U.S. patent application number 13/165580 was filed with the patent office on 2012-01-26 for gas cushion near or around perforating gun to control wellbore pressure transients.
This patent application is currently assigned to SCHLUMBERGER TECHNOLOGY CORPORATION. Invention is credited to Brenden M. Grove, Jeremy P. Harvey.
Application Number | 20120018156 13/165580 |
Document ID | / |
Family ID | 45371790 |
Filed Date | 2012-01-26 |
United States Patent
Application |
20120018156 |
Kind Code |
A1 |
Grove; Brenden M. ; et
al. |
January 26, 2012 |
GAS CUSHION NEAR OR AROUND PERFORATING GUN TO CONTROL WELLBORE
PRESSURE TRANSIENTS
Abstract
A method for perforating a formation material proximate to a
wellbore involving lowering a perforating gun string comprising a
perforating gun and a gas-generating device downhole, providing
wellbore fluid around the perforating gun, and providing a volume
of gas proximate the perforating gun, by the gas-generating device,
wherein the volume of gas is configured to reduce the shock
produced upon firing of the perforating gun.
Inventors: |
Grove; Brenden M.; (Missouri
City, TX) ; Harvey; Jeremy P.; (Houston, TX) |
Assignee: |
SCHLUMBERGER TECHNOLOGY
CORPORATION
SUGAR LAND
TX
|
Family ID: |
45371790 |
Appl. No.: |
13/165580 |
Filed: |
June 21, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61357457 |
Jun 22, 2010 |
|
|
|
Current U.S.
Class: |
166/297 ;
166/55 |
Current CPC
Class: |
E21B 21/085 20200501;
E21B 43/1195 20130101 |
Class at
Publication: |
166/297 ;
166/55 |
International
Class: |
E21B 43/11 20060101
E21B043/11 |
Claims
1. A method for perforating a formation material proximate to a
wellbore, comprising: lowering a perforating gun string comprising
a perforating gun and a gas-generating device downhole; providing
wellbore fluid around the perforating gun; and providing a volume
of gas proximate the perforating gun, by the gas-generating device,
wherein the volume of gas is configured to reduce the shock
produced upon firing of the perforating gun.
2. The method of claim 1, wherein firing the perforating gun
comprises: detonating a plurality of shaped charges in the
perforating gun.
3. The method of claim 1, wherein the gas-generating device is
positioned below the perforating gun on the perforating gun
string.
4. The method of claim 3, further comprising: actuating the
gas-generating device, wherein the gas-generating device is
actuated by one selected from a group consisting of: automatically,
during lowering of the perforating gun downhole, and manually,
using a control signal sent from a surface device to the
gas-generating device.
5. The method of claim 3, wherein the gas-generating device is one
selected from a group consisting of a propellant device, a
high-pressure tank, a stored gas accumulator, a liquid CO.sub.2
generating cylinder device, and a slow bladder inflator.
6. The method of claim 1, wherein the volume of gas comprises gas
bubbles surrounding the perforating gun.
7. The method of claim 1, wherein the volume of gas is an
accumulation positioned directly above the perforating gun.
8. The method of claim 1, wherein the volume of gas completely
surrounds the perforating gun.
9. The method of claim 1, wherein the volume of gas provided is
proportional to a size of the perforating gun.
10. The method of claim 1, wherein the volume of gas is configured
to increase fluid compressibility in the wellbore, resulting in
reducing sudden changes to wellbore pressure transients.
11. The method of claim 1, wherein the volume of gas is introduced
by the gas-generating device immediately before detonation of the
perforating gun.
12. A perforating gun system for perforating a formation material
proximate to a wellbore, comprising: a perforating string
comprising a perforating gun, wherein the perforating string is
lowered into the wellbore; and a gas-generating device operatively
connected to the perforating string and configured to introduce a
volume of gas proximate to the perforating gun, wherein the volume
of gas is configured to reduce the shock produced upon firing of
the perforating gun.
13. The perforating gun system of claim 12, wherein the
gas-generating device is positioned below the perforating gun on
the perforating gun string.
14. The perforating gun system of claim 12, wherein the
gas-generating device is one selected from a group consisting of a
propellant device, a high-pressure tank, a stored gas accumulator,
a liquid CO.sub.2 generating cylinder device, and a slow bladder
inflator.
15. The perforating gun system of claim 12, wherein the volume of
gas is introduced when the gas-generating device is actuated by one
selected from a group consisting of: automatically, during lowering
of the perforating gun string downhole, and manually, using a
control signal sent from a surface device to the gas-generating
device.
16. The perforating gun system of claim 12, wherein the volume of
gas completely surrounds the perforating gun.
17. The perforating gun system of claim 12, wherein the volume of
gas provided is proportional to a size of the perforating gun.
18. The perforating gun system of claim 12, wherein the volume of
gas is configured to increase fluid compressibility in the
wellbore, resulting in reducing sudden changes to wellbore pressure
transients.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application 61/357457, filed on Jun. 22, 2010.
FIELD OF THE INVENTION
[0002] The present invention relates to a gas cushion surrounding a
perforating gun in a wellbore, for purposes of controlling wellbore
fluid compressibility and wellbore pressure transients.
BACKGROUND
[0003] After drilling, oil wells are typically protected with steel
casing that is secured to the wellbore with cement. To produce well
fluid (oil or gas) from a hydrocarbon bearing formation, the
formation is typically perforated from a wellbore to enhance fluid
communication between the reservoir and the wellbore. Perforation
is the process that creates a direct link between the wellbore and
a producing formation by puncturing holes through the casing and
the cement sheath that surrounds it. Perforation guns carrying
shaped charges are typically used for this purpose. These shaped
charges contain explosives. When the explosives are fired, they
produce high pressure and high temperature. As a result, the shaped
charge liners are collapsed and shot out as jets, which can
penetrate the casing and the nearby formation.
[0004] Firing of perforating guns into reservoir rocks, however,
may cause sudden pressure changes to wellbore pressure transients,
which may include the wellbore pressure, the reservoir pressure,
and the perforating gun pressure. Such sudden pressure changes
typically accompany the perforating process, and often cause damage
to the formation surrounding the wellbore and/or to the perforation
system. As drilling depths increase, controlling the sudden changes
to wellbore pressure transients has become an increasing
concern.
SUMMARY
[0005] In general, in one aspect, the invention relates to a method
for perforating a formation material proximate to a wellbore
involving lowering a perforating gun string comprising a
perforating gun and a gas-generating device downhole, providing
wellbore fluid around the perforating gun, and providing a volume
of gas proximate the perforating gun, by the gas-generating device,
wherein the volume of gas is configured to reduce the shock
produced upon firing of the perforating gun.
[0006] In general, in one aspect, the invention relates to a
perforating gun system for perforating a formation material
proximate to a wellbore, comprising a perforating string comprising
a perforating gun, wherein the perforating string is lowered into
the wellbore; and a gas-generating device operatively connected to
the perforating string and configured to introduce a volume of gas
proximate to the perforating gun, wherein the volume of gas is
configured to reduce the shock produced upon firing of the
perforating gun.
[0007] Other aspects of the invention will be apparent from the
following description and the appended claims.
BRIEF DESCRIPTION OF DRAWINGS
[0008] FIG. 1 shows a perforating gun system in a wellbore in
accordance with one or more embodiments of the invention.
[0009] FIG. 2 shows a perforating gun system with gas cushion near
the gun in accordance with one or more embodiments of the
invention.
[0010] FIG. 3 shows a graph of pressure data compared for two
different perforating gun systems in accordance with one or more
embodiments of the invention.
[0011] FIG. 4 shows a perforating gun system surrounded by gas
bubbles in accordance with one or more embodiments of the
invention.
[0012] FIG. 5 shows a perforating gun system completely surrounded
by a gas cushion in accordance with one or more embodiments of the
invention.
[0013] FIG. 6 shows a graph of pressure data corresponding to the
perforating gun system of FIG. 5 in accordance with one or more
embodiments of the invention.
DETAILED DESCRIPTION
[0014] Specific embodiments of the invention will now be described
in detail with reference to the accompanying figures Like elements
in the various figures are denoted by like reference numerals for
consistency.
[0015] In the following detailed description of embodiments of the
invention, numerous specific details are set forth in order to
provide a more thorough understanding of the invention. However, it
will be apparent to one of ordinary skill in the art that the
invention may be practiced without these specific details. In other
instances, well-known features have not been described in detail to
avoid unnecessarily complicating the description.
[0016] In general, embodiments of the invention provide a method
and apparatus for creating a gas "cushion" in the vicinity of a
perforating gun system lowered into a wellbore, either before or
just at the time or perforation. More specifically, the purpose of
the gas cushion is to increase the apparent compressibility of the
wellbore fluid in the vicinity of the perforating gun. The effect
of this increased compressibility is to reduce both the magnitude
and rate of any sudden pressure changes which accompany the
perforating process. Controlling wellbore compressibility, and by
extension the magnitude and rate of pressure change in the
wellbore, is useful to accomplish one or more of the following
desirable objectives: [0017] a. minimize transient loading on gun
string and completions equipment (i.e. "gun shock"); [0018] b.
minimize initial perforation tunnel and/or wellbore collapse and
sand production; and [0019] c. maintain a static underbalance
condition which has been set prior to perforating.
[0020] FIG. 1 shows, during a typical downhole operation, a long
and tubular device, such as a perforating gun 15, is run into the
wellbore 11 in preparation for production. After the perforating
gun 15 has been deployed at its appropriate downhole position on a
line 17 or tubing, e.g., wireline, e-line, slickline, coiled
tubing, etc., perforating charges are fired. Typical perforating
charges, such as shaped charges 14, may be radially disposed and
outwardly directed toward the region of the formation rock to be
perforated. Firing these shaped charges 14, may generate extremely
high-pressure jets 28, which can produce perforation tunnels 18
through both casing 12 (if the wellbore is cased) and surrounding
rock formation 13. The skin 19 of the tunnel 18 may then be made
more permeable for releasing its hydrocarbons from the formation
13.
[0021] In one or more embodiments of the invention, the perforating
system described above may include a gas generating device 30. The
gas generating device 30 may be displaced below the perforating gun
15, and operatively connected to the perforating string used to
lower the perforating gun 15 into the wellbore. The gas generating
device 30 is configured to introduce a "gas cushion," or an
accumulation of gas, in proximity to the perforating gun some time
before perforation occurs. For example, in one or more embodiments
of the invention, the gas generating device 30 may be a propellant
device, a high-pressure tank, a stored gas accumulator, a slow
bladder inflator, a liquid CO.sub.2 generating cylinder device, or
similar device capable of generating a cushion of gas in proximity
to the perforating gun. In one or more embodiments of the
invention, the gas generating device may be retained under a
packer.
[0022] In one or more embodiments of the invention, the amount of
gas generated by the gas generating device 30 is proportional to
the size of the perforating gun. That is, the range of volume of
gas generated by the device 30 is a function of the length of
perforating interval of the perforating gun. For example, the gas
generating device 30 may generate a certain volume of gas for every
foot diameter of the perforating gun.
[0023] In one or more embodiments of the invention, the gas
generating device 30 may be actuated by a control signal sent from
the surface to a circuit, valve, or other component of the gas
generating device. Alternatively, the gas generating device 30 may
be automatically actuated by conditions encountered as the device
is lowered into the wellbore. For example, the gas generating
device 30 may be depth actuated once the device reaches a
predetermined depth in the wellbore. Similarly, the gas generating
device could also be temperature actuated.
[0024] Those skilled in the art will appreciate that exactly when
the gas generating device begins to introduce gas into the region
surrounding the perforating gun may depend on the type of gas
generating device employed and the method of actuation of the
device. For example, a slow bladder inflator may be depth actuated
and may begin to introduce gas minutes before perforation occurs.
Alternatively, in one or more embodiments, a propellant generator
device may actuate immediately before perforation occurs (e.g., via
a control signal sent from the surface), which would result in
introduction of gas 100.sup.ths of milliseconds before perforation
occurs. Further, those skilled in the art will appreciate that
although the gas-generating device is show in FIG. 1 as being below
the perforating gun and operatively connected to the gun string,
the invention is not limited to such a configuration, as other
alternative arrangements for the gas-generating device may be
employed. For example, in an alternative embodiment of the
invention, the gas cushion may be pumped downhole from the surface,
in which case a gas-generating device located downhole may not be
necessary. Instead, the gas-generating device may be a stand-alone
device positioned on the surface.
[0025] The purpose of introducing an accumulation of gas into the
area surrounding the perforating gun is to increase system fluid
compressibility. The pressure differential between a wellbore and
the reservoir prior to perforation may be described as
under-balanced, balanced or over-balanced. A desirable
under-balance condition exists when hydrostatic pressure inside the
well casing is less than pressure in the formation. The term
"underbalance" (UB) means pressure of the wellbore
(Pwb)<pressure of the reservoir (Pres), and "overbalance" (OB)
means Pwb>Pres. The terms "static UB" or "static OB" are now
used when referring to the pressure condition prior to perforating
because on the sub-second time scale, local pressure conditions can
vary greatly from the initial static condition. These terms
indicate the static pressure conditions which existed prior to
perforating, and were assumed to remain throughout the perforating
process. Exposing the newly-created perforation tunnels to a
sufficiently large UB induces tensile and shear stresses within the
perforation damaged zone; maintaining this UB throughout the
perforating process induces a strong surge flow from the reservoir
into the perforation tunnel and wellbore. These events are
recognized as essential conditions to effectively remove
perforation damage.
[0026] When a hollow carrier perforating gun is detonated, the
equilibrium pressure inside the gun (a few milliseconds after
charge detonation) plays an important role. If this pressure is
less than the wellbore pressure (Pgun<Pwb), then this is called
a relative "gun UB"; wellbore fluid gets sucked into the gun and
Pwb drops. If Pwb even briefly drops to a value less than Pres,
then a dynamic underbalance (DUB) condition is present. If
Pgun>Pwb, this is called "gun OB"; detonation gas exits the gun
and Pwb increases. If Pwb even briefly increases to greater than
Pres, then a dynamic overbalance (DOB) condition is present. Those
skilled in the art will appreciate that DUB or DOB conditions can
be created from either initial static UB or OB conditions, and that
the initial static condition alone (Pwb, Pres) does not predict the
transient conditions which may occur during perforating.
[0027] In addition to the initial pressures involved and described
above, a primary parameter governing wellbore pressure transients
is the wellbore fluid compressibility. If the fluid is stiff, then
pressure changes are large and rapid; if the fluid is compliant,
then pressure changes are small and gradual. For example, consider
the case of gun filling. The "initial" (i.e., before reservoir or
far-field wellbore storage effects come into play) pressure drop in
the wellbore fluid is represented as,
dP=dV/(B*V0),
where dV is the volume of fluid which enters the gun, V0 is the
total wellbore fluid volume influenced by gun filling, and B is the
wellbore fluid compressibility (inverse of bulk modulus or
stiffness). Furthermore, the rate of pressure drop is represented
as,
dP/dt=d[V/(B*V0)]/dt
dP/dt.about.(dV/dt)/(B*V0)
So, for a given system (V0) and given volumetric rate of gun
filling (dV/dt), the above relationships show that both the rate
and magnitude of pressure drop are inversely proportional to the
fluid compressibility B.
[0028] Accordingly, embodiments of the invention provide a
gas-generating device for generating a gas cushion near the
perforating gun inside a wellbore configuration, to increase
overall system compressibility to reduce to rate and magnitude of
pressure drop that occurs after perforation. Specifically,
embodiments of the invention involve a method for perforating a
wellbore, in which the following steps may occur. Initially, a
perforating gun string including a perforating gun and a
gas-generating device may be lowered into a wellbore. Next,
wellbore fluid may be provided around the perforating gun. Finally,
a volume of gas may be provided, proximate the perforating gun. The
volume of gas is configured to reduce the shock produced upon
firing of the perforating gun. Specifically, the volume of gas may
be introduced by actuating the gas-generating device, either by a
control signal or automatically by conditions encountered during
lowering of the perforating gun string into the wellbore. Once
actuated, the gas-generating device may slowly or quickly
(depending on the type of device employed) introduce a volume of
gas physically near the perforating gun. After the perforating gun
is detonated, i.e., the shape charges are detonated and the gun
perforates the wellbore casing, the volume of gas may rupture or
vent/escape into the wellbore, eventually making its way back up to
the surface.
[0029] FIGS. 2-6 show example perforating systems after the method
described above takes place. As such, FIGS. 2-6 show a perforating
system with a gas cushion accumulated nearby, and accompanying
laboratory testing statistics for such perforating systems, in
accordance with one or more embodiments of the invention.
[0030] Specifically, FIG. 2 shows a perforating gun-in-wellbore
configuration 200, with a gas cushion 202 nearby, in accordance
with one or more embodiments of the invention. FIG. 2 shows the
various transient pressures including the pressure inside the
perforating gun 206, the pressure in the wellbore 204, and the
pressure of the reservoir 208. The gas cushion 202 may be created
by a gas-generating device such as that described above with
respect to FIG. 1.
[0031] In one or more embodiments of the invention, the gas cushion
created by the gas-generating device, or the liquid/gas interface,
may be considered "nearby" when the characteristic wave speed
transit time is very short relative to the time scale of gun
filling (or gun expulsion, as the case may be). In other words, the
wellbore may be considered as two series springs--a stiff spring
(liquid) backed by the soft spring (gas). As long as the liquid-gas
interface is "close enough" to the perforating gun string, then the
pressure response will be dominated by the gas.
[0032] FIG. 3 shows an example laboratory experiment demonstrating
the magnitude- and rate-reducing effect on DUB of increased
wellbore fluid compressibility, for the gun filling case
corresponding to the arrangement of FIG. 2 described above.
Differences between traces 302 and 304 were (a) gun volume, and (b)
wellbore fluid compressibility. The trace graphed as 302 had a
smaller gun volume and gas cushion (i.e., liquid/gas interface
several inches away from gun). Both the smaller gun volume and
increased wellbore fluid compressibility contributed to the modest
pressure drop of only .about.500 psi of trace 302. Had the gun
volumes been the same for tests conducted in both cases, the
pressure of the 302 trace would have dropped by approximately
.about.2000 psi, instead of by only .about.500 psi. But this
pressure drop (and the rate) would still be far less than for the
testing conducted in a stiff fluid system (very low fluid
compressibility) exhibited by trace 304.
[0033] In one or more embodiments of the invention, in addition to
generating a gas cushion-on-liquid, there may be other ways to
increase wellbore fluid compressibility. For example, as shown in
FIG. 4, gas may be bubbled around the perforating gun. In this
case, the gas bubbles 402 may be introduced by a standard gas
generator (not shown), or may be bubbling up from a lower producing
hydrocarbon bearing formation. Alternatively, other techniques can
be used to increase wellbore fluid compressibility, such as the
addition of hollow microspheres.
[0034] In yet another alternative embodiment, gas may be placed
everywhere around the gun 502, as shown in FIG. 5. Laboratory tests
have demonstrated that the configuration shown in FIG. 5 has the
potential to virtually eliminate the wellbore pressure transients
altogether. FIG. 6 shows a graph of the pressure transients when a
perforating gun is completely or substantially completely
surrounded by a gas cushion as shown in FIG. 5. As can be seen in
FIG. 6 from the flat curve 600, no pressure drops occur in such a
system, because the gas cushion completely envelopes the
perforating system, and eliminates all sudden changes in wellbore
pressure transients.
[0035] Although FIG. 6 shows a balanced case, those skilled in the
art will appreciate that an initial static underbalance may easily
be created, which would remain throughout the perforating event
until full recovery is brought about by some combination of
reservoir inflow and farfield wellbore storage effects.
[0036] Embodiments of the invention described above may be utilized
in many different scenarios of the perforating process. The below
discussion provides examples for using embodiments of the invention
described above. Those skilled in the art will appreciate that
while particular perforating scenarios are provided as examples,
these are not meant to limit the scope of the invention.
[0037] Typically, for perforation cleanup, large magnitude rapid
pressure drops are generally desirable, and so usually require a
very stiff fluid in the vicinity of the perforating gun. However,
there are instances where the exact opposite effect is desired,
i.e., either a modest pressure drop, or a very modest pressure
increase, accompanied by a gradual rate. These cases call for a
highly compressible wellbore fluid. For example, for some
situations which may produce gun filling and DUB, a very modest
(and gradual) pressure drop may be ideal, such as: [0038] a. when
perforating very weak formations, in order to minimize the tendency
of tunnel failure and/or excessive sand production; [0039] b. when
trying to minimize the net force loading (due to pressure
differentials) on the gun string and completion, in order to reduce
the tendency of damaged equipment, unseated packers, tangled
cables, etc.
[0040] Furthermore, there may be some situations in which it is
desired to: [0041] c. maintain, during the perforating event, an
initial static UB condition (i.e., avoid the DOB which can
accompany a static UB job, especially in situations where DUB is
very difficult to achieve--such as low reservoir pressures and/or
very high shot densities), in order to obtain clean perforations.
[0042] d. minimize the transient loading and hence damage on the
gunstring, conveyance device, and completion equipment (as b.
above)
[0043] All of the above scenarios can be addressed by employing
very compressible fluid in the vicinity of the perforating gun. In
the gun filling cases, the modest DUB will translate to reduced net
loading (and hence displacement) of the gunstring and completion
equipment (packers, tubing, etc.). Also, the reduced magnitude and
rate DUB reduces the pore pressure gradient within the formation,
which will in turn reduce the stresses on the formation (tensile,
shear, and drag force). These may reduce the tendency of both
tunnel collapse and transient sand production. In the case of
openhole (OH) perforating, this may also effectively reduce the
tendency to collapse the wellbore (if that were to be considered a
potential problem).
[0044] For the gun OB cases, moderating the increase in Pwb from an
initial static UB condition may allow one to preserve an initial
static UB, and hence produce clean perforations in accordance with
conventional "static UB" assumptions (pre-PURE; pre-early 2000's).
An initial static UB of, say 2000 psi, may revert (with a very
stiff fluid) to a DOB of a few hundred psi, and hence produce dirty
perforations. With a compliant fluid, on the other hand, the 2000
psi static UB may be reduced to a 1500 psi transient UB, which may
still yield fairly clean perforations.
[0045] The preceding discussions have focused on hollow carrier
guns, wherein the shaped charges are isolated from the wellbore
fluid by the carrier tube. However, embodiments of the present
invention can also be applied in the case of exposed, or capsule
charge perforating systems. These systems typically produce large
overpressures in the wellbore, due to the direct coupling of the
detonating charges and the wellbore fluid. The present invention,
by increasing compressibility of the fluid surrounding the
perforating system, minimizes the magnitude and rate of these
dynamic overpressures. Accordingly, any attendant damage to casing,
formation, or other downhole equipment would be minimized.
[0046] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art, having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed herein. Accordingly, the scope of the invention should
be limited only by the attached claims.
* * * * *