US3369604A

US3369604A - Steam stimulation in-situ combustion backflow process

Info

Publication number: US3369604A
Application number: US501740A
Authority: US
Inventors: William M Black; John H Nichols
Original assignee: Exxon Production Research Co
Current assignee: ExxonMobil Upstream Research Co
Priority date: 1965-10-22
Filing date: 1965-10-22
Publication date: 1968-02-20
Anticipated expiration: 1985-02-20

Description

w. M. BLACK ETAL 3,369,604

STEAM STIMULATION INSITU COMBUSTION BACKFLOW PROCESS Filed Oct. 22, 1965 FIG.

FIG. 2.

INVENTORS. WILLIAM M. BLACK, BY JOHN H. NICHOLS ATTORNEY.

United States Patent 6) W ABSTRACT F THE DISCLOSURE A method for stimulating subsurface oil-containing formations by steam stimulation and in situ combustion. Steam is first introduced into a subsurface formation through an injection-production well to condition the well bore and formation for the following in situ combustion step. The steam is followed by an oxygen-containing gas which causes spontaneous ignition and burning of a portion of the formation oil to raise the temperature of the formation remote from the Well bore and reduce viscosity of the oil therein. Injection of the oxygen-containing gas is then halted and the formation oil is pro duced through the injection-production well.

The present invention concerns stimulation of subsurface oil-containing formations in order to increase production of oil. More particularly, the present invention concerns two known techniques used to stimulate viscous oil wells, steam stimulation and in situ combustion. The same well is used for the injection of stimulation fluids and the production of oil.

Steam stimulation is the addition of thermal energy to a subsurface reservoir by injection of steam, usually of 60 to 90 percent quality, through a producing well for a period, following which the well may be closed in for a period prior to being returned to productive status. This technique is often referred to as steam soaking, and in the stimulation of viscous oil wells it is widely practiced. It has two main effects, reduction of oil viscosity and well bore cleanup, including removal of skin effect around the well bore. For purposes herein, by skin effect is meant a skin or cylinder of reduced fluid conductivity about the well bore as described on page 303 of Applied Petroleum Reservoir Engineering, by Craft and Hawkins, published by Prentice-Hall, Inc., 1959. In general, steam is injected into the subsurface formation to be stimulated in quantities suflicient to heat up to about feet of the formation radially from the well bore. The pressures may range from 500 p.s.i.g. to 2500 p.s.i.g. depending upon depth of the formation and the permeability thereof. Also, in general, steam may be injected in the range of 20,000 pounds/hour, more or less, for seven days to six months. A typical field operation involves 20 to days of steam injection (10 to 15 billion B.t.u. injected). This quantity of heat usually produces a twentyto fifty-fold immediate production increase when the well is placed on production following the steam injection and soak period. However, this stimulation does not last long, and, in addition, the total volume of reservoir heated is not large.

In situ combustion refers to a process in which a portion of the underground oil is burned in place to provide high temperatures and heat and cause a reduction in viscosity of the formation oil adjacent the burned out interval, or which may be caused to enter the burned interval. In this process an oxidizing gas, such as air, or a mixture of air and oxygen or other gases capable of sustaining combustion of the formation hydrocarbons, is introduced into the formation through the production well bore. Then, the combustible mixture is ignited in any desired manner as by electric bore hole heaters or chemical 3,309,504 Patented Feb. 20, 1968 catalysts, such as phosphorus, triethylborane, linseed oil, etc., or, most economically, purely by spontaneous combustion. The oxidizing gas is continuously supplied to the formation to maintain combustion of the subsurface hydrocarbons. When the desired quantity of heat has been generated by such combustion, injection of the oxidizing gas is discontinued, and burning ceases. In the technique of in situ combustion stimulation, the well is then returned to productive status. The rate of injection of air or other oxidizing gas may range from 50 to 2,000 M c.f./d. for seven days to three months. The rate and time period to be employed in a particular case will depend upon characteristics of the reservoir treated in this manner.

In practice, in situ combustion stimulation yields fiveto ten-fold production increases after seven to ninety days injection of air at 2000 M c.f./ d. This stimulation effect is usually not as large in terms of peak fold (multiple) increases as with the steam soaking technique; however, the stimulation often persists for a much longer time with equal or greater ultimate stimulated oil volumes.

The multiple production increases achieved by either the steam soaking technique or in situ combustion technique are larger than theoretical considerations would suggest, unless the existence of a skin effect prior to stimulation is taken into account. The present invention takes into consideration the fact that steam soak relatively safely removes or reduces the skin effect, while the average in situ combustion operation does not (unless ignition techniques are employed which will cause ignition inside or within inches of the well bore, in which event, there is serious hazard of burning up the well casing or liner). Calculations provided later herein illustrate this concept.

Experience with in situ combustion stimulation indicates that even an increased skin effect may result in the average in situ combustion operation where combustion does not or intentionally is not allowed to begin at or in the well liner. This stems from creation of a zone or cylinder of weathered or partially oxidized crude oil Within the pro-ignition radius. In addition, laboratory experiments have shown that spontaneous ignition is very difficult (at least very slow) to achieve when ambient reservoir temperature is less than 200 F., i.e., the autooxidation reaction is very slow for the first of temperature rise from, e.g. 100 to 200 F., after which it proceeds quite rapidly to ignition temperature of 500 to 700 F. Consequently, steam injection is useful to raise the temperature of the reservoir to a level (greater than 200 P.) where the reaction rate is sufficiently high to quickly achieve ignition temperatures via autooxidation. Additionally, a period of steam injection will improve the injectivity profile or uniformity of entry of a gaseous oxidizing material, such as air, that may be injected subsequently.

A primary purpose of the present invention, then, is to produce synergistically, stimulation multiple production increases comparable to steam soak with total stimulation volumes comparable to or larger than in situ combustion or steam soak stimulation operations alone.

Briefly, then, the invention comprises a method for stimulating viscous oil producing wells by injecting steam and air in sequence. More particularly, the invention cornprises the steps of introducing steam into a subsurface hydrocarbon-containing reservoir through a production well for a selected period of time sufiicient to remove skin and precondition the completion interval to receive a following oxidizing gas more uniformly; halting introduction of the steam; then, injecting an oxidizing gas into the subsurface formation through the production well, igniting the reservoir oil while continuing injection of the oxidizing gas and burning a portion of the resident reservoir hydrocarbons to raise the temperature of the and in situ combustion for 30 days, after which the hydrocarbons are produced. The well bore skin is removed or reduced by the steam and the oil viscosity remote in the reservoir is reduced by the in situ combustion. The completion interval is preconditioned to receive the oxidizing gas (air) more uniformly by steam cleaning. Further, the oil in the production pay zone may be more quickly spontaneously ignited for in situ combustion because the temperature of the oil is raised by the heat transmitted to it by the injected steam.

A more economical and effective method of treating or stimulating viscousoil producing wells is achieved by combining in sequence the techniques of steam soak and underground combustion than either the steam soak method or cyclic underground combustion method alone afford.

The above object and other objects and advantages of the invention will be apparent from a more detailed description thereof when taken with the drawing in which:

FIGS. 1 and 2 are schematic views of a well bore vanclvarious zones of interest surrounding it.

Calculations to illustrate theadvantages of combining steam soak and in situ combustion according to the invention are set forth below. The calculations demonstrate semiquantitatively the merits of a well stimulation procedure involving a short steam soak followed by conventional in situ combustion stimulation treatment.

Referring to FIG. 1:

Let

r =Radius of the well liner or perforated casing r Radius of a zone of restricted permeability due to a primary depletion plugging mechanism (skin) (asphalteen deposition and fines accumulation, for example) r =Radius of heated zone r =Drainage radius of well.

And from FIG. 6.32 of Craft & Hawkins Ref. PR: 1.87.

(C) PR of well after only conventional in situ stimulation; (skin isnot adequately removed) Let K.,=K and ,t,=0.6/50

The expansion of Craft & Hawkins basic Equation 6.36 on page 295 ofRef. to provide three zones instead of 2 zones, i.e. radial zones (see FIG. 2), is as follows:

(D) PR of well after steaming to remove skin and then in situ combustion Let r =O.33 ft.; r doesnt exist; r =24 ft. K K, and g .3 Virgin wen 58 fold production Increase =47 fold production increase 10 fold production increase The foregoing numerical results are in general agreement with field experience for steam soak and cyclic in situ combustion techniques. In one reservoir 27-day steam soaks onlOO ft. sd for 12 billion B.t.u. injected produced twentyto fifty-fold production increases, which, however, declined to twoto three-fold in a few months. In another reservoir in which an in situ combustion operation was carried out, fiveto ten-fold increases were obtained which took one to two years to decline to twoto three-fold.

The calculated numerical results above are controlled largely by the assumedseverity of skin damage rather than the values assumed for the r or radius of heating; consequently, the (r steam) (r burn) is not an important factor. These values could be considerably different from the assumed ones without substantially affecting the results. Also the ratio of (AP steam/AP burn) is ignored, i.e., assumed equal.

It is also apparent that the skin must exist for the multiple increases to be on the order shown above. Without skin the denominator in A, B, C, under Resulting Stimulation Ratios heading, would be 1.0 and the corresponding multiple increases would be 1.9 and 2.3. for steam soak and steam soak plus combustion, respectively.

Having fully described the method, operation, objectives and advantages of the invention, we claim:

1. A method of well stimulation comprising the steps introducing steam into a subsurface hydrocarbon-containing reservoir through an injection production Well for a selected period of time sufficient to remove skin effect and pre-condition the completion interval to receive a following oxidizing gas more uniformly and to raise the temperature of said reservoir to a reaction temperature sufiicient to ignite spontaneously said hydrocarbons upon introduction of said following oxidizing gas;

halting introduction of steam;

thereafter injecting said oxidizing gas into said subsurface reservoir through said production well and igniting the reservoir oil, continuing injection of said oxidizing gas and burning a portion of the resident reservoir hydrocarbons to raise the temperature of reservoir rock remote from the well how, thereby reducing the viscosity of remaining reservoir hydrocarbons;

thereafter halting injection of said oxidizing gas; and

then producing said reservoir hydrocarbons through said production well.

2. A method as recited in claim 1 in which said oxidizing gas is air.

. about seven days to three months at the rate of from 50' to 2000 million cubic feet per day.

References Cited UNITED STATES PATENTS 6/ 1958 Walter 16639 X 4/1964 Sharp 16611 4/ 1965 Strange 166-11 7/1966 Dietz 16611 FOREIGN PATENTS 8/1939 Great Britain.

STEPHEN J. NOVOSAD, Primary Examiner.