DOWNHOLE PRESSURE COMPENSATING DEVICE
Field of the invention
The present invention relates to a pressure compensating device used for
pressure equalisation in downhole well tools to avoid implosions or explosions of
the tools.
Background art
Downhole tools such as driving units, strokers, perforators etc. are exposed to
extreme pressure differences between the inside and outside of the tools. In
order to avoid collapses by implosion or explosion of the tools, which might
damage both tools and well structure and furthermore lead to production stops in
the wells, pressure compensating devices have been well-known for decades
within this field. To accommodate pressure compensation, borehole fluid is
typically allowed inside the tool on one side of the pressure compensating device
and hydraulic fluids typically maintained inside a downhole tool will be on the
other side, thereby equalising the two pressures on each side of the pressure
compensating device.
A variety of pressure compensating devices are known using rubber bags,
diaphragms, bellows and springs in the pressure compensating mechanism.
However, they suffer from being designed to withstand a certain pressure
difference, which when exceeded leads to a breakdown of the mechanism.
Increased reliability and a more fail-safe mechanism of a pressure compensating
device for use in boreholes would therefore lead to optimised drilling and
production performance, thereby minimising costs and maximising return of well
operations. Since several types of tools require pressure compensation during
borehole operations, various different processes would benefit from an improved
pressure compensating device, all leading to a minimised risk of limitation in
production time.
Thus, there is a need to be able to compensate pressures in downhole tools
during exploration, production and monitoring of subsurface deposits, such as oil
and gas deposits.
Summary of the invention
It is an object of the present invention to wholly or partly overcome the above
disadvantages and drawbacks of the prior art. More specifically, it is an object to
provide an improved system for compensating pressures in downhole tools during
exploration, production and monitoring of subsurface deposits, such as oil and
gas deposits.
The above objects, together with numerous other objects, advantages, and
features, which will become evident from the below description, are accomplished
by a solution in accordance with the present invention by a downhole pressure
compensating device for use in combination with a downhole tool, comprising:
- a housing with a chamber and an internal hollow section,
- a first piston dividing the chamber into a first section and a second
section, the first section being in fluid communication with a first fluid port, the
second section being in fluid communication with a borehole through a second
fluid port, and
- a first spring disposed within the second section to exert a pressure on the
first piston to enable the conservation of an overpressure in the first
section,
wherein the device further comprises:
- a second piston,
- a second spring disposed between the first piston and the second piston,
and
- an overpressure channel arranged in the first or the second piston, which
overpressure channel, when the second spring is in a compressed condition,
provides fluid communication between said first and second sections.
In one embodiment, the downhole pressure compensating device may comprise
at least a pressure connection to a mating tool in a tool string.
Said mating tool may be a driving unit.
In another embodiment, the second section of the compensating device may be
in fluid communication with the inside of an electrical motor unit and/or a
hydraulic pump unit.
Moreover, the first spring, the second spring, the first piston and the second
piston may be arranged coaxially with the longitudinal centre axis of the
compensating device.
Further, the at least one of the first spring, the second spring, the first piston and
the second piston may have been arranged non-coaxially with the longitudinal
centre axis of the compensating device non-circumscribing the internal hollow
section.
The compensating device according to the invention may be arranged noncoaxially
with a longitudinal centre axis of the tool.
Additionally, the second piston may be partly arranged inside the first piston.
Also, the first piston may be partly arranged inside the second piston.
In one embodiment, the first section of the chamber may be filled with a
pressurised hydraulic fluid such as oil with predetermined characteristics
(matching the conditions of the borehole).
Moreover, the first and second springs may be coil springs, helical springs,
bellows, volute springs, leaf springs, gas springs or disc springs.
The downhole pressure compensating device according to the invention may
further comprise electrical sensors for monitoring a temperature inside the device
and/or pressures in the first and second sections and/or positions of the first and
second pistons for producing a feedback signal to a control system.
Said downhole pressure compensating device may further comprise at least a
switch wherein the compensating device can be controlled by the at least a
switch connected to the control system to adapt to changes in environmental
conditions based on the feedback signal.
Furthermore, the device may comprise a plurality of first and/or second springs.
Furthermore, the device may comprise a plurality of spring guides.
Also, the second spring may be arranged within the first piston.
The device may comprise a plurality of first springs arranged concentrically in the
housing.
In an embodiment, the second spring may be arranged within the first piston in
an overpressure valve, the overpressure valve comprising the second spring and
the second piston.
Additionally, the housing may comprise a tubular member and two end members
detachably connected.
The present invention furthermore relates to a downhole system comprising:
-a wireline,
-a mating tool such as a driving unit and/or an operational tool, and
-a downhole pressure compensating device according to the invention.
The present invention also relates to a downhole tool system comprising:
-at least a mating tool such as a driving unit and/or an operational tool, and
-a downhole pressure compensating device according to the invention.
Brief description of the drawings
The invention and its many advantages will be described in more detail below
with reference to the accompanying schematic drawings, which for the purpose of
illustration show some non-limiting embodiments and in which
Fig. 1 shows a cut-through view of a pressure compensating device,
Figs. 2a-2d show schematic diagrams of a pressure compensating device during
filling of a first section with hydraulic fluid,
Figs. 3a-3d show schematic diagrams of a pressure compensating device during
filling of a second section with borehole fluid,
Figs. 4a-4d show schematic diagrams of various embodiments of pressure
compensating devices,
Fig. 5 shows a compensating device comprising non-coaxially arranged springs,
Fig. 6 shows a compensating device arranged non-coaxially with a centre axis of
the tool,
Fig. 7 shows a downhole system comprising a pressure compensating device,
Fig. 8 shows a downhole tool string comprising a pressure compensating device,
Fig. 9 shows a cut-through view of a pressure compensating device,
Fig. 10 shows a schematic diagram of a pressure compensating device during
filling of a first section with hydraulic fluid,
Fig. 11 shows a schematic diagram of a pressure compensating device during
filling of a second section with borehole fluid, and
Fig. 12 shows a cut-through view of a pressure compensating device.
All the figures are highly schematic and not necessarily to scale, and they show
only those parts which are necessary in order to elucidate the invention, other
parts being omitted or merely suggested.
Detailed description of the invention
Fig. 1 shows a pressure compensating device 20 for compensating pressure
differences between the inside and outside of a downhole tool to avoid implosion
or explosion of such a tool due to pressure differences. The pressure
compensating device 20 is attached to a downhole tool 115 in order to
compensate for changes in pressure. The pressure compensating device 20
comprises a housing 100 with a chamber 101 and an internal hollow section 102.
The hollow section 102 may facilitate electrical connections 112 between two
tools 115 arranged in each end of the compensating device 20 and connected to
the compensating device 20 by connecting means 116. The pressure in the
hollow section 102 is regulated by a first piston 103, a second piston 109, a first
spring 108 and a second spring 110. An interior of the two tools connected to
each end of the compensating device may be in fluid communication with the
interior 113 of the hollow section 102 whereby the internal pressure of the two
tools may be regulated by the compensating device 20. The first piston 103 and
second piston 109 seal the first section 104 from the second section 105 of the
chamber 101. When the first spring 108 is arranged between a second end face
101b of the chamber and a second face 103b of the first piston 103, the first
spring 108 thereby applies a force on the second end face 101b of the chamber
101 and a second face 103b of the first piston 103. The second spring 110 is
arranged between the first piston 103 and the second piston 109, the second
spring 110 applying a force on the first piston 103 and the second piston 109. An
overpressure channel 111 is arranged in the first and/or second piston to provide
fluid connection between the first and second sections 104, 105 of the chamber
101, when the first and second pistons 103, 109 are displaced towards their
extremum positions in each end of the chamber 101. Fig. 1 shows a compressed
state of the first spring 108, and if the first and second pistons 103, 109 are
moved further towards the second end face of the chamber 101, the second
piston 109 will, when the first spring is compressed to a certain degree, engage
the second end face, thereby stopping the movement of the second piston 109
towards the second end face of the chamber 101. When the first piston 103
continues to move towards the second end face, the second spring 110 will start
to compress, and at a given point the overpressure fluid channel will then provide
access between the first and second sections 104, 105 of the chamber 101, and
fluid from the first section 104 of the chamber 101 will start to flow through the
overpressure fluid channel entering the second section 105 of the chamber 101.
In Figs. 2 and 3, the activation of the overpressure channel in both ends of the
chamber 101 is shown step by step.
Figs. 2a-d show the displacement of the first and second pistons towards the
second end face 101b due to a pressurisation of the first section 104 of the
chamber 101. Prior to lowering the compensating device 20 into a borehole 4,
the first section 104 may be filled with fluid by removing a plug 124 from a first
fluid port 106 and filling the first section 104 with fluid, whereby the first section
104 will be pressurised. Fig. 2a shows the first and second springs 108, 110 in
relaxed positions with the first and second pistons 103, 109 displaced towards
the first end face 101a and the overpressure channel 111 closed. When a
pressurising fluid enters the first section 104 through the first fluid port 106, the
first spring 108 is compressed as shown in Fig. 2b. As can be seen in Fig. 2b, the
second spring 110 is still uncompressed in this condition and therefore the
overpressure channel is still closed, resulting in no fluid connection between the
first and second sections 104, 105. If, however, the first section 104 is further
pressurised, the second spring 110 will start to compress resulting in movement
of the second piston 109, while the first piston 103 has stopped moving, which is
seen in Fig. 2c. As indicated by an arrow in Fig. 2c, the overpressure channel
provides fluid communication between the first and second sections 104, 105,
when the second piston 109 is displaced beyond a certain point, thereby allowing
fluid from the first section 104 to flow into the second section 105, thus relieving
the overpressure of the first section 104. In Fig. 2d, the first fluid port 106 is
closed, thereby stopping inflow of pressurised fluid into the first section 104.
When the first fluid port 106 is closed, the second piston 109 will move back
towards its relaxed position as fluid exits the first section 104 through the
overpressure channel 111. When the second piston 109 reaches a position in
relation to the first piston 103, the overpressure fluid channel is once again
closed as shown in Fig. 2d, and then the second piston 109 will stop moving. This
mechanism therefore provides a restriction of the pressure in the first section 104
so it does not exceed a certain maximum pressure. Furthermore, it allows the
user to pressurise the first section 104 to a predetermined pressure every time
the first section 104 is pressurised before lowering the compensating device 20
into the borehole. The actual spring constants of the first and second springs
108, 110 are chosen to correspond to the predetermined pressure. Thus the
predetermined pressure may be controlled by changing springs or preloading
springs to a certain degree in order to accommodate special pressure
requirements for the compensating device 20 matching special downhole
conditions.
Figs. 3a-d show how the pressure is compensated during a pressure build-up in
the borehole. As explained above, the first section 104 is pressurised before
lowering the compensating device 20 into the borehole. Therefore, the initial
condition of the compensating device 20 when lowered into the borehole is the
situation depicted in Fig. 2d. When the compensating device then subsequently
enters the borehole, the pressure from the borehole is transferred to the second
section 105 through the second fluid port 107, and the pressure in the second
section 105 increases as the pressure in the borehole increases. In Fig. 3a, the
borehole pressure has forced the first and second pistons 103, 109 towards the
first end face of the chamber 101 decompressing the first spring 108. By this
movement of the first piston 103, the pressure is compensated, i.e. the pressure
is equalised in the first and second sections of the pressure compensating device
20. Since the first section 104 is in fluid communication with the inside of a tool,
the tool will, in this way, be pressure compensated and thereby not destroyed
during a pressure build-up in the borehole. The problem is that if the pressure
inside the tool becomes much higher or much lower than outside the tool, the
tool will either increase or decrease in volume. To avoid this change in volume of
the tool, the inside of the tool is connected to a pressure compensating device, so
that if the pressure in the borehole, i.e. in the second section 105, becomes
much higher than in the tool, which is in fluid communication with the first
section 104, the first section 104 may decrease in volume. If on the other hand
the pressure in the borehole is much lower than in the tool, the first section 104
may increase in volume. Fig. 3b shows the situation in which the first piston has
reached its maximum displacement towards the first end face and abuts the first
end face due to increasing pressure in the second section 105 stemming from the
pressure in the borehole increasing. If the pressure continues to increase in the
second section 105 beyond the point demonstrated in Fig. 3b, the second piston
109 will begin to move towards the first end face and the second spring 110 will
begin to compress. As shown in Fig. 3c, the overpressure in the second section
105 opens the fluid connection between the first and second sections 104, 105
when the second piston 109 has moved sufficiently long towards the first end
face, which allows fluid from the second section 105 to enter the first section
104. In general, this is an undesirable situation since dirty fluid from the borehole
is allowed to enter the inside of the compensating device 20 and thereby the
inside of the tool being in fluid communication with the first section 104 of the
compensating device 20. However, the alternative may be much worse since the
tools may be completely destroyed by implosion if they are unable to compensate
the borehole pressure. Furthermore, the deformation caused by such implosion
might cause the pressure compensating device and/or tool attached thereto to
jam inside the borehole, leading to complete production stop of the well.
Therefore, the flooding of the first section 104 of the compensating device 20 and
thus the tool with dirty borehole fluid protects both the pressure compensating
device and the tool being pressure compensated from collapsing. Therefore, the
possibility of allowing borehole fluid inside the first section 104 acts as a fail-safe
to the pressure compensating device 20. In case the fail-safe is activated and the
hydraulic fluid of the first section 104 is polluted with dirty borehole fluid, both
the pressure compensating device 20 and the potentially polluted tool will
normally be retracted from the borehole and thoroughly cleaned.
In Fig. 3d, the second piston 109 has moved back towards the second end face,
thereby closing the overpressure channel after the pressure has been equalised
in the first and second sections 104, 105.
The compensating device 20 serves another purpose with respect to
compensating the pressure. When the compensating device 20 is lowered into
the borehole, the temperature is increasing depending on the depth and the
proximity of the borehole to the magma layers. When a volume of the
pressurised fluid in the first section 104 increases due to the increase in
temperature, the pressure on the first and second pistons 103, 109 increases. In
case the pressure exceeds a pressure defined by the first and second springs
108, 110 for opening the overpressure channel, the hydraulic fluid from the first
section 104 is released into the second section 105 and into the borehole. Again
the compensating device 20 acts as a fail-safe against collapse or bulging of the
compensating device and/or the tool attached to the compensating device due to
thermal expansion of the hydraulic fluid in the pressure compensating device 20.
Conventionally, this problem has been dealt with by only filling prior
compensating devices partially to avoid bulging. This prior approach has the
following two main drawbacks. The first drawback is that even though the
compensating device is only filled partially to avoid bulging due to thermal
expansion, it still depends on the temperature being below a critical temperature.
This is due to the fact that temperatures may fluctuate locally, e.g. near magma
layers, to very high temperatures. Thus, the safety of the compensating device
might be compromised even with conservative fillings of the hydraulic fluid in the
compensating device so that the tool will bulge anyway if the compensating
device cannot withstand the pressure of the thermally expanded hydraulic fluid.
The second drawback is that the hydraulic fluid serves the purpose of
withstanding the pressure stemming from the borehole pressure which also
increases with depth and local conditions in the borehole. By only filling prior
compensating devices partially, i.e. decreasing the amount of hydraulic fluid in a
compensator, the ability to compensate the pressure in a tool is reduced since
less hydraulic fluid is available in the first section. The ability is reduced since the
volume of the hydraulic fluid may decrease during pressure compensation
through leaks in fluid communication with the first section of the chamber, e.g.
through leaks in the tool, which is typically occurring during operation in the well.
Figs. 4a-d show different embodiments according to the invention. Fig. 4a shows
a compensating device 20 according to the invention, where the overpressure
channel 111 is a bore within the first piston 103. By placing the overpressure
channel internally in the first piston 103, an opening of the overpressure channel
may be arranged distant to the second spring 110. Fig. 4b shows a compensating
device 20, wherein the overpressure channel has been arranged partly in the
second piston 109 and partly in the first piston 103, and when the second spring
110 is adequately compressed, the overpressure channels are aligned and fluid is
allowed to flow from one section 104, 105 of the chamber 101 to the other. Fig.
4c shows a compensating device 20, wherein the first piston has been arranged
partly inside the second piston 109 and the overpressure channel has been
arranged in the housing 100 of the compensating device 20. Fig. 4d shows a
compensating device 20, wherein the first piston 103 has been arranged partly
inside the second piston 109 and the overpressure channel has been arranged
partly in the second piston 109 and partly in the first piston 103, and when the
second spring 110 is adequately compressed, the overpressure channels are
aligned and fluid is allowed to flow from one section 104, 105 of the chamber 101
to the other.
Fig. 5 shows a compensating device wherein two second springs 110 have been
arranged non-coaxially with the centre axis of the tool for two second pistons 103
away from second end face 101b of the chamber 101.
Fig. 6 shows a compensating device 20 wherein the compensating device is
arranged non-coaxially with the centre axis of the tool. In this way, the
compensating device 20 may be arranged in parallel with another device, tool or,
as shown in Fig. 6, an empty space 121. The freedom to arrange the
compensating device non-coaxially from the centre axis increases the versatility
of the compensating device in the design optimisation of space in the downhole
tool string. In Fig. 6, the empty space 121 may provide a possibility to facilitate a
hydraulic pressure fluid to pass a compensating device without entering neither
the compensating chamber 101 nor the interior 113 of the hollow section 102.
Furthermore, Fig. 6 shows an embodiment of a compensating device which
comprises a plurality of first and/or second springs. Other embodiments may
comprise a greater number of separate springs. The compensating device shown
in Fig. 6 comprises a one-way valve 122 arranged in the first fluid port 106 and a
set of switches 123 to enable a feedback signal to a control system, which allows
the user to check when pistons and springs reach extremum positions during
compression or decompression of the springs.
When the compensating device is installed, it forms part of a downhole tool string
10 as shown in Figs. 7 and 8. In Figs. 7 and 8, the tool string may comprise
driving units 11, compensating devices 20 and operational tools 12 etc. The tool
string 10 comprises the tool 115, such as a driving unit 11, arranged in a casing
6, having an inside 4, in a well or borehole 5 in the formation 2. The downhole
tool string 10 is powered through a wireline 9 which is connected with the tool via
a top connector 13. The downhole tool further comprises an electronic section
having mode shift electronics 15 and control electronics 16 before the electricity
is supplied to an electrical motor 17 driving a hydraulic pump 18. The driving unit
11 may be connected with an operational tool 12 through a connecter 14.
As shown in Figs. 9-10, the second spring 110 may be arranged within the first
piston 103 in an overpressure valve 120, the overpressure valve comprising the
second spring 110 and the second piston 109. Since a typical overpressure valve
120 only opens to flow in one direction, a recess 119 in the hollow section 102
may facilitate release of overpressure in the first section 104 as will be explained
below. An overpressure channel 111 is arranged in the first piston to provide fluid
connection between the first and second sections 104, 105 of the chamber 101,
when the second pistons 109 are displaced towards maximum compression of the
second spring 110.
Fig. 9 shows the first spring 108 in an uncompressed state such as before filling
the compensating device. The first piston 103 is forced towards the end of the
chamber 101 before filling the first section 104 with pressurised fluid as
explained in Fig. 2b.
Fig. 10 shows the compensating device of Fig. 9 during filling of the first section
104 with pressurised fluid. When the first piston 103 reaches the position of the
recess 119, the pressurised fluid is allowed to flow from the first section 104 into
the second section 105, thus relieving the overpressure of the first section 104.
When pressure is released to the second section 105, pressurised fluid will exit
the second fluid port 107 and the user knows that the pressure in the first section
104 has reached a desired level.
In Fig. 11, the compensating device 20 of Figs. 9 and 10 is shown during
pressure build-up in the second section 105 when borehole fluid enters the
second section 105 through the second fluid port 107 and the pressure in the
second section 105 increases as the pressure in the borehole increases. In Fig.
11, the borehole pressure has forced the first pistons 103 towards the first end
face 101a decompressing the first spring 108. By this movement of the first
piston 103, the pressure is compensated, i.e. the pressure is equalised in the first
and second sections of the pressure compensating device 20. Since the first
section 104 is in fluid communication with the inside of a tool, the tool will, in this
way, be pressure compensated and thereby not destroyed during a pressure
build-up in the borehole. Fig. 11 shows the situation in which the first piston has
reached its maximum displacement towards the first end face 101a and abuts the
first end face 101a due to increasing pressure in the second section 105
stemming from the pressure in the borehole increasing. If the pressure continues
to increase in the second section 105, the second piston 109 will begin to move
towards the first end face and the second spring 110 will begin to compress. As
shown, the overpressure in the second section 105 opens the fluid connection
through the overpressure channel 111 between the first and second sections 104,
105 when the second piston 109 has moved sufficiently long towards the first end
face 101a, which allows fluid from the second section 105 to enter the first
section 104.
Fig. 12 shows another compensating device 20 comprising two rows of first
springs 108 arranged concentrically in the compensating device 20. The first row
of first springs 108a is arranged within the second row of first springs 108b. Each
row of springs contains four separate springs, only separated by a number of
spring guides 129. The number of spring guides 129 has been placed along the
two first springs 108 to avoid undesired bending of the springs during
compression which may lead to entangling of the two concentrically arranged
rows of first springs 108.
In some embodiments of the invention, the spring may be of another type than
the conventional coil spring shown in the figures. Such types may be helical
spring type, bellow type, volute spring type, leaf spring type, gas spring type or
disc spring type.
The first and second fluid ports may be controllably sealed off by a valve such as
a ball valve, butterfly valve, choke valve, check valve or non-return valve,
diaphragm valve, expansion valve, gate valve, globe valve, knife valve, needle
valve, piston valve, pinch valve or plug valve.
Although the invention has been described in the above in connection with
preferred embodiments of the invention, it will be evident for a person skilled in
the art that several modifications are conceivable without departing from the
invention as defined by the following claims.
Claims
1. A downhole pressure compensating device (20) for use in combination with
a downhole tool, comprising:
- a housing (100) with a chamber (101) and an internal hollow section
(102),
- a first piston (103) dividing the chamber into a first section (104) and a
second section (105), the first section being in fluid communication with a first
fluid port (106), the second section being in fluid communication with a borehole
(4) through a second fluid port (107), and
- a first spring (108) disposed within the second section to exert a pressure
on the first piston to enable the conservation of an overpressure in the first
section,
wherein the device further comprises:
- a second piston (109),
- a second spring (110) disposed between the first piston and the second
piston, and
- an overpressure channel (111) arranged in the first or the second piston,
which overpressure channel, when the second spring is in a compressed
condition, provides fluid communication between said first and second sections.
2. A downhole pressure compensating device according to claim 1 comprising
at least a pressure connection to a mating tool (11, 17, 18, 20) in a tool string
(10).
3. A downhole pressure compensating device according to claim 2 wherein the
mating tool is a driving unit (11).
4. A downhole pressure compensating device according to any of claims 1-3
wherein the second section of the compensating device is in fluid communication
with the inside of an electrical motor unit (17) and/or a hydraulic pump unit (18).
5. A downhole pressure compensating device according to any of claims 1-4
wherein the first spring, the second spring, the first piston and the second piston
are arranged coaxially with the longitudinal centre axis of the compensating
device.
6. A downhole pressure compensating device according to any of claims 1-4,
wherein at least one of the first spring, the second spring, the first piston and the
second piston has been arranged non-coaxially with the longitudinal centre axis
of the compensating device non-circumscribing the internal hollow section.
7. A downhole pressure compensating device according to claim 6, wherein
the compensating device is arranged non-coaxially with a longitudinal centre axis
of the tool.
8. A downhole pressure compensating device according to any of claims 1-7,
wherein the second piston is partly arranged inside the first piston.
9. A downhole pressure compensating device according to any of claims 1-8,
wherein the first piston is partly arranged inside the second piston.
10. A downhole pressure compensating device according to any of claims 1-9,
wherein the first section of the chamber is filled with a pressurised hydraulic fluid
such as oil with predetermined characteristics.
11. A downhole pressure compensating device according to any of claims 1-10,
further comprising electrical sensors for monitoring a temperature inside the
device and/or pressures in the first and second sections and/or positions of the
first and second pistons for producing a feedback signal to a control system.
12. A downhole pressure compensating device according to any of claims 1-11,
further comprising at least a switch wherein the compensating device can be
controlled by the at least a switch connected to the control system to adapt to
changes in environmental conditions based on the feedback signal.
13. A downhole pressure compensating device according to any of claims 1-12,
wherein the device comprises a plurality of first and/or second springs.
14. A downhole system comprising:
-a wireline (9),
-a mating tool such as a driving unit and/or an operational tool, and
-a downhole pressure compensating device according to any of claims 1-13.
A downhole tool system comprising:
-at least a mating tool such as a driving unit and/or an operational tool, and
-a downhole pressure compensating device according to any of claims 1-13.