U.S. patent application number 11/677263 was filed with the patent office on 2007-08-23 for method of intensification of natural gas production from coal beds.
Invention is credited to Alexey Evgenievich Barykin, J. Ernest Brown, Matthew Miller.
Application Number | 20070193737 11/677263 |
Document ID | / |
Family ID | 38426982 |
Filed Date | 2007-08-23 |
United States Patent
Application |
20070193737 |
Kind Code |
A1 |
Miller; Matthew ; et
al. |
August 23, 2007 |
METHOD OF INTENSIFICATION OF NATURAL GAS PRODUCTION FROM COAL
BEDS
Abstract
A method of intensification of natural gas production from the
coal beds, comprising exposure of the coal bed to pressure pulsing
is disclosed. An embodiment comprises exposure of the coal bed to
pressure pulsing in combination with hydraulic fracturing of the
coal bed. Another embodiment comprises exposure of the coal bed to
pressure pulsing in combination with creation of a cavity in the
coal bed by cyclically increasing pressure of the fluid in the well
and depressurizing rapidly the fluid.
Inventors: |
Miller; Matthew; (Moscow,
RU) ; Brown; J. Ernest; (Katy, TX) ; Barykin;
Alexey Evgenievich; (Novosibirsk, RU) |
Correspondence
Address: |
FITCH EVEN TABIN AND FLANNERY
120 SOUTH LA SALLE STREET
SUITE 1600
CHICAGO
IL
60603-3406
US
|
Family ID: |
38426982 |
Appl. No.: |
11/677263 |
Filed: |
February 21, 2007 |
Current U.S.
Class: |
166/249 ;
166/263; 166/280.1; 166/312 |
Current CPC
Class: |
E21B 43/003 20130101;
E21B 43/006 20130101; E21B 43/267 20130101; E21B 28/00
20130101 |
Class at
Publication: |
166/249 ;
166/263; 166/280.1; 166/312 |
International
Class: |
E21B 43/267 20060101
E21B043/267; E21B 37/00 20060101 E21B037/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 22, 2006 |
RU |
2006105514 |
Claims
1. A method of intensification of natural gas production from the
coal beds, comprising exposure of the coal bed to pressure pulsing
and hydraulic fracturing of the coal bed.
2. The method of claim 1, wherein the pressure pulses are low
frequency high-amplitude pressure pulses.
3. The method of claim 1, wherein the hydraulic fracture is created
in the coal bed by the steps of injecting into the coal bed a
fracture fluid at high flow rates and pressures to create the
fracture, further injecting a proppant in the fracture fluid to
prevent closing the fracture created, and finally injecting the
fracture fluid without proppant for washing the well.
4. The method of claim 3, wherein the pressure pulsing is applied
to the coal bed either before creating the fracture, or during
injecting the fracture fluid to create the fracture, or during
injecting a proppant in the fracture fluid, or during finally
injecting the fracture fluid without proppant, or during some or
all said steps.
5. The method according to claim 4, comprising, before starting the
hydraulic fracturing of the coal bed, the following steps: running
a pressure pulse generator into the wellbore approximately at the
level of treated interval; loading the wellbore with an
incompressible fluid in order to couple the coal bed to the
pressure pulse generator; adjusting the generator to create
pressure pulses of required frequency and amplitude; transmitting
the pressure pulses created by the pressure pulse generator to the
coal bed through the incompressible fluid; injecting the fracture
fluid to the coal bed.
6. A method of intensification of natural gas production from the
coal beds, comprising exposure of the coal bed to pressure pulsing
and hydraulic fracturing of the coal bed by injecting into the
wellbore a fracture fluid without proppant at high flow rates and
pressures.
7. A method of intensification of natural gas production from the
coal beds, comprising exposure of the coal bed to pressure pulsing
and creation of a cavity in the coal bed by cyclically increasing
pressure of the fluid in the well and depressurizing rapidly the
fluid.
8. The method of claim 7, wherein the pressure pulses are low
frequency high-amplitude pressure pulses.
9. The method of claim 8, wherein the low frequency high-amplitude
pressure pulses impact the coal bed during increasing the pressure
of the fluid in the well.
10. The method of claim 8, further comprising the step of cleaning
the well, wherein the fluid used for transferring pressure pulses
is replaced by an efficient foamy cleaning medium.
11. The method of claim 10, wherein a fluid driven generator is
used for creation of the pressure pulses and the foamy cleaning
medium is pumped through said generator.
12. The method of claim 10, wherein a generator for creation of the
pressure pulses is placed in a nearby well, and the pressure
pulsing of the coal bed is simultaneously accompanied with
injecting the foamy cleaning medium in the treated well.
13. A method of intensification of natural gas production from the
coal beds, comprising exposure of the coal bed to pressure pulsing,
creation of a cavity in the coal bed by cyclically increasing
pressure of the fluid in the well and depressurizing rapidly the
fluid, and well cleaning, comprising the steps of: running a
pressure pulse generator into the wellbore at the level of treated
interval in the wellbore; loading the wellbore with an
incompressible fluid in order to couple the coal bed to the
pressure pulse generator; adjusting the generator to create
pressure pulses of required frequency and amplitude and
transmitting the pressure pulses created through said
incompressible fluid to the coal bed; creating a pressure gradient
in the near wellbore zone by injecting fluid above the pressure of
the coal bed; depressurizing rapidly the fluid in the wellbore to
cause breakdown of the coal bed and produce rubblized coal;
cleaning the well by circulating efficient foamy cleaning medium
across the treated interval to aid fluidization and transport of
the rubblized coal to the surface.
14. The method of claim 13, comprising using a fluid discharge
pressure pulse generator without a resonator, so that a pulsing jet
produces cutting action on the coal bed.
15. The method of claim 13, wherein said steps are repeated after
refilling the wellbore with the incompressible fluid.
Description
[0001] The present invention relates to methods for intensification
of natural gas production from the coal beds.
[0002] The reserves of natural gas associated with coal deposits
worldwide are estimated at 8,000 trillion cubic feet. This gas is
adsorbed onto the coal surface and desorbs upon reducing pressure.
The desorbed natural gas can flow towards production wells through
the butt and cleat network of the coal bed. Because of the low
effective permeability of coal beds, stimulation methods are
applied to increase the rate of production. Another complicating
factor is that some coal beds are wet, and require dewatering in
order to reduce the pore pressure and allow the desorption process
to begin. Water is more difficult to remove than gas because its
viscosity is two orders of magnitude higher, this causing greater
flow resistance in the low permeability butt-cleat porosity
network. In addition, water must be pumped out of the well using
some mechanical lift assistance. Moreover, water reduces the
effective permeability of the butt-cleat network by creating
interfaces that resist entry into small constrictions in the flow
path and by occupying a large fraction of the pore space.
[0003] The most common existing methods of methane extraction
include the creation of a cavity in the coal seam (cavitation),
hydrological fracturing of the coal bed, or directional drilling
parallel to the bedding of the seam.
[0004] The method of creation of a cavity in the coal seam reduces
the amount of damage to the surrounding structure that may have
resulted during drilling, and gives an area from which the methane
can be extracted. Typically cavitation is performed on open-hole
completed wells (no casing across the production interval) in a
cyclic manner. Thus the treatment is usually referred to as
cavitation cycling.
[0005] Conventional hydraulic fracturing technique is performed on
cased-hole perforated completion wells typically when the coal
permeability is less than 20 mD. As an option of this method, the
hydraulic fracture drilling technique might be used; it involves
drilling an initial cased borehole and a second borehole some
distance away from the original one which acts to collect loose
coal material and as a dewatering location of the formation.
[0006] Directional drilling involves angling the drill stem so that
drilling is not vertical, but will parallel the coal seam.
[0007] Once the appropriate borehole is completed by using whatever
method, dewatering must also occur to reduce the pressure in the
formation. Pressure drop promotes methane release from within the
coal into the cleats directed along the beds. If the cleats contain
a high enough permeability, that is, inter-connectivity, then the
methane will flow from the coal into the well bore and can be
extracted.
[0008] Cavitation cycling, the most characteristic method for
methane extraction, uses several mechanisms to link the wellbore to
the coal fracture system: creates a physical cavity in the coals of
the open-hole section (up to 10 feet in diameter); propagates a
self-propping, vertical fracture that extends up to 200 feet away
from the wellbore (parallel to the direction of least stress), and
creates a zone of shear stress failure that enhances permeability
in a direction perpendicular to the direction of least stress, as
disclosed e.g. in book by Palmer, I. D., Lambert, S. W., and
Spitter, J. L. "Coalbed methane well completions and stimulations",
Chapter 14 in AAPG Studies in Geology 38, pp. 303-341, and
publication by Khodaverian, M. and McLennan, "Cavity completions: a
study of mechanisms and applicability", Proceedings of the 1993
International Coalbed Methane Symposium (Univ. of
Alabama/Tuscaloosa), 1993, pp. 89-97.
[0009] Cavitation is accomplished by applying pressure to the well
using compressed air or foam, and then abruptly releasing the
pressure. The over-pressured coal zones provide a pressure surge
into the wellbore, and the resulting stress causes dislodgement of
coal chips, while abrupt fluid flowback carries the chips up the
well. These cycles of pressure and blowdown are repeated many times
over a period of hours or days, and the repeated alternating
stress-shear failure in the coal formation creates effects that
extend laterally from the wellbore, as disclosed e.g. in
publication by Kahil, A. and Masszi, D. "Cavity stress-relief
method to stimulate demethanation boreholes", SPE Paper No. 12843.
Proceedings 1984 SPE Unconventional Gas Recovery Symposium,
1984.
[0010] Conventional hydraulic fracturing technique is described in
many literature sources, as well as when applied to coal rocks,
e.g. in publication by Holditch, S. A. 1990. "Completion methods in
coal seam reservoirs", SPE 20670. Proceedings 65.sup.th SPE Annual
Technical Conference (New Orleans), p. 533.
[0011] Directional drilling cannot be considered as a pure
stimulation technique for natural gas production. It is worth to
note that both fracturing (either conventional hydraulic fracturing
or cavitation) and directionally drilling simply increase the
amount of the coal seam which is associated with the well bore, but
fail to increase the original porosity of the formation.
[0012] The prior art contains a tool which utilizes mechanical
cutting action by means of jets to create cavity in the bed and
clean its volume, as disclosed in U.S. Pat. No. 6,609,688 (Aug. 26,
2003). The tool is provided with a plurality of ports for jetting a
combination of air, water and/or drilling foam pumped through each
of the ports while the tool rotates. The tool is designed to clean
and flush out coal wells not in production.
[0013] The benefit of this tool is that it opens the hole,
enlarging the hole size, and cleans out the well in one run. But
this tool does not use pulsing.
[0014] The following references disclose methods that involve
pressure pulsing in oil and gas industry for oil recovery
enhancement. However none of these applications were aimed at
enhancement of coal-bed methane production.
[0015] The principle of pumping and vibrations simultaneously is
described in U.S. Pat. No. 4,164,978 (1978); RU patent 16074
(1962); book "Research Activities", M, MGI, 1975, p. 61: "Use of
vibration in oil production", M., Nedra, 1977; EP patent No.
0512331 (1992); U.S. Pat. No. 4,164,498 (1978); RU patents Nos.
1165801 (1985); 2084705 (1993); 2085721 (1997); 2100571 (1992);
2085721 (1994); 2175718 (1997); 2193649 (2002).
[0016] Pumping and vibrations simultaneously, and vibrations with
special frequencies (calculated resonance frequencies) are
described in U.S. Pat. Nos. 4,702,315 (1987); 3,863,717 (1975);
3,744,017 (1973); RU patents Nos. 1143150 (1994), 2231631 (2002).
Pumping and vibrations simultaneously, pulse regime of pumping are
described in U.S. Pat. No. 4,456,068 (1984); RU patents Nos.
2221141 (2004); 2176728 (2000); 2200832 (2001).
[0017] Pumping and vibrations simultaneously, vibrations with
special frequencies, burning of solid/liquid fuel-oxidizer mixtures
are described in U.S. Pat. Nos. 3,520,362 (1970); 3,768,520 (1973);
RU patents Nos. 2128770 (1994); 2191896 (2002); 2003111855 (2004);
1434831 (1999); 1639127 (1987); 2087756 (1994).
[0018] Pumping and vibrations in period of pressure decline, cycle
regime of pumping, vibrations with special frequencies are
described in U.S. Pat. No. 3,520,362 (1970), RU patents Nos.
2128770 (1994); 219896 (2002); 2003111855 (2004); 1434831 (1999);
1639127 (1987); 2087756 (1994); 2066746 (1996); 94023110
(1994).
[0019] Pumping and vibrations simultaneously, pulse regime of
pumping, vibrations with special frequencies, injection of chemical
agents, proppants, are described in U.S. Pat. Nos. 5,197,543
(1993); 5,662,165 (1997); RU patents Nos. 2193649 (2002); 2186953
(2000); 2243364 (2002); 2003111855 (2004); 2066746 (1996); 94023110
(1994).
[0020] Pumping and vibrations simultaneously, vibrations with
special frequencies, injection of chemical agents are described in
U.S. Pat. No. 5,718,289 (1998); RU patents Nos. 2175058 (1999);
2186953 (2000), 2111348 (1994), 2078200 (1994).
[0021] Pumping, vibrations, injection of foam agents,
simultaneously vibrations with special frequencies, injection of
foam agents are described in U.S. Pat. Nos. 6,467,542 (2000);
6,015,010 (2000); 6,405,797 (2000); 6,241,019 (2000).
[0022] The object of the present invention is to provide methods of
intensification of natural gas production from the coal beds,
comprising exposure of the coal bed to pressure pulsing.
[0023] The object of the invention is attained in a method of
intensification of natural gas production from the coal beds,
comprising exposure of the coal bed to pressure pulsing and
hydraulic fracturing of the coal bed.
[0024] The pressure pulses can be low frequency high-amplitude
pressure pulses.
[0025] The hydraulic fracture can be created by the steps of
injecting into the coal bed a fracture fluid at high flow rates and
pressures to create the fracture; further injecting a proppant in
the fracture fluid to prevent closing the fracture created, and
finally injecting the fracture fluid without proppant for washing
the well. The pressure pulsing can be applied either before
creating the fracture, or during injecting the fracture fluid to
create the fracture, or during injecting a proppant in the fracture
fluid, or during finally injecting the fracture fluid without
proppant, or during some or all said steps.
[0026] An embodiment of the method can comprise, before starting
hydraulic fracture of the coal bed, the following steps:
[0027] running a pressure pulse generator into the wellbore at the
level of treated interval;
[0028] loading the wellbore with an incompressible fluid in order
to couple the coal bed to the pressure pulse generator;
[0029] adjusting the generator to create pressure pulses of
specific frequency and amplitude;
[0030] transmitting the pressure pulses created by the generator to
the coal bed through the incompressible fluid;
[0031] injecting the fracture fluid to the coal bed.
[0032] The aforementioned object of the invention is further
attained in a method of intensification of natural gas production
from the coal beds, comprising exposure of the coal bed to pressure
pulsing and hydraulic fracturing of the coal bed by injecting into
the coal bed a fracture fluid without proppant at high flow rates
and pressures.
[0033] The object of the invention is also attained in a method of
intensification of natural gas production from the coal beds,
comprising exposure of the coal bed to pressure pulsing and
creation of a cavity in the coal bed by cyclically increasing
pressure of the fluid in the well and depressurizing the fluid.
[0034] The pressure pulses can be low frequency high-amplitude
pressure pulses. The low frequency high-amplitude pressure pulses
can impact the coal bed during increasing the pressure of the fluid
in the well.
[0035] The method can further comprise the step of cleaning the
well, wherein the fluid used for transferring pressure pulses is
replaced by the efficient foamy cleaning medium.
[0036] A fluid driven generator can be used for creation of the
pressure pulses and the foamy cleaning medium is pumped through
said generator.
[0037] In another embodiment, a generator for creation of the
pressure pulses can be placed in nearby well, and pressure pulsing
of the coal bed can be simultaneously accompanied with injecting
the foamy cleaning medium in the treated well.
[0038] The object of the present invention is further attained in a
method of intensification of natural gas production from the coal
beds, comprising exposure of the coal bed to pressure pulsing,
creation of a cavity in the coal bed by cyclically increasing
pressure of the fluid in the well and depressurizing rapidly the
fluid, and well cleaning, comprising the steps of: running a
pressure pulse generator into the wellbore at the level of treated
interval; loading the wellbore with an incompressible fluid in
order to efficiently couple the coal bed to the pressure pulse
generator; adjusting the generator to create pressure pulses of
specific frequency and amplitude and transmitting the pressure
pulses created through said fluid to the coal bed; creating a
pressure gradient in the near wellbore zone by injecting fluid
above the pressure of the coal bed; depressurizing rapidly the
fluid in the wellbore to cause breakdown of the coal bed and
produce rubblized coal; running well cleaning by circulating
efficient foamy cleaning medium across the treated interval to aid
fluidization and transport of the rubblized coal to the
surface.
[0039] In this method, a fluid discharge pressure pulse generator
without a resonator can be employed, so that a pulsing jet produces
cutting/reaming action on the coal bed.
[0040] The aforementioned method steps can be repeated after
refilling the wellbore with the incompressible fluid.
[0041] Application of pressure pulses to coal seams promotes
opening the butt-cleat network and enhancing its permeability. The
pressure pulses can be applied alone, or they can be combined with
hydraulic fracturing and cavitation operations.
[0042] Application of stand alone high amplitude, low frequency
pressure pulses to the coal bed will cause tensile and shear
failure of the formation fabric. These tensile and shear cracks may
form preferentially along the butt and cleat network, as in the
case of a coal seam, or they may form along new failure planes. The
cracks will tend to become somewhat misaligned due to the
relaxation of anisotropic stress that pre-exists in some coal
formations. What's more, spalling, rubblization, or shear
displacement of the reservoir material may occur which prevents the
tensile and shear cracks from healing completely. In all cases, the
apparent permeability of the formation will increase, enabling
higher gas production rates, and higher dewatering rates (if water
is present).
[0043] Application of pressure pulses can be used to enhance the
cavitation process which is limited by the strength of the coal
formation and the ability to create tensile and shear failure deep
around the wellbore during each over-pressure/depressurization
cycle. Applying high amplitude, low frequency pressure pulses
during the over-pressure/depressurization process will expand the
failure zone during each cavitation cycle. Thus, either fewer
cycles are required to cause the desired growth of the effective
wellbore diameter, or the ultimate diameter of the effective
wellbore can be significantly larger than in the standard
cavitation process.
[0044] Application of pressure pulses can be used to enhance the
hole cleaning process. After a cavitation cycle the use of water or
other efficient medium for propagating pressure pulses is replaced
by an efficient foamed cleaning fluid. This cleaning cycle would
then fluidize and transport the small coal fragments out of the
wellbore and enable unrestricted evolution of gas from the coal
formation. The use of pressure pulses is superior to a conventional
steady-pressure process because the pulsing will cause the foam gas
cells to periodically compress and expand. During compression, the
foam will invade the porosity of the formation debris. During
expansion the foam will dilate the bed of formation debris,
facilitating its entrainment in the flowing stream. This
fluidization process is required for efficient borehole cleaning
and is enhanced by pressure pulses.
[0045] Application of pressure pulses can be used to enhance
hydraulic fracturing. Hydraulic fracturing is accomplished by
injecting a fracture fluid into the wellbore at high flow rates and
pressures to create the fracture, and further injecting a proppant,
such as sand, in the fracture fluid. The fracture that is created
becomes filled with the proppant, which prevents the fracture from
closing. This high conductivity channel enables both water and gas
to flow at a high rate from the coal formation into the wellbore.
Important parameters of the hydraulic fracture include the fracture
conductivity and the fracture geometry. The artificial fractures
have a capacity to conduct the produced fluids. Fractures with the
regular geometrical characteristics will connect to a vast number
of natural cracks, penetrating the reservoir and coupling with the
well. Applying pressure pulses to the formation, either before the
fracture treatment, during the pad stage of the fracture treatment,
during injecting a proppant in the fracture fluid, or during the
entire fracturing treatment, may have the following effects.
[0046] The cracks (cleats) in the coal bed arranged longitudinally
that are intersected by the fracture become dilated and misaligned,
thereby increasing their permeability and ability to feed the
fracture.
[0047] The coal formation fabric along the fracture face undergoes
tensile and shear failure. Coal becomes rubblized and the resulting
fracture is wider than a fracture created by normal processes. The
wider fracture has higher hydraulic conductivity than a more narrow
fracture. Possibly, the fracture will not require proppant to
maintain the fracture faces separate. Eliminating proppant from a
channel will increase the channel porosity (if the channel does not
close due to effective stress), thereby increasing hydraulic
conductivity of the fracture. The fracture may propagate more
easily because the pressure pulses have the effect of fatigue
breakdown of the rock.
[0048] To create pressure pulses in these methods for
intensification of natural gas production from coal beds the
following several pressure pulse generator variants can be
employed. A downhole hydrodynamic generator can be employed that
creates pressure oscillations when fluid is pumped through the
generator. Some mechanism to temporarily block and release the
fluid flow creates oscillations. The mechanism can include
mechanical valves, swirl type inertial valves, rotating ports that
discharge fluid upon alignment and block flow upon misalignment. In
a fully mechanical pressure generator, shock waves are created by
two objects striking each other. An electrodynamic pulse generator
utilizes electric discharge to create strong shock wave. Use could
be also made of water hammering generators, and combustion pulse
generators.
[0049] Pressure pulse generators that can be used in methods
according to the present invention are disclosed in the following
patents: U.S. Pat. Nos. 4,164,978 (1978); 4,164,4978 (1978);
4,702,315 (1987); 3,863,717 (1975); 4,456,068 (1984); 3,520,362
(1970); 3,768,520 (1973); 5,197,543 (1993); 5,662,165 (1997);
5,718,289 (1998); 6,467,542 (2000); 6,015,010 (2000); 6,405,797
(2000); 6,241,019 (2000); RU patent documents Nos. 16074 (1962);
1165801 (1985); 2084705 (1993); 2085721 (1997); 2100571 (1992);
2085721 (1994); 2175718 (1997); 2193649 (2002); 1143150 (1994);
2231631 (2002); 2221141 (2004); 2176728 (2000); 2200832 (2001);
2128770 (1994); 2191896 (2002); 2003111855 (2004); 1434831 (1999);
1639127 (1987); 2087756 (1994); 2066746 (1996); 94023110 (1994);
2193649 (2002); 2186953 (2000); 2243364 (2002); 2003111855 (2004);
2175058 (1999); 2186953 (2000); 2111348 (1994); 2078200 (1994);
2175058 (1999); 2186953 (2000); 2111348 (1994); 2078200 (1994) and
the aforementioned publication "Research Activities", 1975, p.
61.
[0050] The preferred, but non-limiting embodiment for the claimed
methods includes the use of downhole pressure pulses to generate
pulses in production well. The downhole generators which can be
used in methods according to the present invention include e.g. a
generator described in U.S. Pat. No. 6,015,010 for producing a
shock wave in a borehole including a pumping unit arranged at the
wellhead, a tubing string extending downwardly into the production
casing of the well, a hollow cylinder assembly connected with the
bottom of the tubing string, and a pair of plungers arranged within
the cylinder assembly and connected with the pumping unit with
sucker rods and a polish rod for compressing liquid contained
within the cylinder assembly and discharging the compressed liquid
into the production casing, thereby generating a shock wave. The
cylinder assembly includes a hollow upper cylinder, a hollow lower
cylinder arranged below the upper cylinder, a crossover cylinder
arranged between the upper and lower cylinders, and a compression
cylinder containing a compression chamber arranged between the
crossover cylinder and the upper cylinder. The lower cylinder has a
larger inner diameter than the upper cylinder inner diameter, and
the lower plunger has a larger diameter than the upper plunger. In
addition, the lower cylinder is adapted to receive the lower
plunger and the upper cylinder is adapted to receive the upper
plunger. When the plungers are displaced upwardly in the cylinder
assembly, the lower plunger travels into the compression chamber
and the upper plunger travels out of the compression chamber. Due
to the lower plunger having a greater diameter than the upper
plunger, the volume of the compression chamber is reduced and the
liquid contained therein becomes compressed. When the pumping unit
reaches the top of its stroke, the lower plunger allows the
compressed liquid contained in the compression chamber to be
discharged into the well. At this moment the stored pressure is
suddenly released and a very amplitude shock is created in the
wellbore, with amplitude on the order of 200 to 250 bars.
[0051] A method of intensification of natural gas production from
the coal beds, comprising exposure of the coal bed to pressure
pulsing can be accomplished by applying low frequency
high-amplitude pressure pulses to the coal bed to cause fracturing
in the bed structure. The method is performed in the following
manner.
[0052] To prepare the wellbore for the operation, the pressure
pulse generator is run into the wellbore to approximately the
treatment zone and the wellbore is loaded with an incompressible
fluid, if needed, in order to efficiently couple the formation to
the pressure pulse generator and transmit pressure pulses to the
formation. Then the pressure pulse generator is adjusted to create
pressure pulses of desired frequency and amplitude. The low
frequency waves, preferably close to resonant frequency of
formation, should be used in the treatment for down to a few meters
penetration depth. References on selection of proper frequency and
amplitude of pressure pulses for a particular type of formations
might be found in many patents, e.g. EP patent No. 0512331 (1992);
U.S. Pat. No. 4,164,4978 (1978); RU patents Nos. 1165801 (1985);
2084705 (1993); 2085721 (1997); 2100571 (1992); 2085721 (1994);
2175718 (1997); 2193649 (2002).
[0053] Time of treatment is defined empirically depending on rock
properties and regime of treatment. Recommendations on the
operation duration might be also found in the aforementioned
patents.
[0054] After termination of the pressure pulsing, dewatering is
run, if needed, using a pump to remove excessive water from well,
or to dewater the well completely. Then, the well may be cleaned,
if needed, from spalled material by any known method, e.g. by
injecting expanding agents (nitrogen, foam, or energized
liquid).
[0055] The aforementioned steps may be repeated for the desired
number of cycles. If required, the generator is removed and
production equipment is installed to start gas production.
[0056] A method of intensification of natural gas production from
the coal beds can comprise exposure of the coal bed to pressure
pulsing to enhance cavitation in the coal bed.
[0057] To perform this method, the wellbore is prepared for the
operation by running the pressure pulse generator, such as a
fluid-driven generator, into the wellbore to approximately the
treatment zone, and the wellbore is loaded with an incompressible
fluid, if needed, in order to efficiently couple the formation to
the pressure pulse generator and transmit pressure pulses to the
formation. The pressure pulse generator is adjusted to create
pressure pulses of desired frequency and amplitude. Regime and time
of treatment are chosen as mentioned above. One may operate fluid
discharge pressure pulse generators without resonator, so that a
pulsing jet produces cutting action on the formation. Then pressure
is increased in the well above the pore pressure by injecting
fluids such as nitrogen, foam, or energized liquid, which may be
performed through said generator. The wellbore is rapidly
depressurized, this resulting in production of rubblized coal
pieces. A surface valve is opened to a "blooie pit". The well is
then cleaned if needed by circulating energized fluid or foam
across the treated interval to aid fluidization and transport the
rubblized formation to the surface.
[0058] In case of another cavitation cycle, the wellbore is
refilled with an incompressible fluid and the aforementioned steps
are repeated until no additional formation material is produced
during the depressurization process. If required, the generator is
removed and production equipment is installed and production is
started.
[0059] Application of pressure pulses can be used to enhance the
process of cleaning the borehole from coal pieces. To this end,
after a cavitation cycle fluids are switched from water or other
efficient medium for propagating pressure pulses to an efficient
foamed cleaning fluid. During compression, the foam will invade the
porosity of the coal bed debris and fluidize the debris,
facilitating thereby its entrainment in the flowing stream. This
fluidization process is required for efficient borehole cleaning
and is enhanced by pressure pulses.
[0060] The method combines application of pressure pulses and
injection of cleaning fluid. For fluid driven generators the
cleaning fluid is pumped through the generator. For generators
placed in nearby wells, pulsing is continued while injecting
cleaning fluid in the treated well. Circulation of cleaning fluids
is finished if no more spalled material is produced on to
surface.
[0061] The pulse pressure treatment can be used to enhance
hydraulic fracturing.
[0062] The method involves preparing the wellbore for the operation
by running the pressure pulse generator into the wellbore to
approximately the treatment zone, and loading the wellbore with an
incompressible fluid, if needed, in order to efficiently couple the
coal formation to the pressure pulse generator.
[0063] Preparation is carried out for injecting a fracture fluid
into the formation and the pressure pulse generator is adjusted to
create pressure pulses of desired frequency and amplitude. Regime
and time of treatment are chosen as mentioned above.
[0064] Hydraulic fracturing of the coal bed is accomplished by the
steps of injecting into the wellbore a fracture fluid at high flow
rates and pressures to create the fracture, further injecting a
proppant in the fracture fluid to prevent closing the fracture
created, and finally injecting the fracture fluid without proppant
for washing the well. Pressure pulsing is applied to the coal bed
either before creating the fracture, or during injecting the
fracture fluid to create the fracture, or during injecting a
proppant in the fracture fluid, or during finally injecting the
fracture fluid for washing the well or during some or all said
steps. The selected regime of pressure pulsing at different steps
of hydraulic fracturing affects the resulting fracture
geometry.
[0065] An embodiment of the method comprises hydraulic fracturing
steps without a proppant. The possibility of this embodiment has
been explained above.
[0066] Upon completion of the fracturing job, if required, the
generator is removed and production equipment is installed and
production is started.
[0067] Some particular examples of implementing the methods in
accordance with the invention will be described below.
[0068] To determine the appropriate pressure pulse generator
frequency for the various applications in the above methods, the
following algorithms are employed that will be referred to as
"Algorithm" in the following examples.
[0069] Algorithm 1 comprises using experimental data that indicates
that the effective tensile and shear crack formation occurs within
a range of pulse frequency (number of pulses per second) of between
0.1 to 500 Hz.
[0070] Algorithm 2 comprises installing, before the treatment, a
measuring device in an observation well to measure characteristics
of formation vibrations. The measuring device should be coupled to
the productive interval of interest. Then a pressure pulse
generator is installed in a nearby well and activated so that it
produces pressure pulses; the frequency of the pressure pulse
generator is incremented; the formation movement in the observation
well is recorded, and the most effective frequency is selected
through evaluation of the measured formation vibrations in the
observation well corresponding to specific frequencies in the
nearby well.
[0071] Algorithm 3 comprises writing a system of equations for a
linearly elastic porous medium representing the formation
surrounding the wellbore; solving the equations for the condition
that the formation is exposed to a periodic pressure pulse or a
pressure shock; estimating the zone of tensile and shear failure in
the formation surrounding the wellbore, and selecting the pressure
pulsing frequency and amplitude that maximize the extent of the
failure zone.
[0072] Algorithm 4 comprises writing a system of equations for a
linearly elastic porous medium representing the formation
surrounding the wellbore; solving the equations for the condition
that the formation is exposed to a periodic pressure pulse or a
pressure shock; estimating the zone of tensile and shear failure in
the formation surrounding the hydraulic fracture, and selecting the
frequency and amplitude that maximize the extent of the failure
zone.
[0073] A pressure pulse generator employed in the Examples
presented below is disclosed in U.S. Pat. No. 6,015,010; the
generator is mechanical and creates pressure pulses by up and down
strokes of pump rods of the pumping jack operating on surface, and
is capable to produce a hydraulic impact in the wellbore with
amplitude of 200-250 bars.
EXAMPLE 1
Application of Pressure Pulsing to Coal Formation to Enhance
Efficiency of Cavitation
[0074] (a) A well penetrates a coal formation containing adsorbed
methane gas at a depth of approximately 800 meters. The production
interval is 10 meters thick, has a porosity of 2%, effective
permeability of 25 mD, temperature is 25.degree. C., reservoir
pressure is 50 bars, Young's modulus is 0.5 million psi, and
Poisson's ratio is 0.34.
[0075] The Algorithm is used to determine that it will be suitable
to treat this well using a generator that generates 50 bar pressure
pulses at a 60 Hz frequency.
[0076] The well is filled with a 4 wt % KCI brine to control
pressure. A workover unit is used to remove the sucker rod pump,
pump rods, and production tubing and tubing packer. The pressure
pulse generator is attached to the production tubing and the
production tubing is run back into the wellbore. Injection of
treatment fluid for the near wellbore region begins when the
generator reaches the depth corresponding to the top of the
production interval. Filtered solution (of the same 4 wt % KCI
concentration) is injected through the generator. Injected liquid
flowing through the generator periodically pressurizes an
accumulator. During pressurization, the accumulator receives some
of the injected fluid, and the net injection rate into the
formation is less than the average and the bottom hole injection
pressure falls. Once the accumulator is filled, it builds an
overpressure which initiates a rapid discharge of the stored
liquid. This discharge causes the net injection rate into the
formation to exceed the average rate, and the bottom hole injection
pressure increases. By this periodic oscillation mechanism, the
required pressure pulse amplitude and frequency is generated. The
injection is continued until the "break-down" pressure or fracture
pressure of the formation is reached. This hydraulic fracture
pressure is the pressure at which a hydrostatic column of fluid
will initiate a fracture at the wellbore. The pressure pulses
generated by the generator periodically exceed the fracture
pressure. As the injected fluid flows into the formation, a
pressure gradient is developed in the near wellbore region. This
pressure gradient, coupled with the periodic pressure pulses leads
to tensile failure of the rock in the near wellbore region. These
pressure pulses disaggregate the matrix blocks of the coal
formation, effectively shattering the coal seam along the network
of face and butt cleats. This pressure pulse treatment continues
for 15 minutes to several hours.
[0077] At the end of pressure pulsing, the brine is displaced into
the formation with foamed nitrogen (a mixture of 70% by volume
nitrogen and 30% by volume water containing viscosifier and
surfactant to stabilize the foam structure; volume percentage is
measured at the bottom hole conditions). The foamed nitrogen
injection continues in an effort to pressurize the near wellbore
formation to a pressure much greater than the reservoir pressure.
The foam is able to easily penetrate the weakened, disaggregated
coal formation. After the complete foam volume is injected (the
volume should be sufficient to fill the entire production interval
to a radial depth of 3 meters), the wellbore is rapidly
depressurized. Depressurization mobilizes the weakened, rubblized
coal, and those coal fragments are conveyed out of the formation
through the well into a "blooie pit." This depressurization
continues until no more coal and nitrogen gas are produced. The
process enlarges the wellbore radius and cleans pores in the rock
immediately around the wellbore. A total of 2 to 10 or more
treatment cycles including pressure pulsing, foam pressure
charging, and vigorous well depressurization steps may be carried
out over a period of a few hours to one or more days. The act of
applying pressure pulses significantly enhances the efficiency of
the coal rubblization process and the subsequent cavity creation
process.
[0078] A well cleaning operation is performed by injection and
circulation of the foamed fluid through the wellbore in order to
dilate and carry out the slurry away from the wellbore. The
circulation occurs by injecting fluid down a coiled tubing lowered
to the top of the production interval, and then in rather small
measured increments, the coiled tubing is lowered progressively to
the lowest extreme of the well. Periodically the coiled tubing
depth is held constant to insure that debris is thoroughly removed
from one segment of the wellbore. Coiled tubing cleaning continues
until the lowest extreme of the well is tagged. Then the coiled
tubing is removed from the wellbore, continuing circulation. After
removal of the coiled tubing, the pump rods and pump reinstalled.
The well is put back onto production.
[0079] (b) An embodiment of Example 1 is carried out similarly to
embodiment (a) except that the well cleaning operation is
modified.
[0080] Similarly to the described embodiment (a), the well cleaning
operation is performed at the end of the pressurizing and
depressurization cycles. Circulation occurs by injecting fluid down
a coiled tubing lowered to the top of the production interval, and
then in rather small measured increments, the coiled tubing is
lowered progressively through any fill that has accumulated.
Periodically the coiled tubing depth is held constant to insure
that debris is thoroughly removed from one segment of the wellbore
before proceeding deeper into the wellbore. Coiled tubing cleaning
continues until the lowest extreme of the well is tagged. In this
embodiment, the foam cleaning is enhanced during this treatment by
operating a pressure pulse generator during foam circulation. The
action of the pressure pulse generator is to cause foam bubbles to
pulsate, alternatively becoming larger and smaller. During
compression, the foam invades the porosity of the coal debris that
resides in the wellbore. During expansion, the foam dilates the bed
of coal debris, facilitating its entrainment into the flowing
stream. The pressure pulse generator is installed at the end of the
coiled tubing. It has the same pressure pulse generation mechanism
and the same pulse amplitude and frequency as the generator used
during the cavity completion treatment. The major difference is
that the wellbore pressure is reduced during the cleaning operation
to a level less than the formation hydraulic breakdown pressure and
a level low enough to allow the fluids to be produced up the well.
At the end of the cleaning operation the coiled tubing is removed
from the wellbore, continuing circulation. After removal of the
coiled tubing, the production tubing is restored, pump rods and
pump reinstalled, and the well is put back onto production.
EXAMPLE 2
Application of Pressure Pulsing to Coal Formation to Amplify
Hydraulic Fracturing without Proppant
[0081] (a) A well penetrates a coal formation containing adsorbed
methane gas at a depth of approximately 800 meters. The production
interval is 10 meters thick, has a porosity of 2%, effective
permeability of 25 mD, temperature is 25.degree. C., reservoir
pressure is 50 bars, Young's modulus is 0.5 million psi, and
Poisson's ratio is 0.34.
[0082] The Algorithm is used to determine that it will be suitable
to treat this well using a generator that generates 75 bar pressure
pulses at a 20 Hz frequency.
[0083] The well is filled with a 4 wt % KCI brine to control
pressure; the pressure pulse generator is installed, and treatment
fluid injection begins when the generator reaches the depth
corresponding to the top of the production interval.
[0084] During the hydraulic fracturing operation, a fracture fluid
is injected down both the tubing and the tubing-casing annulus.
Additives are incorporated into the fracture fluid so that it has
increased viscosity. The composition in this treatment is 0.4 wt %
polymer viscosifier, such as guar, 0.014 wt % boric acid, enough
sodium hydroxide to raise the fluid pH to 9.6, 0.003 wt % biocide,
0.2% surfactant to cause the coal surface to be hydrophobic
(ethoxylated butoxylated alcohols) plus 0.25 wt % encapsulated
oxidizer for fluid degradation. This fluid will be called
crosslinked gel.
[0085] Crosslinked gel injection begins down the annulus and the
tubing simultaneously. The total injection rate increases slowly
until the formation break-down pressure is reached. The injection
rate is then increased from approximately 0.5 m.sup.3/min to a
steady rate of 3 to 4 m.sup.3/min to extend the fracture a great
distance away from the well. The flow rate of the crosslinked gel
down the annulus is steady, whereas that discharged from the end of
the tubing oscillates under the action of the downhole pressure
pulse generator. The crosslinked gel flowing through the generator
periodically pressurizes an accumulator. During pressurization, the
accumulator receives some of the injected fluid, and the net
injection rate into the formation is less than the average, and the
bottom hole injection pressure falls. Once the accumulator is
filled, it builds an overpressure, which initiates a rapid
discharge of the stored liquid. This discharge causes the net
injection rate into the formation to exceed the average rate, and
the bottom hole injection pressure increases. By this periodic
oscillation mechanism, the required pressure pulse amplitude and
frequency is generated. During this treatment, crosslinked gel only
is being injected: a total of 200 m.sup.3 of crosslinked gel is
injected. The pressure pulse generator operates the entire time of
injection. As the fracture fluid injection creates and extends a
fracture away from the wellbore to a length of 300 to 400 feet, the
pressure pulses act to cause tensile failure of the formation at
the fracture face combined with shear failure in planes parallel to
the fracture face. The pressure pulse induced cracks are orientated
along the face and butt cleat network in the coal matrix,
longitudinally and perpendicular to coal shims. These pressure
pulses disaggregate the matrix blocks of the coal formation,
effectively shattering the coal seam along the network of face and
butt cleats. This combined hydraulic fracturing and pressure pulse
treatment continues for approximately 1 hour as the entire fluid
volume is injected into the formation.
[0086] At the end of the crosslinked gel injection, the crosslinked
gel is flushed to the bottom of the casing with 4% KCl water
containing only 0.2 wt % guar. Displacement is then transitioned to
foamed 0.2% guar and KCl solution being injected to lighten the
hydrostatic column in the wellbore (bottom hole foam
characteristics are 70% by volume nitrogen and 30% viscous water
containing 0.2 wt % guar, and 0.1 to 1.0 volume % surfactant).
Injection is stopped in the annulus once the crosslinked gel is
entirely displaced into the formation (and the foamed uncrosslinked
guar solution fills the entire annulus and tubing). Foam injection
continues down the tubing. An annulus valve is opened allowing the
wellbore fluids to flow from the wellbore through a valve to a
waste collection pit. During this flowback procedure, the
bottomhole pressure in the wellbore is reduced to a level of
approximately 5 to 10 bars and the injected fracturing fluid is
recovered out of the well. During the flowback process, the
disaggregated coal blocks reorientate in a manner that prevents the
fracture and dilated cleats from closing entirely. The resulting
fracture has high hydraulic conductivity.
[0087] A well cleaning operation is performed at the end of the
flowback process in a manner similar to that in Example 1.
[0088] (b) A well penetrates a coal formation containing adsorbed
methane gas at a depth of approximately 800 meters. The production
interval is 10 meters thick, has a porosity of 2%, effective
permeability of 25 mD, temperature is 25.degree. C., reservoir
pressure is 800 psi, Young's modulus is 0.5 million psi, and
Poisson's ratio is 0.34.
[0089] The Algorithm is used to determine whether it will be
suitable to treat this well using a tool that generates a 200 bar
pressure shocks at a 0.1 Hz frequency.
[0090] In this embodiment, the method is accomplished similarly to
the embodiment (a), except that the pressure pulse generator is
lowered into the well to a position 5 meters above the top of the
production interval, and the fluid used as a fracturing fluid
comprises 4 wt % KCl plus 0.5 to 3 volume % viscoelastic
surfactant, plus 0.2 to 0.4 volume % of an organic surfactant of
the type ethoxylated-butoxylated glycols having 1 to 5 ethylene
oxide groups and 5 to 10 butylene oxide groups. This composition is
a non-damaging viscoelastic surfactant solution. A well cleaning
operation is performed without a pulse generator.
[0091] (c) A well penetrates a coal formation that contains
adsorbed methane gas at a depth of approximately 800 meters. The
production interval is 10 meters thick, has a porosity of 2%,
effective permeability of 25 mD, temperature is 25.degree. C.,
reservoir pressure is 50 bars, Young's modulus is 0.5 million psi,
and Poisson's ratio is 0.34.
[0092] The Algorithm is used to determine whether it will be
suitable to treat this well using a generator that generates a 70
bar pressure shocks at a 300 Hz frequency.
[0093] The method is carried out similarly to embodiment (a) except
that the pressure pulse generator comprises a venturi that is built
into the tool. The crosslinked gel flowing through the venturi
produces cavitation. The collapse of gas bubbles in the expanding
part of the venturi is the source of the pressure shocks. By this
periodic oscillation mechanism, the required pressure pulse
amplitude and frequency is generated. During this treatment,
crosslinked gel only is being injected. A total of 200 m.sup.3 of
crosslinked gel is injected. The pressure pulse generator operates
the entire time. The pressure shocks generated by the tool are
focused down the fracture and cause high stress concentrations as
the bubbles collapse near the fracture walls. As the crosslinked
gel creates and extends a fracture away from the wellbore to a
length of 300 to 400 feet, the pressure shocks act to cause tensile
failure of the formation at the fracture face combined with shear
failure in planes parallel to the fracture face. As mentioned
earlier, these pressure shocks disaggregate the matrix blocks of
the coal formation, effectively shattering the coal seam along the
network of face and butt cleats. This combined hydraulic fracturing
and pressure pulse treatment continues for approximately 1 hour as
the entire fluid volume is injected into the formation.
EXAMPLE 3
Application of Pressure Pulsing to Coal Formation Only During
Injection of Fracture Fluid with Proppant to Amplify Hydraulic
Fracturing
[0094] A well penetrates a producing coal formation containing
adsorbed methane gas at a depth of approximately 800 meters. The
production interval is 10 meters thick, has a porosity of 2%,
effective permeability of 25 mD, temperature is 25.degree. C.,
reservoir pressure is 50 bars, Young's modulus is 0.5 million psi,
and Poisson's ratio is 0.34.
[0095] The Algorithm is used to determine that it will be suitable
to treat this well using a tool that generates 75 bar pressure
pulses at a 20 Hz frequency.
[0096] The well is filled with a 4 wt % KCI brine to control
pressure; the pressure pulse generator is installed in the wellbore
and a treatment fluid injection begins when the generator reaches
the depth corresponding to the top of the production interval.
[0097] During the hydraulic fracturing operation, fluid is injected
down both the tubing and the tubing-casing annulus. The fracture
fluid in this treatment is the aforementioned crosslinked gel or a
solution of viscoelastic surfactant. The hydraulic fracturing
process is accomplished as in other examples, wherein the injected
fluid flowing through the generator periodically pressurizes an
accumulator. During pressurization, the accumulator receives some
of the injected fluid, and the net injection rate into the
formation is less than the average, and the bottom hole injection
pressure falls. Once the accumulator is filled, it builds an
overpressure, which initiates a rapid discharge of the stored
liquid. This discharge causes the net injection rate into the
formation to exceed the average rate, and the bottom hole injection
pressure increases. By this periodic oscillation mechanism, the
required pressure pulse amplitude and frequency is generated.
During this treatment, 70 m.sup.3 of crosslinked gel is injected
without proppant; this step is called the pad stage. The pressure
pulse generator operates only during the pad stage. As the
crosslinked gel creates and extends a fracture away from the
wellbore to a length of 300 to 400 feet, the pressure pulses act to
cause tensile failure of the formation at the fracture face
combined with shear failure in planes parallel to the fracture
face. This combined hydraulic fracturing and pressure pulse
treatment continues for approximately 1 hour as the entire fluid
volume is injected into the formation.
[0098] After the pad, proppant is added to the remainder of the
injected crosslinked gel. The proppant concentration begins at a
low concentration and is progressively ramped up to a high
concentration. In this treatment, the proppant concentration begins
at 1% by weight of fracture fluid and ends at 50% by weight of
fracture fluid. The fracture fluid/proppant slurry is only injected
down the annulus. A total of 140 m.sup.3 of slurry is injected. The
pressure pulse generator is not operated during slurry injection.
The subsequent steps are carried out as in Example 2.
EXAMPLE 4
[0099] Application of Pressure Pulsing To Coal Formation
Continuously During Both Pad and Proppant Injection Stages of
Hydraulic Fracturing
[0100] (a) A well penetrates a producing coal formation that
contains adsorbed methane gas at a depth of approximately 800
meters. The production interval is 10 meters thick, has a porosity
of 2%, effective permeability of 25 mD, temperature is 25.degree.
C., reservoir pressure is 50 bars, Young's modulus is 0.5 million
psi, and Poisson's ratio is 0.34.
[0101] The Algorithm is used to determine whether it will be
suitable to treat this well using a tool that generates 75 bar
pressure pulses at a 20 Hz frequency.
[0102] The well is filled with a 4 wt % KCl brine to control
pressure; the pressure pulse generator is lowered into the
wellbore. Treatment fluid injection begins when the tool reaches
the depth corresponding to the top of the production interval.
[0103] Fluid is injected down both the tubing and the tubing-casing
annulus. The composition in this treatment is the aforementioned
crosslinked gel or a viscoelastic surfactant solution.
[0104] Injection of fracture fluid without proppant under pressure
generator pulses is carried out as in Example 3.
[0105] This combined hydraulic fracturing and pressure pulse
treatment continues for approximately 1 hour as the entire fluid
volume is injected into the formation.
[0106] After the first injection stage, proppant is added to the
remainder of the fracture fluid. The proppant concentration begins
at a 1% by weight of crosslinked gel and ends at 50% by weight of
crosslinked gel. The fracture fluid/proppant slurry is only
injected down the annulus. A total of 140 m.sup.3 of slurry is
injected. The pressure pulse generator operates during the step of
injecting the fracture fluid with a proppant also. The remainder of
the process is accomplished as in the previous Examples.
* * * * *