Note: Descriptions are shown in the official language in which they were submitted.
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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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.
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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
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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
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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
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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.
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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
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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.
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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
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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.
4. 10,080 gallons of foam with 1 lb/gal.
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
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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:
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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
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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.
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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
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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.
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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.
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