Note: Descriptions are shown in the official language in which they were submitted.
DIRECT WELLHEAD MEASUREMENT SKID AND RELATED USES AND
METHODS OF OPERATION
TECHNICAL FIELD
[0001] The present disclosure relates to industrial equipment for
measuring the
amount of liquid and gas product extracted from a wellhead, and in particular
to a
direct wellhead measurement skid and related uses and methods of operation.
BACKGROUND
[0002] In the upstream oil and gas industry there are regulatory and
operational
requirements related to measurement and reporting of the oil, water and gas
production from each production well.
[0003] The traditional method to measure production is with a large
volume test
separator. The fluid produced out of each well is diverted into a test
separator on a
scheduled basis and allowed a given retention time to separate into its
various liquid
and gas components. The natural gas production flows out of the top of the
separator
and is measured with a flow meter while the liquid production flows out of the
bottom
with the oil and water volumes measured with their own, respective flow
meters. Each
well could be tested through this system weekly, or even as far apart as
monthly.
[0004] These traditional test separators come outfitted in large
buildings with
numerous types of instrumentation. They are installed on pilings and have
multiple
.. runs of piping and valving to be able to cycle through each of the wells.
All of this is
capital intensive and only gives an operator only a macro view of well
production.
[0005] Accordingly, systems and methods that enable accurate and real-
time
measurement of liquid and gas product extracted from a wellhead remains highly
desirable.
SUMMARY OF THE INVENTION
[0006] According to one aspect of the invention, there is provided a
measurement skid for measuring an amount of liquid and gas produced at a
wellhead,
the measurement skid including: a first skid inlet adapted to continuously
receive an
oil/water emulsion and wellhead gas from the tubing of the wellhead; a second
skid
inlet adapted to continuously receive wellhead gas from the casing of the
wellhead; a
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gravity separator connected to the first skid inlet, the gravity separator
including: a
vapour outlet disposed substantially at the top of the gravity separator, the
vapour
outlet having a vapour control valve disposed thereon and being fluidly
connected to
the second skid inlet; a liquid outlet disposed substantially at the bottom of
the gravity
separator, the liquid outlet having a liquid control valve disposed thereon; a
liquid level
switch adapted to determine a liquid height in the gravity separator; and a
pressure
meter adapted to determine a pressure in the gravity separator; a gas flow
meter
adapted to determine a flowrate of a combined stream from the second skid
inlet and
the vapour outlet; a liquid flow meter adapted to determine a flow rate of the
liquid
outlet; and at least one skid outlet adapted to continuously discharge the
combined
stream and the liquid outlet from the measurement skid, wherein when the
liquid
height in the gravity separator is below a threshold liquid level the liquid
control valve
is selectively closed and the vapour control valve is selectively opened to
allow vapour
to vent from the gravity separator by the vapour outlet, and wherein when the
liquid
height in the gravity separator is above the threshold liquid level the vapour
control
valve is selectively closed, and wherein when the pressure in the gravity
separator is
above a threshold pressure the liquid control valve is selectively opened to
allow liquid
to flow from the gravity separator by the liquid outlet.
[0007] In one embodiment the gravity separator is a cyclone
separator. In a
further embodiment a bypass line is adapted to permit all or a portion of the
oil/water
emulsion and wellhead gas from the tubing of the wellhead to bypass the
measurement skid. In yet a further embodiment a pressure safety valve is
fluidly
connected to the gravity separator. In yet a further embodiment the skid
outlet consists
of a first skid outlet and a second skid outlet, the first skid outlet adapted
to
continuously discharge the combined stream, and the second skid outlet adapted
to
discharge flow from the liquid outlet. In yet a further embodiment a check
valve is on
the first skid outlet and/or the second skid outlet. In yet a further
embodiment the liquid
flow meter comprises a Coriolis tube. In yet a further embodiment the liquid
control
valve is a bladder type control valve.
[0008] According to another aspect of the invention the measurement ski
described herein may be use for measuring the amount of liquid and gas
produced
by the wellhead.
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[0009] According to another aspect of the invention, there is
provided a method
of measuring an amount of liquid and gas produced at a wellhead using the
measurement skid described here, the method including: receiving an oil/water
emulsion and wellhead gas from the tubing of the wellhead at the first inlet;
receiving
wellhead gas at the second inlet of the measurement skid; separating the
oil/water
emulsion and wellhead gas received at the first inlet by the gravity
separator;
selectively discharging wellhead gas from the gravity separator by the vapour
outlet;
selectively retaining the oil/water emulsion in the gravity separator;
selectively
retaining wellhead gas in the gravity separator by closing the vapour control
valve
when the liquid level of the oil/water emulsion in the gravity separator
exceeds the
threshold liquid height; selectively discharging the oil/water emulsion from
the gravity
separator by opening the liquid control valve when the pressure in the gravity
separator exceeds the threshold pressure; measuring a flow rate of the
combined
stream; and measuring a flow rate in the liquid outlet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Further features and advantages of the present disclosure will
become
apparent from the following detailed description, taken in combination with
the
appended drawings, in which:
[0011] FIG. 1 shows a schematic layout of upstream oil and gas
production
including a measurement skid according to one embodiment;
[0012] FIG. 2 shows two view of a computerized model of a measurement
skid
according to one embodiment; and
[0013] FIG. 3 shows a flow chart for a method of measuring an amount
of liquid
and gas produced at a wellhead according to one embodiment.
[0014] It will be noted that throughout the appended drawings, like
features are
identified by like reference numerals.
DETAILED DESCRIPTION
[0015] Embodiments are described below, by way of example only, with
reference to Figs. 1-3.
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[0016] A direct wellhead measurement skid and related uses and
methods of
operation are provided. The measurement skid connects directly to the
wellhead,
preferably, although not necessarily, before any oil and gas processing steps
are
completed at the upstream facility. The measurement skid provides real-time
measurement of process variables including gas and liquid flows from the
wellhead
while requiring less material when compared to traditional methods to measure
wellhead production. In certain embodiments the liquid flow may be further
subdivided
into a water flow and an oil flow. The control methodology for a gravity
separator of
the wellhead measurement skid permits continuous measurement of flow from the
wellhead while relieving gas or liquid accumulation as needed. By manually
setting a
maximum drive gain for a liquid flow meter, measurement difficulties
encountered with
certain Coriolis flow meters may be overcome leading to a more accurate
measurement result.
[0017] With reference to FIG. 1, an example upstream production
system 100
is shown including an example measurement skid 110. The system 100 has a
wellhead 102 including a casing 102a and a tubing 102b. A surface pump 102c is
depicted although other artificial lift systems may be used where preferred.
In certain
wells where there is sufficient motive pressure no artificial lifting system
may be
needed.
[0018] Tubing 102b conveys an oil/water emulsion and/or wellhead gas to
pipe
104, which is in turn fluidly connected to a first skid inlet 110b. Casing
102a conveys
annulus wellhead gas to pipe 106, which is in turn fluidly connected to a
second skid
inlet 110a. Valve 104a on pipe 104 and valve 106a on pipe 106 may selectively
control
or regulate fluid flow from the wellhead. Bypass line 108 is depicted but is
not required
in all embodiments, and may connect pipe 104 and/or pipe 106 directly to
further
processing facilities 112 permitting the flow in pipe 104 and/or pipe 106 to
bypass the
measurement skid 110.
[0019] Measurement skid 110 includes the first skid inlet 110a and
the second
skid inlet 110b. First skid inlet 110a conveys wellhead gas through a gas flow
meter
110f towards skid outlet 110h. Second skid inlet 110b conveys the oil/water
emulsion
and/or wellhead gas to a gravity separator 110c. In certain embodiments the
gravity
separator 110c may be a cyclone separator allowing the liquid to be quickly
separated
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from the vapor without substantial foaming; however, alternative gravity
separators
such as a packed horizontal separator may be used in certain embodiments.
[0020] The gravity separator 110c has a vapour outlet 110d located
substantially at the top of the gravity separator, a liquid outlet 110e
located
substantially at the bottom of the gravity separator, a liquid level switch
and a pressure
meter. The vapour outlet 110d functions to selectively discharge vapour from
the
gravity separator 110c by operation of a vapor control valve on the vapour
outlet.
Flowmeter 110f measures a combined flow from the vapour outlet 110d and the
first
skid inlet 110a. Similarly to the vapour outlet 110d, the liquid outlet 110e
functions to
selectively discharge liquid from the gravity separator 110c by operation of a
liquid
control valve on the liquid outlet. A pressure safety valve 114 is fluidly
connected to
the liquid outlet 110d, or alternatively to the gravity separator 110c, and
allows for
selective venting of vapor if the pressure in the gravity separator or the
vapour outlet
110d exceeds a maximum set pressure. The liquid level switch and the pressure
meter cooperate to selectively open and close the liquid control valve and the
vapor
control valve using the control methodologies described below.
[0021] During start-up, the vapor control valve is open and the
liquid control
valve is closed allowing liquid to accumulate in the gravity separator 110c.
The level
of the retained liquid rises until it exceeds a threshold liquid level as
determined by,
for example, the height of the liquid level switch on the gravity separator
110c, at
which time the vapour control valve is closed preventing further vapor from
escaping
through the vapor outlet 110d. At this point both the vapor control valve and
the liquid
control valve are closed. The pressure in the gravity separator 110c begins to
rise
until the pressure meter on the gravity separator measures a threshold
pressure, at
which time the liquid control valve is opened allowing liquid to escape
through the
liquid outlet 110e. Liquid in the liquid outlet 110e flows through a liquid
flow meter
110g and may be discharged from the measurement skid 110 by the skid outlet
110i.
In certain embodiments a check valve may be present downstream of the liquid
flow
meter 110g to prevent back flow of fluid towards the liquid flow meter.
[0022] In one control strategy, when or if the liquid level in the gravity
separator
110c falls below the threshold liquid level, for example if a slug of wellhead
gas is
received at the gravity separator, the pressure in the gravity separator falls
and the
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liquid control valve is subsequently closed preventing further flow of liquid
through the
liquid outlet 110e and the vapor control valve is opened re-establishing flow
of vapor
through the vapor outlet 110d until such time that the liquid level again
exceeds the
threshold liquid level. Following the process described above, the vapor
control valve
may thereafter be closed allowing the pressure in the gravity separator 110c
to rise,
and the liquid control valve is opened once the pressure in the gravity
separator
reaches the threshold pressure.
[0023] In an alternative control strategy, when or if the liquid
level in the gravity
separator 110c falls below the threshold liquid level, the liquid control
valve is closed
and the vapor control valve is opened for a pre-set length of time. In a
preferred
embodiment the vapor control valve will remain open and the liquid control
valve will
remain closed for between about 0.3 seconds and 30 seconds, and in a
particularly
preferred embodiment the vapor control valve will remain open and the liquid
control
valve will remain closed for between about 1.0 seconds and 1.5 seconds, after
which
time the liquid control valve will be automatically re-opened and the vapor
control
valve will be closed. This procedures may be repeated as required.
[0024] Described above is an example where then tubing 102b is
producing a
mixture of oil/water emulsion and wellhead gas, but where the tubing 102b is
producing predominantly wellhead gas then the above described control
methodology
may still be followed except the system will predominantly operate in the
state with
the vapor control valve open and the liquid control valve closed. This
configuration
allows wellhead gas to substantially free-flow from the first skid inlet 110b
to the vapor
outlet 110d where it rejoins with the second skid inlet 110a. Conversely, if
the tubing
102b is producing predominantly an oil/water emulsion then the system will
predominantly operating in the state with the vapour control valve closed and
the liquid
control valve open, allowing liquid to substantially free-flow from the first
skid inlet
110b to the liquid outlet 110e.
[0025] It will be appreciated that while flows are described with
respect to the
tubing and casing of a wellhead, the oil/water emulsion and wellhead gas flows
may
be delivered to the measurement skid by other wellhead components or piping
particular to the wellhead configuration or production methodology.
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[0026] The geometry of the gravity separator 110c, the threshold
liquid height,
and the threshold pressure depend on the circumstances of each well. For
example,
in certain embodiments it may be desirable to have the threshold pressure be
sufficiently high to aid liquid flow from the gravity separator 110c but low
enough to
minimize the chance or amount of liquid flashed as a vapour should the
pressure in
the gravity separator begin to decrease as liquid flows from the gravity
separator. In
other embodiments it may be desirable to have the threshold pressure be
sufficiently
low to permit more continuous flow of liquids out of the liquid outlet 110e.
[0027] The term pressure meter, as used in the present description,
broadly
indicates a component that can determine the pressure in the gravity separator
110c.
The pressure meter may be a pressure gauge or a pressure transmitter in
certain
embodiments. In one embodiment the liquid control valve may be a bladder type
control valve allowing the liquid control valve to automatically open when a
bladder
set pressure is reached. In this embodiment the bladder set pressure would
correspond to the threshold pressure in the gravity separator 110c, and the
pressure
meter would be the bladder on the bladder type control valve as the bladder is
able to
determine the pressure in the gravity separator.
[0028] A skilled person will appreciate that a certain liquid
fraction may be
present in vapor flows and certain vapor fractions may be present in liquid
flows.
Liquid entrained in vapor may be managed or removed using known techniques,
such
as by including bleeder valves in the measurement skid 110 or by retaining
sufficient
vapor pressure in the measurement skid 110 to purge the liquid fraction along
with
the vapor flow. A significant vapor fraction in the liquid flow of the liquid
outlet 110e
may affect the flow rate measurement of the liquid flow meter 110g,
particularly in the
embodiment where the liquid flow meter is a Coriolis flow meter because the
entrained
vapor may dampen oscillation of the Coriolis tube leading to imprecise flow
measurement.
[0029] Certain flow meters, such as, for example, a Micro MotionTM
Advanced
Phase Measurement flow meter, may correct measured flow rates of liquids
having
non-negligible vapour flow using a drive gain parameter. The drive gain is a
measure
of the amount of power input required to power the drive coils for a Coriolis
tube to
ensure consistent amplitude and vibration. The drive gain is useful for
correcting
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measured flow rates because when biphasic flow is present in a system, or when
there is significant entrained gas in a Coriolis tube, more energy is required
power the
drive coils. The Micro MotionTM Advanced Phase Measurement flow meter compares
a measured drive gain to a maximum drive gain expected for single-phase fluid
flow
.. under the measured operating conditions. When the ratio of the measured
drive gain
to the maximum drive gain becomes too high the flow meter will switch to using
the
measured mass flow rates along with a remediation functionality based on
historical
density values, as opposed to measured density values, to calculate fluid flow
properties.
[0030] The inventors of the present application have noticed that because
of
process fluctuations and variations in process conditions present in
applications
where the measurement skid 110 is intend to operate, Coriolis flow meters may
not
accurately switch to using remediation functionality when appropriate and
therefore
may report incorrect flow properties. Therefore, a maximum drive gain value is
set for
the liquid flow meter by evaluating when data from a liquid flow meter becomes
overly
chaotic or inconsistent with flow properties expected for given process
conditions.
[0031] Turning now to FIG. 2, FIG. 2 shows a first view 200a and a
second
view 200b of a measurement skid according to one embodiment. The first view
200a
shows a first skid inlet 210a and a second skid inlet 210b, which correspond
to the
first skid inlet 110a and second skid inlet 110b, respectively. The second
view 200b
shows the skid outlet 210h and the skid outlet 210i, which correspond with the
skid
outlet 110h and the skid outlet 110i, respectively. Also shown is a pressure
safety
valve 214, which corresponds to the pressure safety valve 114, and a liquid
flow meter
210g, which corresponds to the liquid flow meter 110g.
[0032] Additionally shown in FIG. 2 is a liquid level switch 210j on a
gravity
separator, a liquid control valve 210k on a liquid outlet from the gravity
separator, and
a drain valve 2101. The liquid level switch 210j and the liquid control valve
210k have
the same functionality as described for the liquid level switch and the liquid
control
valve with respect to Figure 1. The drain valve 2101 may selectively open to
allow
draining of the gravity separator during maintenance procedures, for example.
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[0033] FIG. 3 a flow chart for an example method 300 of measuring an
amount
of liquid and gas produced at a wellhead using a measurement skid described
herein.
The method includes, at step 302, receiving an oil/water emulsion and wellhead
gas
from the tubing of the wellhead at the first inlet and at step 304 receiving
wellhead gas
at the second inlet of the measurement skid. As described herein, how the flow
components are received at the measurement skid may be determined by the
piping
configuration of the wellhead or by operating philosophy.
[0034] At step 306 the oil/water emulsion and wellhead gas are
separated at
the gravity separator and at step 308 wellhead gas is selectively discharged
from the
gravity separator by selectively opening the vapour control valve on the
vapour outlet
and the oil/water emulsion is selectively retained in the gravity separator by
selectively
closing the liquid control valve on the liquid outlet.
[0035] Using the control methodology described herein, at step 310
wellhead
gas is selectively retained om the gravity separator by selectively closing
the vapour
control valve on the vapour outlet and the oil/water emulsion is selectively
discharged
from the gravity separator by selectively opening the liquid control valve on
the liquid
outlet. At step 312, flow meters on the discharged liquid and gas streams can
measure
flow properties of the discharged fluids.
[0036] It would be appreciated by one of ordinary skill in the art
that the system
and components shown in Figures 1 to 3 may include components not shown in the
drawings. For simplicity and clarity of the illustration, elements in the
figures are not
necessarily to scale, are only schematic and are non-limiting of the elements
structures. It will be apparent to persons skilled in the art that a number of
variations
and modifications can be made without departing from the scope of the
invention as
defined in the claims. A system of one or more computers can be configured to
perform particular operations or actions described herein by virtue of having
software,
firmware, hardware, or a combination of them installed on the system that in
operation
causes or cause the system to perform the actions.
[0037] Although certain components and steps have been described, it
is
contemplated that individually described components, as well as steps, may be
combined together into fewer components or steps or the steps may be performed
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sequentially, non-sequentially or concurrently. Further, although described
above as
occurring in a particular order, one of ordinary skill in the art having
regard to the
current teachings will appreciate that the particular order of certain steps
relative to
other steps may be changed. Similarly, individual components or steps may be
provided by a plurality of components or steps. One of ordinary skill in the
art having
regard to the current teachings will appreciate that the system and method
described
herein may be provided by various combinations of software, firmware and/or
hardware, other than the specific implementations described herein as
illustrative
exam pies.
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