can be dissolved directly in the aqueous polymer solution to form the gelation solution in a single step.

The aqueous solvent of the gelation solution may be fresh water or a brine having a total dissolved solids concentration up to the solubility limit of the solids in water. Inert fillers such as crushed or naturally fine rock material or glass beads can also be added to the gelation solution to reinforce the gel network structure although a solid-free gelation solution is preferred.

The present process enables the practitioner to customize or tailor-make a gel having a predetermined gelation rate and predetermined gel properties of strength and stability from the above-described composition. The gelation rate is defined as the degree of gel formation as a function of time or, synonymously, the rate of crosslinking in the gelation solution. The degree of crosslinking may be quantified in terms of gel viscosity and/or strength. Gel strength is defined as the coherence of the gel network or resistance to deformation under external forces. Stability is defined as either thermal or phase stability. Thermal stability is the ability of a gel to withstand temperature extremes without degradation. Phase stability is the ability of a gel to resist syneresis which can detract from the gel structure and performance.

Tailor-making or customizing a gel in the manner of the present invention to meet the demands of a particular wellbore is provided in part by correlating the independent gelation parameters with the dependent variables of gelation rate and resultant gel strength and stability. The independent gelation parameters are the surface and in situ gelation conditions including: temperature, pH, ionic strength and specific electrolytic makeup of the solvent, polymer concentration, ratio of the weight of polymer to the combined weight of chromium III and carboxylate species in the mixture, degree of polymer hydrolysis, and average molecular weight of the polymer.

The operable ranges of the gelation parameters are correlated with the dependent variables of gelation rate and resultant gel properties by means including qualitative bottle testing and quantitative viscosimetric analysis. The operable ranges of a number of gelation parameters and their correlation with the dependent variables are described below.

The lower temperature limit of the gelation solution at the surface is the freezing point of the solution and the upper limit is essentially the thermal stability limit of the polymer. The solution is generally maintained at ambient temperature or higher at the surface. The temperature may be adjusted by heating or cooling the aqueous solvent. Increasing the temperature within the prescribed range increases the gelation rate.

The initial pH of the gelation solution is within a range of about 3 to 13 and preferably about 6 to 13. Although gelation can occur at an acidic pH, lowering the initial pH of the solution below 7 does not favor gelation. The initial pH of the solution is most preferably alkaline, i.e., greater than 7 to about 13. Increasing the pH within the prescribed range increases the rate of gelation.

The polymer concentration in the solution is about 500 ppm up to the solubility limit of the polymer in the solvent or the rheological constraints of the polymer solution, preferably about 1000 to about 200,000 ppm, and most preferably about 3000 to about 100,000. Increasing the polymer concentration increases the gelation rate and ultimate gel strength at a constant ratio of polymer to crosslinking agent.

The ionic strength of the solvent can be from that of deionized distilled water to that of a brine having an ion concentration approaching the solubility limit of the brine. Increasing the ionic strength of the solution can increase the gelation rate.

The weight ratio of polymer to chromium III and carboxylate species comprising the mixture is about 1:1 to about 500:1, preferably about 2.5:1 to about 100:1, and most preferably about 5:1 to about 40:1. Decreasing the ratio generally increases the gelation rate and up to a certain point generally increases the gel strength, especially at a constant high polymer concentration.

The degree of hydrolysis where an acrylamide polymer is employed is about 0 to 60% and preferably about 0 to 30%. Within the preferred range, increasing the degree of hydrolysis increases the gelation rate. Increasing the molecular weight of the polymer increases the gel strength.

It is apparent from these correlations that one can produce gels across a very broad range of gelation rates and gel properties as a function of the gelation conditions. Thus, to effect an optimum plugging job according to the present process, the practitioner predetermines the gelation rate and properties of the resultant gel which meet the demands of the given wellbore and thereafter produces the gel having these predetermined characteristics. The demands of the wellbore include the in situ gelation conditions such as temperature, connate water properties, size of the treatment volume, the pressure drop and permeability of the adjoining matrix as well as the post treatment conditions such as injection and production pressures. Analytical methods known to one skilled in the art are used to determine these demands which provide criteria to predetermine the gelation rate and resultant gel properties in the manner described above and continuing hereafter.

The gelation rate is advantageously sufficiently slow to enable preparation of the gelation solution at the surface and injection of the solution as a uniform slug into the wellbore annulus. Too rapid a gelation rate produces excessive gelation of the solution at the surface which results in a solution that may be difficult, if not impossible, to inject into the annulus to be plugged due to its rheological properties. At the same time, the gelation rate must be sufficiently rapid to enable completion of the reaction within a reasonable period of time so that the well may be activated after the plugging job.

The solution may be substantially ungelled before reaching the annulus. However, at least partial gelation of the solution may be advantageous before the solution reaches the annulus being plugged. Partial gelation prevents the solution from penetrating permeable rock in fluid communication with the annulus. Substantial penetration of permeable rock by the solution and its ensuing permeability reduction may be counterproductive to the plugging of the annulus. The solution advantageously gels to completion in the annulus. The values of the independent variables in the process are carefully selected to achieve a gelation rate meeting these criteria.

The amount of solution injected into the wellbore is a function of the volume of the annulus to be plugged. One skilled in the art can determine the required amount of a gel for a given volume to be plugged. Placement of the gelation solution in the annulus may be facilitated by zone isolation means such as packers and the like.

The injection rate is a function of the gelation rate and operational constraints of injection pressure and pumping limits. The required injection rate is fixed such that all of the solution can be practically injected into the annulus before it becomes unpumpable. The gelation time of the gel ranges from near instantaneous up to 48 hours or longer. Longer gelation times are limited by practical considerations of lost production when injection and production wells are shut in.

Gels having a predetermined gelation rate and resultant gel properties to meet the demands of a given well are produced by adjusting and setting the surface gelation conditions as they correlate to the gelation rate and gel properties. Accordingly the gels are produced in a manner which renders them insensitive to most extreme formation conditions. The gels can be stable at formation temperatures as high as 130.degree. C. or more and at any formation pH contemplated. The gels are relatively insensitive to the stratigraphy of the rock, metal tubulars and other materials and chemicals employed in cementing operations. The gels can be employed in carbonate and sandstone strata and unconsolidated or consolidated strata having varying mineralogy. Once the gels are in place, it is extremely difficult to displace the gels by physical or chemical means other than total destruction of the crosslinked network. The gels may be reversible on contact with hydrogen peroxide or sodium hypochlorite, but are substantially insoluble in the formation fluids.

The process is applicable to most oil field wells having a tubing string within a cased wellbore. The process is employed as a remedial treatment process in wellbores having leaking tubulars to displace unwanted brine from the tubing/casing annulus. The process also prevents the subsequent encroachment of brine into the annulus. The process is further employed as a preventive treatment process in new or non-leaking wellbores to preclude brine from entering the annulus should tubular leaks subsequently develop.

The strength of the gel can vary from an elastic jelly-like material to a rigid rubber-like material depending upon the performance demands of the wellbore. The gel is designed to be sufficiently strong not to flow under the maximum rheological stress encountered in flow conduits of the wellbore. Yet, the gel is advantageously not so strong that the tubing cannot be subsequently pulled after treatment if desired. Pulling of the tubing can be facilitated by initially coating the tubular surfaces to contact the gel with a friction-reducing material, such as Teflon, plastic, or grease, prior to applying the process of the present invention.

Stronger rigid gels are generally preferred where extreme drawdown pressures are encountered during production of a well or where extreme injection pressures are encountered during injection of fluids into a well which could cause a weak gel to fail. PA is often preferred for such formulations because it has a slower gelation rate than PHPA which enables one to inject it into a volume before it sets up.

The following examples demonstrate the practice and utility of the present invention but are not to be construed as limiting the scope thereof.

Most of the examples are formatted as tables of data which describe the formulation and maturation of one or more gels. Each gel is represented in a table by a single experimental run. Data include the conditions for producing the gel and the quantitative or qualitative strength of the produced gel. The tables display data in a three-tier format. The first tier is the values of the fixed gelation conditions which are constant and common to every run in the table. The second tier is the values of the gelation conditions which vary among the different runs in the table but are constant for any given run. The third tier is the gel strength which varies as a function of time within each run. Qualitative gel strength is expressed in alphabetic code. Quantitative gel strength is simply the numerical value of apparent viscosity.

The following gel strength code and nomenclature are useful for interpreting the tables.

A No detectable continuous gel formed: the bulk of the solution appears to have the same viscosity as the original polymer solution although isolated local gel balls may be present.

B Highly flowing gel: the gel appears to be only slightly more viscous than the initial polymer solution.

C Flowing gel: most of the gel flows to the bottle cap by gravity upon inversion.

D Moderately flowing gel: only a small portion (5-10%) of the gel does not readily flow to the bottle cap by gravity upon inversion (usually characterized as a tonguing gel).

E Barely flowing gel: the gel can barely flow to the bottle cap and/or a significant portion (>15%) of the gel does not flow by gravity upon inversion.

F Highly deformable nonflowing gel: the gel does not flow to the bottle cap by gravity upon inversion.

G Moderately deformable nonflowing gel: the gel deforms about half way down the bottle by gravity upon inversion.

H Slightly deformable nonflowing gel: only the gel surface slightly deforms by gravity upon inversion.

I Rigid gel: there is no gel surface deformation by gravity upon inversion.

J Ringing rigid gel: a tuning fork-like mechanical vibration can be felt upon tapping the bottle.

% Hydrolysis:

% of carboxylate groups on the acrylamide polymer based on the total number of acrylamide groups

Polymer MW:

average molecular weight of the acrylamide polymer

Polymer Conc:

acrylamide polymer concentration in the polymer solution (ppm)

Polymer Solvent:

aqueous solvent in the polymer solution

Polymer pH:

pH of the polymer solution

Total Ion Conc:

total concentration of chromium III and acetate ions in the gelation solution (ppm)

Weight Ratio Polymer:Ions:

weight ratio of acrylamide polymer to chromium III and acetate ions in the gelation solution

Metal Ion Conc:

chromium III ion concentration in the gelation solution

Temp:

gelation temperature (.degree.C.)

Time:

gelation time (hr)

Gel Code:

gel strength code

Viscosity:

apparent viscosity of the gelation solution at 0.1 sec.sup.-1 shear rate (cp)

Pressure:

viscometer pressure (kPa)

The polymer solutions of the following examples are prepared by diluting aqueous acrylamide polymer solutions with an aqueous solvent. Where qualitative data are obtained, the dilute polymer solution is combined with a crosslinking agent solution in a 0.12 liter widemouth bottle to form a 0.05 liter sample. The sample is gelled in the capped bottle and the qualitative gel strength is determined by periodically inverting the bottle.

Where quantitative data are obtained, the polymer solution and crosslinking agent solution are combined in a variable pressure and temperature rheometer (viscometer), having an oscillatory mode of 0.1 rad/sec and 100% strain. The apparent viscosity at a shear rate of 0.1 sec.sup.-1 is recorded as a function of time.

In all of the examples, the crosslinking agent solution is that used in the present invention (i.e., a complex or mixture of complexes comprised of chromium III and acetate ions) unless the example is comparative and specifically describes a crosslinking agent solution different from that of the present invention. The crosslinking agent solution of the present invention is prepared by dissolving solid CrAc.sub.3.H.sub.2 O or CrAc.sub.7 (OH).sub.2 in water or diluting a solution obtained commercially under the label of "Chromic Acetate 50% Solution". In some examples, the actual method of preparing the crosslinking agent solution is specified.

A gelation solution is prepared by combining a 2.2% by weight solution of 11,000,000 MW PA in fresh water with a chromic/acetate complex crosslinking agent at a PA complex weight ratio of 20:1. The gelation solution is placed at the bottom of an annulus between a 6.0 cm O.D. (5.1 cm I.D.) tubing and a 13 cm diameter casing. The annulus is filled with the gelation solution up to a level of 61 m from the bottom. The resulting gel is matured for 48 hours. The tubing is pulled from the wellbore, which requires a force of 13,300 Newtons in addition to the tubing weight.

152 m of gel is placed in the same well, having a packer at the bottom of the tubing. 26,700 Newtons of force is required to pull the tubing and packer assembly in addition to the weight of the assembly.

A gel is allowed to mature in the 5.1 cm I.D. tubing. 2070 to 2760 kPa water pressure per 9.1 mefters of tubing length is required to dislodge the mature gel. Gel returned to the surface is degraded using hydrogen peroxide.

The data show that gelation rate and gel strength increase as the temperature increases and as the weight ratio of polymer to ions decreases.

The data show that gelation rate and gel strength decrease as pH of the polymer solution decreases.

Viscosity data confirm the observations of Examples 2 and 3 that decreasing the weight ratio of polymer to ions increases the gelation rate.

The mature gel of Run 2 exhibits properties of a Bingham plastic. Its yield stress is determined by attempting to flow the gel through a 1.4 mm diameter orifice at an applied pressure for four hours. The gel does not flow through the orifice at applied pressures up to 3450 kPa for tests conducted at temperatures ranging from 22.degree. C. to 124.degree. C.

Viscosity data confirm that increasing the temperature increases the gelation rate.

Viscosity data show that gelation rate is a weak function of pressure. The rate increases slightly with pressure.

The data show that the rate of gelation of partially hydrolyzed polyacrylamide is considerably faster than that of substantially unhydrolyzed polyacrylamide. Thus, the gelation rate of an acrylamide polymer solution can be slowed by reducing the degree of hydrolysis of the acrylamide groups.

Viscosity data confirm the observations of Example 7.

The data show that there is a functional relation between gelation rate and temperature for polyacrylamide gels as well as partially hydrolyzed polyacrylamide gels.

The data show that decreasing the polymer concentration while maintaining the same weight ratio of polymer to ions substantially decreases the gelation rate and gel strength.

Viscosity data confirm the observations of Example 10 that gelation rate decreases as polymer concentration decreases.

The data show that relation rate and gel strength increase as the molecular weight of the polymer increases.

Viscosity data confirm the observations of Example 12.

Examples 2-13 show that the gelation rate of the polymer and crosslinking agent of the present invention can be adjusted to any desired rate and gel strength by selecting the values of the independent variables such as polymer or crosslinking agent concentration, polymer molecular weight, temperature, pH, etc. This is particularly useful in customizing a gel for a specific plugging application.

Various salts and gases commonly found in oil field brines are added to the gelation solution of Examples 14 and 15 to determine the sensitivity of the gelation relation to in situ fluids.

The data of Examples 14 and 15 show that the gelation reaction is relatively insensitive to these additives.

Examples 16-18 utilize actual or synthetic field injection waters in the gelation solutions.

Examples 16-18 show that gels can be formulated in saline actual field waters. The concentration and proportions of gel components can be selected to form stable gels even in complex injection waters such as B and C at the formation temperature.

In Example 19-21, the gelation solutions are physically modified during aging.

The gel of Run 1 is aged without agitation. The gel of Run 2 is stirred after 1 hour of aging for 5 minutes at 1750 rpm with a 4 mm propeller in a 6 mm inside diameter bottle. The gel of Run 3 after 1 hour of aging is forced through a 10 cm long nylon tube 0.16 cm in diameter at a rate of 100 cc in 7.2 sec and a pressure drop of 410 kPa.

Example 19 shows that shearing the gelation solution after 1 hour of aging has very little negative impact on the strength of the final gel. This is an important property because the gelation solution can encounter shear forces during injection into voids present in the well-bore. The sheared solution matures to approximately the same final gel strength as the unsheared solution.

The 50 cc gelation solution is filtered through an 8 micron millipore cellulose-acetate filter at a filtration pressure of 345 kPa after 1 hour of aging. The diameter of the filter is 47 mm. Although the solution has no visually detectable gel formation, only 0.8 cc of the solution pass through the filter after 10 minutes. In comparison, a polymer solution absent the crosslinking agent is filtered according to the same procedure and the 50 cc sample passes through the filter in 2.6 minutes.

The experiment shows that the gelation solution containing crosslinking agent is sufficiently crosslinked to render it unfilterable. A gel that cannot pass through an 8 micron filter would not be expected to enter a competent formation matrix having a permeability below 1,000 md. Nonetheless, the gelation solution is highly pumpable after 1 hour of aging and capable of entering a void. Thus, the present gel is particularly suited for cement jobs. The gel enters voids and reduces the flow capacity of the voids, but does not enter the matrix and reduce matrix permeability. A further advantage of the present gel is that the greatest shear forces are usually encountered when the gelation solution is injected into the voids. Since the gel is highly pumpable at this point, it suffers little shear degradation.

The gelation solution is aged in a graduated cylinder for 48 hours. 50 additional milliliters of the PHPA solvent are added on top of the gel. The change in the position of the interface between the gel and the solvent in the graduated cylinder is recorded as a function of time.

The gel of the present invention appears substantially resistant to water expulsion (syneresis) or swelling upon contact with additional formation water.

While the foregoing preferred embodiments of the invention have been described and shown, it is understood that the alternatives and modifications, such as those suggested and others, may be made thereto and fall within the scope of the invention.