CA1202882A - Method of removing gas from an underground seam - Google Patents
Method of removing gas from an underground seamInfo
- Publication number
- CA1202882A CA1202882A CA000422533A CA422533A CA1202882A CA 1202882 A CA1202882 A CA 1202882A CA 000422533 A CA000422533 A CA 000422533A CA 422533 A CA422533 A CA 422533A CA 1202882 A CA1202882 A CA 1202882A
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- CA
- Canada
- Prior art keywords
- borehole
- pressurized fluid
- seam
- fracture
- proppant
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Abstract
ABSTRACT OF THE INVENTION
An improved method of removing gas from an underground seam, such as a coal seam, between adjacent strata, involving (1) forcing pressurized fluid into a borehole at an initial low rate and gradually increasing the rate to a final treatment rate such that a bottom borehole pressure is achieved which will selectively fracture the seam as compared to adjacent strata in a manner that produces a vertically confined fracture zone, and (2) adding proppant to the pressurized fluid being forced into said borehole in a controlled manner in order to prevent proppant flowing back into the borehole on removal of the pressurized fluid.
An improved method of removing gas from an underground seam, such as a coal seam, between adjacent strata, involving (1) forcing pressurized fluid into a borehole at an initial low rate and gradually increasing the rate to a final treatment rate such that a bottom borehole pressure is achieved which will selectively fracture the seam as compared to adjacent strata in a manner that produces a vertically confined fracture zone, and (2) adding proppant to the pressurized fluid being forced into said borehole in a controlled manner in order to prevent proppant flowing back into the borehole on removal of the pressurized fluid.
Description
METHOD OF REMOVING GAS FROM AN ~NDERGROUND SEAM
Field of the Invention This invention relates to a method of removing gas from an underground seam such as a coal seam. More specifically, the invention is directed to a method for producing vertically confined fracture zones in such underground seams by means of forcing pressurized fluid into the top of a borehole which penetrates the underground seam and adjacent strata. More particularly, the invention relates to a method whereby the rate of addition of the pressurized fluid into the borehole is increased gradually, and whereby proppant is added in a controlled manner such that flowback of proppant into the borehole from the fracture is substantially eliminated upon removal of the pressurized fluid from the borehole.
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Field of the Invention This invention relates to a method of removing gas from an underground seam such as a coal seam. More specifically, the invention is directed to a method for producing vertically confined fracture zones in such underground seams by means of forcing pressurized fluid into the top of a borehole which penetrates the underground seam and adjacent strata. More particularly, the invention relates to a method whereby the rate of addition of the pressurized fluid into the borehole is increased gradually, and whereby proppant is added in a controlled manner such that flowback of proppant into the borehole from the fracture is substantially eliminated upon removal of the pressurized fluid from the borehole.
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2 --21~
DESCRIPTION OF THE PRIOR ART
Hydraulic stimulation of oil and natural-gas wells is one of the major developments in petroleum engineering in the last 35 years. The technique was introduced in 1948 and since then, its use has progressively expanded in the petroleum industry. The function of stimulation is to overcome the permeability deficiencies of potentially productive formations by creating a highly permeable channel reaching into the formation from the borehole. The length and height of the channel depends upon the formation strength and the stimulation design. The channel is created by applying force to that section of the well bore passing through the production zone. The force is generated at the surface, transferred down the well hydraulically, and applied to the borehole in the form of hydraulic pressure.
Thus, hydraulic fracturing requires transfer of force by a hydraulic fluid.
Much effort has gone into removing gas from underground seams, such as methane from coal seams. This gas is a very valuable energy source which in the past has been wasted. Additionally, this gas presented a serious hazard when mining the seam due to explosions, toxic fumes, etc.
38~
One of the early efforts at hydraulic stimulation of underground seams used high flow rates, such as 10 barrels per minute, of pressurized fluids containing a proppant such as sand. It was possible to produce fractures in the coal seams and remove some gas from such seams.
However, serious problems were encountered using this method since vertical fractures were often created in the strata adjacent to the sea~. In the case of a coal seam, ~or example, when the seam was to be mined subsequent to the removal of the gas, these vertical fractures into adjacent strata could cause difficulties in conducting the mining out of the seam either due to extra costs in reinforcing the adjacent strata above the coal seam, or due to cave-ins, work stoppage, danger to workers, etc.
An aclditional problem which arose was that when the pressuri~ed fluid was removed from the fracture and borehole, the proppant flowed back into the borehole plugging equipment which, in turn, resulted in closing down of the gas removal operations and expensive maintenance and equipment replacement problems.
In an effort to solve these problems, a procedure was developed whereby proppant was eliminated from the pressurized fluid being injected into the i~Z~
borehole to create the fracture in the underground seam.
Additionally, pressurized fluid was added at a very low rate initially, and increased gradually in a controlled manner in order that a vertically confined fracture is produced in the underground seam. Thus, for example, the pressurized fluid was first injected into the borehole at a rate of about 2 barrels per minute, then increased gradually over 3 or 4 hours to a rate of about 6 barrels per minute. The injection of pressurized fluid was then continued at this final treatment rate until the fracturing process was completed. This improved method eliminated the fouiing problems caused by the proppant flowing back into the borehole since no proppant was utilized in the fracturing process. Additionally, this method of gradually increasing the rate of addition of pressurized fluid to the borehole accomplished the objective of producing a fracture in the underground seam which did not penetrate into the adjacent strata in a significant way.
One of the problems with this improved method was that by not having any proppant, the fractures initially produced by the pressurized fluid would not remain open as well as they would with proppant when the pressurized fluid was removed and gas removal operations 8~
began. Therefore, gas production from the borehole would ultimately be decreased. Thus, the cost of recovering the gas from the underground seam was increased due to the need for more boreholes to remove the gas in the underground seam.
SUMMA~Y OF THE INVENTION
The method of this invention which overcomes the above-discussed and numerous other disadvanta~es and deficiencies of the prior art relate to a method for removing gas from an underground seam, such as a coal seam, between adjcent strata comprising (1) forcing the pressurized fluid into a borehole at an initial low rate and gradually increasing the rate to a final treatment rate such that a bottom borehole pressure is achieved which will selectively fracture the seam as compared to adjacent strata in a manner that pro~uces a vertically confined fracture zone, and (2) adding proppant to the pressurized fluid being forced into said borehole in a controlled manner to thereby prevent proppant flowing back into the borehole on removal of the pressurized fluid from the borehole.
This method does eliminate the vertical fracture problem, as well as the equipment fouling problem caused by backflow of proppant upon removal of the pressurized fluid from the borehole prior to initiating gas production.
Additionally, the method of this invention does provide a 12~
proppant which enables the fracture in the underground seam to be propped open during the gas production phase, thereby increasing production from that borehole, and consequently reducing the cost of removing the gas from the underground seam.
BRIEF DESC~IPTION OF THE DRAWING
FIG. 1 shows a cross-sectional view of a coal seam and system for application of the method of this invention.
PREFERRED ~BODIMENT(S) The underground seam of this invention is located between two adjacent strata. The pressure required to propagate a fracture from the seam to each of these adjacent strata is significantly greater than the pressure required to propagate the fracture of the seam. The seam is preferably a coal seam which contains methane gas. The objective is to produce a fracture zone which does not significantly penetrate into either of the adjacent strata.
Preferably, the pressure required to propagate the fracture into adjacent strata is a minimum of about 200 psia greater than the pressure required to propagate the fracture in the underground seam containing the gas to be removed.
The pressurized fluid of this invention may be any suitable fluid capable of being transmitted down the borehole to the underground seam to cause a fracture to take place in the seam. A preferred fluid comprises an inert gas, such as nitrogen, water and a foaming agent. Alternative pressurized fluid comprises a gel, water and an agent that with time will cause the gel to break down, thereby allowing the fluid to be removed from the fracture.
The proppant of this invention can be any solid particulate material which will be readily carried by the pressurized fluid to the fracture in the underground seam, andwhich will remain in the seam when the pressurized fluid is removed from the fracture to thereby prop the fracture open as much as possible to allow maximum production of gas from the seam. A preferred proppant is sand. The proppant preferably has an average particle size between about 10 mesh and about 150 mesh, and more preferably, between about 20 mesh and about 40 mesh.
The proppant is added to the pressurized fluid being added to the borehole in a controlled manner to thereby allow the proppant to be carried to the fracture in a manner that will minimize or substantially eliminate backflow of the proppant into the borehole upon removal of the fluid from the fracture and borehole. Preferably, the proppant is not added to the pressurized fluid until the pressurized fluid rate going into the borehole reaches a level such that the proppant will readily be carried into the fracture in the underground seam.
The initial low rate of the pressurized fluid going into the borehole is generally less than about 4 barrels per minute, and preferably between about 1 to about 2 barrels per minute. The final treatment rate of the pressurized fluid going into the borehole is preferably between about 5 and about 9 barrels per minute, and more preferably, between about 6 and about 8 barrels per minute.
In a preferred embodiment, the pressurized fluid is started into the borehole at a rate of about 1 to 2 barrels perminute, and runs ~or a period of 3 to 4 hours, the rate being gradually increased over this time period to about 6 to 8 barrels per minute. When the final rate is reached, a proppant comprising sand having a particle size between about 20 and about 40 mesh is added to the pressurized fluid which is a water and nitrogen foam containing about 75 percent nitrogen.
Preferably, the pressure required to propagate the fracture in the underground seam is between about 800 psi and about 1800 psi.
_ 9 _ ~;~a3z88~
Preferably, the final treatment rate of adding the pressurized fluid to the borehole is reached after a time period of between about 2 and about 5 hours, and more preferably between about 3 and about 4 hours. Preferably, the total treatment time is less than about 6 hours.
In a preferred treatment, S0,000 gallons of pressurized fluid is added to the borehole to complete a treatment. In another preferred embodiment, 62,000 yallons of pressurized fluid is added to the borehole to complete the treatment.
The amount of proppant added to the pressurized fluid being added to the borehole is dependent upon the type of proppant and its characteristics such as the density, size and nature of the proppant. However, when sand is utilized which has an average particle size of between about 10 mesh and about 150 mesh, an amount of less than about 2 pounds of sand per gallon of pressurized fluid is preferred.
In accordance with ~IG~ 1, the ground surface 1 forms a top to overlying strata 2, which strata generally has a thickness of up to about 5000 feet and may be made up of subsidiary strata. Generally, when working with coal seams, the overlying strata will );28~3~
generally have a thickness between about 300 and about 25~0 feet. Under the overlying strata 2 is the seam 3, which in turn is over underlying strata 4. Borehole 5 is a means of transporting pressurized fluid from the ground surface 1 through the adjacent strata 2 and to the seam 3, as well as a means of removing gas after the fracture is properly formed. When a suitable nozzle is placed in the borehole 5 within the seam 3, notching of the seam can be accomplished, for example, by forcing high pressure sand and water mixture against the surface of seam 3 exposed to the borehole 5.
When pressurized fluid is ~hen forced down the borehole in accordance with the method of this invention, a fracture zone 6 is produced in seam 3. As described in the method of this invention, proppant such as sand is added in a controlled manner to the pressurized fluid and transported to the fracture zone 6 by means of borehole 5. Apparatus 7 is a means for preparing and delivering the pressurized fluid and proppant of this invention to the borehole 5 by means of conduit 8. Apparatus 9 is a means for monitoring and recording treatment rates and pressures for the operation of the method of this invention.
8;~
Example I
Conventional Fractu~ing Technique 50,000-Gallon Foam Treatment A conventional hydraulic fracture technique using a 75 percent nitrogen foam is applied to a gas producing coal seam which before treatment had gas production xates of 2,000 to 8,000 cubic feet per day. The holes are drilled into the strata immediately overlying the coal seam using a rotary drill with a 6-1/4 inch bit. Four and one-half inch casing is installed to the top of the seam, then the hole drilled through the 5~5 foot coal seam. Prior to stimulation, the coal seam is notched using a jet-slotting tool and jetting at a rate of 2.5 bbl/min. using a sand-water jet. Coal cuttings and sand are left in the 30-foot sump below the coal seam to inhibit fracture initiation be:Low the desired coal interval. The treatment is conducted through a 4-1/2 inch casing. The stimulation design is Hydraulic Pad Volume, gal. water 5,000 Total Volume, gal. foam 50,000 Foam Quality, % N~ '75 100-mesh Sand, lb 25,000 20-40 mesh sand, lb 45,000 Pumping Rate, bbl/min. 10 3B~
The 100-mesh sand is used to control leak-off of the fracturing fluids and the 20 by 40 mesh sand is added to serve as a propping agent to keep the created fracture from healing. The borehole is stimulated stepwise at a rate of 10 bbl/min. as follows:
1. 5040 gallons of foam without sand.
2. 5040 gallons of foam with 1 lb/gal.
DESCRIPTION OF THE PRIOR ART
Hydraulic stimulation of oil and natural-gas wells is one of the major developments in petroleum engineering in the last 35 years. The technique was introduced in 1948 and since then, its use has progressively expanded in the petroleum industry. The function of stimulation is to overcome the permeability deficiencies of potentially productive formations by creating a highly permeable channel reaching into the formation from the borehole. The length and height of the channel depends upon the formation strength and the stimulation design. The channel is created by applying force to that section of the well bore passing through the production zone. The force is generated at the surface, transferred down the well hydraulically, and applied to the borehole in the form of hydraulic pressure.
Thus, hydraulic fracturing requires transfer of force by a hydraulic fluid.
Much effort has gone into removing gas from underground seams, such as methane from coal seams. This gas is a very valuable energy source which in the past has been wasted. Additionally, this gas presented a serious hazard when mining the seam due to explosions, toxic fumes, etc.
38~
One of the early efforts at hydraulic stimulation of underground seams used high flow rates, such as 10 barrels per minute, of pressurized fluids containing a proppant such as sand. It was possible to produce fractures in the coal seams and remove some gas from such seams.
However, serious problems were encountered using this method since vertical fractures were often created in the strata adjacent to the sea~. In the case of a coal seam, ~or example, when the seam was to be mined subsequent to the removal of the gas, these vertical fractures into adjacent strata could cause difficulties in conducting the mining out of the seam either due to extra costs in reinforcing the adjacent strata above the coal seam, or due to cave-ins, work stoppage, danger to workers, etc.
An aclditional problem which arose was that when the pressuri~ed fluid was removed from the fracture and borehole, the proppant flowed back into the borehole plugging equipment which, in turn, resulted in closing down of the gas removal operations and expensive maintenance and equipment replacement problems.
In an effort to solve these problems, a procedure was developed whereby proppant was eliminated from the pressurized fluid being injected into the i~Z~
borehole to create the fracture in the underground seam.
Additionally, pressurized fluid was added at a very low rate initially, and increased gradually in a controlled manner in order that a vertically confined fracture is produced in the underground seam. Thus, for example, the pressurized fluid was first injected into the borehole at a rate of about 2 barrels per minute, then increased gradually over 3 or 4 hours to a rate of about 6 barrels per minute. The injection of pressurized fluid was then continued at this final treatment rate until the fracturing process was completed. This improved method eliminated the fouiing problems caused by the proppant flowing back into the borehole since no proppant was utilized in the fracturing process. Additionally, this method of gradually increasing the rate of addition of pressurized fluid to the borehole accomplished the objective of producing a fracture in the underground seam which did not penetrate into the adjacent strata in a significant way.
One of the problems with this improved method was that by not having any proppant, the fractures initially produced by the pressurized fluid would not remain open as well as they would with proppant when the pressurized fluid was removed and gas removal operations 8~
began. Therefore, gas production from the borehole would ultimately be decreased. Thus, the cost of recovering the gas from the underground seam was increased due to the need for more boreholes to remove the gas in the underground seam.
SUMMA~Y OF THE INVENTION
The method of this invention which overcomes the above-discussed and numerous other disadvanta~es and deficiencies of the prior art relate to a method for removing gas from an underground seam, such as a coal seam, between adjcent strata comprising (1) forcing the pressurized fluid into a borehole at an initial low rate and gradually increasing the rate to a final treatment rate such that a bottom borehole pressure is achieved which will selectively fracture the seam as compared to adjacent strata in a manner that pro~uces a vertically confined fracture zone, and (2) adding proppant to the pressurized fluid being forced into said borehole in a controlled manner to thereby prevent proppant flowing back into the borehole on removal of the pressurized fluid from the borehole.
This method does eliminate the vertical fracture problem, as well as the equipment fouling problem caused by backflow of proppant upon removal of the pressurized fluid from the borehole prior to initiating gas production.
Additionally, the method of this invention does provide a 12~
proppant which enables the fracture in the underground seam to be propped open during the gas production phase, thereby increasing production from that borehole, and consequently reducing the cost of removing the gas from the underground seam.
BRIEF DESC~IPTION OF THE DRAWING
FIG. 1 shows a cross-sectional view of a coal seam and system for application of the method of this invention.
PREFERRED ~BODIMENT(S) The underground seam of this invention is located between two adjacent strata. The pressure required to propagate a fracture from the seam to each of these adjacent strata is significantly greater than the pressure required to propagate the fracture of the seam. The seam is preferably a coal seam which contains methane gas. The objective is to produce a fracture zone which does not significantly penetrate into either of the adjacent strata.
Preferably, the pressure required to propagate the fracture into adjacent strata is a minimum of about 200 psia greater than the pressure required to propagate the fracture in the underground seam containing the gas to be removed.
The pressurized fluid of this invention may be any suitable fluid capable of being transmitted down the borehole to the underground seam to cause a fracture to take place in the seam. A preferred fluid comprises an inert gas, such as nitrogen, water and a foaming agent. Alternative pressurized fluid comprises a gel, water and an agent that with time will cause the gel to break down, thereby allowing the fluid to be removed from the fracture.
The proppant of this invention can be any solid particulate material which will be readily carried by the pressurized fluid to the fracture in the underground seam, andwhich will remain in the seam when the pressurized fluid is removed from the fracture to thereby prop the fracture open as much as possible to allow maximum production of gas from the seam. A preferred proppant is sand. The proppant preferably has an average particle size between about 10 mesh and about 150 mesh, and more preferably, between about 20 mesh and about 40 mesh.
The proppant is added to the pressurized fluid being added to the borehole in a controlled manner to thereby allow the proppant to be carried to the fracture in a manner that will minimize or substantially eliminate backflow of the proppant into the borehole upon removal of the fluid from the fracture and borehole. Preferably, the proppant is not added to the pressurized fluid until the pressurized fluid rate going into the borehole reaches a level such that the proppant will readily be carried into the fracture in the underground seam.
The initial low rate of the pressurized fluid going into the borehole is generally less than about 4 barrels per minute, and preferably between about 1 to about 2 barrels per minute. The final treatment rate of the pressurized fluid going into the borehole is preferably between about 5 and about 9 barrels per minute, and more preferably, between about 6 and about 8 barrels per minute.
In a preferred embodiment, the pressurized fluid is started into the borehole at a rate of about 1 to 2 barrels perminute, and runs ~or a period of 3 to 4 hours, the rate being gradually increased over this time period to about 6 to 8 barrels per minute. When the final rate is reached, a proppant comprising sand having a particle size between about 20 and about 40 mesh is added to the pressurized fluid which is a water and nitrogen foam containing about 75 percent nitrogen.
Preferably, the pressure required to propagate the fracture in the underground seam is between about 800 psi and about 1800 psi.
_ 9 _ ~;~a3z88~
Preferably, the final treatment rate of adding the pressurized fluid to the borehole is reached after a time period of between about 2 and about 5 hours, and more preferably between about 3 and about 4 hours. Preferably, the total treatment time is less than about 6 hours.
In a preferred treatment, S0,000 gallons of pressurized fluid is added to the borehole to complete a treatment. In another preferred embodiment, 62,000 yallons of pressurized fluid is added to the borehole to complete the treatment.
The amount of proppant added to the pressurized fluid being added to the borehole is dependent upon the type of proppant and its characteristics such as the density, size and nature of the proppant. However, when sand is utilized which has an average particle size of between about 10 mesh and about 150 mesh, an amount of less than about 2 pounds of sand per gallon of pressurized fluid is preferred.
In accordance with ~IG~ 1, the ground surface 1 forms a top to overlying strata 2, which strata generally has a thickness of up to about 5000 feet and may be made up of subsidiary strata. Generally, when working with coal seams, the overlying strata will );28~3~
generally have a thickness between about 300 and about 25~0 feet. Under the overlying strata 2 is the seam 3, which in turn is over underlying strata 4. Borehole 5 is a means of transporting pressurized fluid from the ground surface 1 through the adjacent strata 2 and to the seam 3, as well as a means of removing gas after the fracture is properly formed. When a suitable nozzle is placed in the borehole 5 within the seam 3, notching of the seam can be accomplished, for example, by forcing high pressure sand and water mixture against the surface of seam 3 exposed to the borehole 5.
When pressurized fluid is ~hen forced down the borehole in accordance with the method of this invention, a fracture zone 6 is produced in seam 3. As described in the method of this invention, proppant such as sand is added in a controlled manner to the pressurized fluid and transported to the fracture zone 6 by means of borehole 5. Apparatus 7 is a means for preparing and delivering the pressurized fluid and proppant of this invention to the borehole 5 by means of conduit 8. Apparatus 9 is a means for monitoring and recording treatment rates and pressures for the operation of the method of this invention.
8;~
Example I
Conventional Fractu~ing Technique 50,000-Gallon Foam Treatment A conventional hydraulic fracture technique using a 75 percent nitrogen foam is applied to a gas producing coal seam which before treatment had gas production xates of 2,000 to 8,000 cubic feet per day. The holes are drilled into the strata immediately overlying the coal seam using a rotary drill with a 6-1/4 inch bit. Four and one-half inch casing is installed to the top of the seam, then the hole drilled through the 5~5 foot coal seam. Prior to stimulation, the coal seam is notched using a jet-slotting tool and jetting at a rate of 2.5 bbl/min. using a sand-water jet. Coal cuttings and sand are left in the 30-foot sump below the coal seam to inhibit fracture initiation be:Low the desired coal interval. The treatment is conducted through a 4-1/2 inch casing. The stimulation design is Hydraulic Pad Volume, gal. water 5,000 Total Volume, gal. foam 50,000 Foam Quality, % N~ '75 100-mesh Sand, lb 25,000 20-40 mesh sand, lb 45,000 Pumping Rate, bbl/min. 10 3B~
The 100-mesh sand is used to control leak-off of the fracturing fluids and the 20 by 40 mesh sand is added to serve as a propping agent to keep the created fracture from healing. The borehole is stimulated stepwise at a rate of 10 bbl/min. as follows:
1. 5040 gallons of foam without sand.
2. 5040 gallons of foam with 1 lb/gal.
3. 10,080 gallons of foam with 2 lb/gal.
100 mesh sand.
100 mesh sand.
4. 10,080 gallons of foam with 1 lb/gal.
20-40 mesh sand.
20-40 mesh sand.
5. 8,240 gallons of foam with 1.5 lb/gal 20 by 40-mesh sand.
6. 8,810 gallons of foam with 2 lb/gal. 20 hy 40 mesh sand.
7. 840 gallons of foam without sand.
The treatment lasts about 2 hours with pumping pressures ranging from 1200 to 2000 psi. Following treatment, pumping of the borehole is begun and production rates of 80,000 cubic feet per day obtained.
In subsequent mine through of boreholes stimulated in this manner, cracks have been observed in the strata immediately overlying the coal seam. Also, excessive operating expenses have been experienced with this type treatment. Sand flowback into the well bore has resulted inpremature degradation of the dewatering equipment with corresponding downtime and maintenance costs.
Example II
Unpropped, Increasing Rate, 50,000-Gallon Foam Treatment A borehole into the same coal seam as used in Example I is prepared as in Example I. The coal is first notched using a notching jet and moving it vertically and rotating it so that the jet impacts the exposed coal face.
Successful notching is determined by the amount of coal returned in the discharge fluid. In contrast to the conventional type treatments, this stimulation is conducted without a proppant and at significantly lower injection 15 rates. The stimulation design is Treatment Volume 53,100 Foam Quality ~ N2 75 Pumping rate range 2 to 6 bbl*/min Foam injection begins at 2 bbl/min and is increased every half hour in l-bbl/min increments until 5-bbl/min was obtained. At this point the injection rate is maintained constant for 1-1/2 hours. The injection sequence is completed at 6-bbl/min for a simular 1-1/2-hour period. The pumping schedule is as follows:
lZ~Z88;~
1. 2700 gallons of foam without sand @ 2 bbl/min.
2. 3780 gallons of foam without sand @ 3 bbl/min.
3. 5040 gallons of foam without sand @ 4 bbl/minO
4. 18900 gallons of foam without sand @ 5 bbl/min.
5. 22680 gallons of foam without sand @ 6 bbl/min.
Th~ treatment lasts for about 4-1/2 hours. ~ollowing treatment, the wellhead valve is opened to allow the stimulation fluids to flow back through a choke for several days. The hole is then flooded, if necessary, and dewatering equipment installed. Once the borehole is dewatered, production rates averaging 63,000 cubic feet a day ~ere obtained. Observations made after the mine through of the hole indicated that no penetration of the ad~acent strata occurred throughout the length of the observed fracture.
*1 barrel equals 42 gallons ~2~ 2 Example III
Increasing Rate, 50,000 Gallon Foal Treatment with Proppant In the same coal seam as used in Examples I and II, the improved process of this in~ention is employed using a modification of Example II. The holes are prepared as in Example I and the treatment is conducted through a 4-1/2 inch casing. The stimulation design is Hydraulic Pad Volume, gal water 40,320 Total Volume, gal foam 50,570 - Foam Quality, % N2 75 20 by 40-mesh sand, lb 10,000 Pumping Rate, initial, bbl/min. 3 Pumping Rate, final, bbl/min. 7 The borehole is stimulated stepwise starting at an initial rate of 3 bbl/min and increasing the rate to 7 bbl/min. The sand proppant is added in the latter part of the final step which is at 7 bbl/min. The pumping schedule is as follows:
1. 2010 gallons of foam without sand @ 3 bbl/min.
2. 2570 gallons of foam without sand @
bbl/min.
3. 6220 gallons of foam without sand @ 5 bbl/min.
Z
4. 7560 gallons of foam without sand @ 6 bbl/min.
5. 22,000 gallons of foam without sand @ 7 bbl/min.
6. 10,250 gallons of foam with 1 lb~gal 20 by 40-mesh sand Q 7 bbl/min.
The treatment lasts for about 4 hours with pumping pressure ranging from 700 to 950 psi. Following the treatment, pumping of the borehole is begun and average production rates of 80,000 cubic feet of gas per day are obtained before mining through occurs. Observations made following mine through of the hole indicated that no penetration of the adjacent strata occurred through the length of the observed fracture.
In addition to the elimination of fracture into the adjacent strata to the coal seam, much less proppant was required than in prior art methods. Also, no problems took place due to flowback of proppant into the well bore with its attendant equipment problems, downtime and maintenance costs. Thus, the serious drawbacks of the prior art use of proppant have been avoided, thus making it possible to take advantage of the chief reason for using proppant, i.e.
keeping the fracture open as much as possible after 1~2~8~
withdrawal of the fractured fluid to thereby increase gasproduction for each borehole and, therefore, reduce the number of boreholes and hence the cost of gas production for a given field.
Example IV
Increasing Rate, 62,500 Gallons Treatment with Proppant The preparation and treatment is conducted in the same manner as Example III: however, it is modified in that the size of the treatment is increased from 50,000 gallons to 62,500 gallons. The borehole is stimulated stepwise starting at an initial rate of 3 bbl/min and increasing the rate to 7 bbl/min, and a sand proppant is added in the latter part of the final step. The pumping schedule is as 15 follows:
1890 gallons of foam without sand @ 3 bbl/min.
2520 gallons of foam without sand @ 4 bbl/min.
6300 gallons of foam without sand @ S
bbl/min.
10,080 gallons of foam without sand @ 6 bbl/min.
29,400 gallons of foam without sand @ 7 bbl/min.
11,760 gallons of foam with 2 lb/gal 20 by 40 mesh sand @ 7 bbl/min.
The treatment lasts about 4 hours.
Four boreholes were completed using the practice cited in Example II and three boreholes using the practice cited in Example III. The highest performing boxehole that was completed, as outlined in Example II, without the use of proppant, produced gas at an average rate of about 63,000 cubic feet per day (cfd), and the highest performing borehole completed, as in Example III using proppant, produced an average rate of about 80,000 cubic feet per day, indicating that both kinds of treatment enhance gas production, but that the use of a proppant in the system does give improved production rates. Experience obtained with boreholes stimulated, as outlined in Example III, has indicated a minimum of problems associated with proppant sand flowback into the wellbore. This is a significant improvement compared to the severe problems associated with the flowback of sand that occurred with boreholes stimulated using conventional techniques, as outlined in Example I.
19 ~
The treatment lasts about 2 hours with pumping pressures ranging from 1200 to 2000 psi. Following treatment, pumping of the borehole is begun and production rates of 80,000 cubic feet per day obtained.
In subsequent mine through of boreholes stimulated in this manner, cracks have been observed in the strata immediately overlying the coal seam. Also, excessive operating expenses have been experienced with this type treatment. Sand flowback into the well bore has resulted inpremature degradation of the dewatering equipment with corresponding downtime and maintenance costs.
Example II
Unpropped, Increasing Rate, 50,000-Gallon Foam Treatment A borehole into the same coal seam as used in Example I is prepared as in Example I. The coal is first notched using a notching jet and moving it vertically and rotating it so that the jet impacts the exposed coal face.
Successful notching is determined by the amount of coal returned in the discharge fluid. In contrast to the conventional type treatments, this stimulation is conducted without a proppant and at significantly lower injection 15 rates. The stimulation design is Treatment Volume 53,100 Foam Quality ~ N2 75 Pumping rate range 2 to 6 bbl*/min Foam injection begins at 2 bbl/min and is increased every half hour in l-bbl/min increments until 5-bbl/min was obtained. At this point the injection rate is maintained constant for 1-1/2 hours. The injection sequence is completed at 6-bbl/min for a simular 1-1/2-hour period. The pumping schedule is as follows:
lZ~Z88;~
1. 2700 gallons of foam without sand @ 2 bbl/min.
2. 3780 gallons of foam without sand @ 3 bbl/min.
3. 5040 gallons of foam without sand @ 4 bbl/minO
4. 18900 gallons of foam without sand @ 5 bbl/min.
5. 22680 gallons of foam without sand @ 6 bbl/min.
Th~ treatment lasts for about 4-1/2 hours. ~ollowing treatment, the wellhead valve is opened to allow the stimulation fluids to flow back through a choke for several days. The hole is then flooded, if necessary, and dewatering equipment installed. Once the borehole is dewatered, production rates averaging 63,000 cubic feet a day ~ere obtained. Observations made after the mine through of the hole indicated that no penetration of the ad~acent strata occurred throughout the length of the observed fracture.
*1 barrel equals 42 gallons ~2~ 2 Example III
Increasing Rate, 50,000 Gallon Foal Treatment with Proppant In the same coal seam as used in Examples I and II, the improved process of this in~ention is employed using a modification of Example II. The holes are prepared as in Example I and the treatment is conducted through a 4-1/2 inch casing. The stimulation design is Hydraulic Pad Volume, gal water 40,320 Total Volume, gal foam 50,570 - Foam Quality, % N2 75 20 by 40-mesh sand, lb 10,000 Pumping Rate, initial, bbl/min. 3 Pumping Rate, final, bbl/min. 7 The borehole is stimulated stepwise starting at an initial rate of 3 bbl/min and increasing the rate to 7 bbl/min. The sand proppant is added in the latter part of the final step which is at 7 bbl/min. The pumping schedule is as follows:
1. 2010 gallons of foam without sand @ 3 bbl/min.
2. 2570 gallons of foam without sand @
bbl/min.
3. 6220 gallons of foam without sand @ 5 bbl/min.
Z
4. 7560 gallons of foam without sand @ 6 bbl/min.
5. 22,000 gallons of foam without sand @ 7 bbl/min.
6. 10,250 gallons of foam with 1 lb~gal 20 by 40-mesh sand Q 7 bbl/min.
The treatment lasts for about 4 hours with pumping pressure ranging from 700 to 950 psi. Following the treatment, pumping of the borehole is begun and average production rates of 80,000 cubic feet of gas per day are obtained before mining through occurs. Observations made following mine through of the hole indicated that no penetration of the adjacent strata occurred through the length of the observed fracture.
In addition to the elimination of fracture into the adjacent strata to the coal seam, much less proppant was required than in prior art methods. Also, no problems took place due to flowback of proppant into the well bore with its attendant equipment problems, downtime and maintenance costs. Thus, the serious drawbacks of the prior art use of proppant have been avoided, thus making it possible to take advantage of the chief reason for using proppant, i.e.
keeping the fracture open as much as possible after 1~2~8~
withdrawal of the fractured fluid to thereby increase gasproduction for each borehole and, therefore, reduce the number of boreholes and hence the cost of gas production for a given field.
Example IV
Increasing Rate, 62,500 Gallons Treatment with Proppant The preparation and treatment is conducted in the same manner as Example III: however, it is modified in that the size of the treatment is increased from 50,000 gallons to 62,500 gallons. The borehole is stimulated stepwise starting at an initial rate of 3 bbl/min and increasing the rate to 7 bbl/min, and a sand proppant is added in the latter part of the final step. The pumping schedule is as 15 follows:
1890 gallons of foam without sand @ 3 bbl/min.
2520 gallons of foam without sand @ 4 bbl/min.
6300 gallons of foam without sand @ S
bbl/min.
10,080 gallons of foam without sand @ 6 bbl/min.
29,400 gallons of foam without sand @ 7 bbl/min.
11,760 gallons of foam with 2 lb/gal 20 by 40 mesh sand @ 7 bbl/min.
The treatment lasts about 4 hours.
Four boreholes were completed using the practice cited in Example II and three boreholes using the practice cited in Example III. The highest performing boxehole that was completed, as outlined in Example II, without the use of proppant, produced gas at an average rate of about 63,000 cubic feet per day (cfd), and the highest performing borehole completed, as in Example III using proppant, produced an average rate of about 80,000 cubic feet per day, indicating that both kinds of treatment enhance gas production, but that the use of a proppant in the system does give improved production rates. Experience obtained with boreholes stimulated, as outlined in Example III, has indicated a minimum of problems associated with proppant sand flowback into the wellbore. This is a significant improvement compared to the severe problems associated with the flowback of sand that occurred with boreholes stimulated using conventional techniques, as outlined in Example I.
19 ~
Claims (27)
1. In a method of removing gas from an underground seam between adjacent strata, the improvement comprising (1) forcing pressurized fluid into a borehole at an initial low rate and gradually increasing said rate to a final treatment rate such that a bottom borehole pressure is achieved which will selectively fracture said seam as compared to adjacent strata in a manner that produces a vertically confined fracture zone, and (2) adding proppant to said pressurized fluid being forced into said borehole in a controlled manner in order to prevent significant proppant flowback upon removal of said pressurized fluid from said borehole.
2. Method as in claim 1 wherein the pressure required to propagate the fracture into said adjacent strata is significantly greater than the pressure required to propagate the fracture in said seam.
3. Method as in claim 1 wherein said proppant is not added until a pressurized fluid rate in said borehole reaches a level such that proppant will readily be carried into the fracture in said seam.
4. Method as in claim 1 wherein said pressurized fluid comprises (1) an inert gas, water and a foaming agent, or (2) a gel, water, and an agent that with time will cause the gel to break down thereby allowing the fluid to be removed from the fracture.
5. Method as in claim 1 wherein said seam is first notched prior to forcing said pressurized fluid into said borehole.
6. Method as in claim 1 wherein said initial very low rate is less than about 4 barrels per minute and wherein said final treatment rate is between about 5 and about 9 barrels per minute.
7. Method as in claim 1 wherein said proppant is not added to said pressurized fluid until said final treatment rate is achieved.
8. Method as in claim 1 wherein said proppant is a solid particulate material.
9. Method as in claim 8 wherein said proppant has an average particle size between about 10 mesh and about 150 mesh.
10. Method as in claim 9 wherein said proppant comprises sand having an average particle size between about 20 and about 40 mesh.
11. Method as in claim 1 wherein said gas is methane.
12. Method as in claim 1 wherein said seam comprises a coal seam.
13. Method as in claim 1 wherein the pressure required to propagate the fracture in said coal seam is between about 800 psi and about 1800 psi.
14. Method as in claim 1 wherein said seam is associated with a mine and wherein the major fracture created in said seam does not penetrate the mine roof or mine floor.
15. Method as in claim 1 wherein said initial very low rate is between about 1 and about 2 barrels per minute of said pressurized fluid into said borehole.
16. Method as in claim 15 wherein said final treatment rate is between about 6 and about 8 barrels per minute of said pressurized fluid into said borehole.
17. Method as in claim 16 wherein said pressurized fluid comprises a high quality water-nitrogen gas foam.
18. Method as in claim 17 wherein a proppant is added to said pressurized fluid when it reaches said final treatment rate.
19. Method as in claim 17 wherein said final treatment rate is reached after a time period of between about 2 and about 5 hours.
20. Method as in claim 17 wherein said final treatment rate is reached after a time period of between about 3 and about 4 hours, and wherein said total treatment time is less than about 6 hours.
21. In a method of removing gas from an underground coal seam between adjacent strata, the improvement comprising (1) forcing pressurized fluid into a borehole at an initial very low rate and gradually increasing said rate to a final treatment rate of between about 5 and about 9 barrels per minute such that a bottom borehole pressure is achieved which will selectively fracture said coal seam as compared to adjacent strata, and (2) when a rate at or near the final treatment rate, adding proppant to said pressurized fluid being forced into said borehole, and wherein the pressure required to propagate the fracture into adjacent strata is a minimum of about 200 psia greater than the pressure required to propagate the fracture in said coal seam.
22. Method as in claim 21 wherein said initial rate is between about 1 and about 2 barrels per minute, and wherein said final treatment rate is between about 6 and about 8 barrels per minute.
23. Method as in claim 21 wherein said coal seam is notched prior to forcing said pressurized fluid into said borehole to selectively fracture said coal seam.
24. Method as in claim 21 wherein said coal seam is located at a depth between about 300 and about 5000 feet below ground level.
25. Method as in claim 21 wherein said gas is methane and said proppant is sand.
26. Method as in claim 25 wherein said sand has an average particle size between about 20 and about 40 mesh.
27. Method as in claim 21 wherein the pressure required to propagate the fracture in said coal seam is between about 800 and about 1800 psia.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US35371082A | 1982-03-01 | 1982-03-01 | |
| US353,710 | 1982-03-01 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1202882A true CA1202882A (en) | 1986-04-08 |
Family
ID=23390238
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000422533A Expired CA1202882A (en) | 1982-03-01 | 1983-02-28 | Method of removing gas from an underground seam |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1202882A (en) |
Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4913237A (en) * | 1989-02-14 | 1990-04-03 | Amoco Corporation | Remedial treatment for coal degas wells |
| US7754659B2 (en) | 2007-05-15 | 2010-07-13 | Georgia-Pacific Chemicals Llc | Reducing flow-back in well treating materials |
| US8003214B2 (en) | 2006-07-12 | 2011-08-23 | Georgia-Pacific Chemicals Llc | Well treating materials comprising coated proppants, and methods |
| US8058213B2 (en) | 2007-05-11 | 2011-11-15 | Georgia-Pacific Chemicals Llc | Increasing buoyancy of well treating materials |
| US8133587B2 (en) | 2006-07-12 | 2012-03-13 | Georgia-Pacific Chemicals Llc | Proppant materials comprising a coating of thermoplastic material, and methods of making and using |
| US20140058686A1 (en) * | 2012-08-22 | 2014-02-27 | Baker Hughes Corporation | Natural fracture injection test |
| CN109236262A (en) * | 2018-10-15 | 2019-01-18 | 中国地质大学（北京） | A kind of pressure break rear support agent reflux analysis method considering proppant wetability |
| CN111270987A (en) * | 2020-01-20 | 2020-06-12 | 中国矿业大学 | Method for accurately preventing and controlling rock burst in remote area under coal mine |
-
1983
- 1983-02-28 CA CA000422533A patent/CA1202882A/en not_active Expired
Cited By (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4913237A (en) * | 1989-02-14 | 1990-04-03 | Amoco Corporation | Remedial treatment for coal degas wells |
| US8003214B2 (en) | 2006-07-12 | 2011-08-23 | Georgia-Pacific Chemicals Llc | Well treating materials comprising coated proppants, and methods |
| US8133587B2 (en) | 2006-07-12 | 2012-03-13 | Georgia-Pacific Chemicals Llc | Proppant materials comprising a coating of thermoplastic material, and methods of making and using |
| US8058213B2 (en) | 2007-05-11 | 2011-11-15 | Georgia-Pacific Chemicals Llc | Increasing buoyancy of well treating materials |
| US7754659B2 (en) | 2007-05-15 | 2010-07-13 | Georgia-Pacific Chemicals Llc | Reducing flow-back in well treating materials |
| US20140058686A1 (en) * | 2012-08-22 | 2014-02-27 | Baker Hughes Corporation | Natural fracture injection test |
| CN103628865A (en) * | 2012-08-22 | 2014-03-12 | 贝克休斯公司 | Natural fracture injection test |
| US9366122B2 (en) * | 2012-08-22 | 2016-06-14 | Baker Hughes Incorporated | Natural fracture injection test |
| CN109236262A (en) * | 2018-10-15 | 2019-01-18 | 中国地质大学（北京） | A kind of pressure break rear support agent reflux analysis method considering proppant wetability |
| CN111270987A (en) * | 2020-01-20 | 2020-06-12 | 中国矿业大学 | Method for accurately preventing and controlling rock burst in remote area under coal mine |
| CN111270987B (en) * | 2020-01-20 | 2020-12-25 | 中国矿业大学 | Method for accurately preventing and controlling rock burst in remote area under coal mine |
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