US3630281A

US3630281A - Explosive fracturing of petroleum bearing formations

Info

Publication number: US3630281A
Application number: US875843A
Authority: US
Inventors: Clarence R Fast; George C Howard; Morton A Mallinger
Original assignee: BP America Production Co
Current assignee: BP America Production Co
Priority date: 1969-11-12
Filing date: 1969-11-12
Publication date: 1971-12-28
Anticipated expiration: 1988-12-28

Abstract

This is a method for increasing the permeability of an underground formation adjacent a well bore by explosive fracturing. In one embodiment an explosive slurry, having selected fracturing fluid characteristics, is used to hydraulically fracture the formation and such explosive slurry is subsequently detonated. In another embodiment a sorbent solvent slug is injected down the tubing preceding the injection of the explosive slurry and a second such sorbent solvent immediately follows the explosive slurry. This avoids trapping of air or other gas and therefore prevents premature ignition or detonation of the explosive fracturing fluid.

Description

United States Patent Clarence R. Fast;

George C. Howard; Morton A. Mallinger, all of Tulsa, Okla.

[21] Appl. No. 875,843

[22] Filed Nov. 12, 1969 [45] Patented Dec. 28, 1971 [73] Assignee Amoco Production Company Tulsa, Okla.

[72] Inventors [54] EXPLOSIVE FRACTURING OF PETROLEUM BEARING FORMATIONS 8 Claims, 4 Drawing Figs.

52 us. Cl 166/290, 102/23, 166/299 [51] Int. Cl E2lb 43/26 [50] Field of Search 166/299,

[56] References Cited UNITED STATES PATENTS 3,561,532 2/1971 Fletcher et al 166/299 2,481,422 9/1949 Haynes et al. 166/70 X 2,708,876 5/1955 Nowak 166/299 2,892,405 6/1959 Chesnut 166/299 2,892,406 6/1959 Hradel et al. 102/23 3,075,463 1/1963 Eilers et al. 166/299 3,191,678 6/1965 l-linson 166/299 3,336,982 8/1967 Woodward et a1 166/299 OTHER REFERENCES Frac Shot, Richmond, Va., Reynolds Metals Co., 1961 Primary Examinerlan A. Calvert Attorneys-Paul F. Hawley and John D. Gassett ABSTRACT: This is a method for increasing the permeability of an underground formation adjacent a well bore by explosive fracturing. In one embodiment an explosive slurry, having selected fracturing fluid characteristics, is used to hydraulically fracture the formation and such explosive slurry is subsequently detonated. In another embodiment a sorbent solvent slug is injected down the tubing preceding the injection of the explosive slurry and a second such sorbent solvent immediately follows the explosive slurry. This avoids trapping of air or other gas and therefore prevents premature ignition or detonation of the explosive fracturing fluid.

36\ NITROGEN EXPLOSIVE SORBING SOLVENT PATENTEU DEE28I97| I 3530.2 1

SHEET 1 UF 3 6\ NITROGEN EXPLOSIVE 49 54 52 I SORBfIENG SOLV NT 5 7O 64 22 66 67 23 72 WATER IO s I2 3% E 1;; CLARENCE R. FAST g GEORGE c. HOWARD MORTON A. MALLINGER INVENTORS FIG. I ATTORNEY PATENTEDBEB28|97| 3,830,281

SHEET 2 OF 3 EXPLOSIVE SORBING N SOLVENT 23 WATER E 29 30 A 62 mi l c g Q I 2 CLARENCE R. FAST GEORGE C. HOWARD MORTON A. MALLINGER INVENTORS ATTORNEY SHEET 3 BF 3 FIG.

CLARENCE R FAST GEORGE C. HOWARD MORTON A. MALLINGER INVENTORS ATTORNEY PATENTED [H28 191:

EXPLOSIVE FRACTURING OF PETROLEUM BEARING FORMATIONS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of increasing the permeability in a formation adjacent an oil and gas well which penetrates therethrough. It relates especially to a system in which an explosive fracturing fluid is used as a hydraulic fracturing fluid and is subsequently detonated. It also relates to ways of preventing premature ignition of the explosive fracturing fluid and a safer way of igniting the explosive fluid.

2. Setting of the Invention Many oil and gas wells are drilled into petroleum bearing fonnations which have low permeability. That is, the formations are of a character that is difficult for the oil to be recovered; in other words, the formation is very reluctant to give up its petroleum. When such formation rocks are encountered, it becomes necessary to resort to one or more means to increase the permeability. One means which has been quite successful is the hydraulic fracturing technique. In this system a special hydraulic fracturing fluid is injected through the well bore adjacent to the formation. Fluid is continued to be pumped into the well bore so as to increase the pressure to the point where the formation is fractured. When this fracturing occurs there are usually propping agents left in the fractures so that the fractures will remain open, forming new paths for the oil to flow from the rock into the well bore. Frequently, this type stimulation does not result in desired production increases as it does not reach the unfractured matrix of the formation. Although hydraulic fracturing has permitted the production of billions of barrels of oil not heretofore recovered, there is still some room for improvement.

BRIEF DESCRIPTION OF THE INVENTION An explosive slurry having fracturing fluid characteristics is injected into the well bore at a pressure and rate sufficient to fracture the formation. This explosive fracturing fluid is detonated in situ deep within the formation rock. In addition to the fracture caused by an explosive fluid acting as a fracturing fluid, many side fractures are formed when an explosive is detonated. Special precautionary steps are also taken to avoid trapping of air or other displacement gas which sometimes causes premature ignition or detonation of the explosive fluid. This is accomplished by injecting a slug of sorbent solvent, which is a desensitizer for the explosive used, immediately preceding the injection of an explosive slurry and a second such slug immediately following the injection of the explosive slurry.

DESCRIPTION OF THE DRAWINGS Various objects and a better understanding of the invention can be had from the following description taken in conjunction with the drawings.

FIG. 1 is a vertical view, partly in section and partly schematic, of a well penetrating an underground producing formation.

FIG. 2 illustrates the lower end of a well bore which penetrates a producing formation in which the explosive liquid has penetrated the formation.

FIG. 3 illustrates stemming in a well in which an explosive liquid has been placed under pressure.

FIG. 4 is an enlarged view of one plug used in the tubing of FIGS. 1, 2 and 3.

DETAILED DESCRIPTION OF THE INVENTION Attention is first directed to FIG. 1 which illustrates schematically a well equipped to carry out this invention. Shown thereon is a well bore which penetrates a producing formation 12. An upper part of borehole 10 is cased with a casing 14 cemented in place in a conventional manner. In this particular setting, casing 14 does not extend through the producing formation l2. Suspended within casing 14 is a string of tubing 16 having a sub 18 at the bottom and a removable plug insertion head 21 on top containing a conductor cable-packing gland 58. Sub 18 has plug catcher 20 on the inside thereof. As will be explained more fully hereinafter, catching means 20 permits plug 22 to pass therethrough but catch plug 24. Plug 22 is shown retained in the upper part of the tubing by plug retainer pin 23 and is just immediately ahead of the explosive fluid. Plug 24, retained by plug retainer pin 25, trails the fracturing fluid. The details of

plugs

22 and 24 and catching means 20 will be described in detail hereinafter. The lower end of the annulus between tubing string 16 and casing 14 is sealed by packer 26 which can be a drillable production packer. The upper or main part of tubing string 16 can be disconnected from sub 18 by simply rotating and lifting. This, as will be seen, is useful for placing cement in the well bore above packer 26.

A pipeline 29 having valve 30 therein connects the annulus 32 of the well bore with a pit or tank not shown. This is useful in passing a fluid such as cement slurry into the casing 14 above packer 26.

Attention will now be directed briefly toward that part of the equipment located on the surface which is used for injecting the liquid explosive into the well bore. This includes a container or source 34 having an explosive slurry or liquid therein. A nitrogen source 36 is connected through line 38 having valve 40 to drive the liquid explosive from container 34. The outlet of container 34 is connected through line 42 to a wellhead manifold 49. Line 42 has a valve 48 adjacent container 34. The outlet of pump 44 is connected through line 46 to tubing string 16 through manifold 49. Pump 44 is a highpressure pump such as a triplex plunger positive displacement pump capable of exerting sufficient pressure and rate to fracture formation 12. A sorbent solvent source indicated as container 50 is provided. This has an outlet line 52 which connects into line 64 leading to pump 44. Line 52 has valve 54 therein adjacent container 50. A water source 62 connects through conduit 67 to conduit 64. Conduit 64 is connected to pump 44.

An electric-conducting line 56 extends through a packing 58 in the upper end of tubing string 16. This line 56 extends downwardly through top plug 24 and is connected to and supports an igniter or detonator 60.

Characteristics of Liquid Explosive In a preferred embodiment we propose to use the explosive liquid itself as the fracturing fluid. SUch fluid should have a fracturing fluid coefficient, C, of 0.01 feet per square root a minute, or less, (fluid coefficient, C, is a term which means volume lost per distance per unit of time or feet cubed/feet squared/time in square root of minutes) and a fluid loss of less than cc. per 30 minutes (API), be essentially water immiscible and somewhat oil miscible. The water immiscibility allows the displacement of the liquid explosive down the tubing string with water. On the other hand, the oil miscibility insures desensitization of the explosive during the production phase. Additionally, the liquid explosive should be capable of transporting propping agents, such as sand or predominantly aluminum, into the fracture. This means that the liquid explosive should have a plastic viscosity of greater than 100 cp. This liquid explosive must be able to support detonation in the narrow confines of the fracture of a formation which may, in some cases, be less than about one thirty-second inch. A suitable liquid explosive is one in which nitromethane is the predominant composition and contains a thickening agent, sensitizer and other additives tailored to the bottom-hole temperature, pressure and type formation. A suitable commercially available liquid explosive is Tal-l005, which is available from Talley Industries, Inc., Mesa, Arizona. Nitromethane-based liquid explosives are especially desirable as they can be ignited and if the pressure builds up, the deflagration will progress to high-order detonation and unburned nitromethane will detonate, causing the desired explosion. We show means to confine the nitromethane-based liquid explosive in the well bore. We can then use a flame igniter. The liquid explosive then can be flame squibbs and a propellant. This is much safer than a time bomb containing a high explosive.

Operation System A sorbent solvent from container 50 is released through line 52 with valve 54 open to line 64. This goes in through manifold 49 on top of bottom plug 22. In the past, attempts to displace liquid explosives in oil wells, resulted in the explosives being placed in the tubing string and then followed by a displacing plug. During displacement operations, air, displacing gas, or vapors from the explosive are injected on top of the bottom plug. Under some conditions, these mixtures of air, displacement gas, and vapors when compressed under bottom-hole temperature can cause explosives to ignite or detonate prematurely. We overcome this problem by injecting a slug of sorbent solvent into the tubing just above bottom plug 22 and into the tubing string just below top plug 24 after all liquid explosive has been pumped into the tubing string. A sorbent solvent suitable for this type operation is one that desensitizes the explosive and, for example, can be dibutyl phthalate, toluene, benzene, or the like. This sorbent combination or solvent absorbs or dissolves any gas and desensitizes any explosive with which it comes into contact. No compounds containing an explosive sensitizer such as amines or acids should be used as sorbents.

Having described the suitable liquid explosives and a suitable sorbent solvent, attention will now be directed toward their injection. As mentioned, a slug of sorbent solvent is injected on top of plug 22. This volume should be enough to occupy several feet of tubing string. At this time valve 54 is closed and valve 48 is opened, thus connecting the liquid explosive container 34 with manifold 49. This liquid is displaced by nitrogen into the tubing string at the selected pressure and rate of flow. Continued pressuring of the liquid explosive forces the bottom plug 22 downward through the bottom end of the tubing string when it drops down to the borehole bottom. The injection of the liquid explosive continues as desired. If it is desired that the liquid explosive be used as a hydraulic fracturing fluid, then sufficient fluid is injected under sufficient pressure to cause such fracture. Quality and characteristics of hydraulic fracturing fluid are well known. When the total amount of liquid explosive is injected into the well, valve 48 is closed, closing off the liquid explosive source, and valve 54 is opened and a second slug of sorbent solvent is injected into the tubing, this time on top of the explosive liquid just below top plug 24. This is done by opening valve 70 and closing

valves

68 and 72. Top plug 24 is then displaced downwardly by closing valve 54 and opening valve 66 connecting the pump to the water source 62 and valve 68 is opened and

valves

70 and 72 are closed. after which water is pumped on top of top plug 24. When the top plug reaches locking means 20 it is locked in position which confines the liquid explosive in the borehole below packers 26 as shown in FIG. 2.

Bottom plug 22 is a conventional cementing tubing wiper plug with a soft outer elastomer body and a hard metal or plastic core. The plug is designed to wipe the pipe walls clean as it moves down the tubing. The dimensions of plug 22 are such that the plug will pass through top plug latching means 20 shown in FIG. 4.

Top plug 24, sub 18 and plug latching means 20 are shown in detail in FIG. 4. Latching means 20 is simply a reduced diameter steel ring that is fitted in sub 18. Top plug 24 consists of

elastomer wiper portions

82 and 84 connected by steel center portion 86. The dimensions of center portion 86 are such that the bottom part 88 of the center portion 86 of plug 24 will pass through latching means 20. The top part 90 of center portion 86 of plug 24 is preferably a metal ring which will not pass through latching means 20. Plug 24 also contains latching ring 92 which is a split spring steel ring that compresses on passing through latching means 20 and expands after passing through so that latching ring 92 bears against shoulder 94, thereby locking plug 24 in place so that it cannot move upward. Top part of the plug prevents downward movement. Top plug 24 and catcher 20 are commercially available from Baker Oil Tools, lnc., Los Angeles, California, and are identified as Model C Cementing Plug and Full Flow Fill Up Collar.

As shown in FIG. 2, the liquid explosive 70 fills the borehole and extends out into fracture 72. The flame igniter or explosive detonator 60 is positioned in the liquid explosive 70. It is also seen that the bottom plug 22 has dropped down to the bottom of the borehole. At this point, the upper part of tubing string 16 has been disconnected from sub 18. At this time, cement slurry is displaced down tubing string 16 and out the lower end thereof and onto the upper side of packer 26 and upper plug 24. This placing of the cement 74 is the first step of the important stemming of the explosive.

Attention is next directed to FIG. 3 which illustrates the liquid explosive extending out into the fractures filled by the liquid explosive and in which such liquid explosive is stemmed. Show thereon is the liquid explosive 70 with a sorbent solvent 76 just below packer 26. Above packer 26 is about feet of cement 74. It has been found that about 100 feet will ordinarily be adequate. Immediately above cement 74, sand 80 has been placed and extends a few hundred feet up into the casing. The remainder of the hole is filled with water to the surface. This stemming does two things: 1. It prevents the explosive from damaging the casing 14 and keeps the explosive force at the formation. 2. This confines the liquid explosive and in the case where the liquid explosive is predominantly sensitized nitromethane, detonation can be accomplished by the use of a flame igniter. Thus, when it is time to set off the explosive composition, an electrical current is sent down wire 56 to ignite the flame igniter or explosive detonator. In the case where a flame igniter is used, the buming nitromethane reverts to high-order detonation if the pressure generated is not relieved. As this liquid 70 is confined below the stemming, the pressure is confined, thus high order detonation occurs. There is quite an advantage to the technique of igniting nitromethane mixtures because an igniter is much safer to handle than a detonator, which is a high explosive initiated by a dangerous blasting cap.

Tests were conducted to evaluate the use of a sorbent solvent preceding and following the injection of a flammable liquid explosive to the well to prevent ignition. The tests were conducted in a pressure chamber fitted with an electrical heating unit and a thermocouple. A l milliliter sample of TAL-IOOSC liquid explosive (produced by Talley Industries lnc., Mesa, Arizona) was placed in the chamber and gas pressure held in the chamber with the thermocouple immersed in the explosive. Heat was applied to the chamber at a low rate and the temperature of the explosive measured until it ignited. lgnition was observed by a very sharp increase in temperature. The following results were observed:

These tests show that at a given pressure, the ignition temperature of a liquid explosive, in this test an explosive identified as TAL-lOOSC, produced by Talley Industries, lnc., Mesa, Arizona, is appreciably lower with oxygen present than with nitrogen. They also show that the ignition temperature of the explosive decreased with increasing temperature when oxygen was present but did not when the explosive was in a nitrogen atmosphere. These tests then suggest that a liquid sorbent should be injected preceding and following the liquid explosion to sorb any volatilized explosive that is produced when the well is on vacuum during explosive displacement.

While the above embodiments of the invention have been described with considerable detail, it is to be understood that various modifications of the device can be made without departing from the scope or spirit of the invention.

We claim:

1. A method of explosively fracturing a formation penetrated by a well bore which comprises:

a. injecting a first slug of a sorbent solvent into said well bore, the sorbent solvent used in this method being capable of desensitizing said liquid explosive which it contacts;

b. immediately following step (a) with the injection of a liquid explosive;

c. following step (b) immediately with injection of a second slug of a sorbent solvent; and

d. thereafter detonating said explosive liquid.

2. A method as defined in claim 1 including the step of stemming said liquid explosive above said second slug of sorbent solvent.

3. A method as defined in claim 1 in which said sorbent solvent is selected from the group consisting of dibutyl phthalate, toluene and benzene.

4. A method of using a liquid explosive to fracture an underground formation penetrated by a well bore in which a string of tubing is suspended which comprises:

inserting a bottom plug in said string of tubing;

placing a packer about the lower end of said string of tubing to seal the annulus between the string of tubing and the well bore;

injecting a first slug of sorbent solvent on top of said bottom plug, the sorbent solvent used in this method being capable of desensitizing said liquid explosive which it contact;

injecting said liquid explosive into said tubing string on top of said first slug, continuing injecting said liquid explosive until a selected amount has been injected and driving said bottom plug through the lower end of said tubing string; injecting a second slug of sorbent solvent into said string of tubing on top of said liquid explosive;

inserting a top plug on top of said second slug;

displacing said liquid explosive, said second slug and said top plug down said tubing string until said top plug reaches the lower end of said tubing string;

retaining said top plug in the lower end of said tubing string;

placing stemming material on top of said annulus packer and said top plug;

thereafter detonating said explosive liquid.

5. A method as defined in claim 4 wherein said step of placing stemming material comprises separating the tubing string from a lower section thereof and injecting cement down through said tubing string and onto the top of said top plug and said packer; and

removing said tubing string as said cement is displaced into said well.

6. A method as defined in claim 5 in which there is about feet of cement placed above said annulus packer and said top plug.

7. A method of explosively fracturing a formation penetrated by a well bore which comprises:

a. injecting a liquid explosive into said well bore;

b. immediately following step (a) injecting a slug of sorbent solvent, the sorbent solvent used in this method being capable of desensitizing said liquid explosive which it contacts; and

c. thereafter detonating said liquid explosive.

8. A method as defined in claim 7 wherein said liquid explosive and said sorbent solvent is directed down through a string of tubing and including the step of inserting a top plug in said string of tubing above said slug of sorbent solvent.

Claims

2. A method as defined in claim 1 including the step of stemming said liquid explosive above said second slug of sorbent solvent.
3. A method as defined in claim 1 in which said sorbent solvent is selected from the group consisting of dibutyl phthalate, toluene and benzene.
4. A method of using a liquid explosive to fracture an underground formation penetrated by a well bore in which a string of tubing is suspended which comprises: inserting a bottom plug in said string of tubing; placing a packer about the lower end of said string of tubing to seal the annulus between the string of tubing and the well bore; injecting a first slug of sorbent solvent on top of said bottom plug, the sorbent solvent used in this method being capable of desensitizing said liquid explosive which it contact; injecting said liquid explosive into said tubing string on top of said first slug, continuing injecting said liquid explosive until a selected amount has been injected and driving said bottom plug through the lower end of said tubing string; injecting a second slug of sorbent solvent into said string of tubing on top of said liquid explosive; inserting a top plug on top of said second slug; displacing said liquid explosive, said seCond slug and said top plug down said tubing string until said top plug reaches the lower end of said tubing string; retaining said top plug in the lower end of said tubing string; placing stemming material on top of said annulus packer and said top plug; thereafter detonating said explosive liquid.
5. A method as defined in claim 4 wherein said step of placing stemming material comprises separating the tubing string from a lower section thereof and injecting cement down through said tubing string and onto the top of said top plug and said packer; and removing said tubing string as said cement is displaced into said well.
6. A method as defined in claim 5 in which there is about 100 feet of cement placed above said annulus packer and said top plug.
7. A method of explosively fracturing a formation penetrated by a well bore which comprises: a. injecting a liquid explosive into said well bore; b. immediately following step (a) injecting a slug of sorbent solvent, the sorbent solvent used in this method being capable of desensitizing said liquid explosive which it contacts; and c. thereafter detonating said liquid explosive.
8. A method as defined in claim 7 wherein said liquid explosive and said sorbent solvent is directed down through a string of tubing and including the step of inserting a top plug in said string of tubing above said slug of sorbent solvent.