US3123158A - sealing porous sukfaces - Google Patents

Description

United States Patent T he present invention relates to methods for preventing fluid flow through porous media and more particularly relates to a process for coating porous surfaces on which polyvalcnt cations are present in order to render them impermeable. In a preferred embodiment, the invention relates to an improved core drilling process during which the core is continuously coated with a film impervious to water, oil and gas in order to prevent the escape of fluids originally present in the formation.

Studies of cores brought to the earths surface as a result of core drilling operations yield much useful information concerning subsurface strata and the fluids contained in such strata. Porosity, permeability and similar formation characteristics otherwise dificult to evaluate can be measured directly. Oil, gas and water recovered from the cores can be measured and analyzed. The information thus obtained provides a basis for predicting the productive life of oil and gas reservoirs, facilitates the selection of primary recovery techniques most suitable for particular reservoirs, and later permits the susceptibility of such reservoirs to various secondary and tertiary recovery processes to be assessed. Experience has shown, however, that the results of such studies are not wholly reliable from the standpoint of the amount of fluids present in the reservoir and generally must be adjusted to compensate for fluid losses. These losses are primarily attributable to the methods used to cut and recover cores from subterranean formations.

Conventional core drilling operations are carried out with an annular bit and core barrel rotated from the earths surface by means of a rotary drill string. A coring fluid is circulated downwardly through passages in the drill string, barrel and bit in order to maintain pressure on the formation. Cuttings produced by the bit are entrained in the fluid and returned to the surface through the annulus surrounding the drill string. As the bit cuts away the formation, the central core left by the bit is encased in the barrel. The barrel is generally provided with means for breaking off the core after it has been filled and with closures which protect the core against contarm'nation by fluids present in the borehole. Pressure core barrels which can be sealed against changes in pressure are also used. After the core has been cut, the drill string is Withdrawn from the hole and the core is recovered.

Studies have shown that the pressure maintained at the bottom of the borehole during a coring operation has a profound effect upon the fluids content of cores subsequently recovered. If this pressure is less than the formation pressure, fluids contained in the formation will tend to flow out of the core into the borehole until equilibrium is established. If, on the other hand, the bottomhole pressure exceeds the formation pressure, the coring fluid will tend to flow into the interstices of the formation and displace any oil, gas or Water contained therein. In either case, the result is a change in the fluids content of the core such that subsequent measurement of the amount of fluids present will not accurately reflect the amount originally present in the formation. Since this change occurs continuously as the core is cut, the use of a pressure core barrel does not prevent it.

Several methods for avoiding the difliculty outlined above have been proposed in the past. The most obvious of these involves carrying out the coring operation with olefinically-unsaturated monomers.

no pressure differential between the coring fluid and the formation. This is impractical because the formation pressure cannot be conveniently measured during core drilling and, even if it could, because the fluid pressure cannot be controlled with the necessary precision. The use of coring fluids which will not invade the formation under pressures well in excess of the formation pressure has been suggested but efforts to develop a satisfactory fluid have been unsuccessful. Mercury, plastics, molten metals and other materials advocated in the past all invade the formation to an appreciable extent and hence lead to changes in fluids content. In addition, the materials proposed have generally been costly and difficult to use and in many cases required highly specialized coring bits and core barrels. Other systems, including the use of tracers to permit determination of the extent to which core invasion occurs, have been proposed but have not been found effective.

The present invention provides an improved core drilling system which obviates the difliculties outlined above and permits the recovery of core samples having fluid contents much closer those of the cored formations than has been possible heretofore. In accordance with the invention, it has now been found that certain unstable polymeric latices can be used to form an impermeable film on the surface of cores as they are cut, thus sealing connate fluids within the cores and preventing their es cape. The latices do not invade the cores to a significant extent and hence do not displace fluids therefrom in significant quantities. On recovery of the sealed cores, the film can readily be removed from the core surface. Little change in core permeability due to deposition and subsequent removal of the film occurs.

In addition to being useful as coring fluids, the latices may be employed in accordance with the invention for a number of other applications. They may be used as lost circulation fluids for sealing the walls of boreholes to prevent subsequent loss of drilling fluid to porous strata. They may be used to coat strata in order to prevent their breakdown due to hydration. They may be employed prior to well cementing operations to prevent the cement from penetrating into porous Zones. They are useful as fracturing fluids in situations where low fluid losses and non-permanent plugging are desired. They may be used to line mud pits, storage caverns and similar cavities in order to improve their ability to retain fluids. They may be used as Water-shutoff agents during air drilling operations. Other uses will readily suggest themselves to those skilled in the art.

The polymeric latices utilized for purposes of the invention are emulsions of oil-resistant elastomers which readily coagulate in the presence of polyvalent cations. When contacted with porous surfaces upon which polyvalent cations are present, such latices form films which are impervious to oil, gas, Water and other fluids. Suitable latices may be derived from natural rubber or from synthetic elastomers prepared by the polymerization of unsaturated monomers. Studies have indicated that the chemical compositions of the elastomers contained in the latices generally have little effect upon their suitability for purposes of the invention and that latices containing a wide variety of synthetic elastomers may be successfully employed.

Latices suitable for purposes of the invention may be derived from synthetic elastomers prepared by the polymerization of olefinically-unsaturated hydrocarbons or by the copolymerization of such hydrocarbons with other The olefinically-unsaturated hydrocarbons utilized may be olefins such as isobutylene and the pentylenes; diolefins such as butadiene, isoprene, piperylene, dimethyl butadiene and 2- methyl pentadiene; or vinyl aromatics such as styrene, methyl styrene and vinyl toluene. Mixtures of such hydrocarbons may also be used. Olefinically-unsaturated monomers which may be copolymerized with the hydrocarbons include halogenated olefinically-unsaturated compounds such as vinyl chloride, allyl chloride and chloroprene; unsaturated esters such as vinyl acetate, allyl propionate, methy methacrylate, ethyl acrylate, methyl fumarate, ethyl maleate and propyl itaconate; unsaturated nitriles such as acrylonitrile, methacrylonitrile, ethyl acrylonitrile and chloroacrylonitrile; unsaturated ketones such as methyl vinyl ketone; cyclic vinyl compounds such as vinyl pyridine; and mixtures thereof.

Specific examples of elastomers prepared from the foregoing monomers suitable in the form of latices for purposes of the invention include polyisobutylene, polystyrene, polybutadiene, polyisoprene, butadiene-isoprene copolymers, isoprene-isobutylene copolymers, isobutylenestyrene copolymers, piperylene-vinyl acetate copolymers, butadiene-styrene-vinyl chloride copolymers, butadieneacrylonitrile copolymers, butadiene-methacrylonitrile copolymers, and isoprene-chloroprene-vinyl acetate copolymers.

Latices containing the foregoing elastomers may be prepared by the emulsion polymerization of suitable monomers or by the emulsification of organic solvent solutions of dry elastomers with water or other liquid, followed by removal of the solvent. The method utilized will depend primarily upon the elastomer used. Many conjugated diolefin polymers and copolymers of conjugated diolefins with monomers containing a vinylidene linkage, polybutadiene and copolymers of 1,3-butadiene with styrene, acrylonitrile or vinyl chloride for example, can readily be prepared by emulsion polymerization and can be recovered in latex form. Other elastomers, styrene-isobutylene and isobutylene-isoprene copolymers for example, are best prepared by bulk or solution polymerization processes which do not result in the formation of latices. Elastomers prepared in the latter manner must subsequently be emulsified with the aid of a solvent to produce latices. Processes for preparing latices by both methods are widely described in the chemical and patent literature. A typical emulsion polymerization process is described in US. Patent 2,460,038, issued to George E. Serniuk on January 25, 1949. A description of one method for preparing latices from dry elastomers utilizing an organic solvent may be found in US. Patent 2,799,662, issued to John L. Ernst et al. on July 16, 1957.

Latices consisting of aqueous emulsions of suitable elastomers are normally employed in the practice of the invention but in some cases emulsions in which the con tinuous phase is a liquid other than water may be preferred. One instance of this occurs in coring operations carried out in very high temperature strata where a liquid having a boiling point above that of water must be used. Another instance occurs in the case of arctic operations where an aqueous emulsion would quickly freeze during storage. Any of a number of chemically nonreactive liquids which have the proper temperature characteristics and do not act as solvents for the elastomers employed may be used. Normal decane, for example, will be suitable for use with certain elastomers. Nonaqueous latices are available commercially and methods for their preparation are known to those skilled in the art.

The latices employed in accordance with the present invention are preferably homopolymers prepared by the emulsion polymerization of a conjugated diolefin containing from 4 to 6 carbon atoms per molecule or copolymers obtained by the copoly-merization of such a diolefin with one or more olefinically-unsaturated monomers containing from 4 to 8 carbon atoms per molecule. Preferred latices include polybutadiene, polyisoprene, butadiene-styrene copolymer, butadiene-acrylonitrile copolymer, butadienemethacrylonitrile copolyrner, butadiene-vinyl acetate copolymer and but-adiene-isoprene copolymer latices. Butadiene-acrylonitrile copolyrner latices are particularly preferrcd.

The latices useful for purposes of the invention are characterized by the fact that they are rapidly coagulated upon contact with calcium, magnesium and other polyvalent cations found in subterranean formations. They thus differ from latices employed in the past as additives for improving the viscosit gel strength and fluid loss characteristics of drilling muds. Conventional drilling muds generally contain clays, barytes, gypsum, lime and similar materials in relatively high concentrations and hence are rich in polyvalent cations. Latices used as additives in this manner must therefore be highly stable in the presence of polyvalent cations in order to avoid coagulation and loss of the improved properties which they are intended to confer. Such latices are generally stabilized by the addition of stabilizing agents after they are formed. They do not coagulate during use. The invention does not contemplate the use of stable latices as additives to fluids containing polyvalent cations and instead is predicated upon the use of unstable latices substantially unadulterated by the presence of other materials. These unstable latices do not come into contact with appreciable quantities of polyvalent cations until they contact subsurface strata. Polyvalent cations are always present on the surfaces of such strata, although sometimes in low concentrations, and hence the latices coagulate on contact and form impermeable films. The rapid film formation which occurs restricts the coagulation to that part of the latex in contact with the strata and prevents the entire latex stream from setting up as a solid.

Although the latices employed in accordance with the invention are relatively unstable and quickly coagulate in the presence of polyvalent cations in very low concentrations, they should nevertheless have sufficient shear stability to permit them to be pumped without fear of coagulation. This requires that the stability characteristics of the latices be carefully controlled. Latex stability depends upon a number of factors, including the amount of emulsifier, dispersing agent or stabilizer used in preparing the latex, the pH of the latex product, and the particle size of the dispersed elastomer in the latex. All of these factors are somewhat interrelated and may be Varied widely depending upon the particular emulsifier used, the elastomer employed, and the conditions under which the latex is prepared. it is therefore impractical to attempt to prescribe specific properties which characterize latices suitable for purposes of the invention. The suitability of any particular latex can readily be determined, however, by

simply contacting it with a surface coate with a very weak solution of polyvalent cations and by circulating it through a laboratory pump to simulate the shear stresses encoun tered in a well circulation system, observing in each case whether coagulation of the latex occurs.

In most cases it is preferred that the latices employed in accordance with the invention have pH values between 8.5 and about 10, although as pointed out above latices of equal stability prepared with different elastomers and different emulsifiers may vary in pH over a considerable range. Studies have indicated that many latices having pH values below about 8.5 have insufficient shear stability to permit their circulation through pumps and that those whose pH exceeds about 10 are generally too stable to coagulate readily in the presence of very dilute polyvalent cation solutions.

Typical latices found suitable for purposes of the invention have average elastomer particles ranging between about 0.05 and about 0.1 micron in diameter. Latices having average particle sizes outside this range may in some cases be used, since a highly eflicient emulsifier may result in a latex of the proper stability containing considerably larger particles; while on the other hand a poor emulsifier may necessitate smaller elastomer particles. The 0.05 to 0.1 micron range is, however, generally to be preferred.

Laboratory studies have shown that the solids contents of the latices useful for purposes of the invention are not highly critical but that dilution with water tends to decrease their shear stability and increase their resistance to coagulation in the presence of polyvalent cations. In some cases this provides a means for controlling the stability properties of the latices. Latices containing from about 5 to about 70% solids are generally suitable for purposes of the invention, those containing from about 20 to about 50 weight percent solids being preferred.

The process of the invention may be carried out with conventional apparatus familiar to those skilled in the art. A variety of commercially available core bits and core barrels may be used in core drilling operations wherein the unstable latices are employed to seal cores and prevent the loss of fluids contained therein. Diamond core bits and pressure core barrels are particularly attractive for use in such operations. In applications of the process aside from coring operations, conventional equipment similarly available may be utilized. A wide range of tools for contacting fluids with the walls of boreholes and with other porous surfaces are described in the literature and in catalogs available from commercial suppliers and manufacturers. Further description of the apparatus employed in the practice of the invention is therefore unnecessary.

The nature and objects of the invention can be more fully understood by referring to the results of experimental work carried out to test and demonstrate the use of unstable latices as coring fluids.

The apparatus utilized in carrying out the experimental work closely resembled that employed in full scale core drilling operations. A 6 /2 inch diameter core bit and a core barrel of conventional design were attached to a length of drill pipe and rotated with respect to a block of porous rock measuring 12" x 12" x 24" by rotating the pipe. The block, bit and barrel were encased in a pressure tight chamber in order to permit the simulation of high formation pressures. Fluid was circulated through the drill pipe, barrel and bit from a fluid reservoir by means of a high pressure pump. Fluid containing cuttings was withdrawn through the annulus surrounding the drill string, cuttings were removed, and the fluid was returned to the reservoir. Facilities for measuring the fluids contents of cores recovered from the core barrel were provided. This particular method and apparatus was selected because earlier studies had shown that the results obtained were comparable to actual field tests.

In a first series of experiments, a block of Berea sandstone was prepared by first completely dehydrating the block, hermetically sealing its surfaces and applying a high vacuum. A synthetic brine closely resembling a typical formation Water was forced into the sealed and evacuated block. Oil was then forced in under pressure. The volumes of brine and oil injected into the block and the volume of brine displaced by the oil were precisely measured. The saturated block contained 65% oil and 35% brine. The oil used was a 12 centipoise white oil. The brine employed contained 2,500 p.p.m. of calcium chloride, 1,000 p.p.m. of magnesium chloride, and 26,000 p.p.m. of sodium chloride and thus had an average polyvalent cations content. A core about 17 inches long and 2 /2 inches in diameter was cut from the block using the equipment described above and a conventional bentonite drilling mud as the coring fluid. The mud was circulated through the system at a rate of 97 gallons per minute. Mud pressure at the bit was maintained at 35 pounds per square inch, 30 pounds per square inch in excess of the pressure Within the block. Upon recovery of the core, it was found that the bentonite mud had invaded it and had largely displaced the oil and water originally present therein. Displacement of volume percent of the connate fluids is considered to be the maximum displacement that can be tolerated if analysis of the fluids recovered from cores is to be significant. Screening tests showed that considerably more than 20 volume percent of the fluids originally present had been displaced by the bentonite mud and therefore further measurements of the displacement were not made. The results thus obtained are typical of field core drilling operations, where 50 volume percent or more of the oil and water contained in the cored strata is generally displaced by the coring fluid.

Additional tests using bentonite drilling muds at pressure difierentials ranging from about 40 to about pounds per square inch were made and in every case the cores recovered had been invaded by the mud so that more than 20 volume percent of the fluids originally present therein were displaced.

Following the tests using bentonite drilling muds as the coring fluid, a test was run in which an unstable latex was employed. The latex used was one prepared by the emulsion polymerization of 65 Wt. percent butadiene and 35 wt. percent of acrylonitrile in the presence of a persulfate catalyst and about 8 wt. percent of a potassium rosin soap emulsifier. The latex contained about 8.0 Wt. percent of an antioxidant but had not been chemically stabilized. It had a 40 wt. percent solids content, an average elastomer particle size of 0.07 micron, and a pH of about 9.5. The surface tension was 50 dynes per centimeter at 25 C. This latex was employed as the coring fluid during the cutting of a core 17 inches long and 2 /2 inches in diameter from a block of Berea sand stone identical to that used in the previous tests. The sandstone was saturated with 65 vol. percent oil and 35 vol. percent brine as in the earlier tests. The coring fluid pressure at the bit exceeded the simulated formation pressure by 50 pounds per square inch. A fluid circulation rate of 13 gallons per minute was used. Analysis of the recovered core showed that the latex had formed a film impermeable to oil, gas and water on the core surfam and that no appreciable invasion of the core occurred. Only 4.6 vol. percent of the fluids initiallly contained in the cored sandstone were lost from the core. The film formed was easily removed from the recovered core. Tests showed no change in the permeability of the sandstone to water due to formation and subsequent removal of the film. No difficulties due to premature coagulation of the latex Were encountered.

The above results demonstrate that the use of unstable latices as coring fluids permits the recovery of cores substantially unaltered with respect to their fluids content. Cores containing fluids which are representative, both qualitatively and quantitatively, of those present in the formations from which the cores are taken can readily be obtained by sealing the cores with such latices as they are cut. it will be apparent to those skilled in the art that the properties which render the latices useful as coring fluids make them equally attractive for use in a variety of other operations wherein it is desired to prevent the flow of fluids into or out of porous rock and similar earthen formations in which polyvalent cations are present. The latices may also be employed for sealing other porous surfaces normally free of polyvalent cations by pretreating such surfaces with a solution containing polyvalent cations.

What is claimed is:

l. A process for recovering a core from subterranean strata on which polyvalent cations are present which comprises cutting the core from said strata with an annular drill bit connected to the lower end of a drill string in a borehole, injecting an ion-sensitive latex into said drill string at the earths surface, continuously depositing a polymeric film on the surfaces of said core as said surfaces are exposed by said bit by discharging said latex beneath said bit in contact with said strata, and thereafter recovering said core from said borehole with the connate fluids retained therein by said polymeric film.

2. A process as defined by claim 1 wherein said latex is an emulsion containing a butadiene-acrylonitrile c0 polymer.

3. A. process as defined by claim 1 wherein said latex contains natural rubber.

4. A process as defined by claim 1 wherein said latex is an aqueous emulsion containing from about 5 to about 70 wt. percent of an elastomer prepared by the copolymerization of a conjugated diolefin and an olefinically unsaturated monomer.

5. A process as defined by claim 1 wherein said latex is an aqueous emulsion of a conjugated diolefin-unsaturated nitrile copolymer having a pH between about 8.5 and about 10.

References Cited in the file of this patent UNITED STATES PATENTS Irons June 21, 1938 Johnston Feb 7, 1939 Thomas Feb. 16, 1943 Golden Mar. 3, 1953 Menaul Feb. 23, 1954 Davis et a1 July 8, 1958 Booth et a1 Sept. 30, 1958 Williams Apr. 7, 1959 Harrison Dec. 1, 1959 Loofbourow Aug. 2, 1960

Claims (1)

1. A PROCESS FOR RECOVERING A CORE FROM SUBTERRANEAN STRATA ON WHICH POLYVALENT CATIONS ARE PRESENT WHICH COMPRISES CUTTING THE CORE FROM SAID STRATA WITH AN ANNULAR DRILL BIT CONNECTED TO THE LOWER END OF A DRILL STRING IN A BOREHOLE, INJECTING AN ION-SENSITIVE LATEX INTO SAID DRILL STRING AT THE EARTH''S SURFACE, CONTINUOUSLY DEPOSITING A POLYMERIC FILM ON THE SURFACES OF SAID CORE AS SAID SURFACES ARE EXPOSED BY SAID BIT BY DISCHAGING SAID LATEX BENEATH SAID BIT IN CONTACT WITH SAID STRATA, AND THEREAFTER RECOVERING SAID CORE FROM SAID BOREHOLE WITH THE CONNATE FLUIDS RETAINED THEREIN BY SAID POLYMERIC FILM.