Method Of Increasing Productivity And The Injectivity Of Oil Wells

Abstract

During injection into an injection tubing string or production from a production tubing string contained within an oil well, the tubing string is fitted with tubing joints or other couplings means which may be activated to allow the introduction therethrough of a friction reducing fluid, for example a polymer. The friction reducing fluid may be introduced at various lengths along the tubing string to reduce the opposing friction head of the fluid injected into or produced from the well. In particular, the invention is utilized when producing viscous crudes or for the reduction of friction losses when injecting treating fluids.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for increasing the productivity or injectivity of oil wells. More particularly, the method and apparatus of the present invention comprises means for the introduction of friction reducing fluid at periodic intervals along a production or injection tubing string.

Well injectivity and productivity are the main criteria of any well completion. For a production well, the productivity of the tubing string determines the economics of the oil producer as does the injectivity of an injection well. Various and sundry problems may be encountered in well productivity and, in particular, in the production of viscous crudes, through production tubing, for example those crude oils having a viscosity greater than 10 centipoise. Productivity in an oil well generally is inhibited due to friction loss or friction head encountered in the production string. Therefore, with viscous crude oil containing reservoirs, the productivity to the producing wellbore may be high and of economic magnitude, but, however, there exists the inherent problem of producing the viscous crude through a relatively small diameter production string. Typically, production strings of two or three inches diameter are utilized. With this constraint, the economics of wellbore production may dictate the total economics for the reservoir. A similar problem is incurred in injection wells wherein the injection fluid is a relatively viscous medium, for example those fluids utilized for hydraulically fracturing formation. High injection rates are required for adequate hydraulic fracturing. Often these rates may not be achievable in a relatively small injection tubing string in that the friction losses due to wall friction drag may limit the injectivity for the treatment desired.

Many methods and apparatus have been disclosed for the reduction of friction losses within the wellbore tubing strings. In particular, in the case of viscous crude oils, solvents are often introduced at the bottomhole of the well and intermingled with the viscous crude in order to reduce the viscous crude oil's viscosity to a point where its pumpability and, therefore, the productivity of the well becomes economic. Wellbore heaters have been applied to entire tubing strings or bottomhole in order that the lower viscosities incurred at higher temperatures may be used advantageously. In injection wells, friction reducing fluids have been introduced in the injection treatment fluid to lower the viscosity of the treatment fluid. Alternately, the friction wall losses have been reduced by coating the walls of the injection tubing strings with a viscosity reducing fluid such that wellhead pressures are reduced and suitable injection rates may be obtained. The inherent problem in both productivity and injectivity methods for reducing wall friction losses is the tremendous amount of heat, solvent or friction reducing fluids required for the treatment thereof. Especially paramount to these problems are the requirement of separating solvents from producing oils, the tremendous heat losses incurred in wellbores in the heating of viscous crude oils, the chemical derrogation of friction reducing fluids in injection wells and, in particular, the rapid deterioration of chemicals injected into those wells requiring a heated injection medium. Therefore, what is required is a method for reducing friction loss in injection or production tubing strings without a tremendous expenditure of energy or materials, while avoiding complicated separational devices for the removal of these materials therefrom.

It is an object of the present invention to provide a method and apparatus for increasing the injectivity and productivity of oil wells.

It is still a further object of the present invention to provide a method and apparatus for the introduction of a friction reducing fluid at various depths into an injection or production tubing string.

It is still a further object of the present invention to reduce frictional losses in injection and production wells by the introduction, under controlled conditions, of a friction reducing fluid at various lengths along the tubing string.

With these and other objects in mind, the present invention may be more fully understood by referral to the accompanying drawing and description.

SUMMARY OF THE INVENTION

The objects of the present invention are accomplished through a method of reducing wall frictional forces within a tubing string. The method comprises the introduction of a friction reducing fluid at various lengths along the tubing string. The friction reducing fluid is introduced at a pressure greater than that contained within the tubing string such that no bleeding of the injection or production fluid into the friction reducing stream occurs.

Generally, the friction reducing fluid is introduced into a production well, which is producing a viscous crude oil, so as to reduce the wall frictional forces of the crude oil in the production string and thereby allow adequate production therefrom. The friction reducing fluid may also be introduced into an injection well through which is injected, for example a viscous hydraulic fracturing fluid, and, in particular, those fluids being detrimental to a fluid reducing fluid and degrading them rapidly such that intermediate introduction of a friction reducing fluid is required.

The present invention also comprises apparatus for introducing a friction reducing fluid into a tubing string contained within a normally cased and completed wellbore, thereby forming an annulus between the casing and tubing string. The apparatus comprises means for injecting the friction reducing fluid into the annulus formed between the completed casing and tubing string. Also, within the apparatus of the present invention are one or more tubing joints containing injection slots joining the tubing string together in conjunction with means for segregating the tubing string from the completed casing below the lowest injection slotted tubing joint in the wellbore. In general, the means for injecting the friction reducing fluid may comprise a pressure pump or other suitable injection apparatus with the means for segregating the tubing string from the completed casing comprising a hydraulic set packer or other means for isolating the lower portion of the wellbore such that fluid injected into the annulus, formed between the casing and tubing string, must traverse the annulus and be injected through the injection slots within the tubing joints into the tubing string. For particular applications of the apparatus of the present invention, means for controlling the injection of friction reducing fluids through the injection slots may be provided so that an on-off series of injection ports are exposed to the friction reducing fluid and intermittent injection of friction reducing fluids to the tubing string may be conducted. When particularly high pressures are to be encountered within the tubing and casing strings; a concentric casing string may be hung about and isolated from the tubing string to provide added strength for the increased pressures.

BRIEF DESCRIPTION OF THE DRAWING

The present invention may be more fully understood by referral to the accompanying drawing in which the FIGURE represents one embodiment of the present invention wherein apparatus is shown for performing the method of increasing the injectivity of an injection well during the injection of a hydraulic fluid.

DETAILED DESCRIPTION OF THE INVENTION

The inherent problem solved by the apparatus and method of the present invention is the reduction of the opposing friction head on production or injection wells, for example those wells producing a viscous crude oil and a reduction of friction losses while injecting treating fluids into injection wells, for example in an acidizing treatment of a formation. The invention is particularly advantageous wherein, by the injection of a friction reducing fluid at the surface, the friction reducing fluid would be degraded by the production or injection fluid at some point within the tubing string such that friction losses over the entire tubing string would not be incurred.

The particular advantages of the present invention may be more fully understood by referral to the accompanying FIGURE which represents one embodiment of the present invention wherein apparatus and a method are depicted for increasing the injectivity of an injection well during the injection of a hydraulic fluid for the hydraulic fracturing of a formation. In particular, a well 14 is completed open hole into a formation 13 through overburden 12 from the earth's surface 11. The well is cased and completed by casing string 15 extending from the surface to the top of the formation 13. Injection tubing string 16 is hung in the well intact or isolated from the casing string 15 by isolation means, for example hydraulic packer 23. The tubing string contains a series of slotted tubing joints 17 or couplers in which the slots 24 may be normally in the closed position in the completion of the tubing string 16, but many be opened by means for controlling the rate of friction fluid injection, for example by rotation of the tubing string 16 at the earth's surface such that the slots 24 may be activated upon demand. One or more of the tubing joints 17 having slots 24 may be activated, either sequentially or simultaneously by the amount of rotation of the tubing string 16, to regulate the flow of friction reducing fluid therethrough. Connected to an annulus 25, formed between the casing 15 and tubing string 16, is a fluid injection means, for example pump 18 which has an inlet 19 through which a friction reducing fluid 22 may be introduced. The tubing string 16 similarly has an inlet 20 through which hydraulic fluid 21 may be introduced.

Therefore, in operation of the process of the present invention hydraulic fluid 21 is normally introduced into inlet 20 and through injection tubing string 16 into the formation 13 of interest so as to cause hydraulic fracturing as depicted by hydraulic fractures 26 through the introduction of the hydraulic fluid at the bottomhole 23. Because of the large amount of friction loss normally encountered in hydraulic fracturing of viscous fluids, a friction reducing fluid 22 is introduced into the inlet 19 of pump 18, traverses the annulus 25 and the well 14 and is introduced through the tubing joint 17 and the slots 24 contained therein so as to commingle with the hydraulic fluid 21 and thereby reduce the head or friction losses due to the wall intervention with the viscous hydraulic fluid 21 and allow a lower pumping rate requirement for the injection of the hydraulic fluid 21. The apparatus of the present invention allows for continuous incremental injection of friction reducing fluid along the tubing string. In conventional injection of a friction reducing fluid, the friction reducing fluid would be destroyed by, for example, an acidic treating fluid, such as utilized in wellbore acidizing or hydraulic fracturing before the friction reducing fluid and the treating fluid had traversed the length of the injection tubing string. Through spaced injection within the tubing string, the degradation of polymers, which may be used as a friction reducing fluid, is counteracted by the reintroduction of further polymer or other fluid friction reducing material lower in the injection string. The friction reducing fluid enters the tubing through the ports, opened by manipulating the tubing, and serves to supplement the friction reducing component originally contained in the treating fluid which has undergone attrition due to chemical reaction and thermal degradation.

It is preferred that friction reducing fluids, for example aqueous solutions selected from the group consisting of the synthetic polymers sodium carboxymethyl cellulose, acrylate-acrylamide copolymer, guar gun, and other natural polymers or oil solutions of polyisobutylene, polymethacrylates, etc., be utilized in the method and apparatus proposed. Particularly in the production of a viscous crude, a solvent may be injected down the annulus and through the slotted production tubing joints to reduce the fluid's viscosity and therefore allow greater pumpability or gas lift, should the solvent be volatile. The solvent may be recovered from the subsequently produced fluids and recycled by again injecting down the annulus and through the tubing joint ports. Similarly, a friction reducing agent, not necessarily a solvent, such as polyisobutylene, may be injected and recycled in the same manner to both change the desirable flow properties of the crude and form a friction reducing film on the inner surface of the production tubing to effect greater pumpability. In reducing the surface pressure required to inject acid or water base well treating fluids, oil or aqueous solution of the friction reducing polymers treated with 1-2% KCl or CaCl may be utilized to prevent clay dispersion and other forms of formation permeability reduction.

It is desirable for the friction reducing fluid and the injected or produced fluid to be relatively insoluble. Hence, it is better to use oil solutions of polyisobutylene, polypropylene or polymethacrylates to reduce friction when producing or injecting oils, oil based emulsions, etc. It is also desirable that the friction reducing fluid have approximately the same density as the injected or produced fluid in order to prevent gravitational segregation during possible intermittent pumping operations.

To overcome the restrictive frictional pressure losses experienced in simultaneously pumping very viscous acid and oil emulsions down the total length of tubing, the concentrated acid with an inhibitor may be injected down the tubing while simultaneously injecting oil and surfactant down the annulus. The shearing action induced by the flow of the annulus fluid into the inhibited acid through properly sized tubing joint ports, placed near the depth of the formation to be treated, will cause emulsification. Thus the extreme pressure losses associated with the flow of the emulsion through the tubing are only generated over the short interval of tubing between the point of introduction of the oil into the tubing, through the tubing joint ports, and the treated zone. The surface pressure required to inject the acid emulsion are thereby reduced significantly below that required to inject the prepared emulsion at the surface.

The particular advantages derived from use of the present invention may be more fully illustrated through the following Example:

EXAMPLE

During normal operations, the natural polymer formed by guar gum becomes totally ineffective as a friction reducing agent in 15 percent hydrochloric acid after a contact time of 5-10 minutes at temperatures as low as 120.degree.F. Using a dissipation time of 5 minutes for design purposes, a treatment requiring 3 BPM total fluid at the formation face down 27/8 inches tubing inside 7 inches casing at a depth of 12,000 foot is designed as follows: ##SPC1##

This phenomena may be described for the lth spacing by the following equations:

Q.sub.1 = Q.sub.o - q.sub.o = Q.sub.o [1 - (Ct/Ca)]

V.sub.1 = (5.6146 Q.sub.1 /A.sub.1) = (5.6146 Q.sub.o /A) [1 - (Ct/Ca)]

l.sub.1 = (5.6146Q.sub.1 /A)t = (5.6146/A.sub.1) .DELTA.t.Q.sub.o [1 - (Ct/Ca)] = l.sub.o [1 - (Ct/Ca)]

q.sub.1 = Q.sub.1 (Ct/Ca) = Q.sub.o [1 - (Ct/Ca)] (C.sub.i /C.sub.o)

wherein:

C.sub.t = 10 pounds guar gum/1000 gallons;

C.sub.o = 100 pounds guar gum/1000 gallons;

Q.sub.o = 3 BPM;

v.sub.o = (5.6146 Q.sub.o /A) = 520 fee/minutes;

l.sub.o = V.sub.o t = 2590 feet;

q.sub.o = Q.sub.o (Ct/Ca) = 0.3 BPM;

a = cross sectional area. of TBG., FT.sup.2 ;

.DELTA.t = Dissipation time;

C.sub.i = Concentration of friction reducing agent required in the tubing string; and

C.sub.o = Concentration of friction reducing agent in solution.

Using these general equations and the given values, the following quantities are calculated for each step of l.sub.k values to locate the slotted tubing joints as shown on the above diagram.

l.sub.o = 2590' Q.sub.o = 3 BPM q.sub.o = .3 BPM l.sub.1 = 2330' Q.sub.1 = 2.7 BPM q.sub.1 = .27 BPM l.sub.2 = 2100' Q.sub.2 = 2.43 BPM q.sub.2 = .243 BPM l.sub.3 = 1890' Q.sub.3 = 2.185 BPM q.sub.3 = .219 BPM l.sub.4 = 1700' Q.sub.4 = 1.97 BPM q.sub.4 = .197 BPM l.sub.5 = 1390' Q.sub.5 = 1.77 BPM

then using friction data for 15% HCl with 10 pounds of guar gum per 1000 gallon entering the tubing and water with 1000 pounds of guar gum per 1000 gallon entering the annulus, the pressure drop across the ports in the tubing joints are determined. The ports are then sized using a modified orifice equation:

d = (3.08 Q.sup.2 /.DELTA.P) .sup.1/4

wherein:

.DELTA.P = Pressure differential between annulus and tubing at the slotted tubing joint, psi; and

Q = Flow Rate per Hole, BPM.

The number of tubing joint openings and size of each opening are as follows:

d.sub.o = 0.216" - 1 port d.sub.1 = 0.219" - 1 port d.sub.2 = 0.162 - 2 ports d.sub.3 = 0.125 - 4 ports d.sub.4 = 0.827 - 4 ports

The tubings joint ports are thus located in the string and sized.

The treatment would be initiated by opening the ports and circulating to reduce borehole temperature. The ports would then be closed and pressure applied through the tubing to open the formation. After "breakdown" is observed, the ports would be opened and friction reducing solution injected down the annulus a.SIGMA.q.sub.k flow rate or 1.239 BPM as the treating fluid is injected down the tubing at Q.sub.5 or 1.77 BPM.

It is understood that various and sundry means are available for the introduction of the friction reducing fluid into either the injection or production tubing string; for example, the normal bubble cap valve assemblies utilized in gas lift operations may be also utilized for the introduction of the friction reducing fluid into the tubing strings in the present invention. The valves may be hydraulically activated through increased pump pressure or may be electrically activated from the surface so as to be in a normally closed position, but be open upon responsive demand for the introduction of friction reducing fluid. The apparatus may be utilized conjunctively with gas lift operations and for the hydraulic fracturing of a low permeability hydrocarbon containing reservoir having relatively low pressures with no measurable open flow. Friction reducing fluid may be introduced in the tubing string conjunctively with the injection of a hydraulic fracturing fluid to increase the effective wellbore radius of the reservoir, but during a production cycle, gas may be introduced into the tubing string, through tubing joint ports, for gas lift and production of the hydrocarbons.

Through the utilization of the present invention it becomes apparent that both the apparatus and method described herein allow the reduction of friction losses primarily due to wellbore wall effects of viscous materials in production and injection tubing strings. Therefore, through utilization of the invention of the present invention, a simplified and economic solution to wellbore friction loss in production and injection wells is provided conjunctively in the utilization of friction reducing fluids which would normally degrade upon introduction into a tubing string. A method and apparatus is provided for the continuous injection of a friction reducing fluid such that continual makeup for the degraded friction reducing fluid is obtained.

The invention has been described herein with respect to particular embodiments and aspects thereof. It will be appreciated, however, by those skilled in the art, that various changes and modifications may be made without departing from the scope of the invention as presented.

Claims

Therefore, I claim:

1. A method of reducing wall frictional forces within an oil well tubing string which comprises introducing a friction reducing liquid at various depths along the tubing string.

2. The method of claim 1 wherein the friction reducing liquid is introduced at a pressure greater than that contained within the tubing string.

3. The method of claim 2 wherein;

a. the friction reducing liquid is introduced into a production well which is producing a viscous crude oil; and

b. the friction reducing liquid is selected from the group consisting of aqueous solutions of sodium carboxymethyl cellulose, guar gum and acrylate-acrylamide copolymer and oil solutions of polyisobutylene and polymethacrylates.

4. The method of claim 2 wherein:

a. the friction reducing liquid is introduced into an injection well through which is injected a viscous hydraulic fracturing fluid; and

b. the friction reducing liquid is selected from the group consisting of aqueous solutions of carboxymethyl cellulose, guar gum and acrylate-acrylamide copolymer and oil solutions of polyisobutylene and polymethacrylates.