Note: Descriptions are shown in the official language in which they were submitted.
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SYSTEM FOR CONTROLLING PRODUCTION FROM A
GAS-LIFTED OIL WELL
The invention relates to a system for controlling
production of crude oil through a production tubing which
extends into a gas-lifted oil well whereby lift-gas is
injected at a downhole location.
In such gas-lifted oil wells the pressure in the
production tubing may fluctuate which may lead to an
irregular in-flow of lift-gas that is injected into the
production tubing. Such irregular injection of lift-gas
may eventually cut off production of oil all together.
Consequently such unstable gas-lifted wells tend to see-
saw between oil-producing and non-oil-producing states
whereby slugs of crude oil and lift-gas are produced.
It is common practice to adjust the flow of lift-gas
that is injected into the well by means of a choke to
such a level that the production of crude oil is
maximized and stabilized.
The article "Wellhead monitors automate Lake
Maracaibo gas-lift" published by J C Adjunta and A Majek
on pages 64-67 of the Oil and Gas Journal of 28 November
1994 discloses that an automatic choke may be used which
varies the flow of lift-gas such that it stays close to a
calculated optimum.
In the system known from this prior art reference the
choke is located at the earth surface near the wellhead
of the gas-lifted well. A problem encountered with the
known system is that the gas injection conduit, which is
usually formed by the annular space between the
production tubing and the well casing, may have a length
of several kilometres and may have such a large volume
that it is not possible to accurately control the amount
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of lift-gas which is injected downhole into the
production tubing by adjusting the flow of lift-gas that
enters the lift-gas injection conduit via the variable
choke at the wellhead.
It is also known, for example from International patent
application PCT/EP 95/00623 (International Publication
Number: WO 95/22682), to adjust the flow of lift-gas which is
injected into the oil production tubing by means of a surface
controlled variable downhole orifice via which the lift-gas is
injected into the production tubing.
Such a variable downhole orifice enables an accurate
control of the amount of lift-gas into the well such that
always a steady flow of lift-gas is injected and a stable
and optimum gas-lift is created.
However, the installation, operation and maintenance
of such a variable downhole orifice is expensive. In
particular if the well is equipped with a dual
completion, which may consist of two concentric
production tubings that extend to various depths in the
well, and gas is injected via the surrounding annulus and
orifices near the bottom of each of these tubings, then
the installation of a set of two downhole valves in the
well may not be economical.
International patent application PCT/AU87/00201
(International publication Number: WO 88/00277) discloses
a method of start-up of oil production in a gas lifted
oil well wherein the inlet flow of the injected gas is
maintained substantially constant by means of a vortex
flowmeter.
UK patent application GB 2252797 discloses a gas-
lifted oil production system wherein a production choke
and an inlet valve in the gas injection conduit are
adjusted simultaneously in a pre-programmed parametric
logic sequence to improve the control of oil production.
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A drawback of this known system is that said pre-
programmed sequence generates a fixed regime for
operating the two valves and which does not use feed-back
of operating conditions to adjust said sequence.
Furthermore the simultaneous adjustment of the two
valves may lead to oscillations in the lift gas flow,
particularly if lift gas stemming from a single source is
injected into a=plurality of wells.
It is an object of the present invention to provide a
system which is able to further enhance the accuracy of
control of the injection of gas into a gas-lifted oil
production well in such a manner that the crude oil
production is maximized and stabilized and which does not
require the use of downhole control equipment.
The system according to the invention thereto
comprises the control module comprising a PID controller
which is set to dynamically control the opening of a
choke in a production tubing in such a manner that the
fluid pressure within said lift-gas injection conduit is
minimized and stabilized.
It will be understood that a PID controller is a
controller which gives an output signal which is
proportional to the input signal,. but which also
integrates and differentiates the input signal to adjust
the characteristics of the output signal.
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According to one aspect of the present invention,
there is provided a system for controlling production of
crude oil through a production tubing which extends into a
gas-lifted oil production well and into which lift-gas is
injected at a downhole location, the system comprising: a
variable choke for adjusting the flow of crude oil through
the production tubing; and a control module for dynamically
controlling opening of the choke, which control module uses
pressure measured by a pressure gauge in a lift-gas
injection conduit as input signal; wherein the control
module comprises a proportional-integral-derivative (PID)
controller which is set to dynamically control the opening
of the choke in such a manner that the fluid pressure within
the lift-gas injection conduit is minimized and stabilized.
The control module may further comprise a master
controller which incorporates a fuzzy logic algorithm to
generate for the PID controller a setpoint for the pressure
in the lift-gas injection conduit.
The concept of a fuzzy logic control and of a
fuzzy controlled PID controller is known per se and
described for example in chapter 3 of the Handbook of
Intelligent Control: Neural, Fuzzy and Adaptive Approaches,
written by A White and D A Sofge and issued by van Nostrand
Reinhold, New York, 1992.
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Conveniently the variable choke and control module
are located at the earth surface at a location near the
wellhead of the gas-lifted oil production well.
The location of the choke and control module at the
earth surface allows installation and maintenance thereof
outside the well and without interruption of oil
production operations which saves significant cost and
effort. This is-particularly relevant if the well
comprises a plurality of crude oil production tubings and
lift-gas is injected at various downhole locations into
the various production tubings via a common gas injection
conduit which is at least partly formed by an annular
space between the production tubings and a well casing,
and wherein each production tubing is equipped with a
production control system according to the invention.
These and other features, objects and advantages of
the system according to the invention will become
apparent from the accompanying claims, abstract and
drawings.
In the drawings:
Fig. 1 shows a schematic longitudinal sectional view
of a crude oil production well in which the crude oil
production is controlled by a system according to the
invention;
Fig. 2 shows a flowscheme of the control logic for
the control module CM of the control system shown in
Fig. 1;
Fig. 3 shows a flowscheme which further explains the
operation of the control logic for the control module CM
of the control system shown in Fig. 1; and
Fig. 4 is a graph which shows the results of an
experiment which indicates that the control system according to the invention
is able to optimize and
stabilize production from a gas-lifted well.
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Referring now to Fig. 1 there is shown a gas-lifted
crude oil production well comprising a variable choke 1
and a control module CM according to the invention.
The choke 1 is mounted in a production tubing 2 which
extends from near the bottom of an oil production well 3
= through the wellhead 4 towards processing facilities (not
shown) at the earth surface 5.
Oil is produced via perforations 6 that have been
shot into an oil bearing formation. A packer 7 is mounted
near the lower end of the production tubing 2 which
provides a fluid barrier between the inflow zone 8 at the
bottom of the well and the annular space 9 that is formed
between the outer surface of the production tubing 2 and
the inner surface of a well casing 10.
To stimulate the production of crude oil via the
production tubing 2 lift-gas is injected via the
annulus 9 and a downhole orifice 11 into the production
tubing 2.
The lift-gas is fed into the annulus via a gas
injection conduit 12 and an annular chamber 13 at the
wellhead 4. The gas injection conduit 12 is equipped with
a choke 14 which serves to adjust the flow of lift-gas.
However, the considerable volume and length of the
annular space 9 result in a significant delay between the
moment at which the position of the choke 14 is varied
and the moment that this results in a variation of the
flow of gas that passes through the downhole orifice 11.
The variable choke 1 and the control module CM
according to the invention serve to avoid that swift
variations in the fluid pressure in the production
tubing 2 would result in an unstable lift-gas injection
regime whereby the lift-gas is injected in slugs via the
downhole orifice 11 into the production tubing 2 and the
= well would start to produce irregular slugs of crude oil
and lift-gas.
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The control module CM according to the invention is
continuously or intermittently fed with data concerning
the casing head pressure CHP measured by a pressure gauge
at the top of the annular space 9 and the tubing head
pressure THP measured by a pressure gauge at the top of
the tubing 2. Also data are fed to the control module CM
concerning the temperature T of the produced fluid
mixture and the,flow of lift gas Qlg and of the produced
fluid mixture Qprod measured by flowmeters that are
mounted in the lift-gas injection conduit 12 and the
production tubing 2, respectively. In the embodiment
shown the control module CM does not only control the
opening of the production choke 1, but also the opening
of the lift-gas injection choke 14.
The principal operation of the control module CM is
that it adjusts the opening of the production choke 1
such that the flow of lift-gas through the downhole
orifice 11 remains approximately constant. This is
achieved by maintaining a constant differential pressure
across the downhole orifice. The pressure downstream of
the orifice can be influenced by varying the backpressure
at the wellhead, i.e. the tubing head pressure THP. In
this way the backpressure exerted by the tubing head
pressure THP on the produced fluid mixture is varied such
that the backpressure increases in response to a decrease
in the measured casing head pressure CHP and vice versa.
This variation of the tubing head pressure THP is an
adequate measure to accomplish a substantially constant
rate of injection of lift-gas at the downhole orifice 11.
The control module CM aims to minimize the casing
head pressure CHP by variation of the opening of the
production choke 1.
Without constraints, however, further and further
opening of the production choke 1 would lead to '
instability. Therefore, the control module CM is set to
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obey another rule which dictates that the lower the lift-
gas injection rate Qlg is the wider the control margin
Cm(t) on the production choke 1 needs to be. Setting this
control margin Cm(t) requires some empirical judgement
which is incorporated into a fuzzy control unit FCU which
is described in more detail with reference to Fig. 2 and
Fig. 3.
Referring now to Fig. 2 there is shown a block-scheme
which shows the operation of the control module CM.
The heart of the control module CM is formed by a
conventional PID controller, in the block-scheme referred
to as PID, which adjusts the position Cp(t) of the
production choke 1 in response to variations of the
measured casing head pressure CHP.
The flowscheme shows that the casing head pressure
CHP is dependant of the tubing head pressure THP, the
pressure Pres of the fluid in the pores of the reservoir
formation RES, and also on the lift-gas injection rate
Qlg via the lift-gas choke 14 and the downhole orifice
11.
The fuzzy control unit FCU provides a casing head
pressure setpoint CHP sp(t) for the PID controller and
also adjusts the position of the lift-gas injection
choke 14 on the basis of empirical data, represented by
arrow 20, which identify classes of suitable positions of
the chokes 1 and 11 for various production rates.
Accordingly the fuzzy control unit FCU acts as a
master controller for the PID controller.
The interaction between the fuzzy control unit FCU
and the PID controller will be described in more detail
with reference to the flowscheme shown in Fig. 3.
= The flowscheme will be described from top to bottom
and the actions of the fuzzy control unit FCU and PID
. controller PID are contained within phantom lines.
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The first box at the top indicates that a control
cycle starts with a measurement at a certain moment in
time (t) of the lift-gas injection rate Qlg(t), the
casing head pressure CHP(t) and the actual position Cp(t)
of the production choke 1.
The next box indicates that on the basis of the
measured gas flow rate Qlg(t) the fuzzy control unit
calculates the choke margin Cm(t).
Subsequently the fuzzy control unit FCU verifies
whether the actual choke position Cp(t) is below the
choke margin Cm(t).
If this is indeed the case the fuzzy control unit FCU
will decrease the setpoint for the casing head pressure
CHPsp(t) for the PID controller, whereas if this is not
the case said setpoint CHPsp(t) will be increased.
The PID controller subsequently verifies whether the
measured casing head pressure CHP is lower than the
setpoint CHPsp(t) supplied by the fuzzy control unit FCU.
If this is indeed the case the PID controller will
decrease the choke opening Cp(t), whereas if this is not
the case the PID controller will increase the choke
opening.
The measurement and control cycle is then repeated
after a selected interval of time and the same steps of
the procedure set out in the flowscheme are taken again.
The performance of the control module according to
the invention was tested in a miniaturized well in which
water was produced via an 18 metres high riser pipe and
in which air was injected as lift-gas via an annulus
surrounding the pipe to enhance the flow of water through
the riser pipe.
During the experiment the lift-gas injection rate Qlg was 15 m3 per day and
the productivity index PI,
simulated with a variable restriction, was 10 m3 per day
per bar.
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The graph shown in Fig. 4 shows the response of the
casing head pressure CHP and fluid production rate Qprod
to various settings of the production choke at the top of
the riser pipe. The horizontal axis of the graph
represents elapsed time, in seconds. The vertical axis
contains a scale of 0-100 units which represent both the
opening Cp of the production choke (in %), the measured
casing head preSsure CHP times a factor 50 (in bar) and
the fluid production rate Qprod times a factor 10 (in
m3 per day).
At the start of the experiment, between t = 0 and
240 s, the production choke position Cp was fixed at 600
open. Without dynamic control, a fixed choke setting of
60% was required to achieve stable production.
The graph shows that at this choke setting the
production rate Qprod was stable and averaged 1.9 m3/day.
At t = 240 s the control module according to the
invention was switched on and accomplished an optimum
choke setting Cp = 910 open at t = 420 s.
At this point the average production Qprod equalled
3 m3 per day, which represents a production increase of
550.
At t = 660 s the control module according to the
invention was switched off and the choke setting remained
fixed at 910 open. The graph shows that the production
became unstable and the production rate Qprod dropped to
about 1.4 m3 per day.
At t = 960 s the control module according to the
invention was switched on again. It detected that there
was no gas injection downhole because the casing head
pressure CHP did rise and the control module fully opened
the production choke. When downhole lift-gas injection
started again and the casing head pressure CHP thus
dropped the control module partly closed the choke and
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opened it again to reach stable and optimum production
again at a rate of about 3 m3 per day.
It will be understood that the continuous or
intermittent variation of the production choke opening
consumes a significant amount of power.
If the well is located at a remote location and
electrical power is not readily available, power for
actuating the production choke could be generated by a
positive displacement motor or other rotary power
generator which utilizes the elevated pressure of the
lift-gas within the lift-gas injection conduit as a power
source. Preferably the inlet of the motor or generator is
connected to the lift-gas conduit and the outlet thereof
to the oil production tubing.
The control system according to the invention is also
suitable for use on a well which comprises a plurality of
crude oil production tubings which produce crude oil from
various locations in a reservoir. Such a well with
multiple completions may produce crude oil either from
various inflow regions along a single wellbore or from
various inflow regions along different downhole branches.
In such case various production tubings may be arranged
concentrically within the upper part of the well and
lift-gas may be injected at different depths into the
production tubings via the annular space formed between
the outermost tubing and the well casing. If in such case
each production tubing is provided with a control system
according to the invention which adjusts the opening of a
production choke near the top of the production tubing in
question in the manner described with reference to the
drawings then a stable gas injection and an optimum crude
oil production is accomplished in each of the production tubings.
