CA1054783A - Secondary recovery methods - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method for increasing the production of fluids from a subterranean fluid-bearing formation containing acid-soluble components which comprises injecting into said formation an aqueous acid solution comprising (a) from 0.5 to 28% by weight of a non-oxidizing mineral acid or from 0.01 to 5% by weight of carbon dioxide, and (b) from 0.005 to 2% by weight of at least one sulfonated anti-scale compound of the formula

Description

This invention relates to a method for increasing the production of fluids from subterranean fluid-bearing formations. More particularly, this invention relates to a method in which the productivity of a hydrocarbon-bearing formation containing acid-soluble components, and with or without water-sensitive clays or shales, is improved upon treatment of the formation with an aqueous solution of a non-oxidizing mineral acid (or carbon dioxide) and an anti-scale as hereinafter described, said anti-scale compound effect-ing the elimination of plugging of capillary openings due to post-precipitat-ion of dissolved salts subsequent to the acidization, as well as effecting elimination of mineral scale on production equipment such as pumps, or tubing, caused by such precipitation.
The technique of increasing the permeability of a subterranean hydrocarbon-bearing formation and of removing obstructing acid-soluble mineral scale for the purpose of stimulating the production of fluids therefrom has long been practiced in the art. One such method, known as acidizing, is widely utilized in treating sub-surface acid-soluble geological formations, e.g., limestone, or dolomite. The technique is not limited to application in fonmations of high acid solubility. Sandstone and gypsum-containing fonmations may require acidization if the produced water is unstable with respectcto CaC03. In the usual well-acidizing procedure, a non-oxidizing mineral acid is introduced into the well and under sufficient pressure is forced into the adjacent subterranean formation, where it reacts with for-mation components, and deposited mineral scale, particularly, the carbonates such as calcium carbonate, and magnisium carbonate to form the respective salt of the acid, carbon dioxide and water. The usual mineral acid employed in such acidization procedures is hydrochloric acid.
During the acidizing process, passageways for fluid flow are created or existing passageways therein are enlarged, thus stimulating the production of oil, water, brines and various gases. If desired, the acidization may be carried out at an injection pressure sufficiently great to create fractures in the strata or formation which has the desired advantage of opening up passageways into the fonmation along which the acid can travel to more remote areas from the well bore. The salt formed upon neutralization of the acid is extensively water-soluble and is readily removed by reverse flow from the formation via the well bore.
There are, however, troublesome complications attending the use of hydrochloric acid or other similar non-oxidizing mineral acids. In the acidizing process, the following primary beneficial reaction occurs:
CaC03 + 2HCl ) CaC12 + H20 + C02.

Under the higher pressures required to conduct an acidization, the C02 is dissolved in the reaction mixture consisting of spent acid and connate water:

C0 + H20 < ~ H2C0 ~ ~ H + NC03 ~ ) 2H + C0 The equilibria may be summarized and written:

Ca(HC03) ~ ) CaC0 + H2C03~ ~ H 0 + C02.

After acidization is completed, the well is often back-flowed in the case of a water injection well (in order to clean out fonmation and tub-ing) and put back on production in the case of a producing oil or gas well.
In both cases, pressure diminishes, and C02 breaks out of solution, inducing CaC0 to precipitate. Such precipitation, when it occurs within the capil-laries of a tigh formation or on the tubing or annulus as a mineral scale, can severely lessen the rate of production or injection by plugging such capillaries or well equipment.

It is known that polyphosphates are effective in retarding CaC03 precipitation. Theæe polyphosphates are unsatisfactory in acidic solutions because they undergo rapid hydrolysis in the presence of mineral acid, and, as a result, lose their scale inhibiting properties. In addition, one hydro-lytic reaction product, the phosphate ion (P04 3), can precipitate with calcium or barium ions present in the produced water, causing additional plugging or scale deposition, further aggravating the problem. Other known scale inhibitors, are the "glassy" phosphates, which are unsatisfactory be-cause of their slight solubility in acidic media and the tendency to form objectionable hydrolytic reaction products.
It is also known to employ various organic polymers to prevent the precipitation of mineral salts, but many of these polymeric materials are unstable in mineral acids, undergoing spontaneous depolymerization to an ineffective species. Such a polymeric material which undergoes hydrolysis in the presence of acids is polyacryla~ide, which is unstable in aqueous media at temperatures of about 250 F. and upwards. Many wells that may be treated by the method of the present invention have bottom hole temperatures of 250-300 F. or higher.
Chemically altered natural polymers, and natural polymers themselves, are effective inhibitors to prevent the precipitation of mineral salts.
However, some materials such as sodium carboxymethylcellulose precipitate or decompose in the presence of mineral acids. Other known sequestering agents such as citric or tartaric acids, and/or complexing agents such as ethylened-iaminetetraacetic acid and its water-soluble salts, are known inhibitors to prevent the deposition of boiler scale in aqueous media. Such materials cannot, however, be employed in the treatment of subterranean formations, because they are not appreciably surface active and do not adsorb on the fonmation face.
The present invention provides a method for increasing the product-ion of fluids from a subterranean fluid-bearing formation containing acid-soluble components which comprises injecting into said fonmation an aqueous acid solution comprising (a) from 0.5 to 28% by weight of a non-oxidizing mineral acid or from 0.01 to 5% by weight of carbon dioxide, and (b) from 0.005 to 2% by weight of at least one sulfonated anti-scale compound of the formula Rl

2 2 2 3 (1) wherein A+ is an aIkali metal or ammonium ion, and Rl and R2 which may be the same or different, are each a saturated or unsaturated aliphatic hydro-carbon group containing up to 20 carbon atoms, the total number of carbon atoms in Rl and R2 being from 9 to 30.
According to one embodiment of this invention, the production of fluids from a subterranean fluid-bearing formation containing acid-soluble components is increased by injecting the aqueous acid solution down the well bore to said formation, and therefrom into said formation under a pressure greater than the formation pressure, maintaining the solution in contact with the formation strata for a time sufficient for the acid to react chemically with the acid-soluble components of the formation and/or acid-soluble mineral scale deposited on production equipment, to etch or enlarge passageways through the strata and remove the scale, and thereby to increase substant-ially the flow capacity of the subterranean formation.
In accordance with another embodiment, the formation is penetrated by at least one production well and one injection well, and said solution is injected into said formation, and displaced through said formation, and fluids from said fornation are recovered through the production well. The anti-scale compound prevents precipitation of compounds fonmed by the reaction of the acid component, thereby permitting a substantial increase of production of hydrocarbons from the formation via the production well.
When carrying out the method of the invention, carbon dioxide is concomitantly released, whereby a beneficial effect, due to the mutual miscibility of carbon dioxide in the fluid phases, is realized as a reduction in viscosity and retentive capillary forces, while another beneficial effect is an increased fonmation energy, due to the pressure generated by the released carbon dioxide.
Another advantage resulting from the employment of the method of this invention in acidizing fluid-bearing formations i9 that the post-precip-itation of dissolved carbonates is prevented or materially decreased. Such post-precipitation occurs because of the nature of the dissolution reaction:

Ca(HC0 ) ( ) CaC03 + H D + C02~.

When pressure is released 90 that spent reaction products from the acidi~ation process can be removed, carbon dioxide gas can break out of solution, causing post-precipitation of calcium carbonate. Such post-precipi-tation occuring within the fonmation matrix near the bore hole can decrease permeability by plugging the fornation capillaries, particularly those near the well bore, and result in a lower production rate. Furthermore, such post-precipitation can occur in the tubing or annulus of the well itself and manifest itself as mineral scale, reducing their diameter(s) and resulting in a lower production rate.
The anti-scale compounds are water-soluble substituted taurines of the fonmula (1) defined above. Representative substituted taurine compounds (I) include those wherein either the R2 group or the Rl group is methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, or eicosyl, including the branched chain and unsaturated variants thereof, such as oleyl. It is to be understood that mixtures of these above named R2 and Rl groups can be used, such as those obtained from coconut, tall oil, tallow and palm oils.
The preferred substituted taurines are those wherein the Rl substitu-ent is a relatively low molecular weight aliphatic hydrocarbon group, e.g.
one having 1 to 4 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl or isobutyl and the other substituent R2, is a saturated or unsaturat-ed straight or branched chain, aliphatic hydrocarbon containing between 8 and 20 carbon atoms, including more specifically such hydrocarbons derived from the coconut, palm and tall oil acids etc., high in oleyl groups.

The aqueous acid composition used in this invention is one compris-ing an aqueous solution, which may include brine, and from about 0.5 to about 28% by weight preferably 3 to 15% by weight of a non-oxidizing mineral acid, such as hydrochloric acid or from about 0.01% to about 5% by weight prefer-ably 1 to 3% of carbon dioxide, and which contains therewith between from about 0.005% to about 2% preferably from about 0.05% to about 1% by weight of the aforesaid compound (I).
Generally, the aqueous non-oxidizing mineral acid solution will contain an inhibitor to prevent or greatly reduce the corrosive attack of the acid on metal. Any of a wide variety of compounds known in the art and employed for this purpose can be used, e.g., certain compounds of arsenic, nitrogen or sulfur as described in United States Patent No. 1,877,504. The amount of the inhibitor is not highly critical and it may be varied widely.
Usually this amount is defined as a small but effective amount, e.g., from 0.02% to about 2.0~o by weight.
In carrying out one embodiment of the method of this invention a solution containing the de9ired amount of the non-oxidizing mineral acid or carbon dioxide dissolved in water is first prepared. An inhibitor to prevent corrosion by the mineral acid of the metal equipment associated with the well is usually added with mixing in the next step. The anti-scale compound in an amount within the stated concentration range is then admixed with the aqueous acid solution. The thus-prepared acid solution is forced, usually via a suitable pumping system, down the well bore and into contact with the production equipment and formation to be treated. As those skilled in the art will readily understand, the pressure employed is detenmined by the nature of the formation, the viscosity of the fluid, and other operating variables. The acidization method of this invention may be carried out at a pressure sufficient merely to penetrate the formation, or it may be of sufficient magnitude to overcome the weight of the overburden and create fractures in the formation. Propping agents to prop open the fractures, as created, for example 20 to 60 mesh sand, in accordance with known fractur-ing procedures, may be employed in admixture with the aqueous acidic solution.
Generally, it is advisable to allow the aqueous acid solution to remain in contact with the formation and production equipment until the acid therein has been substantially depleted by reaction with the acid-soluble components of the fonmation and the deposited scale. After this, the substantially spent treating solution is reversed out of the well, i.e., it is allowed to flow back out of, or to be pumped out of, the fonmation. Further, as those skilled in the art will understand, the concentrations of the compound and acid components should be chosen to provide an acidizing fluid of the desired rheological properties.
Another embodiment of the method of this invention can be carried out with a wide variety of injection and production systems which will comprise one or more wells penetrating the producing strata or fonmation.
Such wells may be located and spaced in a variety of patterns which are well-known to those skilled in the art. For example, the so-called "line-flood" pattern may be used, in which the injection and producing systems are composed of rows of wells spaced from one another. The recovery zone, i.e., that portion of the producing formation from which hydroca~bona are dis-placed by the drive fluid to the production system will, in this instance,be that part of the fonmation underlying the area between the spaced rows.
Another pattern which is frequently used i9 the so-called "circular flood"
in which the injection system comprises a central injection well while the production system comprises a plurality of production wells spaced about the injection well. Likewi9e, the injection and production systems each may consist of only a single well and here the recovery zone will be that part of the producing strata underlying a roughly elliptical area between the two wells which is subject to the displacing action of the aqueous drive fluid.
For a more elaborate description of such recovery patterns, reference is made to Uren, L.C., Petroleum Production EnRineerinR-Oil Field ExDloitation, Second Edition, McGraw Hill Book Company, Inc., New York, 1939, and to United States Patents Nos. 3,472,318 and 3,476,182.
In carrying out this embodiment of the invention, the aqueous acidic solution of the anti-scale compound is forced, usually via a suitable pumping system, down the well bore of an injection well and into the producing fo~mation, through which it is then displaced together with hydrocarbons of the formation in the direction of a production well. As in the other embod-iment, the pressure employed is determined by the nature of the formation, viscosity of the fluid, and other operating variables, and the acidization method of this invention may be carried out at a pressure sufficient merely to penetrate the formation or it may be of sufficient magnitude to fracture the formation, in which case propping agents, as described above, may also be included in the aqueous acidic solution.
The formation may be treated continuously with the acidic solution, or such treatment may be temporary. If desired, however, after a time, conventional flooding may be resumed. The aqueous acidic solution of the anti-scale compound also may be applied in a modified water flood operation in which there is first injected into the well bore a slug of the aqueous acidic solution which is forced under pressure into the subterranean format-ion. The first step is then followed by a similar injection step wherein a slug of an aqueous drive fluid, such as water, is injected, which is thereafter followed by a repetition of the two steps. This sequence may be repeated to give a continuous cyclic process. The size of the slugs may be varied within rather wide limits and will depend on a number of c~nditions, including the thickness of the formation, its characteristics and the conditions for the subsequent injection of the aqueous drive medium.
The anti-scale compound provides means whereby ions, produced by the reaction of the acid component of the solution with the fonmation and having tendencies to precipitate as salts such as CaC0 , hydrous iron oxides and CaS04.2H20, combine with the compound to form a highly stable complex so that solid salts do not precipitate from the spent treating solution. This binding up of the aforementioned ions from weakly ionizable compounds permits the formed complex to remain dissolved in the treating solution and pass through the formation pores. Further, the anti-scale compound dissolved in the composition provides means whereby the nucleation and growth of the solid itself is thwarted, so that qolid salts do not precipitate from the spent treating solution. Finally, the anti-scale compound in the composition provides means whereby continuous protection against post-precipitation of salts is obtained for a considerable time after treatment due to continuous slow desorption of the compound from the formation faces. In contrast, use of surfactants having merely dispersant and suspending properties, and not possessing the capability of molecularly binding these produced ions or thwarting the necleation and growth of solid salts, will permit post-precip-itation of said salts from such treating solution with the likelihood of plugging of the fonmation passageways during subsequent recovery of desirable formation hydrocarbons.
It should be understood that the concentrations of the compound and the acid components are chosen to provide a displacing fluid of the desired rheological properties. Similarly, the appropriate compound is selected on the basis of the formation being treated as well as other operating conditions employed.
If desired one can also add to the aqueous mineral acid solution containing the anti-scale compound, a polymeric material to retard the tendency of the acid to attack the calcareous components of the formation.
A polyvinylpyrrolidone as more particularly described in United States Patent No. 3,749,169 is particularly suitable for this purpose.
Example 1 A producing well in East Texas can be treated in the following manner.
A treating mixture is prepared by mixing 10 barrels of salt water containing about 2.6% by weight of sodium chloride and 12 barrels of 20% by weight aqueous hydrochloric acid, and 0.15 barrel of sodium N-methyl-N-oleoyltaurate is added.
The treating mixture is squeezed into the formation at a rate of about 1/2 BPM at 450 psig. The shut-in tubing pressure is 450 psig which is bled down to zero in a short time. The well can then be returned to product-ion.
Example 2 A treating mixture is prepared from 10 barrels of salt water (2.6%
by weight of sodium chloride) and 10 barrels of 12% by weight aqueous hydroch-loric acid solution containing 0.2 barrel of sodium N-methyl-N-oleoyltaurate.
The aqueous acidic solution is injected into the producing formation in the manner approximating that used in Example 1. Thereafter 20 barrels of water are used to overflush the treated formation by injection down the tubing, followed by injection of 5 barrels of water down the casing. The well can then be returned to production.
Example 3 The aqueous acidic solution of Example 2 is injected into another producing fonmation. An overflush of 10 barrels of water is used to force the aqueous acidic solution into the formation by injection down the tubing.
The well is able to be returned to production.
It is significant that the admixture is an effective material in the presence of high calcium ion concentrations of the order of up to 10,000 ppm or more.
Examples 4 to 9 The procedure set forth in Examples 1 to 3 above is repeated using:
Examples 4 to 6 - Sodium N-methyl-N-palmitoyltaurate.
Examples 7 to 9 - Sodium N-methyl-N-tall oil acid taurate.
The compound of Examples 1 to 3 above can be prepared in the follow-ing manner:

The sodium salt of taurine, NH CH CH SO Na, is reacted with methy-lamine to provide the intermediate sodium N-methyltaurate. This intermediate is reacted with the acid chloride of oleic acid to complete the preparation of sodium N-methyl-N-oleoyltaurate. The conditions under which this known reaction is conducted is well known in the art, including obvious variations thereof.
Example 10 Through a water injection well drilled into a limestone formation there is displaced under pressure down the tubing and into the formation an aqueous acidic solution containing 1% by weight of carbon dioxide and 1% by weight of sodium N-methyl-N-oleoyltaurate. The pressure required to inject the required volume of water declines considerably and no increase in said pressure is noted subsequent to treatment, indicating that post-precipitat-ion of CaCO within the fonmation leading to permeability reduction is prevented or materially lessened. The well is then returned to conventional water injection. After about 30 days the production of hydrocarbons from an adjacent prcducing well is substantially increased.
Example 11 A flooding operation is carried out in an oil-containing reservoir in accordance with the process of this invention. Four injection wells are arranged in a rectangular pattern around a single centrally located product-ion well in this system. A slug consisting of 75 barrels of an aqueous acidic solution containing 2~o by weight of carbon dioxide and 0.5% by weight of sodium-N-methyl-N-oleoyltaurate is displaced via each of the four injection wells into the fonmation at a rate of about 50 bbl/day. In the next step, 100 barrels of water are injected under pressure into the producing formation through each injection well at a rate of about 55 bbl/day. This sequence of operations is repeated numerous times and the result is an increased injection rate of the drive streams into the injection wells and a subsequent increase in the rate of production of hydrocarb~ns via the production well.

Example 12 An injection well in a formation containing abou~ 30% by weight of HCl-soluble material is treated with 1500 gallons of 1.5% by weight aqueous carbon dioxide containing 0.7% by weight of sodium N-methyl-N-oleoyltaurate.
The aqueous acidic solution is displaced from the tubing into the formation with water and the well shut in for 24 hours. Thereafter the well is returned to water injection. The injectivity of the well is materially increased for a sustained period of time resulting in enhanced hydrocarbon recovery.
Examples 13 to 18 The procedure of Examples 10 to 12 is repeated using Examples 13 to 15 - Sodium N-methyl-N-palmitoyltaurate Examples 16 to 18 - Sodium N-methyl-N-tall oil acid taurate It has been found that the compositions of the present invention are especially effective in the presence of high calcium ion concentrations, e.g. above 0.5% by weight and particularly and somewhat uniquely in applicat-ions where the aqueous solutions may have temperatures above 100 C. The compositions of the present invention are temperature stable and effective as scale inhibitors at temperatures up to 150 C. e.g. at 100 to 150 C.

Claims (13)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for increasing the production of fluids from a subterranean fluid-bearing formation con-taining acid-soluble components which comprises injecting into said formation an aqueous acid solution comprising (a) from 0.5 to 28% by weight of a non-oxidizing mineral acid or from 0.01 to 5% by weight of carbon dioxide, and (b) from 0.005 to 2% by weight of at least one sulfonated anti-scale compound of the formula wherein A+ is an alkali metal or ammonium ion, and in which R1 and R2 which may be the same or different, are each a saturated or unsaturated aliphatic hydrocarbon group con-taining up to 20 carbon atoms, the total number of carbon atoms in R1 and R2 being from 9 to 30.

2. A method as claimed in claim 1 wherein com-ponent (a) of the solution is a mineral acid.

3. A method as claimed in claim 1 wherein com-ponent (a) of the solution is carbon dioxide.

4. A method as claimed in claim 1 wherein R1 represents an alkyl group having 1 to 4 carbon atoms and R2 represents an acyl group derived from coconut oil, palm oil, tall oil or tallow fatty acid.

5. A method as claimed in claim 1, wherein the solution comprises 3 to 15% by weight of mineral acid.

6. A method as claimed in claim 1 wherein the solution comprises 1 to 3% by weight of carbon dioxide.

7. A method as claimed in claim 1, 2 or 3 wherein the solution comprises from 0.05 to 1% by weight of the anti-scale compound.

8. A method as claimed in claim 1, wherein the solution is injected into the formation under a pressure greater than formation pressure and maintained in contact with the formation for a period sufficient for chemical reaction between the solution and acid-soluble components of the formation to etch passageways through the formation.

9. A method as claimed in claim 8 wherein the solution is injected into the formation under a pressure sufficient to fracture the formation.

10. A method as claimed in claim 8 wherein the solution is injected into the formation at a pressure above the formation pressure but insufficient to create fractures in the formation.

11. A method as claimed in any of claims 1, 2 or 3 wherein the formation is penetrated by at least one production well and one injection well, and said solution is injected into said formation, and displaced through said formation, and fluids from said formation are recovered through the production well.

12. A method as claimed in claim 8 wherein the solution is injected into the formation under a pressure sufficient to fracture the formation and said solution contains a propping agent.

13. A method as claimed in any of claims 1, 2 or 3 wherein the formation is penetrated by at least one production well and one injection well, and said solution is injected into said formation, and displaced through said formation, and fluids from said formation are recovered through the production well and said solution contains a propping agent.