US3525400A - Method for decreasing water production by gas injection in a single well operation - Google Patents

Description

Aug.- 2 19. I c B. POLLOCK ET I 3,525,400 METHOD FOR DECREASING WATER PRODUCTION BY GAS INJECTION IN A SINGLE WELL OPERATION Filed Nov. 18, 1968 com K C M O O NT m m mw Q Q H R Q A W HA 2 CJ 7 ud l & MB 0 w 2 mm 19 VS mm WSW w m NN w mm. Hm z mm mm I Mm ATTORNEY United States Patent 3,525,400 METHOD FOR DECREASING WATER PRODUC- TION BY GAS INJECTION IN A SINGLE WELL OPERATION Charles B. Pollock, Littleton, Colo., and Jack L. Shelton, Tulsa, Okla., assignors to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed Nov. 18, 1968, Ser. No. 776,566 Int. Cl. E21b 43/25 US. Cl. 166-305 9 Claims ABSTRACT OF THE DISCLOSURE Where water is produced with crude oil, either commingled with the oil or from a different level in the formation, it has been found that water production rates can be substantially lessened by injection of a gas having a relatively high preferential solubility in oil into the formation. Under these conditions a trapped gas saturation is developed inthe water, reducing its mobility and thereby bringing about a substantial decrease in the fiow thereof to the well bore. This effect is generally only temporary lasting for several months. After water production again approaches pre-gas injection levels the procedure may be repeated.

INTRODUCT ION The present invention relates to an improved method of oil production and more particularly is concerned with a method for reducing the flow of water which is either commingled with the oil or is produced from a separate zone.

BACKGROUND Production of water along with crude oil is a problem that generally is tolerated so long as the lifting and separating costs do not become uneconomical. Where water entry occurs largely in a level of the formation different from that yielding the oil, the water flow is oftentimes controlled by squeeze cementing or casing the producing formation and perforating through only to the oil production zone. These methods while generally effective have not necessarily met with unqualified success and of course cannot be used with any positive efiect where the oil and water are commingled and are produced essentially from the same zone.

BRIEF DESORIPTION OF THE INVENTION In the accompanying drawing the solubility of light hydrocarbon gases in oils of different gravities and water, as affected by pressure, are shown.

We have now discovered a method that is not only selective for the reduction of the water flow'whether commingled with or essentially separate from the oil producing zone(s)but which lowers the oil viscosity substantially thus increasing the oil producing rate. Briefly, this is accomplished by first discontinuing production from the well and then injecting into the formation a gas or gaseous mixture having a substantially higher solubility in oil than in water. Under these conditions a trapped gas saturation is created in the water thereby de creasing the relative permeability of the formation to water and, accordingly, resulting in decreased water production rates. On the other hand, the injected gas or gaseous mixture which is highly soluble in the oil goes into solution causing a marked reduction in viscosity. Thus, it is seen that by such injection the mobility of the oil is increased while that of the water is decreased resulting in a marked change in the relative oil and water production rates.

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gas should be injected, e.g., at least about 100,000 cubic feet per foot of net pay involved in order to establish a large trapped gas saturation in the water-containing region of the formation. When the well is put back on production the water rate is reduced because of the resistance to flow due to the trapped gas.

The process of our invention is more effective the greater the degree of separation of the oil and water producing levels in the formation. Gas trapped in the water can diffuse into the oil and be absorbed eventually depleting the gas saturation in the water in which case the cycle is repeated by halting production and again injecting gas into the formation. Although best results are usually obtained when the oil and water producing zones are separated, benefits can also be secured when oil and water are produced from the same strata. This is particularly true in viscous oil reservoirs where channeling and bypassing of oil by injected or natural water results in localized regions of high water saturation.

The choice of the gas depends mainly on the properties of the crude oil and the pressure. Many undersaturated crudes have a high solubility for methane and in such cases a dry field gas can be used. By the expression dry field gas we mean one which is principally methane, i.e., a minimum of 90-95% methane and the balance being predominantly ethane. Usually, however, a mixture of methane with ethane, propane and/or butane, etc., is ordinarily preferable to insure good solubility. This is especially true with reservoirs having pressures of less than 600 psi. In no case is it necessary to use a gas which exceeds the requirements for miscibility with the reservoir oil. Generally speaking the composition of the gas injected may vary rather widely. For example, in the case of methane-propane mixtures we have found that 20 mol percent of propane may be used to advantage with asphalt-containing crudes. With non asphaltic crudes the proportion of propane may be as high as 25 mol percent. The injected gas, however, in order to be effective should be at least 13 times more soluble in the reservoir oil than in water.

The effects of oil gravity and pressure on the solubility of methane or propane in such oil as well as in water are ShOIWIl in the accompanying drawing. This series of fluids illustrates the influence of oil gravity, pressure and composition of injected gas on the relative solubility of such gas in both oil and water. Thus it is seen that the relative solubility is greater for the higher API gravity oils and the 16 API gravity oils represents the approximate lower limit of applicability of our process although it should be noted in this connection that the example hereinafter described shows that the process of our invention can be employed successfully where the API gravity of the oil is not more than about 14. Further the curves demonstrate that as the oil gravity increases, the solubility of gas therein becomes greater. Also, it is seen that with any given oil the solubility of methane, propane or equivalents thereof in such oil can be substantially increased by the addition of one of the lighter hydrocarbons such as propane or butane. The lower most curve based on 16 AJPI gravity oil illustrates the approximate minimum relative solubility of methane, i.e., a ratio of 13:1 which will bring about a distinct decrease in water mobility with no apparent loss of oil mobility. In fact, as previously stated, oil mobility can be increased particularly where hydrocarbons such as propane and/or butane are present.

The process of our invention will be further illustrated by means of the following specific example.

EXAMPLE The well in which our invention was employed had To accomplish this result, relatively large volumes of about feet of net pay beginning at a depth of 1220 feet. The formation had a porosity of 24%, a water saturation of 25%, and a permeability of about 640 millidarcies. The original displacement mechanism in the field in which this well was located was by Water drive. The oil had an API gravity of 14.4", a viscosity of 1,000 centipoises and contained little or no dissolved gas. Oil production amounted to about 25 bbls./day. Water production had historically been 40-50% of the producing well fluids. Production was discontinued and thereafter for a period of two weeks 11,700,000 cubic feet of gas consisting of about 85% methane and 15% propane was injected into the pay zone of this well. Thereafter, injection of gas was discontinued and the well placed on production. Increased oil production rates were obtainedwith an average during the first month of about 75 bbls./ day-as a result of lowering the viscosity of the oil to about 625 centipoises caused primarily from solution of propane into the crude. A very significant decrease in water production also resulted. The water cut after resumption of production was less than Over the next two months the water content rose slowly to a value of about 12% The average oil production during this period was about 40 bbls./day.

The data appearing in the tables below were calculated using Standings Correlation of properties of natural hydrocarbon mixtures of gas and liquid (Volumetric and Phase Behavior of Oil Feld Hydrocarbon Systems, Reinhold Publishing Co., New York, 1952). The data shown for water solubility were based on the original data of Vapor- Liquid Elqiuibrium for Binary Hydrocarbon-Water Systems, by Riki Kobazashi and Donald L. Katz, Industrial and Engineering Chemistry, 45, p. 440 (1953).

Table I gives the volume of methane and propane dissolved in a 16 API stock tank oil at 80" F. and bubble point pressure between 200 and 2000 p.s.i.a. In addition the incremental volume of methane or propane dissolved above the base pressure of 200 lbs. is given.

Table II gives the same information as Table I except that a 26 API stock tank oil is considered.

Table III tabulates the cubic feet of a hydrocarbon gas such as methane or propane that is dissolved in water in the pressure range of 200-2000 lbs. at 90 F. This table also gives the incremental volume of gas that is dissolved in water above a base pressure of 200 lbs.

Table IV takes the incremental volume figures or numbers of Tables I-III and ratios the incremental volumes of methane and propane to the incremental volume of Water dissolved in the particular API stock tank oil crude at the designated pressures. This is the information on which the curves on the accompanying drawings are based.

TABLE I MCF of either methane or propane that a 16 API stock tank oil will dissolve less the amount that will dissolve at 200 Total MCF of either methane or propane that a 16 API stock Bubble point tank oil will dissolve p.s.i.a. pressure, p.s.i.a. Methane Propane Methane Propane TABLE II MCF of either methane or Total MOF of either propane that a 26 API methane or propane that stock tank oil will dlssolve a 26 API Stock tank oil less the amount that w1ll Bubble point will dissolve I dissolve at 200 p.s.1.a. pressure, p.s.i.a. Methane Propane Methane Propane 20 32 0 0 45 68 25 36 71 110 51 78 140 210 120 178 200 300 180 268 275 410 255 378 TABLE III MCF of gas that water Total MCF of gas will dissolve less the Bubble point (methane or propane) amount that will pressure, p.s.i.a. that water will dissolve dissolve at 200 p.s.i.a..

TAB LE IV Ratio of additional gas the oil will dissolve to the additional gas the water will dissolve Bubble point 16 API 26 API Methane Propane Methane Propane From the foregoing description and example it will be seen that the process of our invention is elfective to bring about a distinct reduction in water flow coming from an oil bearing formation. Such flow may be either commingled with the oil or from a separate level in the same formation in which the oil is present. Thus, it is seen that by our invention we are able to improve substantially the efficiency of a single producing well having water influx problems, merely by the periodic injection of a gas or gas mixture having a preferential solubility in oil at least about 13 times as great as in water. After suflicient gas has been injected the well is placed back on production to realize a sustained decrease in water production. Whenever the water cut increases to the point where eflicient operation of the well becomes difficult, the above cycle should be repeated.

We claim:

1. In a process for recovering oil from an oil-bearing formation having a production well extending into said formation and wherein a substantial water flow from said formation accompanies the flow of oil into said well, the method of substantially reducing said water flow by injecting into said formation hydrocarbon gas which is at least about 13 times more soluble in said oil than in water under the conditions of the reservoir to develop a trapped gas saturation in said formation water thereby decreasing the relative permeability thereof to water, whereby the oil-water ratio produced into said well is substantially greater than prior to said injection step and thereafter producing fluids from said well.

2. The method of claim 1 in which the gas injected consists of at least about 75 mol. percent methane.

3. The method of claim 1 in which the crude oil involved is a substantially non-asphaltic crude and wherein the propane content of the injected gas is not more than about 25 mol. percent.

4. The method of claim 1 wherein the cycle of steps outlined is repeated after water production approaches the water production rate prior to treatment.

5. The method of claim 1 wherein the API gravity of the oil produced is not less than about 14.

6. The method of claim 1 wherein the amount of gas injected corresponds to at least about 100,000 cubic feet per foot of net pay.

7. The method of claim 1 wherein the oil involved is an asphaltic crude and the propane content of the injected gas is not more than about 20 mol. percent.

8. The method of claim 1 wherein the oil involved is an undersaturated crude and the hydrocarbon gas is a dry field gas.

6 9. The method of claim 1 wherein the water flow is 2,909,224 10/1959 Allen 166-305 commingled Water. 3,252,512 5/1966 Baker et a1. 166-305 X 3,283,818 11/1966 Santourian 166-305 References Cited UNITED STATES A S 5 ERNEST R. PURSER, Primary Examiner 2,788,855 4/1957 Peterson 166-306 X I. A. CALVERT, Assistant Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,525,h00 Dated Au ust 25, 1970 Inventor) Charles B. Pollock and Jack L. Shelton It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 27, "Feld" should read --Field--; line 30, "Elqiuibrium" should read --Equilibrium"; line &2, "90" should read --80--.

Column h, in TABLE IV, below "Bubble point" insert -pressure, p.s.i.a.--.

SIGNED ANu SEALED Atlmt:

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