US3463232A - Method of acid treating wells - Google Patents

Description

United States Patent 3,463,232 METHOD OF ACID TREATING WELLS George E. Scofield, 19 Wakeman Road, Darien, Conn. 06820 No Drawing. Filed Nov. 3, 1967, Ser. No. 680,327 Int. Cl. E21b 43/27 U.S. Cl. 166307 6 Claims ABSTRACT OF THE DISCLOSURE The productive capacity of a well is greatly increased by injecting a dilute acid solution containing the equivalent of from about 0.00003 percent by weight to 0.5 percent by weight hydrochloric acid continuously into the well for a period of at least several hours. When so treated, the solution scours and enlarges the pores of the formation for a substantial distance from the well, and not merely in the vicinity of the well bore.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a method for treating a well in order to increase its productive capacity by injecting an aqueous solution of an acid into the well, thereby enlarging the pores of the acid-soluble material surrounding the well bore.

Description of the prior art It has been previously though necessary to use strong solutions of acids to treat wells, as exemplified by the solutions containing from about to about 35% by weight hydrochloric acid employed in the processes disclosed in US Patents Nos. 2,038,956, 2,308,414 and 2,640,810. In these prior art methods, the strong acid solutions tend to attack the tubing and other metal parts of the well causing serious operating and maintenance problems. Further, when the treatment of a well is attempted by injecting a strong acid in conjunction with an inhibitor to retard metal attack, the desired enlargement of the pores of the well formation is often not obtained. Treatment with the strong acid solutions described in the prior art has the further disadvantage that the increased permeability of the acid-soluble material of the formation has been restricted to the area immediately surrounding the well bore, as disclosed in US. Patents Nos. 2,308,414 and 3,160,207. Moreover, the use of the strong acid solutions of the prior art limits the shut-in time during which the acid can attack the well formation to about 3 or 4 hours, as disclosed in US. Patent No. 2,640,810. Other means of treating wells include well fracturing, as disclosed in US. Patent No. 3,104,706, and the methods of acidization and fracturing are sometimes combined.

SUMMARY OF THE INVENTION The present invention is based upon my discovery that the injection of a large volume of a dilute acid solution into a well, when continued for a period of at least several hours, greatly increases the productive capacity of a well. Specifically, I have discovered that the injection of an aqueous solution containing the equivalent of from about 0.00003 to about 0.5 percent by weight of hydrochloric acid for a period of at least several hours, and preferably for twelve or more hours, extends the beneficial treatment deep into the well formation and thereby greatly increases the productivity of the well as compared with the acidization processes of the prior art. The improved results obtained appear to be due to the fact that Patented Aug. 26, 1969 "ice DESCRIPTION OF THE PREFERRED EMBODIMENT The practice of my invention is exemplified by the advantageous embodiment hereinafter described.

A portable pump having a capacity that roughly approximates the desired capacity of the well is connected to an available source of water. For example, if the well to be treated has shown a decline in capacity from 2000 g.p.m. to 500 g.p.m., it would be desirable to select a pump with a capacity of about 2000 g.p.m. The discharge of the pump is connected to the well to be treated, and if this is a well that is equipped with a pump, the well pump is preferably removed, A connection at the discharge of the pump provides water to operate an eductor which draws concentrated acid from a tank-car and injects it by means of a sparger into the main stream of Water being pumped into the well. Recording instruments comprising, for example, a flowmeter, a pressure gage, and a pH meter, enable the operators to control the acidity of water pumped into the well and to observe the progress of the treatment. In one form of the invention, the flow of concentrated acid is regulated in an automatic manner by the pH meter so as to continuously provide the desired pH in the water being pumped into the Well.

The beneficial effects of the dilute acid can be continuously observed in the response of the flowmeter and pressure gage. At the start of treatment the rate of flow will be relatively low and the solution pressure will be relatively high, and the quotient of solution flow divided by solution pressure will be at a minimum. As treatment proceeds and the pores in the formation become larger, the flow will increase and the pressure will decrease leading to an increasing flow to pressure ratio. The percentage increase in this quotient or ratio is a rough indication of the percentage increase in the capacity that is being obtained for the Well. The treatment preferably should be continued as long as the quotient of flow divided by pressure continues to show a significant increase. After treatment has been carried out to an acceptable degree, the procedure is terminated and if the well is the pumped type, its pump is reinstalled. The well is then allowed to flow to Waste, or is pumped, or is drained by other known means, until the water is sufficiently free of acid and dis solved salts to be acceptable for use.

The optimum combination of treatment conditions that will lead to an increase in well capacity varies with the cost of concentrated acid, the quantity of dilution water available, the time available to carry out the treatment, and the reactivity of the walls of the pores in the formation to the acid. As a general rule, the longer the treatment can be carried out, and the higher the flow of dilute acids that can be forced into the formation, the greater is the improvement that can be expected, treatment times of twelve hours or longer generally being preferred.

The acidity of the water that is pumped into the well formation should be maintained between a pH of 1 and 5. Although any clean acid, such as nitric acid, may be used to achieve this pH range, hydrochloric acid is usually the least expensive and most convenient acid to use, The concentration of hydrochloric acid in a solution having a pH within these limits would range from about 0.5 percent to as low as 0.00003 percent by weight ttuuut HCl, and the use of other acids would require the presence of equivalent amounts of these acids in the water. In my work I have found that a pH of about 3 is an effective level of solution acidity.

The rate at which the dilute acid is pumped into the formation should be as high as possible, for in this way the beneficial scouring efiect is achieved and, moreover, the acid is carried far back in the formation before it has expended itself. A rule-of-thumb determination would call for a flow rate of dilute acid that approximates the flow that is expected to be achieved from the well after the treatment. Lesser flow rates for the dilute acid will be less effective. In any case, the rate of flow should not be allowed to drop below 1 percent of the flow that is expected to be achieved from the well after treatment.

It has been found that the duration of treatment may continue for a period of several days without any harmful corrosive effect being observed on the tubing or other metal portions of the well equipment. The actual duration of treatment will obviously depend upon the rate of injection of the dilute acid solution and the measured observable response from the flowmeter and the pressure gage. In general, the treatment is best terminated when the ratio of solution flow to solution pressure no longer shows any significant change.

I claim:

1. In the method for increasing the productive capacity of a well by treating the well with an aqueous solution of an acid, the improvement which comprises injecting a large volume of a dilute acid solution into the well under pressure, and continuing injection of said dilute acid for a period of at least several hours, the concentration of acid in said solution being maintained at that level which is equivalent to a hydrochloric acid concentration within the range of from about 0.5 percent by weight to 0.00003 percent by weight hydrochloric acid, and the pH of the acid solution being maintained between about pH 1 and pH 5.

2. The method according to claim 1, wherein the treatment with said acid solution is continued for a period of at least twelve hours.

3. The method according to claim 1, wherein the injection of the acidic solution into the well is continued throughout the period during which the quotient of solution flow divided by solution pressure continues to show a significant increase.

4. The method according to claim 1 wherein the pH of the solution is maintained at about 3.

5. The method according .to claim 1, wherein the rate of flow of the acidified solution into the well is at least 1 percent of the flow of the Well after said treatment thereof.

6. The method according to claim 1, wherein hydrochloric acid is employed to acidify the treatment solution.

References Cited UNITED STATES PATENTS STEPHEN I. NOVOSAD, Primary Examiner