US2672935A - Method of treating wells - Google Patents

Description

Patented Mar. 23, 1954 METHOD OF TREATING WELLS Frank H. Braunlich, Jr., and Paul H. Cardwell, Tulsa, Okla., assignors to The Dow Chemical Company, Midland, Mich, a corporation of Delaware No Drawing. Application July 2, 1952, Serial No. 296,952

11 Claims.

I The invention relates to methods of treating wells formed in the earth. It mor particularly .concerns an improved method of treating oil and gas wells so as to facilitate their production.

It has long been known that the flow of fiuid, such as oil and gas, to a well drilled into a producing stratum of the earth may be facilitated by providing either additional flow passageways in the stratum or enlarging the existing ones by which the produced fluid reaches the well.

One of the most widely used methods of facilitating the flow of a fluid from a producing stratum of a well'comprises the injection through the well into the stratum of a suitable solvent which tends to form new flow channels to the well hole. The solvent most generally used is an aqueous solution containing hydrochloric acid.

In the use of such solvents in the treatment of wells, it is generally understood that to get the best results from the solvent it should be injected deeply into the stratum from which production is sought. Yet, injection into the earth is not always easy and special pumps providing considerable pressure are oftentimes used to force the treating solution to enter the earth formation. In fact pressures as high as those required to lift the overburden of earth and to break and crack the earth layers oftentimes are used. Operations of this kind are described in an article by Grebe and Stoesser, published in the August 1935 issue of World Petroleum, at page 481, in which it is disclosed that pumps were developed capable of producing suflicient pressure to force the acid into newly made cracks, while the earth is compressed and even lifted. Wells are often completely dry, but by application of acid under high pressure pores are developed which lead to reservoirs and more porous portions of the formation.

Although well treating pressures high enough to fracture and lift the earth formation are oftentimes used in convetnional treatments, earth formations are encountered which take the treating solution too easily to permit high treating pressures to be reached in the bore hole without xtreme rates of injection. As a consequence, cracking or fracturing of the formation to be treated and the lifting of the overburden by the treating fluid is not assured in such cases without high rates of injection. It is apparent that when the treating fluid enters the earth readily or under a relatively low pressure the earth formations are already relatively pervious to fluid at the zones of treatment and but little, if any, beneficial effect on production can :be expected beyond that resulting from mere pore enlargement or cleaning ofexisting flow channels. It is the usual practice to attempt to control the penetration of the solvent into the earth so that the relatively fluid impermeable portions are treated rather than those which are already quite fluid permeable. In order to accomplish this relatively high treating pressures are required and to prevent excessive penetration of the treating agent into the more permeable zones While high pressure is exerted on the treating agent, a pore-plugging agent is used with the treating agent. The pore-plugging agent soused flows into and blocks the pores of the more pervious zones into which the treating agent tends at first to flow the most readily. Thereafter, as the more permeable portions become more or less sealed by the pore-plugging agent, the treating pressure may be desirably raised, thereby forcing the treating agent to find ingress into the tighter and less permeable portions of the formation exposed in the well hole to the treating agent, as by breaking down or fracturing the formation.

In applying the foregoing procedure, certain difficulties arise which have not heretofore been adequately overcome. One of these difficulties is that the sealing agent tends to remain in the pores of the formation, permanently plugging them after the treating pressure is released, unless the plugging material used is dislodged by earth fluids seeking their way to the well. To ensure pore-plugging during treatment followed by dislodgement when the treatment pressure is released, it has been proposed heretofore to use as the pore-plugging agent an organic jellifying material mixed with suitable bacteria which cause the jellifying material to lose its pore-plugging properties through liquefaction after a sufficient period of time has elapsed. Although a generally satisfactory temporary pore-plugging action is thereby obtained in many wells, conditions oftentimes are encountered under which an adequate temporary pore-plugging action cannot be ob-' tained. The conditions involve either relatively high earth temperatures which interfere with the bacterial action, or excessive porosity which permits the jellifying material to be squeezed out of the well hole too easily into the porous parts of the formation before sufiicient pressure can be exerted on the treating agent to force it into the less permeable portions, the treatment of which it is desired to ensure. Insofar as we are aware, there is no commercially available pore-plugging material which can be used effectively at extreme pressures and temperatures in deep wells. Accordingly, it is the principal object of the invention to provide an improved pore-plugging agent which overcomes the disadvantages of those heretofore available. Other objects and advantages will become apparent as the description of the invention proceeds.

We have discovered that by reacting together in aqueous solution a water-soluble magnesium salt and a water-soluble fluoride in the presence of strong mineral acid a jelly-like viscous mass is obtained which may be used as a pore-plugging agent that is free from the foregoing disadvantages. In addition, it possesses the property of losing its jelly-like viscous consistency by contact with the usual earth formations for a suiflcient length of time. Hence, the plugging action is but temporary. The jelly-like material also exhibits the property of thixotropy and thus injection into the well is facilitated. The material also has the ability to suspend particulated solids, e. g. sand particles, ceramic particles, metal particles and the like, and it is, therefore, possible to include such agents in the material in the event that it is desirable to deposit a fluid permeable mass of solid particles in the cracks or fractures which may be formed on applying injection pressures sufliciently great to crack and lift the overburden of earth. The invention then consists of the improved pore-plugging agent and method of treating wells therewith herein fully described and particularly pointed out in the claims.

In carrying out the invention, a soluble magnesium salt is employed which, in solution in Water, furnishes magnesium ions in sufflcient concentration. Examples of suitable magnesium salts are: magnesium chloride, magnesium bromide, magnesium acetate, magnesium nitrate, magnesium sulfate. A water-soluble fluoride capable of reacting in aqueous solution with the magnesium salt in solution, to produce a magnesium fluoride gel, is used. Examples of suitable fluorides are the alkali metal and ammonium fluorides and bifluorides.

The number of equivalent weights of soluble fluoride used should be from about 95 to 105 per cent of the number of equivalent weights of the magnesium salt used or preferably a mole of the fluoride salt for each mole of the magnesium salt. The formation of the desired magnesium fluoride gel takes place in water solution in the presence of mineral acid, in accordance with the relation: Mg+++2F-+acid MgFz+acid. The number of equivalent weights of acid used is from about 45 to 350 per cent of the number of equivalent weights of magnesium salt involved. The necessary acid may be provided by the addition of a mineral acid, such as hydrochloric acid, to either the solution containing the magnesium salt or the solution containing the fluoride salt or apportioned between both solutions. Alternatively, the desired acid is produced during the gel-forming reaction, without the need for supplying another acid constituent, when bifluorides are used as the source of fluoride ion. For example, the use of ammonium bifluoride is illustrative of this action and is expressed in the following equation:

As indicated by the equation, the one equivalent weight of acid is liberated in the gel-forming reaction for each mole of magnesium salt used. If desired, additional HCl, beyond that liberated when bifluorides are used, may be present, such as an amount up to about 250 per cent of the number of moles of magnesium salt so that the total number of equivalent weights of acid involved may be as high as 350 per cent of the number of equivalent weights of the magnesium salt.

In preparing the gel, the solutions to be reacted, that is the solution containing the mag nesium salt, the solution containing the fluoride salt, and additional acid if used, are mixed together just before use. The amount of water used should be sufiicient to dissolve the soluble magnesium salt and fluoride salt used, the total amount in moles being from about 12 to 121 times the number of equivalent weights of the magnesium salt. The mixing of the solutions may be carried out in any convenient manner as by pouring one solution into the other and subjecting the resulting mixture to stirring until the two solutions are intimately mixed. In a few minutes, the mixture becomes viscous as a result of the interaction of the magnesium ions with the fluoride ions in the presence of the required acid. The viscous mixture changes in a few minutes of quiescent standing into a jelly-like viscous material, herein referred to as a gel, which owes its jelly-like consistency to the presence, in the acidcontaining aqueous reaction mixture, of hydrated magnesium fluoride. The jelly-dike properties of the gel are not significantly changed on heating up to about 275 F. but on neutralizing the acid therein the jelly-like properties are destroyed and the gel thins to a consistency like that of the salt solutions from which the gel is prepared. The gel may be caused to revert to the non-gelled condition by subjecting the gel to agitation as by stirring the gel or pumping it through a pipe in which agitation occurs during flow. The property possessed by the gel of reverting to a non-gelled condition on being subjected to agitation and of jelling again on being held quiescent, a property hereinbefore referred to as thixotropy, is highly advantageous in its use in well treatment.

By virtue of the property of thixotropy, the agitated gel may be introduced into the bore of a well through the usual tubing or casing as a thin fluid having a viscosity of not over about cps. and on leaving the thin fluid in the well in a quiescent state it sets to a jelly-like mass which strongly resists displacement into the adjacent earth. Pressures high enough to lift the overburden can be attained in the well hole containing the gel with attendant advantages of forming new channels or cracks in the adjacent formation. The earth formation generally contains or comprises calcareous matter, such as calcium carbonate and magnesium carbonate in various forms, e. g. limestone, dolomite, and these materials neutralize more or less of the acidin the gel while it is in the earth formation. As a result, the gel after a while liquefies or thins in situ to a viscosity of 40 to 60 centipoises and does not form a permanent block in the interstice of the earth.

As already mentioned, particulated solids, e. g. sand particles, which in the aggregate form of a fluid permeable mass, may be carried by the gel into the channels and cracks on applying high pressure to the gel in the well hole as above mentioned. When the gel liquefies, the particles permeable mass, thereby facilitating the flow of, earth fluids to the well through the formed cracks or channels. i

TABLE Gel Components in mole percent Viscosity of 1 Agitated gel M onemo NH FHF H2O Hon cenflpmses *Exclusive of that liberated in the gel-formation reaction.

It is manifest from the foregoing table that the gel, even in the fluid state induced by agitation, possesses a high viscosity which renders it effective to hold in suspension particulated solids, such as sand.

The following example is illustrative of various embodiments of the invention:

Example A well 8000 feet deep is eased, the casing being perforated from 7500 feet to 7525 feet. The formation opposite the perforations is mainly sandstone having a solubility of about 10 per cent by weight in hydrochloric acid. The temperature of the well at the bottom is 225 F. A string of tubing carrying a packer is run into the well and the well filled with oil. The packer is located at 7490 feet and the lower end of the tubing at 7500 feet. In one tank, 3150 pounds of MgCl2.6HzO are dissolved in 247 gallons of water to make about 500 gallons of magnesium chloride solution. In another tank, 900 pounds of NH4F.HF are dissolved in 424 gallons of wa ter to make about 500: gallons of ammonium bifluoride solution. At the well, the two tanks of solution are mixed together in a third tank for about 20 minutes and then 1000 pounds of sand are mixed with the resulting gel. The resulting gel-sand mixture is then pumped into the well tubing followed by oil. As the gel-sand mixture flows downwardly into the well, a corresponding volume of oil is displaced and allowed to flow from the casing. When the displaced oil volume indicates that the gel-sand mixture has reached within 100 feet of the bottom of the tubing the packer is set. The injection of follower oil into the tubing string is continued while escape of fluid up the casing is prevented by the packer. The pressure of the oil in the tubing increases, and pressure is also maintained on the casing so as to assist the packer in holding. The introduction of follower oil into the tubing is continued until the volume introduced indicates that the gel-sand mixture has reached the perforations. The introduction of follower oil then is halted for 20 minutes to allow the gel-sand mixture to set to a jellied mass or become stiffened. After allowing the gell to stiffen, the introduction of follower oil is resumed under sufiicient pressure to force the jelled gel-sand mixture through the perforations into the adjacent earth formation. The injection pressure required rises until the earth cracks or fractures allowing the gel-sand mixture to enter the earth. The introduction of follower oil into the tubing is continued until it is calculated that the entire volume of the gel-sand mixture has been injected into the earth. After allowing the gel to become thinned by contact with the earth formation, the pressure on the tubing is released and the well is put back on production.

Among the advantages of the invention are,

that the gel produces an effective barrier to the passage of fluid into porous earth except under a pressure great enough to crack or fracture the earth; the gel does not permanently plug earth pores as it liquefies or thins in time as the alkaline constituents of the earth neutralize more or less of the acid in the gel; the gel is thixotropic so that it is easily introduced into a well in its fluid state with conventional equipment, yet stiifens or gels on quiescent standing, as in the well hole or bore, when pore-plugging action is desired; the thixotropic property of the gel makes it hold solid particles, such as sand, in suspension with but slight, if any, tendency to settle even at temperatures as high as 400 F.

We claim:

1. The method of treating an earth formation ,penetrated by the bore of a well which comprises introducing into the well an aqueous thixotropic composition comprising magnesium fluoride formed by reacting together an aqueous solution containing a water-soluble magnesium salt and an aqueous solution containing a water-soluble fluoride in the presence of a strong mineral acid, the number of equivalent weights of the fluoride salt being from to per cent of the number of equivalent weights of the magnesium salt and the number of equivalent weights of acid being from 4.5 to 350 per cent of the number of equivalent weights of the magnesium salt, the number of moles of water in the magnesium salt solution plus the number of moles of Water in the fluoride salt solution being between about 12 and 121 .times the number of equivalent weights of the magnesium salt.

2. The method of treating an earth formation penetrated by a well bore which comprises introducing into the well bore a composition comprising an aqueous solution containing a watersoluble magnesium salt and an aqueous solution containing a water-soluble fluoride in the presence of a strong mineral acid, the number of equivalent weights of the fluoride salt being from 95 to 105 per cent of the number of equivalent weights of the magnesium salt and the number of equivalent weights of acid being from 45 to 350 per cent of the number of equivalent weights of the magnesium salt, the number of moles of Water in the magnesium salt solution plus the number of moles of water in the fluoride salt solution being between about 12 and 121 times the number of equivalent weights of the magnesium salt, said composition having a volume sufficient to fill the well bore to a depth above that of the earth formation to be treated, allowing the said composition to remain in the well bore for a, time sufficient to induce jelling of the composition, and following the said composition with a fluid under pressure so as to force the composition after jelling into the earth formation.

3. The method according to claim 1 in which a particulated solid is added to the thixotropic swam composition in: amount up to 4 pounds per thereof.

t. The method according to claim 2 which a particulated' solid is added to the composition in amount up to 4 pounds per gallon thereof.

5. The method of treating an earth formation penetrated-F by the bore of a we'll which comprises introducing into the well an aqueous thixotr'opic composition comprising magnesium fluoride tormed by reacting together an aqueous solution containing magnesium chloride and an aqueous solution containing ammonium bifiuoride", the number of equivalent weights of the ammonium bifluoride being from 95 to 105 per cent of the number of equivalents of the magnesium chloride, the number of moles of water in the magnesium chloride solution plus the number of moles of water in the ammonium bifiuoride solution being between about 12 and 121.

6'. A thi xotropic composition comprising the reaction product of reacting together an aqueous solution of a water-soluble magnesium salt and} a water-soluble fluoride in the presence of a mineral acid the number of equivalent weights of the water-soluble fluoride being from 95 to 105 per cent of the number of equivalent weights of the magnesium salt, and the number of equivalent wei'ghtsof the acid being from 45 to 350 per cent of the number of equivalent weights of the magnesium salt, the number of moles of water in the magnesium salt solution plus the number of moles of water the fluoride solution. being between 12 and 121.

'F. A- thi-xotropic composition according to claim it in which there is included per gallon thereof up to4 pounds of a particulated solid.-

8'. A thix'otropic composition according to claim 7 in which the particulated solid is sand.

9. The method according to claim 1 followed by the application of hydraulic pressure through the well bore suificient to force the composition into theearth formation and fracture it.

10', The method according to claim 2 followed by the application of hydraulic pressure through the well bore sufiicient to force the composition into the earth formation and fracture it.

11. The method according to claim 5 followed by the application of hydraulic pressure through the well bore sufii'ci'ent to force the composition into the earth formation and fracture it.

FRANK H. BRAUNLICH, JR. PAUL H. CARDWELL.

References Cited in the file of this patent UNITED STATES PATENTS Name Date 2,156,220 Dunn Apr. 25, 1939 2,236,147 Lerch et a1 Mar. 25, 1941 2,454,921 Heinemann Nov. 30, 1948 2,596,137 Fast 4. May 13, 1952 2,596,843 Farris a May 13, 1952 OTHER REFERENCES The Oil and. Gas Journal, February 4, 1937, pp. 46, 48, 49 and 50.

Claims (1)

1. THE METHOD OF TREATING AN EARTH FORMATION PENETRATED BY THE BORE OF A WALL WHICH COMPRISES INTRODUCING INTO THE WELL AND AQUEOUS THIXOTROPIC COMPOSITION COMPRISING MAGNESIUM FLUORIDE FORMED BY REACTING TOGETHER AN AQUEOUS SOLUTION CONTAINING A WATER-SOLUBLE MAGNESIUM SALT AND AN AQUEOUS SOLUTION CONTAINING A WATER-SOLUBLE FLUORIDE IN THE PRESENCE OF A STRONG MINERAL ACID THE NUMBER OF EQUIVALENT WEIGHTS OF THE FLUORIDE SALT BEING FROM 95 TO 105 PER CENT OF THE NUMBER OF EQUIVALENT WEIGHTS OF THE MAGNESIUM SALT AND THE NUMBER OF EQUIVALENT WEIGHTS OF ACID BEING FROM 45 TO 350 PER CENT THE NUMBER OF EQUIVALENT WEIGHT OF THE MAGNESIUM SALT, THE NUMBER OF MOLES OF WATER IN THE MAGNESIUM SALT SOLUTION PLUS THE NUMBER OF MOLES OF WATER IN THE FLUORIDE SALT SOLUTION BEING BETWEEN ABOUT 12 AND 121 TIMES THE NUMBER OF EQUIVALENT WEIGHTS OF THE MAGNESIUM SALT.