Formation Treating Methods

This invention concerns well treating methods including in one important embodiment, a method for treating wells completed in subterranean formations containing unconsolidated particulate matter, e.g. unconsolidated sand, in order to bind the unconsolidated sand grains together in the portions of the formation immediately adjacent to the perforations of the well, and thereby form in this embodiment a stable yet still fluid permeable barrier around the wellbore, in order to facilitate production of fluids from the formation while restraining the movement of sand into the wellbore during the fluid production phase. In another embodiment, this method results in the creation of an essentially impermeable barrier around the wellbore to exclude flow of fluids including water from the formation into the well. In yet another embodiment, the area around the well and in the well are filled with an impermeable mass so the well can be abandoned safely. More particularly, this invention pertains to a method for accomplishing sand consolidation, water exclusion or well plugging in producing oil wells utilizing the sand naturally present in the formation and a method which utilizes a substantially reduced number of procedural steps, which reduces the time and cost of treating wells, by use of single fluid containing the polymerizable resins with the catalyst already mixed with the resin in order to achieve more uniform mixing and to reduce the necessity of using multiple fluid injection steps to clean the sand or coat the sand with catalyst. In particular, preferred embodiments of this invention permit treating wells completed in formations whose temperatures are from 60.degree. F. ( 15.5.degree. C.) to 350.degree. F. (176.7.degree. C.) or greater with set times below 24 hours.

Formation treating for purposes of sand consolidation, water exclusion or well plugging for abandonment are well known terms applying to procedures routinely practiced in the commercial production of petroleum. When wells are completed in petroleum-containing which also contain unconsolidated granular mineral material such as sand or gravel, production of fluids from the formation causes the flow of the particulate matter, e.g. sand, into the wellbore, which often leads to any of several difficult and expensive problems. Sometimes a well will "sand up", meaning the lower portion of the production well becomes filled with sand, after which further production of fluid from the formation becomes difficult or impossible. In other instances, sand production along with the fluid results in passage of granular mineral material into the pump and associated hardware of the producing well, which causes accelerated wear of the mechanical components of the producing oil well. Sustained production of sand sometimes forms a cavity in the formation which collapses and destroys the well. All of these problems are known to exist and many methods have been disclosed in the prior art and applied in oil fields in order to reduce or eliminate production of unconsolidated sand from a petroleum formation during the course of oil production.

Another problem encountered in oil production to which the present process offers an attractive solution is encountered when a well is in fluid communication with a section that includes one or more oil producing intervals and one or more water producing intervals. Production from such a zone results in production of some oil and some water. Since the viscosity of water is less than the viscosity of oil, a much larger volume of water than oil is frequently produced.

In yet another embodiment, the resin-catalyst mixture can be used to completely plug a well which is to be abandoned. This embodiment requires a slightly greater volume of resin-catalyst mixture to be used, since the best results for this embodiment require that the interior of the well casing and/or tubing be filled completely with the resin for all or a substantial amount of its length. The long term durability of the material used to plug wells for abandonment is of high current interest because of the danger that salt water or petroleum might enter and contaminate fresh water-containing 15 intervals.

The above-described sand control problems and potential solutions to the problems have been the subject of extensive research by the petroleum industry in the hope of developing techniques which minimize or eliminate the production of sand particles into the producing well and associated equipment during the course of producing fluids from the formation. One general approach suggested in the prior art for consolidating sand to form a fluid permeable zone around the well involves consolidating the porous but unconsolidated sand structure around the wellbore in order to cement the loose sand grains together, thereby forming a permeable mass which will allow production of fluids from the formation into the well but which will restrain the movement of sand particles into the wellbore when used for fluid exclusion, into an impermeable mass if the desired result is water exclusion. Another approach involves removing a portion of the formation around the well and packing specially prepared granular material into the formation around the wellbore which is subsequently caused to be cemented together in a manner which maintains fluid permeability.

It is a primary objective of any successful sand consolidation method that a barrier be formed around the wellbore which restrains the movement of sand particles into the well while offering little or no restriction to the flow of fluids, particularly oil, from the formation into the wellbore where it can be pumped to the surface of the earth. Consolidation only needs to extend into the formation to a depth of 6 to 12 inches around the periphery of the perforations or other openings in the outer casing of the production well.

The objective of a water exclusion of fluid entry control process is to form an impermeable barrier around the well to prevent the flow of sand and formation fluids into the well. A process similar to that used to control sand problems can be used for plugging formation for water exclusion, provided the process is modified to cause creation of an impermeable barrier rather than a permeable barrier.

A very important quality of a satisfactory sand consolidation method, plugging or water exclusion method is durability of the permeable or impermeable barrier formed around the wellbore. Once a permeable barrier is formed and the well is placed on production, there will be a continuing flow of fluids through the flow channels within the permeable barrier, and it is important that the barrier last for a significant period of time, e.g. several months and preferably years, without excessive abrasive wear or other deterioration of the consolidation matrix which would cause the particulate matter to flow once again into the wellbore.

It is also important that the material injected into the formation for all of our formation treating methods described above should be essentially unreactive during the period it is inside the wellbore, i.e. while it is being pumped down the well and positioned where it is desired adjacent to the perforations of the production casing. It is this desire to delay the polymerization reaction that has lead prior art methods to employ multi-step procedures in which first a catalyst is injected into the formation, after which the polymerizable resin-containing fluid is injected. While this reduces the propensity for the fluid to polymerize in the wellbore, it does give rise to several problems which constitute inherent weaknesses in many prior art methods for accomplishing sand consolidation. First, each separate injection step increases the time and cost associated with the well treatment by which sand consolidation, water shut off or well plugging is accomplished. Second, injection of catalyst into the formation in advance of the polymerizable fluid does not accomplish uniform mixing of catalyst with all of the polymerizable fluid which is needed to ensure optimum polymerization of the resin, which is essential for strength and durability of the consolidated mass. Use of aqueous fluids to inject catalyst often gives rise to the need for yet additional steps to clean the sand to remove formation petroleum so the catalyst will be absorbed by the sand and later mix with the subsequently injected resin containing fluid.

Many processes and materials have been utilized for treating formations for consolidating sand in the formation adjacent to production of wellbores or to plug wells to control fluid entry into the well or to prepare the well for to abandonment. One of the more successful agents utilized for this purpose is furfuryl alcohol resin which can be polymerized to form a solid matrix which binds the sand grains together, while at the same time offering superior resistance to high temperatures and to caustic substances which may be encountered in steam flood operations. One of the problems in utilizing furfuryl alcohol resin to polymerize in the formation is in accomplishing uniform catalysis of the polymerization. Many catalysts that are effective for polymerizing furfuryl alcohol resins cannot be admixed with the furfuryl alcohol to permit employing a single fluid containing both the resin and the catalyst to be injected into the formation, because the time of polymerization is so short or unpredictable that there is excessive danger that the resin will polymerize in the injection wellbore.

In U.S. Pat. No. 4,427,069 there is disclosed a procedure for consolidating sand in a formation adjacent to a wellbore using an oligomer of furfuryl alcohol, in which the catalyst used is a water soluble acidic salt, preferably zirconyl chloride, which is injected in an aqueous solution into the formation prior to the resin containing fluid injection. The salt absorbs on the sand grains, and sufficient acidic salt remains adsorbed on the sand grain during the subsequent resin fluid injection stage that adequate polymerization occurs. Although this has been effective in many difficult situations where sand consolidation procedures are utilized, specifically in connection with thermal flooding such as steam injection procedures, the procedure nevertheless requires a multi-fluid injection procedure which requires more time and is more expensive than is desired. Usually a preliminary sand cleaning step is required before injecting the aqueous-catalyst solution in order to remove the naturally-occurring oil film from the sand grains to ensure good catalyst adsorption on the sand. Also, although catalyst mixes with the subsequently injected polymer to a limited degree, usually sufficient to cause some polymerization, it is believed that improved performance would result if the catalyst resin mixture can be made more homogenous prior to polymerization, in order to achieve a dense strong durable consolidation mass.

In U.S. Pat. No. 4,842,072 for "SAND CONSOLIDATION" we disclosed a particularly effective method for consolidating sand utilizing a mixture of a polymerizable resin such as an oligomer of furfuryl alcohol and a diluent such as butyl acetate and an oil soluble, slightly water soluble acid catalyst such as orthonitrobenzoic acid which is injected followed by injection of salt water to reestablish permeability.

In U.S. Pat. No. 4,903,770 for "SAND CONSOLIDATION" we disclosed a preferred process which is more easily removed after a period of use and which is quite inexpensive. The process employs a fluid comprising a polymerizable monomer such as furfuryl alcohol and as a diluent, a polar organic solvent such as methanol and a strong, non-volatile acid catalyst such as sulfuric acid, mixed with steam to form a multiphase or aerosol treating fluid, and injected into the formation to be consolidated. An ester such as ethyl or butyl acetate is incorporated in the fluid when the steam quality is less than 80 percent.

In U.S. Pat. No. 4,669,543 which issued Jun. 2, 1987, there is described a method for consolidating sand using an acid-curable resin and utilizing as a catalyst, the reaction product of an acid, and an alkyl metal or ammonia molybdate. In that instance, the catalyst is incorporated in an aqueous carrier fluid which comprises the continuous phase of an emulsion in which the polymerizable resin is the dispersed or discontinuous phase. Thus this process requires that the emulsion be resolved or broken after it is located in the portion of the formation where the permeable consolidating mass is desired, which is difficult to achieve to the high degree of completion necessary to accomplish the desired strong durable consolidating matrix necessary for a long lasting sand consolidation process.

U.S. Pat. No. 5,010,953 which issued Apr. 30, 1991 teaches a sand consolidating process using a polymerizable compound such as furfuryl alcohol, a diluent such as a low molecular weight alcohol, an acid catalyst and an ester and as an additive to reduce shrinkage, a copolymer of starch and a synthetic polymer such as acrylamide or acrylate.

U.S. Pat. No. 5,005,647 which issued Apr. 9, 1991, discloses a process for shutting off permeable zones in wellbores to reduce excess water flow using fluids similar to that described in U.S. Pat. No. 5,010,953 discussed above.

U.S. Pat. No. 5,005,648 which issued Apr. 5, 1991 describes a method of treating permeable zones in a formation to reduce water flow into a well completed therein by injecting a fluid-containing polymerizable compound, an ester, and an acid catalyst such as orthonitrobenzoic acid or toluenesulfonic acid.

U.S. Pat. No. 4,938,287 which issued Jul. 3, 1990 describes a process in which a preflush such as ethyl or butyl acetate is injected into the sand to be consolidated to remove oily residue, followed by injecting the treating fluid containing the polymerizable resin, diluent, ester and acid catalyst to accomplish sand consolidation.

The above processes have been extremely successful in treating wells in many formations, especially in formations where the temperature exceeds 350.degree. F. This is highly advantageous since many formations being steam stimulated and which cannot be treated by other processes, can be treated by this process with a high success ratio. When the temperature is much below 350.degree. F., however, the set time or time required for polymerization of the furfuryl alcohol often runs several days to one week or more. This often causes poor adhesion of the polymerized furfuryl alcohol to the sand grains, resulting in a weak consolidation job. Thus there is still an unfulfilled need for a well treating method for sand consolidation, fluid exclusion or plugging wells prior to abandonment applicable to formations whose temperatures span a broad range including temperatures up to and exceeding 350.degree. F. which will result in a set time less than 24 hours, preferably from 1-2 hours.

We have discovered methods for treating wells for consolidating sand or for forming a fluid-excluding impermeable barrier in formations over a broad temperature range while still providing a set time less than 24 hours, e.g. from 0.75 to 4.0 hours and preferably in the range of 1.0-2.0 hours. A fluid comprising a polymerizable resin, preferably a derivative such as an oligomer of furfuryl alcohol, a diluent such as ethyl or butyl acetate and an oil soluble internal catalyst which can safely be mixed with the resin on the surface, is injected into the well and in one embodiment, into the surrounding unconsolidated sand. The catalyst action is the key to the success of our process, since this process is applied to formations whose temperature may be less than 350.degree. F. The pK or negative of the log of the ionization constant of the acid catalyst must be chosen carefully to produce a set time in the range of 0.75 to 4 hours and preferably from 1-2 hours at the formation temperature. If the set time is below one hour, especially if it is below 0.75 hours, there is danger that the fluid which contains both the polymerizable compound and the acid catalyst, will polymerize in the surface mixing equipment or in the injection string. If the set time exceeds four hours there is danger that the polymerizable compound will be washed off the sand grains before polymerization occurs, resulting in a poor bond between the polymerized compound and the sand grains and a poor consolidation job or fluid exclusion job. Ideally the oil-soluble acid chosen should have a pK in a fairly narrow range, from 0.4 to 6.0 or greater. The preferred acid for our process varies with the temperature of the formation being treated. The concentration of the acid in the treating fluid must be selected carefully to ensure the set time is from 0.75 to 4.0 and preferably from 1.0 to 2.0 hours. As an example, usually a concentration of toluenesulfonic acid in the range of from 0.2 to 5.0 and preferably from 0.4 to 4.0 will result in a set time in the desired range of from 0.75 to 4.0 and preferably 1 to 2 hours when the formation temperature is from 60.degree. F. to 180.degree. F. The precise concentration of the preferred acid which produces the desired set time in a particular application in a formation whose temperature is known or determinable is defined by the formation temperature. The preferred embodiment for the 60.degree.14 180.degree. F. range involves preparation of a mixture of from 0.2 to 5.0 percent and preferably from 0.4 to 4.0 percent toluenesulfonic acid, the preferred catalyst for this reaction at this temperature, and from 20 to 70%, and ideally around 59% of a polar organic diluent. Our preferred organic diluent is butyl acetate. To this mixture of butyl acetate and toluenesulfonic acid is added from 30 to 80 and preferably about 40% resin, e.g. the furfuryl alcohol oligomer. This homogenous organic fluid can then be injected via the injection string into the formation without danger of premature polymerization. The injected mixture of resin, butyl acetate and toluenesulfonic acid, being oil soluble, simultaneously removes and displaces undesired oil and other oil soluble material coating the sand grains, and ensures a thorough contact between the sand grains and the resin-catalyst mixture. When this well treating process is being applied for sand consolidation, the next step is injection of an aqueous saline solution which is from 70% to 100% saturated with inorganic salt, preferably sodium chloride, into the resin saturated zone of the formation. This injection step accomplishes an opening of flow channels within the void spaces in the formation into which the resin catalyst mixture had been injected without removing the polymerizable resin, an event which would occur with <70% salt solution, which is important to ensure that the resulting polymerized resin bonded sand matrix is sufficiently permeable to permit flow of formation fluids from the formation after the sand consolidation process is completed. The salt water also modifies the resin coating on the sand, removing water therefrom, which increases the strength and durability of the polymerized resin matrix. The brine post-flush is not used when an impermeable barrier is desired. The well is then shut in for a period of from 0.75 to 4.0 hours and preferably from 1-2 hours. This two-step procedure results in the formation of a permeable, durable, consolidated sand mass around the perforations of the wellbore which restrains the movement of sand into the wellbore during production operations, while permitting relatively free flow of formation fluids, particularly formation petroleum, into the wellbore.

When our well treating process is applied for the purpose of plugging zones to prevent undesired fluid entry into the well, or to plug an entire well prior to abandonment, essentially the same polymer, diluent and acid catalyst is injected in essentially the same manner, but the salt water injection step is not used.

We have discovered, and this constitutes our invention, that it is possible to accomplish an improved well treating process such as sand consolidation method, water exclusion or well plugging utilizing the sand naturally occurring in the formation in a process employing a single treating fluid injection step, plus in some embodiments a brine injection step. A mixture of polymerizable resin, having dissolved or dispersed therein the catalyst for the polymerization step, and a organic polar diluent, is injected into the formation to saturate the void space in the portion of the formation adjacent to the production well. This accomplishes coating the formation granular material, e.g. the formation sand, with the mixture of polymerizable resin and catalyst. Since the fluid injected into the formation in this step is organic and contains a diluent, the minor amounts of formation petroleum and other oil-based materials coating and contaminating the surface of the sand grains is effectively removed or dissolved. It is a particular feature of this method that a separate preliminary wash step to remove materials coating the sand grains is not required. We have conducted laboratory tests, using formation sand containing crude oil, to which additional oil was deliberately added, and we still obtained successful consolidation by this method without any preliminary wash step.

In another embodiment of our well treating process, the same fluid is injected to coat the formation granular material and to fill the void spaces between the grains completely, to form an impermeable mass which restricts fluid movement into or out of the well.

The resin which we have found to be especially preferable for use in our well treating process is a furfuryl alcohol oligomer. Any resin which will polymerize upon exposure to heat and contact with an acid catalyst can be used in this process; however, furfuryl alcohol oligomer (C.sub.4 H.sub.3 OCHO).sub.n is the particularly preferred polymerizable resin. This resin has the advantage of being relatively inexpensive and having the characteristic of autopolymerizing on exposure to acid catalyst, forming a thermal-setting resin which cures to an insoluble mass that is highly resistant to chemical attack as well as to thermal degradation. The particularly preferred commercial form in which this resin is available is "QUACORR 1300" marketed by QO Chemicals. This resin is ordinarily obtained commercially in a form containing 90 to 95 percent furfuryl alcohol oligomer.

The furfuryl alcohol oligomer emulsion utilized in our process is so viscous that it must be diluted with an appropriate solvent in order to permit it to be pumped into the formation, and to accomplish relatively complete filling of void spaces in the formation. Any solvent for the furfuryl alcohol oligomer would accomplish this objective. It is possible, however, to accomplish this and another important objective by using as the diluent a hydrolyzable ester. The polymerization of the furfuryl alcohol oligomer produces water and the water produced by polymerization eventually limits the polymerization reaction. If water produced during polymerization of furfuryl alcohol oligomer can be removed, it is possible to force the polymerization reaction to proceed further toward completion and thereby produce longer polymer chains than would result if water were left in the polymer reaction mass. A hydrolyzable ester will remove water as it is produced, leading to the formation of longer chain polymers which result in a stronger, more durable polymer matrix which binds the sand grains together. Accordingly, our preferred diluent for the furfuryl alcohol oligomer is a hydrolyzable ester, and our especially preferred species is butyl acetate. Other preferred esters are methyl acetate, ethyl acetate and propyl acetate.

It is essential for this procedure that the acid catalyst utilized be oil soluble so that it may be incorporated in the resin solvent solution. This permits thorough mixing of the catalyst which is essential in order to ensure that the polymerization reaction occurs uniformly throughout the entire mass of well treating chemical placed in the formation. Prior art methods which utilize a catalyst injected in a non-miscible fluid either before or after injection of the fluid containing the polymerizable resin, or present in a non-miscible phase of an emulsion polymer fluid, do not accomplish uniform reactions such as are possible by use of the present soluble catalyst. The catalyst for use in our invention must also be one which exhibits temperature sensitivity such that the catalytic polymerization does not occur during the time that the fluid is prepared and mixed on the surface of the earth or pumped into the formation. It is equally important that once the fluid is placed in the formation and left in a quiescent state for a period of time sufficient to ensure temperature equalization with the formation, that the polymerization reaction occur rapidly in order to permit completion of the procedure in an relatively brief period of time, so the well can be put on production as soon as possible. Because of this dual requirement, the catalyst choice and concentration are both very critical to the proper function of our invention.

As stated above, the preferred catalyst for use in our process is one which is oil soluble and very slightly water soluble. While we have previously disclosed that the preferred organic acid catalyst is orthonitrobenzoic acid for processes being applied to relatively high temperature (e.g., greater than 350.degree. F.) formations, we have found that at temperatures less than 350.degree. F. and especially when the formation temperature is below 300.degree. F., orthonitrobenzoic acid is so weak and so insoluble that the time required for polymerization to proceed at least sufficiently far that no displacement of polymer from the sand grain occurs, is in the range of several days to a week or more. This long set time causes several problems. The polymerizable compound, e.g. the furfuryl alcohol, may be washed off the sand grains before polymerization proceeds far enough to render the polymer immobile, which greatly weakens the strength of the polymerized mass. Also, the total cost of a well treatment is greatly increased by the extended period which the well is shut in, which delays returning the well to production.

We have found that the desired set time of from 0.75 to 4.0 and preferably from 1-2 hours can be realized for any particular formation temperature in the range of 60.degree. F. to greater than 350.degree. F. and especially from 100 to 350.degree. F. if the pK of the acid catalyst and the concentration of the acid catalyst are carefully selected.

The pK of an organic acid is defined as the negative of the log of the ionization constant of the acid and is essentially an inverse scale measure of the strength of the acid, e.g. strong acids have lower pK values. The acid catalyst for this process is preferably an organic acid which is oil soluble and which has a pK in the range of 0.4 to >6.0. There is an especially preferred acid for our process for each of several temperature ranges. For example, if the temperature is from 60.degree. F. to 180.degree. F., the acid pK should be from 0.4 to 0.8 and the especially preferred catalyst is toluenesulfonic acid, usually p-toluenesulfonic acid, although mixed isomers may also be used. In this range, hydrochloric acid, nitric acid and sulfuric acid are also preferred acids. Mixtures of toluenesulfonic acid with the above may also be used. For convenience, a mixture comprising 95% toluenesulfonic acid with 5% xylenesulfonic acid has been used in the field because the mixture is liquid at field surface conditions and therefore easier to mix with the other fluids in preparing the treating fluid. This is a commercial product available under the trade name WITCAT TX ACID.RTM. from Witco. Other mixtures may also be used, to ensure that the melting point is below ambient temperature.

When the temperature of the portion of the formation being treated is from 180.degree. F. to 250.degree. F., the acid pK should be from 0.8 to 2.1 and the especially preferred acid is oxalic acid. Other preferred acids for this temperature range are iodic acid, maleic acid, dichloroacetic acid and trichloracetic acid.

When the temperature range is from 250.degree. F. to 350.degree. F., the pK should be 2.1 to 4.2 and the especially preferred acid is 0-nitrobenzoic acid. Other preferred acids are chloroacetic acid and phosphoric acid.

When the temperature exceeds 350.degree. F., the acid pK should be greater than 4.2 and the especially preferred acids are acetic acid, benzoic acid and adipic acid. This information is given in Table I below:

Once the acid has been selected, the acid concentration should be determined. The concentration of a particular acid to yield the desired 0.75-4.0 hour set time is solely determined by the formation temperature. It is essential in applying our process to a formation that the temperature of the formation be known or measured. The following table gives the relationship between toluenesulfonic acid catalyst and temperature to produce set time within the preferred 1-2 hour range.

As can be seen from Table II, it is possible to use toluenesulfonic acid over a broader temperature range than the 60.degree. F.-180.degree. F. range for which this acid is the preferred catalyst; however, it is preferred that a weaker acid be employed in the 180.degree. F. to 300.degree. F. range for optimum results and to reduce the sensitivity of the treating fluid to small errors in acid concentration.

Surprisingly, we have found that the above correlation holds for any mixture ratio of resin and ester, e.g. butyl acetate, over the volume ratio 90 to 10 to 40 to 60.

Table III below gives the concentrations of oxalic acid which results in reaction times in the desired range.

In Table IV, the concentration of o-nitrobenzoic acid which produces reaction times in the desired range are given:

In Table V below, the concentrations of benzoic acid to yield reaction times the desired range are given.

One preferred method for forming a particularly effective fluid for use in practicing the well treating process of our invention involves mixing an approximately 50-50 mixture of the resin in its commercial form, which is usually an emulsion, with butyl acetate, after which the acid such as toluenesulfonic acid catalyst is dissolved in this mixture of resin and ester.

The melting points of many of our preferred acids are above surface ambient temperatures. For example, the melting point of toluenesulfonic acid is 223.degree. F., so it is necessary to incorporate the acid in a suitable diluent, usually a low carbon alcohol such as methanol, to facilitate mixing it with the resin emulsion. From 2 to 5 percent methanol is usually adequate for this purpose. This procedure may be used when applying the fluids described above for sand consolidation, when the well treatment is being used to shut off undesired water flow, or when the fluids are injected to completely plug a well, such as when the well is being prepared for abandonment.

The quantity of the fluid comprising the resin, diluent and catalyst injected into the formation varies depending on the purpose to be served by the treatment, e.g. sand consolidation, fluid entry prevention or complete well plugging for abandonment. The volume also varies with the thickness and porosity of the formation to which the well treating process is to be applied, as well as the diameter of the well and the desired thickness of the treated zone in the formation. The thickness and porosity of the formation and the diameter of the well will always be known, and it is ordinarily satisfactory if depth of the penetration is in the range of from 6 to 12 inches from the well bore. As an example, if it is desired to treat a formation whose thickness is 18 feet and porosity is 35% to form a permeable barrier for sand control just outside the perforations of the wellbore which is 8 inches (0.67 ft.) thick, and the well being treated is 10 inches in diameter (radius=5 in.=0.42 ft), then the volume of fluid necessary is calculated according to the example below. ##EQU1##

About the same volume of salt water is used to create the permeable treated zone when the well treating method is being used for sand control.

About the same volume of resin-containing fluid will be required to treat an otherwise identical well to form a fluid impermeable barrier around the well for the purpose of reducing or preventing flow of fluid into the well. No salt water injection is used in this embodiment.

When a well is to be plugged completely as a step in preparation for abandonment, a greater volume of fluid will be required, since it is necessary in this embodiment to treat a portion of the permeable formation outside the well perforations as above, and also to fill all or a major portion of the inside of the well casing or tubing, or of an open hole. In this case, the volume of fluid required for the same well described above is the amount shown in the above example plus enough to fill the inside of the well for 18 feet. The amount required is: ##EQU2##

After the above described quantity of fluid comprising resin, catalyst and diluent are injected into the formation, a second step is needed when our process is being used to form a permeable barrier for sand control. The polymerizable resin must be displaced from the injection string to avoid the possibility that the resin might polymerize in the wellbore, also the fluid injected into the formation occupies essentially all of the void space of the formation, e.g. the volume other than the sand grains themselves in the portion of the formation contacted by the fluid. If this injected fluid polymerized without injecting any second fluid to displace a portion of the resin material from the void spaces of the formation, the resultant barrier would be strong and resistant to chemical attack but it would not be sufficiently permeable to permit flow of fluid through the formation into the wellbore. For this same post-flush fluid must be used to reestablish fluid permeability in the treated zone.

The polymerizable resin used to prepare the sand consolidation matrix is normally available commercially as a mixture containing about 5 percent water. Additional water is formed by the condensation polymerization reaction. The strength of the sand consolidating polymer matrix will be increased if at least a portion of this water is removed before the resin polymerizes. We have found that the desired objective of displacing resin from the injection string for developing permeability within the sand consolidated mass and dewatering the polymer-containing fluid is best accomplished by injecting brine or water containing an inorganic salt, preferably sodium chloride, into the string to displace the residual amount of resin fluid from the injection string, and also to pass through the portion of the formation occupied by the resin fluid. Injection of the brine develops permeability within the treated portion of the formation which ensures that after the resin has polymerized, the resultant barrier will be permeable to the flow of fluids. The salinity of water utilized in this procedure is quite important. The surface of the resin coated sand grains should be dewatered in order to aid in the polymerization reaction and also in order to produce a denser stronger matrix cementing the sand grains together. Fresh water or water containing up to 70 percent salt does not accomplish the drying action necessary to produce the desired strength in the permeable barrier. The desired results will only be achieved if the second fluid injected into the formation is at least 70% saturated with respect to the inorganic salt. It is preferably at least 80% saturated with salt. Our particular preferred embodiment uses essentially saturated brine, specifically water saturated with sodium chloride at the conditions of injection. By using at least 70% saturated brine, the desired development of permeability is achieved without displacing any of the resin from the sand grains and dehydration of the resin necessary for the polymerization reaction to occur in the time and to the extent desired for optimum polymerization is also realized. In practice, the brine usually is not totally effective at displacing set polymer from the inside of the injection string. It is usually desirable to drill out any residual polymer before placing the well back on production.

As a practical matter, the brine utilized will probably be water containing mainly sodium chloride because of the cost and availability of sodium chloride in the field. This is a particularly preferred brine for our purpose. We have discovered that potassium chloride does not work well in this application, and so the fluid injected into the formation after the polymerization fluid has been injected should not contain appreciable quantities of potassium chloride. The quantity of brine injected into the formation for sand consolidation should be sufficient to displace all of the residual resin fluid from the injection string, and also sufficient to pass through the resin saturated portion of the formation. It is generally sufficient if about the same volume of brine as the polymerization fluid is utilized, and the rate at which it is injected is not particularly critical for our purposes. As a practical matter, the brine usually is not totally effective at displacing set polymer from the inside of the injection string. It is usually desirable to drill out any residual polymer before placing the well back on production.

When our well treatment process is used to form an impermeable barrier for fluid entry prevention, e.g. to reduce flow of water into the well, little or no brine is used. Although enough brine could be injected to flush all of the resin from the well interior, this is difficult to control. We have found it preferable to leave the resin-containing fluid in the well until it has set, and then drill out the well to remove the set resin to a point at least as deep as the bottom perforation from which production is taken. In some cases, the well is drilled out for from 10 to 50 feet below the lowest production level to create a void (called a rat hole in the field) into which sand and other solids can fall and accumulate and thereby delay the time when it will become necessary to suspend oil production and bail out the well.

After the above steps of injecting the polymerization fluid and when applying the process for sand consolidation, the sodium chloride solution or brine are completed, the well should be shut in and left to stand for a period of from 1 to 4 and preferably at least 1 to 2 hours.

In application of either the sand consolidation, well plugging or water shut off embodiments of our invention, leaving the well shut in for more than 2 hours will have no adverse effect on the process, and indeed the strength of the polymerized resin may increase in this additional period. The set time as described herein only defines the minimum time in which polymerization of the resin will proceed to a sufficient level to prevent washing the polymer from the sand grains.

A series of experiments were performed under controlled laboratory conditions to determine the concentration of various preferred acids which produced a set time in the preferred 1.0-2.0 hour range over appropriate temperature ranges. The following Tables give the observed results.

For the purpose of complete disclosure, including what is now believed to be as the best mode for applying the process of our invention, the following pilot field example is supplied.

A producing well is completed in a subterranean petroleum containing formation, the formation being from 8560 to 8588 feet. Considerable sand production has been experienced in other wells completed in this formation in the past, and so it is contemplated that some treatment must be applied in order to permit oil production from this formation without experiencing the various problems of unconsolidated sand production. This particular well has not yet been used for oil production, and so little sand has been produced from the formation. It is known that the sand is coated with formation crude, but is otherwise of a reasonable particle size to accommodate sand consolidation process using the natural sand present in the formation. It is decided therefore to inject the treating fluid of our invention into the formation immediately adjacent to the perforation of the producing well in order to bind the naturally occurring sand grains together and form a stable mass which forms a permeable barrier to restrain the flow of formation sand into the well while still permitting the free flow of formation fluids including petroleum through the barrier. It is determined that it is sufficient to treat approximately 12 inches (1 foot) into the formation. Based on experience in this field, it is expected that the porosity of the formation to be treated is approximately 40%. The outside casing diameter of the well being treated is ten inches (radius=5.0 in. or <0.417 ft.). The volume of fluid necessary to treat this portion of formation is determined as follows:

In order to accomplish adequate saturation of the portion of the unconsolidated sand formation adjacent to the production well, a total of 490 gallons of resin treating fluid is required. The resin employed in this procedure is "QUACORR 1300" obtained from QO Chemicals, which is an oligomer of furfuryl alcohol. The 490 gallons of sand consolidation treating fluid is formulated by mixing 245 gallons of the above-described resin with 245 gallons of butylacetate. Since the formation temperature is known to be 200.degree. F., the desired concentration of toluenesulfonic acid is 1.0%. This requires 40 pounds of toluenesulfonic acid. In order to facilitate use of toluenesulfonic acid in this application, since the surface ambient temperature is 85.degree. F., a mixture comprising 40 pounds toluenesulfonic acid and 4.9 gallons of methanol is prepared and then added to the resin-ester mixture. This fluid is injected into the formation at a rate of about 900 gallons per hour. After all of the treating fluid has been injected into the formation, 500 gallons of saturated sodium chloride brine is formulated and injected into the well at the same rate to displace the treating fluid out of the injection string and to force brine through the portion of the formation into which the treating fluid has been injected, displacing a portion of the treating fluid from the void spaces in the formation thereby forming flow channels in the resin zone. This ensures that the residual permeable barrier will exhibit sufficient permeability to permit production of fluids from the well. The well is shut in and is left for a period of 2 hours, which is adequate for this particular formation temperature. At the conclusion of this shut-in soak period, the well is drilled out to remove residual resin from inside the well, and to a point 30 feet below the bottom of the perforations to oreate a rat hole where produced sand can settle. The well is then placed on production and essentially sand-free oil production is obtained.

A producing well is completed in a subterranean petroleum containing formation, the petroleum formation being from 8540 to 8588 feet. This zone is treated as described above to control production of unconsolidated sand. Production of oil occurs without sand but excessive salt water is being produced from a twenty three foot thick zone located thirty feet above the oil production zone. It is decided therefore to inject treatment fluid into the water producing interval of the formation in order to form an impermeable barrier to restrain the flow of water into the well. It is determined that it is sufficient to treat approximately 12 inches (1 foot) into the formation. Based on experience in this field, it is expected that the porosity of the water producing interval formation to be treated is also approximately 40%. The outside casing diameter of the well being treated is ten inches (radius=5 inches or 0.417 ft.). The volume of fluid necessary to treat this portion of formation is determined as follows: ##EQU3##

In order to accomplish adequate saturation of the portion of the unconsolidated sand formation adjacent to the production well, a total of 397 gallons of resin treating fluid is required. The resin employed in this procedure is the same "QUACORR 1300" described above obtained from QO Chemicals, which is an oligomer of furfuryl alcohol. The 397 gallons of treating fluid is formulated by mixing 278 gallons of the above-described resin with 119 gallons of butylacetate. Since the formation temperature is known to be 200.degree. F., the desired concentration of toluenesulfonic acid is 1.0%. This requires 33 pounds of toluenesulfonic acid. In order to facilitate use of toluenesulfonic acid in this application, since the surface ambient temperature is 85.degree. F., a mixture comprising 33 pounds toluenesulfonic acid and 4 gallons of methanol is prepared and then added to the resin-ester mixture. A retrievable plug is set in the well at a point just below the formation being treated. This fluid is injected into the formation at a rate of about 900 gallons per hour. After all of the treating fluid has been injected into the formation, the well is shut in and is left for a period of 2 hours, which is adequate for this particular formation temperature. The interior of the well is drilled out to remove the hardened resin, and the retrievable plug is removed. At the conclusion of this treatment, the well is placed on production and essentially no water production is obtained from the treated zone.

If the above well is to be abandoned, a treatment similar to Example II is employed, except the volume of treating fluid is increased so the well interior is filled from the bottom to the point above the top of the upper most perforations, and allowed to solidify. The residual resin in the well is not drilled out in this embodiment.

The additional volume required to fill the bottom 200 ft. of this well is ##EQU4## This is in addition to the 397 gallons required to seal the formation outside the perforations.

Although our invention has been described in terms of a series of specific preferred embodiments and illustrative examples which applicants believe to include the best mode for applying their invention known to them at the time of this application, it will be recognized to those skilled in the art that various modifications may be made to the composition and methods described herein without departing from the true spirit and scope of our invention which is defined more precisely in the claims appended hereinafter below.