US3557874A

US3557874A - Method of drilling and completing a gas well

Info

Publication number: US3557874A
Application number: US862506A
Authority: US
Inventors: Vaughan W Rhoades; Eugene D Glass
Original assignee: Cities Service Oil Co
Current assignee: Cities Service Oil Co
Priority date: 1969-09-30
Filing date: 1969-09-30
Publication date: 1971-01-26
Anticipated expiration: 1988-01-26

Abstract

The well is drilled and cased to slightly above the gasproducing horizon. Drilling within the gas-producing horizon is performed by use of a nonaqueous drilling fluid. If the well is not sufficiently productive the adjacent formation is subjected to at least one stimulation by an explosive and test for adequate production. A hydraulic fracture network is induced by use of nonaqueous fracturing fluid, if appropriate stimulation is not achieved by the explosive means. The problem of irreversible wellbore damage created by water base drilling muds and aqueous fracturing fluids is avoided. Formation permeability restrictions in the vicinity of the wellbore are overcome by the use of the nonaqueous well completion technique.

Description

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Inventors Vaughan W. Rhoades;

Eugene D. Glass, Tulsa, Okla.

Appl. No. 862,506

Filed Sept. 30, 1969 Patented Jan. 26, 1971 Assignee Cities Service Oil Company Tulso, Okla. a corporation of Delaware METHOD OF DRILLING AND COMPLETING A GAS WELL Primary Examiner-Ian A. Calvert Attorney-J. Richard Geaman ABSTRACT: The well is drilled and cased to slightly above the gas-producing horizon. Drilling within the gas-producing horizon is performed by use of a nonaqueous drilling fluid. If the well is not sufficiently productive the adjacent formation is subjected to at least one stimulation by an explosive and test for adequate production. A hydraulic fracture network is induced by use of nonaqueous fracturing fluid, if appropriate stimulation is not achieved by the explosive means. The problem of irreversible wellbore damage created by water base drilling muds and aqueous fracturing fluids is avoided. Formation permeability restrictions in the vicinity of the wellbore are overcome by the use of the nonaqueous well completion technique.

METHOD OF DRILLING AND COMPLETING A GAS WELL BACKGROUND OF THE INVENTION This invention relates to the recovery of gas from subterranean reservoirs. More particularly it relates to the recovery of gases following the nonaqueous drilling of a gas formation and subsequent nonaqueous stimulation of said formation.

The use of nonaqueous drilling fluids in the completion of gas-bearing reservoirs is known in the art. The gas-bearing formation is drilled or cored by use of nonaqueous drilling fluid so that the cuttings or core section and the gas-bearing formation obtained are not contaminated by water and accurate laboratory tests may be performed to determine the true formation water content. It has been found that initial or subsequent contacting of low permeability gas formation with aqueous drilling fluids creates irreversible wellbore damage. This damage restricts the production of the reservoir fluid and yields an exceptionally low well productivity. Stimulation of the damaged wellbore is normally not effective as there exists presently no satisfactory means of extracting the introduced water trapped in the formation during drilling.

As a result of this irreversible wellbore damage, many gas wells have been abandoned. Many of these reservoirs were extensively stimulated by aqueous fluid fracturing and thought not to be of productive capacity. With the use of a proper completion technique some of these wells would have proved to be productive. Considerably higher well-drilling success ratios would have resulted in enormous reserves of gas having been released for industry consumption. A completion technique is required which can provide an unrestricted wellbore area from which these formations may produce.

It is an object of this invention, therefore, to provide an improved process for the recovery of gas from low permeability natural gas reservoirs.

It is another object of the invention to provide an improved technique for the completion of a subterranean gas reservoir.

It is another object of the invention to provide a technique for recovering gas from a subterranean reservoir by the use of nonaqueous drilling fluids and with proper stimulation obtained by the use of nonaqueous fracturing means.

It is still a further object of the present invention to provide for the proper completion of subterranean gas reservoirs by the use of nonaqueous drilling fluids and subsequent stimulation by nonaqueous fracturing means, and also to provide means for nonintroduction of water base drilling fluids, required for the completion of lower horizons, into the formations previously completed.

With these and other objects in mind, the present invention is hereinafter set forth.

SUMMARY OF THE INVENTION The objects of the invention are accomplished by a technique in which drilling with conventional drilling fluids, for example with water base muds, is ceased slightly above the gas-bearing formation. The wellbore is cased and cemented to the drilled horizon with extreme care so as to seal the formation from the wellbore. Subsequent drilling through the low permeability gas-bearing formation is accomplished by use of a nonaqueous drilling fluid. Formation productivity is determined by conventional well testing means. If stimulation is required a sequence of explosive charges is detonated in the wellbore opposite the gas bearing horizon. If proper stimulation is not achieved with the use of the explosive charges, the well is then fractured by use of a nonaqueous fracturing fluid. Means are provided to protect the gas-bearing formation wellbore should further drilling with water base muds be required.

DETAILED DESCRIPTION OF THE INVENTION The drilling method used prior to penetration of the gasbearing formation may consist of any conventional drilling practice such as rotary drilling with a water base mud. .Ex treme'care is required as the drill bit approaches the gas-bearing formation in order that no drilling mud filtrate is allowed to permeate the gas-bearing strata. All drilling with aqueous drilling fluid should be ceased far enough from the gas-bearing formation such that no aqueous fluid contacts or permeates the formation. Generally discontinued use of aqueous drilling fluids, 5 to l0 feet above the formation, is adequate. Low water loss muds or anhydrous base muds should be used to help alleviate the filtrate invention problem. These muds should have a high colloidal content so as to form a filtrate cake of low permeability which will prohibit circulation fluid loss to the gas-bearing formation. It is preferred in the embodiment of our invention to use an aqueous colloidal bentonitic clay-drilling mud while drilling in the vicinity of the gas-bearing formation whereby the rate of drilling fluid filtrate loss will be substantially reduced.

The casing and cementing operation should be performed with great caution to avoid any gas-bearing formation invasion by cement filtrate. The sealing agent used may be any oil well cement which is pumped down the casing as a slurry and upward into the annulus where it is allowed to harden and set. Mud cake remaining on the walls of the hole from the water base-drilling mud should be washed or eroded away so as to permit an appropriate bonding of the cement between the easing and walls of the hole. A washing procedure with water is customarily used to alleviate the residual mud cake and allow for the proper cementing of the casing to the well wall. Often this operation is supplemented by applying small scrapers attached to the casing, and reciprocating the pipe in the hole to clear any residue caking. However, in accordance with this invention, the mud cake is removed from the well wall by rinsing with a nonaqueous fluid. This procedure is to prevent any invasion of the gas-bearing formation by aqueous fluid which will create irreversible wellbore damage. The nonaqueous washing fluid also forms a barrier about the wellbore which restricts further invasion by the cement filtrate. Squeeze cementing may be required to complete the shutoff which may not have been entirely affected by the normal casing cementing job. Squeeze cementing such as with a low water loss cement is preferred to avoid the water damage so defined.

The gas-bearing formation may be drilled by use of any of the commonly used drill bits. While drilling through the gas bearing formation a suitable nonaqueous fluid should be used as a replacement for the drilling mud. The preferred nonaqueous drilling fluid should be chosen from the group consisting of readily available dry gases such as air, nitrogen, natural gas, and carbon dioxide but low viscosity, nonaqueous liquids having high volatility such as liquified petroleum gas, light naphtha kerosene or other light end hydrocarbon distillation products may be used. Materials of sufiicient volatility are required as they are easily withdrawn from the formation and,

therefore, do not create any irreversible wellbore damage.

The nonaqueous drilling of the gas-bearing formation should be ceased above any bottom water.

First the productivity of the gas reservoir is determined. Conventional open-flow testing and bottomhole pressure meas'urements are the most common form of productivity test, but any applicable method may be employed. If the reservoir is not sufficiently productive, a series of explosions in the wellbore followed by further productivity testing may be required. Should productivity still be insufficient after the explosive stimulation, the gas reservoir is subjected to fracturing with a nonaqueous fluid. Suitable fracturing fluids would include those chosenfrom the group consisting of air, nitrogen, natural gas, carbon dioxide, or any suitable normally gaseous material. Low viscosity nonaqueous liquids having a high volatility such as liquified petroleum gas, light naptha kerosene and other light end hydrocarbon distillation products may also be employed as the fracturing fluid. One or more fracturings may be required with the use of propping agents such as sand, nutshells, polymer beads, etc. in order to render the reservoirproductive.

The particular problem which is overcome by our invention is illustrated by the example. presented'herein. The following table represents the results of the permeability reduction experienced in a low permeability gas formation due to irreversible wellbore damage. The experimental results were obtained by confining a two-inch diameter by two-inch long core plug under different reservoir pressures, injecting nitrogen, a suitable nonaqueous drilling fluid, and measuring the permeability of the core to the nitrogen, the natural permeability of the rock matrix, as would exist if nonaqueous drilling fluids had been used during the well drilling. Subsequently the core plug was subjected to water injection as would exist from the use of water base-drilling fluids and the permeability to nitrogen was thereafter measured at the same reservoir pressures as for the natural permeability test. Therefore, the Table represents the percentage increase in wellbore permeability obtained by utilizing nonaqueous drilling fluid rather than a water base drilling fluid.

TABLE Percentage increase in permeability obtained by utilizing nonaqueous drilling Wellbore Stress, p.s.i.: fluids The above experimental test was conducted using a western Oklahoma sandstone core plug. Similar results have been obtained for other formation types and for a variety of permeability ranges. The results obtained in all tests have shown a marked increase in wellbore permeability and well productivity by the use of nonaqueous drilling fluids and show the utility of our present invention.

Another embodiment of our invention is the multiple drilling and completion of several gasbearing formations in a single production well. The first gas-bearing formation is substantially drilled and completed as discussed above.

If subsequent formations are to be drilled at depths considerably below that of the first gas bearing formation, a solid well liner should be set within the wellbore of the first gas bearing formation before conventional water base-drilling fluids are reintroduced into the hole. The method by which the liner is set is not a constraint upon the present invention. However, this procedure should be consistent with the concept of the invention in that no appreciable aqueous substance should be contacted with the gas-bearing formation. This liner will allow the use of water base-drilling fluids for the drilling of the intermediate structures and protect the first gas-bearing formation from mud filtrate wellbore damage. Each additional gas bearing formation is drilled, stimulated, tested, and lined as described with the last formation left unlined and open. The productivity of each formation is tested by conventional test ing methods. Those formations thought to be productive but which do not so indicate upon original testing are stimulated by explosives and retested. Multiple intervals of stimulation and testing may be required in order to make the well produce sufficiently. If insufficient stimulation is provided by the explosives the formations should be hydraulically fractured with a nonaqueous fracturing fluid. Preferred fluids are from the group consisting of readily available dry gases under pressure such as air, nitrogen, natural gas and carbon dioxide although low viscosity, nonaqueous liquids having a high volatility such as liquified petroleum gas, light naphtha, kerosene and other light end hydrocarbon distillation products may be used. Multiple nonaqueous fracturing jobs may also be required to sufficiently stimulate the well. Standard propping agents may be incorporated with the fracturing fluid to sustain the fracture system created in the formation.

When all formations of possible interest have been drilled any water zones open to the well are shutoff to the production string. The isolation of these water-containing strata may be effected by the use of packers or casing set with a cement liner. As in all cases extreme care should be used so as not to allow any aqueous fluid introduction into the gas-bearing formations. After all gas-bearing formations have been drilled the zones found to be productive are perforated. All gas-bearing formations determined to be productive are connected to one or more production lines and readily produced without production reduction due to irreversible wellbore damage.

The present invention, therefore, provides a highly signifcant practical method for the completion of wells in low permeability gas reservoirs. The irreversible wellbore damage created during conventional drilling such as with water base muds is eliminated by the use of the successful completion method provided. By this method tight reservoirs, which were previously abandoned, may be rendered productive and economic. Recovery of natural gas from what were previously unproductive subterranean gas reservoirs is thereby enhanced.

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

We claim:

1. A method for recovering natural gas from a subterranean gas reservoir comprising:

a. initially drilling a well using an aqueous drilling fluid to a depth slightly above the gas reservoir so that no aqueous drilling fluid permeates the gas reservoir;

b. casing and cementing the well to said depth;

c. subsequently drilling the well through the gas reservoir with a nonaqueous drilling fluid; and

d. stimulating the gas-bearing formation by nonaqueous means.

2. The method of claim 1 in which said nonaqueous drilling fluid is selected from the group consisting of dry gases and low viscosity nonaqueous liquids having high volatility.

3. The method of claim 1 in which said stimulation of the gas-bearing formation by nonaqueous means consists of one or more series of explosive detonations within the wellbore.

4. The method of claim 3 in which said series of explosive detonations is followed by one or more formation fracturings with a nonaqueous fracturing fluid selected from the group consisting of dry gases and low viscosity nonaqueous liquids having high volatility.

5. The method of claim 1 in which said initial drilling method utilizes an aqueous colloidal bentonitic clay in the vicinity of the gas-bearing formation, thereby forming a mud cake on the well wall which restricts the permeation of the mud filtrate into the gas reservoir.

6. The method of claim 5 in which the mud cake formed by said bentonitic clay is removed from the well wall by rinsing with a nonaqueous liquid prior to said casing and cementing.

7. A method for recovering natural gas from several subterranean gas-bearing formations comprising:

a. initially drilling a well using an aqueous drilling fluid to a depth slightly above the first gas-bearing formation so that no aqueous drilling fluid permeates the formation;

b. casing and cementing the well to said depth;

c. subsequently drilling the well through the gas-bearing formation with a nonaqueous drilling fluid;

d. stimulating the gas-bearing formation by nonaqueous means;

e. isolating the gas bearing formation from the well by a solid liner over the depth interval of the gas-bearing formation; and

f. sequentially and repetitiously drilling and isolating subsequent gasebearing formations as described in steps a) through e) leaving the last gas formation to be drilled exposed to the well.

8. The method of claim 7 in which said nonaqueous drilling fluid is selected from the group consisting of dry gases and low viscosity nonaqueous liquids having high volatility.

9. The method of claim 7 in which said stimulation of gas bearing formations by nonaqueous means consists of one or more series of explosive detonations within the wellbore.

10. The method of claim 9 in which said series of explosive detonations is followed by one or more formation fracturings with a nonaqueous fracturing fluid selected from the group consisting of dry gases and low viscosity liquids having high volatility.

11. The method of claim 7 in which said drilling in non gasbearing formations utilizes an aqueous colloidal bentonitic clay in the vicinity of the gas-bearing formations, thereby forming a mud cake on the well wall which restricts the per meation of the mud filtrate into the gas reservoir.

12. The method of claim 11 in which the mud cake formed by said bentonitic clay is removed from the well wall by rinsing with a nonaqueous liquid prior to said casing and cementing.

13. The method of claim 7 further comprising perforating said liners of the productive gas-bearing formations so that gas production may be commenced.

Claims

2. The method of claim 1 in which said nonaqueous drilling fluid is selected from the group consisting of dry gases and low viscosity nonaqueous liquids having high volatility.
3. The method of claim 1 in which said stimulation of the gas-bearing formation by nonaqueous means consists of one or more series of explosive detonations within the wellbore.
4. The method of claim 3 in which said series of explosive detonations is followed by one or more formation fracturings with a nonaqueous fracturing fluid selected from the group consisting of dry gases and low viscosity nonaqueous liquids having high volatility.
5. The method of claim 1 in which said initial drilling method utilizes an aqueous colloidal bentonitic clay in the vicinity of the gas-bearing formation, thereby forming a mud cake on the well wall which restricts the permeation of the mud filtrate into the gas reservoir.
6. The method of claim 5 in which the mud cake formed by said bentonitic clay is removed from the well wall by rinsing with a nonaqueous liquid prior to said casing and cementing.
7. A method for recovering natural gas from several subterranean gas-bearing formations comprising: a. initially drilling a well using an aqueous drilling fluid to a depth slightly above the first gas-bearing formation so that no aqueous drilling fluid permeates the formation; b. casing and cementing the well to said depth; c. subsequently drilling the well through the gas-bearing formation with a nonaqueous drilling fluid; d. stimulating the gas-bearing formation by nonaqueous means; e. isolating the gas bearing formation from the well by a solid liner over the depth interval of the gas-bearing formation; and f. sequentially and repetitiously drilling and isolating subsequent gas-bearing formations as described in steps a) through e) leaving the last gas formation to be drilled exposed to the well.
8. The method of claim 7 in which said nonaqueous drilling fluid is selected from the group consisting of dry gases and low viscosity nonaqueous liquids having high volatility.
9. The method of claim 7 in which said stimulation of gas-bearing formations by nonaqueous means consists of one or more series of explosive detonations within the wellbore.
10. The method of claim 9 in which said series of explosive detonations is followed by one or more formation fracturings with a nonaqueous fracturing fluid selected from the group consisting of dry gases and low viscosity liquids having high volatility.
11. The method of claim 7 in which said drilling in non gas-bearing formations utilizes an aqueous colloidal bentonitic clay in the vicinity of the gas-bearing formations, thereby forming a mud cake on the well wall which restricts the permeation of the mud filtrate into the gas reservoir.
12. The method of claim 11 in which the mud cake formed by said bentonitic clay is removed from the well wall by rinsing with a nonaqueous liquid prior to said casing and cementing.
13. The method of claim 7 further comprising perforating said liners of the productive gas-bearing formations so that gas production may be commenced.