U.S. patent application number 10/797939 was filed with the patent office on 2005-09-15 for high frequency pressure compensator.
Invention is credited to Moriarty, Keith Alan.
Application Number | 20050199423 10/797939 |
Document ID | / |
Family ID | 34920164 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050199423 |
Kind Code |
A1 |
Moriarty, Keith Alan |
September 15, 2005 |
High frequency pressure compensator
Abstract
A downhole pressure compensation system includes a seal housing
disposed in a downhole tool, a dynamic seal disposed on the seal
housing, wherein the dynamic seal seals around a part that is
allowed to move relative to the seal housing, and a flexible
membrane disposed in a sidewall of the seal housing proximate the
dynamic seal.
Inventors: |
Moriarty, Keith Alan;
(Houston, TX) |
Correspondence
Address: |
Tim W. Curington
Stonehouse Technology Centre
Brunel Way, Stroudwater Business Park
Stonehouse
GL 10 3SX
GB
|
Family ID: |
34920164 |
Appl. No.: |
10/797939 |
Filed: |
March 11, 2004 |
Current U.S.
Class: |
175/7 ;
166/358 |
Current CPC
Class: |
E21B 47/017 20200501;
E21B 47/18 20130101 |
Class at
Publication: |
175/007 ;
166/358 |
International
Class: |
E21B 023/08; E21B
007/128 |
Claims
What is claimed is:
1. A downhole pressure compensation system, comprising: a seal
housing disposed in a downhole tool; a dynamic seal disposed on the
seal housing, wherein the dynamic seal seals around a part that is
allowed to move relative to the seal housing; and a flexible
membrane disposed in a sidewall of the seal housing proximate the
dynamic seal.
2. The downhole pressure compensation system of claim 1, wherein
the flexible membrane is in fluid communication with a fluid inside
the seal housing via a fluid passage.
3. The downhole pressure compensation system of claim 1, further
comprising a second pressure compensation system comprising a
piston disposed in a cylinder.
4. The downhole pressure compensation system of claim 1, further
comprising a second pressure compensation system comprising a
bladder.
5. The downhole pressure compensation system of claim 1, wherein
the downhole tool is a mud pulse telemetry pulse generator.
6. The downhole pressure compensation system of claim 1, wherein
the flexible membrane comprises a frustoconical section of the seal
housing.
7. The downhole pressure compensation system of claim 1, wherein
the flexible membrane is constructed of an elastomer.
8. The downhole pressure compensation system of claim 1, wherein
the flexible membrane is constructed of a metal.
9. A method of compensating for a mud pressure signal, comprising:
generating a pressure signal in a mud flow rate; and transmitting
the pressure to the inside of a seal housing through a flexible
membrane disposed on a seal housing proximate a dynamic seal.
Description
BACKGROUND
[0001] Wells are generally drilled into the ground to recover
natural deposits of hydrocarbons and other desirable materials
trapped in geological formations in the Earth's crust. A well is
typically drilled using a drill bit attached to the lower end of a
drill string. The well penetrates the subsurface formations
containing the trapped materials so that the materials can be
recovered.
[0002] During drilling or after a well is drilled, various logging
instruments are used to collect information about the formation
properties. The well may then be completed based on the information
collected about the formation to maximize the production
efficiency. In the processes of drilling, logging, completion, and
production, various tools are used. These tools need to withstand
the harsh conditions downhole, which may include temperatures as
high as 200.degree. C. and pressures as high as 20,000 psi. Often
sensitive parts of the tools are enclosed in chambers (seal
housings) that may be filled with liquids (e.g., oil). The part of
the tools that exit the enclosed chambers are often protected with
seals that isolate the enclosed oil from the outside, while
allowing movement (e.g., rotation) of the extruded parts. These
seals are often referred to as "dynamic seals" because they seal
against a moving part. The following description uses a mud pulse
telemetry system as an example to illustrate the present
invention.
[0003] FIG. 1 shows a typical drilling system 101. A drilling rig
102 at the surface is used to rotate a drill bit 107 using a drill
string 103. Using a mud pump 121, drilling fluid, called "mud," is
pumped to the drill bit 107 through the drill string 103. The
downward flow of mud is represented in FIG. 1 by downward arrow
104. The mud lubricates and cools the drill bit 107 and then it
carries the drill cuttings back to the surface as it flows upwardly
through the annulus. The return flow of mud is represented by the
upward arrow 106.
[0004] The drilling system 101 includes a bottom-hole assembly
("BHA") 110 at the bottom end of the drill string 103. The BHA 110
includes the drill bit 107 and any sensors, testers, tools, or
other equipment (not shown) used in the drilling process. Such
equipment may include formation evaluation tools, directional
drilling tools, and control circuitry.
[0005] Communication between the driller and the BHA 110 is
typically called "telemetry." The data that are collected by the
sensors in the BHA 110 must be relayed to the surface so that the
driller will have the data when making decisions about the drilling
process. Additionally, the driller must be able to communicate with
the BHA 110 so that commands may be sent to the BHA 110. A
"downlink" is a communication from the surface to the BHA.
Likewise, an "uplink" is a communication from the BHA to the
surface.
[0006] There are various prior art telemetry methods. One class of
telemetry methods is called "mud pulse telemetry." Mud pulse
telemetry uses pulses in the mud flow rate or pressure to
communicate between the surface and the BHA.
[0007] One method of downlink mud pulse telemetry uses the mud
pumps at the surface to control the mud flow rate to the BHA. The
flow rate is detected and interpreted by the downlink system.
Methods of uplink mud pulse telemetry typically include a pressure
modulator in the downhole tool. The pressure modulator creates
pressure pulses in the mud flow that may be detected at the
surface. A pressure modulator uses a motor or drive mechanism to
operate a flow control device to generate pressure pulses in the
mud flow. The drive mechanism is enclosed in a seal housing that
includes a dynamic seal to allow the drive shaft to exit the seal
housing.
[0008] Dynamic seals on downhole tools need to function in a wide
range of ambient pressures--from the atmospheric pressure uphole to
the high pressure (up to 20,000 psi) downhole. To overcome such
challenges, a seal housing is often equipped with a pressure
compensation mechanism that permits the pressure inside the seal
housing to adapt to the ambient pressure. Prior art pressure
compensation mechanisms typically use a piston that is allowed to
move in order to change the volume of the seal housing in response
to the ambient pressure.
[0009] Due to the limited diameter (hence, the volume) of the
downhole tools, the piston mechanism may have to be placed at a
distance from the dynamic seal. The distance between the dynamic
seal and the pressure compensation mechanism unnecessarily
introduces a delay between pressure pulse generation and
compensation. It is therefore desirable to have methods and systems
that can provide better pressure compensation.
SUMMARY
[0010] In some embodiments the invention relates to a downhole
pressure compensation system that includes a seal housing disposed
in a downhole tool, a dynamic seal disposed on the seal housing,
wherein the dynamic seal seals around a part that is allowed to
move relative to the seal housing, and a flexible membrane disposed
in a sidewall of the seal housing proximate the dynamic seal.
[0011] In some other embodiments, the invention relates to a method
of compensating for a mud pressure signal that includes generating
a pressure signal in a mud flow rate, and transmitting the pressure
to the inside of a seal housing through a flexible membrane
disposed on a seal housing proximate a dynamic seal.
[0012] Other aspects and advantages of the invention will be
apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0013] FIG. 1 shows a cross section of a typical drilling
system.
[0014] FIG. 2 shows a cross section of a prior art pressure
compensation system.
[0015] FIG. 3A shows one embodiment of a modulator in an open
position.
[0016] FIG. 3B shows one embodiment of a modulator in a closed
position.
[0017] FIG. 4 shows a cross section of a seal in a prior art
pressure compensation system.
[0018] FIG. 5 shows a cross section of a mud port and a piston in a
prior art pressure compensation system.
[0019] FIG. 6 shows a graph of a mud pressure signal and a
compensated pressure signal in a prior art pressure compensation
system at 24 Hz.
[0020] FIG. 7 shows one embodiment of a pressure compensation
system in accordance with one embodiment of the invention.
[0021] FIG. 8 shows a graph of a mud pressure signal and a
compensated pressure signal in a pressure compensation system
operating at 24 Hz in accordance with an embodiment of the
invention.
DETAILED DESCRIPTION
[0022] Embodiments of the invention relate to pressure compensation
systems suitable for applications involving high frequency and high
amplitude pressure pulses. Certain embodiments of the present
invention relate to a system for high frequency/high amplitude
pressure compensation. Other embodiments of the invention may
relate to a method of compensating a high frequency/high amplitude
pressure signal. For clarity, the following description uses a mud
pulse telemetry generator to illustrate the present invention.
However, one of ordinary skill in the art would appreciate that
embodiments of the invention are not limited solely to mud pulse
generator. Instead, embodiments of the invention are generally
applicable in any pressure compensation applications, particularly
for downhole tools. The invention will now be described with
reference to the figures.
[0023] FIG. 2 shows a cross section of a mud pulse modulator 201
that may be used to send an uplink signal. The mud pulse modulator
201 includes a rotor 202 and a stator 203. The rotor 202 rotates
with respect to the stator 203 to generate the pressure pulses, as
will be explained with reference to FIGS. 3A and 3B. The rotor 202
is coupled to a shaft 205 that connects the rotor 202 to a drive
assembly that includes a gear assembly 206 and a servo motor 207.
The shaft 205 passes through a seal housing 216, and seals 204 seal
around the shaft 205 to isolate the working oil inside the seal
housing 216 from the mud that is outside the seal housing 216.
Typically, a servo motor 207 is used to enable precise control of
the rotor 202, although other drive mechanisms may be used.
[0024] FIGS. 3A and 3B show one example of a modulator 301 that may
be used to generate a pressure pulse. In FIG. 3A, the modulator 301
is in an open position. The stator 304 includes four passages, such
as passage 305, that enable mud to flow through the modulator 301.
In the open position, the rotor 306 is positioned so that it does
not cover the openings 305 in the stator 304. The rotor includes
cuts 307 that enable the openings 305 to be uncovered in the open
position. In the open position, the modulator 301 enables free flow
of mud through the modulator 301.
[0025] FIG. 3B shows a modulator 301 in a closed position. Flaps
308 on the rotor 306 partially cover the openings 305 in the stator
304. This presents an impediment to the flow of mud, and the
pressure increases so that a constant flow rate of mud is
maintained. FIGS. 3A and 3B shows the modulator 301 in open and
closed positions, but those having ordinary skill in the art will
realize that the rotation of the rotor 306 causes the modulator 301
to modulate between the open and closed positions.
[0026] Referring again to FIG. 2, the seal 204 provides a dynamic
seal to isolate the oil inside the seal housing 216 from the mud
outside. The oil inside the seal housing 216 lubricates and
protects the drive mechanisms. In order for the seal 204 to
maintain its integrity and proper function under conditions ranging
from the atmospheric pressure (when it is uphole) to the downhole
pressure (up to 20,000 psi), a pressure compensation mechanism is
needed so that the pressure differential across the seal 204 is
minimal, regardless of the outside pressure. The pressure
compensation mechanism typically comprises a piston that is able to
move freely along a cylinder to alter the volume of the oil chamber
in response to the outside pressure, ensuring that the pressures on
both sides of the piston are substantially the same regardless of
the outside pressure. A pressure compensation mechanism typically
used in a downhole tool will be described in detail later.
[0027] Referring to FIG. 2 again, the modulator 201 creates
pressure pulses that travel uphole, or to the left in FIG. 2. For
example, when the modulator 201 is in a closed position (e.g., as
shown in FIG. 3B), a high pressure pulse will travel up hole. In
the closed position, a reduction in pressure is experienced on the
downhole side of the modulator 201. Conversely, when the modulator
201 is in an open position (e.g., as shown in FIG. 3A), a reduction
in pressure is experienced uphole, and an increase in pressure is
experienced on the downhole side of the modulator 201.
[0028] FIG. 4 shows a close-up of the shaft 205 that drives the
rotor (202 in FIG. 2) and a seal assembly 404, 406 that seal around
the shaft 205. The outer seal 404 is a rotating seal that rotates
with the shaft 205, and inner seal 406 is a stationary seal that
also seals around the shaft 205, but it remains fixed with the seal
housing 216. In operation, the rotor 202 is driven by the drive
shaft 205 to rotate with respect to the stator 203, generating
pressure pulses in the mud. These pressure pulses are experienced
on the outboard side of the inner seal 406, in area 410, for
example. The pressure pulses created by the modulator can have an
adverse effect on seal performance and seal life. Thus, it is often
desirable to use a pressure compensation system to balance the oil
pressure on the inboard side of the seal 406.
[0029] A pressure compensation system balances the oil pressure
inside the seal housing 216 (i.e., in area 412) so that is will
fluctuate with the borehole hydrostatic pressure and the mud
pressure signal outside the seal housing 216 (i.e., in area 410).
This will ensure that the pressure differential across the inner
seal 406 will remain close to zero at all times. A balanced
pressure will reduce the leakage across the seal 406 and, more
importantly, increase the life of the seal.
[0030] Referring back to FIG. 2, a pressure compensation system
provides pressure compensation using a port 208, a mud chamber 210,
and a piston 212 to achieve pressure compensation inside the drive
housing 209 that is in fluid communication with the stator seal
406. The piston 212 is free to move along the length of the mud
chamber 210 so that the pressures on both sides of the piston 212
are substantially the same, which in turns ensures that the
pressures across the stator seal 406 are substantially the same,
regardless of the outside pressure. The pressure compensation
system is placed at a distance to the seal 406 due to the limited
diameter (volume) of the downhole tool. The distance between the
pressure compensating piston and the seal 406 necessarily creates a
time delay between the pulse generation and compensation. The
pressure pulses from the mud pulse modulator 201 need to travel
through the mud outside the tool between the modulator 201 and the
mud port 208. At the mud port 208, the change in pressure may enter
the mud chamber 210 in the drive housing 209. Typically, the
pressure compensation piston 212, located inside the drive housing
209, is able to move (e.g., along the length of the mud chamber
210) in response to pressure differences between the mud in the mud
chamber 210 and the oil pressure inside the drive chamber 209. The
oil pressure behind the piston 212 is then relayed to the seal 406
to counter (compensate) the change in pressure on the other side of
the seal 406. However, due to the time needed for the change in
pressure to travel this distance, the pressures across the seal 406
are not equalized during the delay. If the pressure on the outside
is greater than the pressure on the inside, then the fluid on the
outside (e.g., mud) may leak into the oil housing, resulting in
damages to the parts to be protected.
[0031] FIG. 5 shows a close-up view of the mud port 208, the mud
chamber 210, and the pressure compensation piston 212. A change in
pressure enters the drive housing 209 through the port 208 and is
transmitted into the mud chamber 210. The change in pressure then
acts on the piston 212, causing a corresponding change in the oil
pressure. An increase in mud pressure will cause the piston 212 to
move upwardly and increase the oil pressure. Similarly, a decrease
in mud pressure will cause the piston 212 to move downwardly and
decrease the oil pressure.
[0032] In some embodiments, a piston 212 may be coupled to a spring
214. The spring 214 applies a force to the piston 212 that would
create a slightly higher pressure in the oil chamber than the
pressure in the mud chamber 210. Thus, if there were to be any
leakage across the inner seal (406 in FIG. 4), the leakage would be
of oil out of the seal housing (216 in FIG. 4) and not of mud into
the seal housing.
[0033] Referring again to FIG. 2, an increase of pressure in the
mud chamber 210 will cause the piston 212 to move, thereby
transmitting the pressure increase through the drive chamber 209
and to the inboard side of the seal (406 in FIG. 4). This type of
pressure compensation system requires that a pressure pulse travel
from the modulator 201 to a port 208 in the drive housing, before
returning through the interior of the drive housing 209.
[0034] The time delay, t, between the mud pressure pulse and the
resulting pulse in the oil is related to the distance that the
pulse must travel and the speed of sound in the particular fluid
through which the pulse is traveling. The time delay may be
quantified as shown in Equation 1: 1 t = ( d o + d m ) C m + d m C
m + d o C o Eq . 1
[0035] where d.sub.o is the length of the oil cavity in the tool
(shown in FIG. 2), d.sub.m is the length of the mud cavity in the
tool (shown in FIG. 2), C.sub.o is the speed of sound in the oil,
and C.sub.m is the speed of sound in the mud.
[0036] The first term in Equation 1 represents the time it takes
the mud pressure pulse to travel through from the seal and mud
pulse modulator area to the mud port (e.g., 208 in FIG. 2). This
length is represented by the sum of the length of the oil chamber
d.sub.o and the length of the mud chamber d.sub.m. The sum is
divided by the speed of sound in mud C.sub.m, the medium through
which the signal travels in this direction. The middle term
represents the time it takes the pressure pulse to travel back
through the mud chamber (e.g., 210 in FIG. 2) inside the drive
housing. This time is represented by the length of the mud chamber
d.sub.m divided by the speed of sound in mud C.sub.m. The last term
in Equation 1 represents the time it takes the pressure pulse to
travel through the oil chamber of the drive mechanism--the length
of the oil chamber d.sub.o divided by the speed of sound in oil
C.sub.o.
[0037] More sophisticated mud pulse telemetry systems use higher
pulse frequencies to increase and optimize the data transmission
rate of the telemetry system. These can range from less than 1 Hz
to 24 Hz. The higher frequencies have created problems with the
response time of pressure compensation systems. At higher
frequencies, the time that it takes for the pressure signal to
travel to the mud port (e.g., 208 in FIG. 2), travel back through
the mud chamber (e.g., 210 in FIG. 2), and travel back through the
oil chamber to the inboard side of the seal (e.g., 204 in FIG. 2)
may be a significant portion of one cycle. The time delay creates a
compensated pressure that is out of phase with the modulator
pressure.
[0038] FIG. 6 shows a graph of the mud pressure signal 601 along
with the compensated pressure signal 602 in the oil on the inboard
side of the seal in a prior art pressure compensation system. The
signal shown in FIG. 6 is a 24 Hz signal. As shown in FIG. 6, there
is a phase shift between the mud signal 601 and the oil signal, or
compensated pressure signal 602. The compensated signal 602 is
delayed from the mud signal 601, making the compensated signal 602
out of phase with the mud signal 601. The difference between the
mud signal 601 and the compensated signal 602 is plotted at 603.
The pressure difference 603 shown in FIG. 6 may cause the seal
(e.g., 406 in FIG. 4) to oscillate with the pressure fluctuations
(represented by the pressure difference curve 603). Oscillation of
the seal may cause damage to the seal that will reduce seal life.
Additionally, when the pressure on the outside of the seal is
higher than that on the inside, mud may leak into the housing,
leading to damages of the seal and the drive mechanism.
[0039] FIG. 7 shows one embodiment of a pressure compensation
system in accordance with one embodiment of the invention. The seal
housing 716 includes a flexible membrane 710 that enables pressure
to be transmitted to the interior of the seal housing 716. When the
pressure outside of the seal housing 716 increases, the flexible
membrane 710 flexes inwardly, thereby increasing the pressure on
the inboard side of the seal 706. Conversely, when the pressure
outside of the seal housing 716 decreases, the flexible membrane
710 flexes outwardly, thereby decreasing the pressure on the
inboard side of the seal 706.
[0040] The flexible membrane 710 is located in the seal housing 716
to be proximate the seal 706. This significantly reduces the
distance over which the pressure signal must be transmitted to
compensate the pressure on the inboard side of the seal 706. By
reducing the distance over which the signal must travel, the
response time of the pressure compensation system is significantly
increased.
[0041] In the embodiment shown, the flexible membrane 710 is
coupled to a passageway 712 that leads to the interior of the seal
housing 716. In other embodiments, a flexible membrane may be in
contact with both the mud outside the seal housing and with the oil
inside the seal housing without the need for a passage way, i.e.,
the flexible membrane 710 may form part of a wall of a seal
housing.
[0042] The flexible membrane 710 may be made of any material that
will flex enough to transmit pressure to the interior of the seal
chamber 716. For example, the flexible membrane 710 may be
constructed of an elastomer or a thin piece of metal. Additionally,
the geometry (i.e., the shape and size) of the membrane 710 may be
selected based on the particular application or operating
condition. For example, the membrane 710 may extend around the
entire circumference of the seal housing 716, forming a
frustoconical shape. In other embodiments, the membrane 710 may
form a window over only a portion of the seal housing 716. The
geometry and the material of the membrane 710 may be selected for
specific applications and design considerations.
[0043] Those having ordinary skill in the art will realize that any
number of variations of a flexible membrane may be possible without
departing from the scope of the invention. For example, this
description makes reference to a "seal housing," which houses and
protects the seals, and a "drive housing," which houses and
protects the drive mechanisms for the modulator. In practice,
however, these may not be separate components. That is, a drive
mechanism housing may also house and protect the seals.
[0044] Additionally, a flexibly membrane may be constructed of a
material having enough strength that the flexible membrane may be
in direct contact with both the mud on the outside of the seal
housing and the oil on the inside of the seal housing. In such an
embodiment, a passage (i.e., passage 712) between the flexible
membrane and the interior of the seal housing may not be necessary.
Other variations of a flexible membrane may be devised that do not
depart from the scope of the invention.
[0045] FIG. 8 shows a graph of a mud pressure signal 801 along with
a compensated pressure signal 802, using a pressure compensation
system in accordance with the invention. The compensated pressure
signal 802 closely matches the mud pressure signal 801 created by
the modulator (e.g., 202 in FIG. 2). Plot 803 shows the difference
between the mud pulse signal 801 and the compensated pressure
signal 802. The difference 803 shows a constant, slight excess
pressure on the inside of the seal housing. This slight excess
pressure is typically provided by a spring mechanism to ensure that
no mud will leak into the housing.
[0046] Embodiments of the invention use flexible members close to
the dynamic seals to provide better pressure compensation and
improved seal lives. One of ordinary skill in the art would
appreciate that the flexible membrane pressure compensation
mechanism in accordance with the invention may be used together
with the prior art piston pressure compensation mechanism. For
downhole tools, the combined use of these two types of pressure
compensation mechanisms is particularly beneficial--the piston
pressure compensation mechanism ensures that the protected oil
chamber can be used in a wide range of pressure (e.g., from the
atmospheric pressure to the downhole pressure), while the flexible
membrane mechanism ensures that high frequency and/or high
magnitude pressure pulses are effectively compensated.
[0047] It is noted that a piston arrangement is one possible prior
art pressure compensation system that could be used with
embodiments of the invention. Other pressure compensation systems
may include a bellows system or a bladder system. Those having
ordinary skill in the art will be able to devise other types of
pressure compensation systems that may be used with embodiments of
the invention.
[0048] Certain embodiments of the present invention may present one
or more of the following advantages. A pressure compensation system
in accordance with the invention may decrease the phase shift of a
compensated pressure pulse. At a high modulator frequency, the
reduced phase shift may reduce the pressure differential across a
seal in the modulator system.
[0049] Advantageously, a pressure compensation system in accordance
with the invention may reduce or prevent oscillations of a seal in
the modulator system. Reduced oscillation may decrease seal leakage
and increase seal life. The ability of a pressure compensation
system to compensate for high frequency pressure telemetry signals
enables the use of still yet higher frequencies in a telemetry.
Advantageously, a pressure compensation system in accordance with
the invention may enable faster communication in a telemetry
system. Similarly, embodiments of the invention may provide
benefits to other tools that include pressure compensation
mechanisms. It is noted that there are devices that can emit high
frequency and high magnitude pressure changes other than the
telemetry devices described above and the scope of this invention
should not be limited as such.
[0050] 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.
* * * * *