Note: Descriptions are shown in the official language in which they were submitted.
CA 02821155 2013-07-17
Pump for Controlling the Flow of Well Bore Returns
Field of the Invention
[0001] The present invention is directed to oil and gas drilling operations,
and in particular to
a pump for controlling the flow of well bore returns.
Background
[0002] During drilling operations, drilling fluid, also referred to as
drilling mud, is pumped at
high pressure down the well bore. Mud pumps draw drilling fluid from mud tanks
and pump
the drilling fluid at high pressure down the drill string. The drilling fluid
jets out of the drill
bit and cleans the bottom of the hole. The drilling fluid moves back up the
well bore in the
space between the drill sting and the side of the well bore, known as the
annulus, flushing
cuttings and debris to the surface. The pressurized drilling fluid creates
down-hole hydrostatic
pressure which promotes the prevention of formation fluids from entering into
the well bore
during drilling operations and suspends cuttings in the well bore during
interruptions to
drilling.
[0003] The mixture of drilling fluid, formation fluids and debris travelling
back up the well
bore to the surface is referred to as the 'well bore returns' or 'drilling
returns'. The well bore
returns also frequently contain dissolved gas which moves from the formation
surrounding the
well bore being drilled into the drilling fluid in the annulus.
100041 Upon arrival at the surface, a series of valves and pipes are utilized
to controllably
direct the well bore returns to a mud gas separator, or to a de-gasser. A
separator typically
CA 02821155 2013-07-17
comprises a cylindrical or spherical vessel and can be either horizontal or
vertical. It is used
to separate gas from the drilling fluid. In the separator, the drilling fluid
containing gas is
usually passed over a series of baffles designed to separate gas and mud.
Liberated free gas is
moved to a flare line while the mud is discharged to a shale shaker and to a
mud tank.
[0005] A de-gasser is used when the gas content of the drilling fluid is
relatively lower and it
operates on much the same principles as the separator. A vacuum is applied to
the drilling
fluid as it is passed over baffles to increase surface area thereby promoting
the liberation of
dissolved gas.
[0006] During drilling operations, it is extremely important to maintain
constant down-hole
hydrostatic pressure to try and prevent formation fluids and gases from
entering into the well
bore as mentioned above. This can be challenging due to shifting well bore
conditions and
frequent interruptions to drilling operations, such as tripping pipe. To
maintain down-hole
hydrostatic pressure, conventional drilling operations typically utilize one
or more chokes at
the well head. The primary role of the choke is to regulate the flow of well
bore returns from
the well head. The choke comprises an orifice that can be selectively opened
or closed to
control the flow rate of the well bore returns. Controlling the flow of the
well bore returns at
the well head in turn regulates down-hole pressure. There are both fixed and
adjustable
chokes, the latter being more conducive to enabling the fluid flow and
pressure parameters to
be adjusted to suit process and production requirements. However, there are
problems
associated with restricting the cross-sectional area of a conventional choke
orifice to passively
regulate down-hole pressure. First, as the cross-sectional area of the choke
decreases, the
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likelihood of the choke becoming clogged with cuttings and debris increases.
Second, for a given
volumetric flow rate, as the cross-sectional area of the choke decreases, the
velocity of the well
bore returns within the choke increases, which increases the scouring effects
of solids in the well
bore returns on the choke. Third, the chokes do not accurately measure well
bore return volume.
[0007] What is needed is an apparatus and a method of controlling well bore
returns to regulate
down-hole hydrostatic pressure that mitigates the problems of existing choke
devices.
Summary of the Invention
[0008] In one aspect, there is provided a method of controlling a flow of well
bore returns to
regulate a down-hole pressure of a well bore, the method comprising: directing
the well bore
returns to a reversible multiphase, positive displacement pump positioned on a
drilling rig;
and controlling a speed and a direction of the pump to selectively restrict or
reverse the flow
of the well bore returns through the pump.
[0008a] In one embodiment, the method further comprises the step of measuring
the volume
of well bore returns passing through the pump.
[0009] There is also provided an apparatus for controlling a flow of well bore
returns to
regulate a down-hole pressure of a well bore, the apparatus comprising: a
first flow line for
receiving the flow of the well bore returns from a blow-out-preventer stack of
a well head; a
reversible, multiphase, positive displacement pump positioned on a drilling
rig and connected
to the first flow line; a vessel for receiving the well bore returns; and a
second flow line which
connects the pump to the receiving vessel.
[0010] In one embodiment, the apparatus may comprise means for measuring the
volume of well
bore returns passing through the pump. The apparatus may further comprise a
source tank for fluid
pumped by the pump into the well bore.
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Brief Description of the Drawin2s
[0011] In the drawings, like elements are assigned like reference numerals.
The drawings are not
necessarily to scale, with the emphasis instead placed upon the principles of
the present invention.
Additionally, each of the embodiments depicted are but one of a number of
possible
arrangements utilizing the fundamental concepts of the present invention. The
drawings are
briefly described as follows:
[0012] Figure 1 is an elevated diagrammatic depiction of one embodiment of the
present
invention.
[0013] Figure 2 is an elevated diagrammatic depiction of one embodiment of the
present
invention.
[0014] Figure 3 is a sectional diagrammatic depiction of one embodiment of the
pump
utilized in the present invention.
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Detailed Description of Preferred Embodiments
[0015] The invention relates to an apparatus and a method of controlling the
flow of well bore
returns to regulate the hydrostatic force in a well-bore. When describing the
present
invention, all terms not defined herein have their common art-recognized
meanings. To the
extent that the following description is of a specific embodiment or a
particular use of the
invention, it is intended to be illustrative only, and not limiting of the
claimed invention. The
following description is intended to cover all alternatives, modifications and
equivalents that
are included in the spirit and scope of the invention, as defined in the
appended claims.
[0016] Figure 1 depicts the apparatus (10) of the present invention. The
apparatus (10) can be
utilized to control and exert a predetermined force back on the wellbore thus
controlling the
hydrostatic force on the formation surrounding the well-bore, the inflow of
fluids from the
1,
surrounding formation and the drilling circulating medium. In one embodiment,
the apparatus
(10) will also allow an operator to measure the volume of well-bore returns
passing through
the apparatus (10).
[0017] An intake flow line (19) receives the well bore return flow (F) that is
diverted from the
wellhead, blow-out-preventer ("BOP") stack, or rotating flow control diverter
("RFCD") (not
shown in the Figures). As shown in Figure 1, a diversion manifold (26)
provides two
alternate flow paths which can be interchangeably selected by selectively
opening and closing
gate valves (15, 17, 18). For example, if valve (18) is open and valve (15) is
closed, then the
flow will bypass pump (12).
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[0018] The gate valves (18, 15, 17) may also all be closed to block the flow
of well bore
returns if required for safety purposes. As shown in Figure 1, a choke valve
(29) may be used
with the present apparatus (10) and may be employed to quickly kill fluid flow
if required. It
should be understood that the choke valve (29) is present for safety purposes
only and is not
integral to the method of controlling the down-hole hydrostatic pressure
described herein.
[0019] The first flow path passes through valve (18) and leads directly to the
separator flow
line (33) which is connected to a gas/liquid separator (14). Any suitable
separator (14) may
be used with the present invention provided that it has an adequate volume and
pressure
rating. In one embodiment, a gas source (16) is interconnected to the
separator (14). The gas
source may consist of any suitable equipment capable of producing on-site
generated nitrogen,
liquid nitrogen, natural gas, propane or CO2. A liquid outlet line (20) leads
from the separator
(14) to a tank (38) or de-gasser (36) or to a shaker (34). A gas outlet line
leads from the
separator (14) to a flare stack (not shown in the Figures). The gas outlet
line (24) has an
integral back pressure valve (22).
100201 The second flow path follows the pump flow line (32) to a multiphase,
positive
displacement pump (12). In one embodiment, the pump (12) comprises a twin
screw helical
pump comprising a body (40), two pairs of helical screws (50, 55), and a drive
means (60).
Helical screw pumps are well known in the art of multiphase pumps.
100211 The body (40) defines an inlet (42), an outlet (44) and a chamber (46)
in fluid
communication with the inlet (42) and the outlet (44). The inlet (42) and
outlet (44) are
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configured for receiving and releasing, respectively, well bore returns from
an upstream point
and a downstream point, respectively, of the flow path.
[00221 Each of the pairs of helical screws (50, 55) is disposed within the
chamber (46), and
consists of two intermeshing helical screws (50a, 50b; and 55a, 55b). The
helical screws (50a,
50b; and 55a, 55b) within each pair (50, 55) are "opposite-handed" so that,
when viewed from
a common perspective, the threads of one of the helical screws (50a; and 55a)
when axially
rotated in one direction, and the threads of the other helical screw (50b; and
55b) when axially
rotated in the opposite direction, both appear to advance either towards or
away from the
viewer. When the helical screws (50a, 50b; and 55a; 55b) are counter rotated,
they displace
the well bore returns from the inlet (42) to the outlet (44). As will be
apparent to those persons
skilled in the art, the pump (12) may be suited to a particular application by
having as few as
one pair of helical screws or as many pairs of helical screws, as is needed to
achieve the
desired down-hole hydrostatic pressure of the well bore, and the desired
restriction of flow
rate of the well bore returns.
[00231 The drive means (60) drivingly engages the helical screws (50a, 50b;
and 55a, 55b) of
each pair of helical screws (50, 55) so as to rotate them in opposite axial
directions. In one
embodiment, the drive means (60) comprises a drive motor (62) for providing a
torque which
is transmitted to the helical screws (50a, 50b; and 55a, 55b) via a primary
drive shaft (64),
engaging a gear set (66), engaging a pair of secondary shafts (68a, 68b) which
support the
helical screws (50a, 50b; and 55a, 55b). A bearing and seal assembly (48)
isolates the drive
motor (62), the drive shaft (54) and the gear set (66) from the chamber (46)
and the well bore
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returns contained therein. The drive motor (62) may be powered by any suitable
energy source
such as electricity, diesel, gasoline or natural gas. The helical screws (50a,
50b; 55a, 55b) and
the chamber (46) are dimensioned, and the parameters of the drive means (60)
are selected
such that the helical screws (50a, 50b; 55a, 55b) restrict the flow rate of
well bore returns
through the pump (12) over a desired range of operating speeds.
[00241 The function of the pump (12) is to selectively restrict the flow of
well bore returns in
the flow path, but the direction of the pumping effect is not essential. It is
therefore
conceivable that the pump direction may be reversed such that the pump pumps
fluid back
towards and into the well bore. Accordingly, the terms "inlet" and "outlet"
prescribe neither a
specific relationship to upstream and downstream points of the flow path, nor
the direction in
which the helical screws (50a, 50b; and 55a, 55b) displace the well bore
returns.
[0025] Operation of the apparatus (10) depicted in Figure 1 will now be
described.
[00261 If an operator elects to flow the well bore returns through the pump
(12) from the BOP
stack, then the gate valve (18) mounted on the separator flow line (33) will
be closed and the
gate valves (15 and 17) on either side of the pump (12) will be opened. The
flow is
accordingly directed through the pump flow line (32) into an inlet of the pump
(12). As a
positive displacement pump, there will be no substantial flow through the pump
until the
pump is actuated. The flow of the well bore returns through the pump (12) can
be restricted in
a controlled manner by controlling the speed that the pump (12) runs at. The
faster the pump
runs, the less restriction the flow experiences. Conversely, the slower the
pump runs, the
more restricted the flow becomes. Inhibition of the flow of the well bore
returns results in
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back pressure on the wellbore and an increase in down-hole hydrostatic
pressure. In this
manner the down-hole hydrostatic pressure can be controlled and maintained at
a constant
level by the varying the speed of the pump (12). Further to this, if the
drilling program
requires a constant pressure on the wellbore and this cannot be achieved by
simply slowing or
stopping the pump (12), then the pump (12) can be reversed to pump liquid from
the separator
or a designated storage tank back down the wellbore to maintain a constant
pressure.
[0027] Use of a pump (12) also provides the operator with the means to
accurately calculate
the return volume of the wellbore returns. Such information is extremely
important to the
operator as they are continuously trying to achieve a net balance of liquid
injection and liquid
returns during operations. As the pump (12) is a positive displacement pump,
it is possible to
calculate the return volume of the pump during a particular time period, by
multiplying a
known volume of wellbore returns passing through the pump (12) in one
revolution of the
helical screws, the speed of the pump (e.g, in revolutions per minute), and
the time period.
The flow exits the pump (12) though an outlet and is directed to the separator
flow intake line
(33) (as shown in Figure 1).
[0028] While Figures 1 and 2 depict an embodiment of the apparatus having both
a pump and
a separator, one skilled in the art will realize that the present invention
can be practiced using
just a pump connected to a de-gasser and a shaker or using a separator without
a pump.
[0029] If an operator elects to flow the well bore returns directly into the
separator (14) from
the BOP stack, the gate valves (15, 17) on either side of the pump (12) are
closed and the gate
valve (18) mounted on the separator flow line (33) is opened thereby directing
flow directly
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along the separator flow line (33) into the separator (14). Gas is released
from the well bore
returns in the separator (14). If desired, in one embodiment, the back
pressure valve (22) can
be used to restrict the flow of gas out of the separator (14) into the gas
outlet line (24). This
results in a pressure build up in the separator (14) which inhibits the flow
of the well bore
returns into the separator (14) from the separator flow line (33). The
restricted flow of well
bore returns results in back pressure on the wellbore and an increase in down-
hole hydrostatic
pressure. In this manner the down-hole hydrostatic pressure can be controlled
and maintained
at a constant level by the back pressure valve (22) on the gas outlet line
(24).
[0030] In the event that the returns do not have sufficient associated gasses
to create the
required back pressure in the separator (14) to restrict the flow of the well
bore returns into the
separator (14), then the internal pressure of the separator (14) can be
artificially increased as
required by the introducing gas into the separator (14) from the gas supply
(16).
[0031] In the embodiment depicted in Figure 2, an additional flow line (35)
and additional
gate valves (23, 21) may be utilized which allows the operator to direct the
well bore returns
directly to a de-gasser (36), a shaker (34) or to a rig tank (38) without
having to pass through
the separator. Using the apparatus (10) shown in Figure 2, an operator could
selectively run
the well bore returns through the pump (12) and then directly to the de-gasser
(36) and the
shaker (34) by closing the gate valve mounted on the separator flow line (21)
and by opening
the gate valve (23) on flow line (35).
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[0032] It should also be understood that while the pump (12) and the separator
(14) may be
used independently to control the flow of the well bore returns, they may also
be used
cooperatively to control the flow of well bore returns.
[0033] As will be apparent to those skilled in the art, various modifications,
adaptations and
variations of the foregoing specific disclosure can be made without departing
from the scope
of the invention claimed herein.
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