OA10655A

OA10655A - System for controlling production from a gas-lifted oil well

Info

Publication number: OA10655A
Application number: OA9800009A
Authority: OA
Inventors: Kinderen Wilhelmus Johanne Der; Pieter Koornneef
Original assignee: Shell Int Research
Priority date: 1995-07-24
Filing date: 1998-01-23
Publication date: 2002-09-19
Also published as: NO980301L; EP0840836B1; EP0756065A1; WO1997004212A1; NO311450B1; EA000484B1; NO980301D0; MY119607A; CA2226289C; EA199800149A1; CA2226289A1; EP0840836A1

Abstract

A system for controlling production from a gas-lifted oil well comprises a dynamically controlled choke (1) for adjusting the flow of crude oil from a production tubing (2) of the well. The system is capable of maximizing and stabilizing the crude oil production in an accurate manner and without requiring downhole equipment. <IMAGE>

Description

1 010655

SYSTEM FOR CONTROLLING PRODUCTION FROM AGAS-LIFTED OIL WELL

The invention relates to a System for controllingproduction of crude oil through a production tubing whichextends into a gas-lifted oil well whereby lift-gas isinjected at a downhole location.

In such gas-lifted oil wells the pressure in theproduction tubing may fluctuate which may lead to anirregular in-flow of lift-gas that is injected into theproduction tubing. Such irregular injection of lift-gasmay eventually eut off production of oil ail together.Consequently such unstable gas-lifted wells tend to see-saw between oil-producing and non-oil-producing Stateswhereby slugs of crude oil and lift-gas are produced.

It is common practice to adjust the flow of lift-gasthat is injected into the well by means of a choke tosuch a level that the production of crude oil ismaximized and stabilized.

The article "Wellhead monitors automate LakeMaracaibo gas-lift" published by J C Adjunta and A Majekon pages 64-67 of the Oil and Gas Journal of 28 November1994 discloses that an automatic choke may be used which ( varies the flow of lift-gas such that it stays close to acalculated optimum.

In the System known from this prior art reference thechoke is located at the earth surface near the wellheadof the gas-lifted well. A problem encountered with theknown System is that the gas injection conduit, which isusually formed by the annular space between theproduction tubing and the well casing, may hâve a lengthof several kilométrés and may hâve such a large volumethat it is not possible to accurately control the amount 010655 - 2 - 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 Internationalpatent application PCT/EP 95/00623, to adjust the flow oflift-gas which is injected into the oil production tubing(by means of a surface controlled variable downholeorifice via which the lift-gas is injected into theproduction tubing.

Such a variable downhole orifice enables an accuratecontrol of the amount of lift-gas into the well such thatalways a steady flow of lift-gas is injected and a stableand optimum gas-lift is created.

However, the installation, operation and maintenanceof such a variable downhole orifice is expensive. Inparticular if the well is equipped with a dualcompletion, which may consist of two concentricproduction tubings that extend to various depths in thewell, and gas is injected via the surrounding annulus andorifices near the bottom of each of these tubings, thenthe installation of a set of two downhole valves in thewell may not be economical.

International patent application PCT/AU87/00201(International publication Number: WO 88/00277) disclosesa method of start-up of oil production in a gas liftedoil well wherein the inlet flow of the injected gas ismaintained substantially constant by means of a vortexflowmeter. 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. b 1 0655 - 3 - A drawback of this known System is that said pre- programmed sequence generates a fixed régime 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 twovalves may lead to oscillations in the lift gas flow,particularly if lift gas stemming from a single source isinjected into a· plurality of wells.

It is an object of the présent invention to provide aSystem which is able to further enhance the accuracy ofcontrol of the injection of gas into a gas-lifted oilproduction well in such a manner that the crude oilproduction is maximized and stabilized and which does notrequire the use of downhole control equipment.

The System according to the invention theretocomprises the control module comprising a PID controllerwhich is set to dynamically control the opening of achoke in a production tubing in such a manner that thefluid pressure within said lift-gas injection conduit isminimized and stabilized.

It will be understood that a PID controller is acontroller which gives an output signal which isproportional to the input signal, but which alsointégrâtes and differentiates.the input signal to adjustthe characteristics of the output signal.

The control module may further comprise a mastercontroller which incorporâtes a fuzzy logic algorithm togenerate for the PID controller a setpoint for thepressure in the lift-gas injection conduit.

The concept of a fuzzy logic control and of a fuzzycontrolled PID controller is known per se and describedfor example in chapter 3 of the Handbook of IntelligentControl: Neural, Fuzzy and Adaptive Approaches, writtenby A White and D A Sofge and issued by van NostrandReinhold, New York, 1992. 010655

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 theearth surface allows installation and maintenance thereofoutside the well and without interruption of oilproduction operations which saves significant cost andéffort. This is. particularly relevant if the wellcomprises a plurality of crude oil production tubings andlift-gas is injected at various downhole locations intothe various production tubings via a common gas injectionconduit which is at least partly formed by an annularspace between the production tubings and a well casing,and wherein each production tubing is equipped with aproduction control System according to the invention.

These and other features, objects and advantages ofthe System according to the invention will becomeapparent from the accompanying daims, abstract anddrawings.

In the drawings:

Fig. 1 shows a schematic longitudinal sectional viewof a crude oil production well in which the crude oilproduction is controlled by a System according to theinvention;

Fig. 2 shows a flowscheme of the control logic forthe control module CM of the control System shown inFig. 1;

Fig. 3 shows a flowscheme which further explains theoperation of the control logic for the control module CMof the control System shown in Fig. 1; and

Fig. 4 is a graph which shows the results of anexperiment which indicates that the control Systemaccording to the invention is able to optimize andstabilize,production from a gas-lifted well. 018655

Referring now to Fig. 1 there is shown a gas-liftedcrude oil production well comprising a variable choke 1and a control module CM according to the invention.

The choke 1 is mounted in a production tubing 2 whichextends from near the bottom of an oil production well 3through the wellhead 4 towards Processing facilities (notshown) at the earth surface 5.

Oil is produced via perforations 6 that hâve beenshot into an oil bearing formation. A packer 7 is mountednear the lower end of the production tubing 2 whichprovides a fluid barrier between the inflow zone 8 at thebottom of the well and the annular space 9 that is formedbetween the outer surface of the production tubing 2 andthe inner surface of a well casing 10.

To stimulate the production of crude oil via theproduction tubing 2 lift-gas is injected via theannulus 9 and a downhole orifice 11 into the productiontubing 2.

The lift-gas is fed into the annulus via a gasinjection conduit 12 and an annular chamber 13 at thewellhead 4. The gas injection conduit 12 is equipped witha choke 14 which serves to adjust the flow of lift-gas.However, the considérable volume and length of theannular space 9 resuit in a significant delay between themoment at which the position of the choke 14 is variedand the moment that this results in a variation of theflow of gas that passes through the downhole orifice 11.

The variable choke 1 and the control module CMaccording to the invention serve to avoid that swiftvariations in the fluid pressure in the productiontubing 2 would resuit in an unstable lift-gas injectionrégime whereby the lift-gas is injected in slugs via thedownhole orifice 11 into the production tubing 2 and thewell would start to produce irregular slugs of crude oiland lift-gas. 010655

The control module CM according to the invention iscontinuously or intermittently fed with data concerningthe casing head pressure CHP measured by a pressure gaugeat the top of the annular space 9 and the tubing headpressure THP measured by a pressure gauge at the top ofthe tubing 2. Also data are fed to the control module CMconcerning the température T of the produced fluidmixture and the- flow of lift gas Q]_g and of the producedfluid mixture Qprod measured by flowmeters that aremounted in the lift-gas injection conduit 12 and theproduction tubing 2, respectively. In the embodimentshown the control module CM does not only control theopening of the production choke 1, but also the openingof the lift-gas injection choke 14.

The principal operation of the control module CM isthat it adjusts the opening of the production choke 1such that the flow of lift-gas through the downholeorifice 11 remains approximately constant. This isachieved by maintaining a constant differential pressureacross the downhole orifice. The pressure downstream ofthe orifice can be influenced by varying the backpressureat the wellhead, i.e. the tubing head pressure THP. Inthis way the backpressure exerted by the tubing headpressure THP on the produced fluid mixture is varied suchthat 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 anadéquate measure to accomplish a substantially constantrate of injection of lift-gas at the downhole orifice 11.

The control module CM aims to minimize the casinghead pressure CHP by variation of the opening of theproduction 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 010655 obey another rule which dictâtes that the lower the lift-gas injection rate Q]_g is the wider the control marginCm(t) on the production choke 1 needs to be. Setting thiscontrol margin Cm(t) requires some empirical judgement 5 which is incorporated into a fuzzy control unit FCU which is described in more detail with reference to Fig. 2 andFig. 3.

Referring now to Fig. 2 there is shown a block-schemewhich shows the operation of the control module CM. 10 The heart of the control module CM is formed by a conventional PID controller, in the block-scheme referredto as PID, which adjusts the position Cp(t) of theproduction choke 1 in response to variations of themeasured casing head pressure CHP. 15 The flowscheme shows that the casing head pressure CHP is dépendant of the tubing head pressure THP, thepressure Pres of the fluid in the pores of the réservoirformation RES, and also on the lift-gas injection rateQ]_g via the lift-gas choke 14 and the downhole orifice 20 11.

The fuzzy control unit FCU provides a casing headpressure setpoint CHP sp(t) for the PID controller andalso adjusts the position of the lift-gas injectionchoke 14 on the basis of empirical data, represented by 25 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 amaster controller for the PID controller.

The interaction between the fuzzy control unit FCU30 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. 010655

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 Qig(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 themeasured gas flow rate Qig(t) the fuzzy control unitcalculâtes the choke margin Cm(t).

Subsequently the fuzzy control unit FCU vérifiéswhether the actual choke position Cp(t) is below thechoke margin Cm(t).

If this is indeed the case the fuzzy control unit FCUwill decrease the setpoint for the casing head pressureCHPSp(t) for the PID controller, whereas if this is notthe case said setpoint CHPsp(t) will be increased.

The PID controller subsequently vérifiés whether themeasured casing head pressure CHP is lower than thesetpoint CHPsp(t) supplied by the fuzzy control unit FCU.

If this is indeed the case the PID controller willdecrease the choke opening Cp(t), whereas if this is notthe case the PID controller will increase the chokeopening.

The measurement and control cycle is then repeatedafter a selected interval of time and the same steps ofthe procedure set out in the flowscheme are taken again.

The performance of the control module according tothe invention was tested in a miniaturized well in whichwater was produced via an 18 métrés high riser pipe andin which air was injected as lift-gas via an annulussurrounding the pipe to enhance the flow of water throughthe riser pipe.

During the experiment the lift-gas injection rate Q]_g was 15 m3 per day and the productivity index PI, simulated with a variable restriction, was 10 m3 per day per bar. 010655

The graph shown in Fig. 4 shows the response of thecasing head pressure CHP and fluid production rate Qprodto various settings of the production choke at the top ofthe riser pipe. The horizontal axis of the graphrepresents elapsed time, in seconds. The vertical axiscontains a scale of 0-100 units which represent both theopening Cp of the production choke (in %) , the raeasuredcasing head pressure CHP times a factor 50 (in bar) andthe fluid production rate Qprod times a factor 10 (inm3 per day).

At the start of the experiment, between t = 0 and240 s, the production choke position Cp was fixed at 60%open. Without dynamic control, a fixed choke setting of60% was required to achieve stable production.

The graph shows that at this choke setting theproduction rate Qprod was stable and averaged 1.9 m3/day.

At t = 240 s the control module according to theinvention was switched on and accomplished an optimumchoke setting Cp = 91% open at t = 420 s.

At this point the average production Qprod equalled3 m3 per day, which represents a production increase of55%.

At t = 660 s the control module according to theinvention was switched off and the choke setting remainedfixed at 91% op.en. The graph shows that the productionbecame unstable and the production rate Qprod dropped toabout 1.4 m3 per day.

At t - 960 s the control module according to theinvention was switched on again. It detected that therewas no gas injection downhole because the casing headpressure CHP did rise and the control module fully openedthe production choke. When downhole lift-gas injectionstarted again and the casing head pressure CHP thusdropped the control module partly closed the choke and 10 010655 opened it again to reach stable and optimum productionagain at a rate of about 3 m3 per day.

It will be understood that the continuous orintermittent variation of the production choke openingconsumes a significant amount of power.

If the well is located at a remote location andelectrical power is not readily available, power foractuating the production choke could be generated by apositive displacement motor or other rotary powergenerator which utilizes the elevated pressure of thelift-gas within the lift-gas injection conduit as a powersource. Preferably the inlet of the motor or generator isconnected to the lift-gas conduit and the outlet thereofto the oil production tubing.

The çontrol System according to the invention is alsosuitable for use on a well which comprises a plurality ofcrude oil production tubings which produce crude oil fromvarious locations in a réservoir. Such a well withmultiple complétions may produce crude oil either fromvarious inflow régions along a single wellbore or fromvarious inflow régions along different downhole branches.In such case various production tubings may be arrangedconcentrically within the upper part of the well andlift-gas may be injected at different depths into theproduction tubings via the annular space formed between , the outermost tubing and the well casing. If in such caseeach production tubing is provided with a control Systemaccording to the invention which adjusts the opening of aproduction choke near the top of the production tubing inquestion in the manner described with reference to thedrawings then a stable gas injection and an optimum crudeoil production is accomplished in each of the productiontubings.

Claims

TS 6045 PCT 010655 C L A I M S

1. A System for controlling production of crude oilthrough a production tubing which extends into a gas-lifted oil production well and into which lift-gas isinjected at a downhole location, the System comprising: 5 - a variable choke for adjusting the flow of crude oil through said production tubing; and a control module for dynamically controlling theopening of the choke, which control module uses thepressure measured by a pressure gauge in the lift-gas 10 injection conduit as input signal; characterized in that the control module comprises a PIDcontroller which is set to dynamically control theopening of the choke in such a manner that the fluidpressure within said lift-gas injection conduit is 15 minimized and stabilized.
2. The System of claim 1, wherein the control modulefurther comprises a master controller which incorporâtesa fuzzy logic algorithm to generate for the PIDcontroller a setpoint for the pressure in the lift-gas 20 injection conduit.
3. The System of any preceding claim, wherein the i variable choke and control module are located at theearth surface at a location near the wellhead of the gas-lifted oil production well.
4. The System of any preceding claim, wherein the well comprises a plurality of crude oil production tubings andlift-gas is injected at various downhole locations intothe various production tubings via a common gas injectionconduit which is at least partly formed by an annular 30 space between the production tubings and a well casing, and wherein each production tubing is equipped with a 010655 12 production control System according to any precedingclaim.
5. The System of any preceding claim, wherein thevariable choke is equipped with power means whichutilizes the elevated fluid pressure of the lift-gaswithin the lift-gas injection conduit as a power source.
6. The System of claim 5, wherein the power meansconsist of a positive displacement motor of which aninlet is connected to the lift-gas injection conduit andan outlet is connected to the production tubing or one ofthe production tubings. 10