GB2276675A

GB2276675A - Control of gas-lift wells

Info

Publication number: GB2276675A
Application number: GB9404877A
Authority: GB
Inventors: Robert Colin Pearson
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-03-17
Filing date: 1994-03-14
Publication date: 1994-10-05
Anticipated expiration: 2014-03-14
Also published as: GB2276675B; GB9404877D0

Abstract

A production, tube 4 travels inside an annulus 6 or similar passage through which gas is fed through a valve 14 to the production tube, and the supply of gas to the annulus is automatically controlled by a choke valve 12 via a feedback loop. In one embodiment, the supply of gas is regulated to a remotely settable datum value for mass flow rate to the annulus, using sensed pressure drop and temperature at venturi 101. Mass flow rate of the output from the production tube 4 is calculated using sensed pressure drop and temperature at venturi 104, and this is used to vary the datum value. In other embodiments different parameters are sensed and provide the basis by which the choke valve is controlled. Such parameters include the annulus gas pressure or its differential with respect to the production tube pressure; the liquid content or the liquid/gas ratio of the production flow; or the pressure of the production tube flow. <IMAGE>

Description

OILFIELD CONTROLS TECHNICAL FIELD This Invention is generally concerned with the control of product flow from oil wells and is particularly concerned with low pressure wells requiring enhancement of flow by gas-lift. It is also concerned with other gas-lift requirements, as, for example, encouragement of flow in deep risers, as risers that convey petroleum product from seabed to surface facilities for processing.

BACKGROUND Low pressure wells may have the tapped reservoir many feet below a land surface or sea-bed and drillings to tap into the reservoir are lined by metal tubes known as casings. These are installed in the course of drilling to prevent the formation falling into the drilled hole.

An outer casing is cemented in, and is not in contact with flow of product from the reservoir. An innermost casing known as the production tube, however, has its bottom end in the reservoir to convey the hydrocarbon product from the reservoir to the surface, from which the flow is controlled and directed to processing equipment as required.

A further casing, which is external to the production tube but with a clearance from the production tube known as the annulus, may also have its bottom end in contact with reservoir fluid, but product flow into the upper annulus is stopped by a sealing disc which seals on the outside of the production tube and on the bore of the casing. This sealing disc is known as the packer. The production tube, annulus and casing come within the generic term of downhole completions.

The natural pressure of the hydrocarbon in the reservoir many feet below ground may not be sufficient to force the product to the surface due to the weight, or head, of the product between surface and reservoir. In such cases a system known as gas-lift is used to bring the product to the surface.

For gas-lift, the production tube has one or more radial holes well below the surface but above the packer, and the hole is fitted with a valve known as the gas-lift valve. This valve is biased towards the closed position, but may open to allow passage of fluid from the annul us into the production tube when annulus pressure exceeds the pressure in the production tube.

To improve liquid product flow from the reservoir, gas, usually natural gas from a well, is pumped at pressure into the annulus to pass through the gaslift valve into the production tube where it mixes with the liquid product from the reservoir so reducing the density of the column of fluid in the production tube. The reduced head enables the reservoir pressure to deliver liquid product (mixed with the gas) to the surface and on to the processing facility.

The gas pressure required in the annulus to produce optimum delivery of liquid product from the reservoir is a complicated matter. Too much gas pressure causes reduction of flow as the excess gas entering the flow stream reduces liquid flow in flow lines, and too little gas pressure also causes reduction of flow as the increased density.of fluid in the production tube creates a head which resists flow from the reservoir. Current practice is to adjust the rate of gas delivery to the annulus to achieve a precalculated gas pressure. However, the prevailing gas pressure may vary with changes in flow conditions in the production tube, particularly in multi-well installations where all wells supply to a common manifold and flow changes in one well affect flow conditions in other wells.

Another complication arises as reducing pressure on the reservoir encourages gas to come out of solution with the liquid product, which further increases the gas to liquid proportion in the flow from the production tube.

In applications in which gas-lift is used to reduce reservoir pressure for the purpose of increasing gas delivery from the reservoir, the problems of control of delivery from the reservoir by control of gas-lift are much the same.

An object of the present Invention may be viewed as being to optimise flow of liquid product from the reservoir whilst minimising gas usage.

SUMMARY OF THE INVENTION The present invention proposes a gas-lift well having a production tube which travels alongside an annulus or similar passage through which gas is fed to the production tube, in which the supply of gas to the annulus is automatically regulated via a feedback loop.

Although the gas-lift gas will normally be supplied to the production tube through an annular passage surrounding the production tube it will be appreciated that the term "annulus" is intended to include any conduit by which the gas is supplied to the production tube.

BRIEF DESCRIPTION OF THE DRAWINGS The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a diagrammatic sectional view of part of a down hole completion of the invention, including gas-lift control, and Figures 2 to 4 are modified forms 'relating to Fig. 1.

DETAILED DESCRIPTION OF THE DRAWINGS Figure 1 A down-hole completion to tap low-pressure oil from a reservoir 1 below the surface 2 includes a hangar 3 for a production tube 4. The production tube enters the reservoir 1 and is surrounded by a casing 5 to form an annulus 6, 8 between the casing and production tube. A packer 7 separates the lower annulus 8 from the upper annulus 6, sealing on the outside of the production tube 4 and on the bore of the casing 5.

A drilling 9 provides communication through the hangar 3 to the upper annulus 6, and a bore 10 provides for flow from the production tube 4 through the hangar.

By way of example, the reservoir is typically situated some 2,500 ft below the surface 2, with the packer 7 located 2,050 ft below the surface. A reservoir pressure of 1,000 psi would thus be required to overcome the head of liquid in the production tube if any reservoir liquid is to emerge from the top of the production tube. (Calculations are based on the assumption that a head of 2.5 ft of liquid hydrocarbon is equivalent to a pressure of 1 psi). As the reservoir pressure is only 800 psi, clearly there will be no flow from the production tube without enhanced means of delivery.

The means chosen for enhancing flow from the reservoir is gas-lift.

Pressurised hydrocarbon gas delivered through a conduit 11 and a choke valve 12, which will be described later, passes to a conduit 13 and through the bore 9 to the annulus 6.

At 2,000 ft from the surface 2, i.e. above the packer, the production tube 4 has a radial bore 14 which receives a non-return valve 15 biassed towards the closed position by a spring 16. When pressure in the annulus exceeds pressure in the production tube the valve lifts against the action of its spring to allow passage of gas from annulus to production tube. The entry of gas into the production tube 4 displaces liquid product and lessens the head on the reservoir 1.

If, for example, the density of the contents of the production tube above the valve 15 were reduced by half due to gas inclusion, the head acting against flow from the reservoir would be 500 ft (reservoir to valve) + 2,000 ft divided by 2 - 1,500 ft total, which is equivalent to 600 psi. Thus, with reservoir pressure at 800 psi a delivery pressure from the production tube of 200 psi would result.

The situation is however complicated, for if the same gas pressure is maintained in the annulus, greater gas flow will occur through the bore 14 as the pressure in that part of the production tube 4 has been reduced from 2,000 ft head (800 psi) to 1,000 ft head (400 psi) so that more gas will pass into the production tube. This in turn will result in still further pressure reductions due to greater inclusion of gas with liquid flow, leading to further increase in gas flow, such escalation of gas flow eventually being limited by the size of the bore 14. In addition, however, increase in the volume of gas increases back-pressure on flow from the production tube through flow line 16 from the bore 10 to the processing equipment, for as the pressure falls over the length from well-head to processing plant the gas expands and reduces mass flow rate.

Further complications arise in this situation for it is common for several wells, each with gas-lift, to feed into a common manifold to collect all deliveries for transfer to the processing equipment by a common flow line. This situation is illustrated in Fig. 1, where the flow line 16 from the well head described enters a manifold 17 through a control valve 18 (which may be another choke valve). The manifold is also supplied from well heads 19 and 20 respectively through flow lines 21 and 22 and control valves 23 and 24 to be exported to the processing equipment by the common flow line 25. The back pressure interactions of one well delivery on another due to changes in gas content add to the problems of regulating product flow from all of the reservoirs by control of the gas supply.

To improve product flow whilst minimising usage of gas, gas supply to the annulus is controlled automatically by the choke valve 12, as follows: The choke control valve 12 comprises a needle valve 26 which by downward movement (as shown) closes towards a seating 27 to reduce flow through entry port 28 which connects with pressurised gas supply from the conduit 11. Outlet is through exit port 29 which connects with the gas-lift supply line 13 to the annulus 6.

The needle valve 26 is integral with a spindle 30 which carries a drilled flange 31 and has a screw thread portion 32. The body of the valve 12 houses two pegs 33 and 34 which pass through drillings 35 and 36 of the flange 31, to prevent rotation of the spindle 30 and also mount a cage 37 for a bevel gear 38. The bevel gear is internally screw threaded to mate with the screw thread 32 of the spindle 30, axial movement of the gear 38 being prevented by the cage 37.

An electric motor 39 drives a bevel gear 40 which meshes with the bevel gear 38. Thus rotation of the electric motor 39 in one direction moves the needle valve 26 towards the closed position, and rotation in the other direction moves the needle valve 26 towards the open position. The direction of the electric motor 39 is determined by electronic apparatus within a control box 41 which is located in the gas-lift apparatus enclosure 42 and is supplied by electric power from a source 43. The control box receives setting signals from a remote position through a communication line 44 into the comparator boxes 45, 46, 47, 48 and 54, which output to the control box 41.The comparator boxes have interval timings settable through the communications link 42, which enables comparisons followed by any adjustment outputs to be made at preset time intervals which may, if required, be zero for continuous monitoring, comparison, and correction.

Five modes of control are described as follows, any one or more of which may be used to control gas-lift:a) Mode 1 Simple annulus gas pressure control as sensed by transducer 49 which connects with conduit 13. This enables the annul us pressure to be accurately maintained at a preset value in spite of variations'of flow and other changes.

The pressure sensed by the transducer 49 is sent to the comparator box 48 and any error on the setting results in correcting movement of the choke valve through the control box 41 and the motor 39.

b) Mode 2 Differential pressure between annulus and production tube is sensed by transducer 50 which connects with conduits 13 and 16. This enables the differential pressure to be accurately maintained at a preset value in spite of variations of flow and other changes. The differential pressure sensed by the transducer 50 is taken to the comparator box 47 and any error on the setting results in correcting movement of the choke valve through the control box 41 and the motor 39. The interval timer in the comparator box 47 makes comparisons with resulting adjustment only once every minute, at intervals determined by a setting through the communication line, say, to enable the flow conditions resulting from corrections to have settled down to steady values before the next comparison of transducer output with setting is made.

c) Mode 3 Liquid content of the flow from the production tube in conduit 16 is sensed by a through-flow transducer 51. This enables the actual liquid content to be compared and maintained at a preset value set through the communication link 44. It also has the feature of enabling the current liquid content to be compared with a previous reading to enable gas-lift to be regulated to optimise - i.e. to increase, decrease, or maximise - liquid flow from the production tube. The liquid content sensed by the transducer 51 is signalled to comparator box 46 and any error on the setting results in correcting movement of the choke valve 12 through the control box 41 and the motor 39.Comparator box 46 has the additional feature of being settable to maximise liquid content, in that it remembers previous liquid content readings, which are taken at intervals, and the adjustments are made so that on receipt of signal from the transducer 51 it compares the liquid content reading with the previous liquid content reading and then it determines whether to open or to close or to maintain the position of the valve 12 to optimise - i.e. to increase or maintain at maximum - the liquid content in delivery from the production tube 4.

d) Mode 4 The ratio of liquid content to gas content is sensed by transducer 52. This enables the actual ratio to be compared and maintained at a preset value sent through the communication link 44. It also has the feature of enabling the current liquid/gas ratio to be compared with a previous reading to enable the gas-lift to be regulated to increase, decrease, or maximise the liquid/gas ratio of the flow from the production tube. The liquid/gas ratio sensed by the transducer 52 is signalled to the comparator box 45 and any error on the setting results in correcting movement of the choke valve 12. Comparator box 45 is settable to increase the ratio of liquid/gas content to a maximum.

The box memorises previous liquid/gas content readings, taken at intervals, and makes adjustments so that on receipt of signal from the transducer 52 it determines whether to optimise - i.e. to open, close, or make no adjustment to the valve 12 to increase or to maintain at maximum the liquid/gas ratio in delivery from the production tube 4.

e) Mode 5 The pressure at the top of the production tube is sensed by transducer 53 through the connecting branch of the flow-line 16 from the bore 10. This enables the output pressure from the production tube to be accurately maintained at a value set remotely through the communication line 44 in spite of variations of flow and other changes. The output pressure sensed by the transducer 53 is taken to the comparator box 54 and any error on the datum setting results in correcting movement of the choke valve through the control box 41 and motor 39.An interval timer in the comparator box 54 makes comparisons, with resulting adjustments being made at intervals as determined by a setting through the communication line 44, say only once every minute, to enable the flow conditions and pressure resulting from a correction to have settled down to steady values before the next comparisons and any required adjustments are made.

The mode is selected according to signals received from the communication line 44, which includes instructions regarding time intervals, datum settings, and whether the control is to operate on datum settings or is to optimise as described under sub-paragraphs (c) and (d) above.

Figure 2 The apparatus is similar to that of Fig.l except as described below.

The electric motor 39, which adjusts the needle valve 26, is controlled by electronic apparatus within the control box 41 located in the enclosure 42 and supplied by electric power from the source 43. The control box 41 receives setting signals, and other instructions, e.g. mode instructions, from a remote position through the communication line 44 into the comparator boxes 45 and 46. Box 46 outputs to the control box 41 through the line 108.

The comparator boxes 45 and 46 include interval timings which are settable (from a maximum to zero) through the communications link 44, which enables comparisons followed by any adjustment outputs to be made at preset time intervals. Such time intervals may, if required, be zero, thus providing continuous monitoring, comparison and correction.

Four modes of control are described as follows, any one or more of which may be used to control gas-lift: a) Mode 6 Simple regulation of gas-lift flow rate to the annulus in spite of variations of flows, pressures and other parameters. A venturi-type transducer 101 is installed in the flow path 13 of gas-lift from the choke 12 to the drilling 9 connecting with the annulus 6. Pressure is sensed at the throat of the venturi restriction and at the full bore inlet to the restriction to act on the differential pressure transducer 102, the output signal from which represents flow rate of gas-lift gas supply and is taken by the line 103 to the comparator box 46. In mode 6 the desired datum setting for gas-lift flow rate, the first datum, is alterable according to signals from differential transducer 102 which are compared with the datum setting.Any error results in instructions being sent through the line 108 to the control box 41, leading to adjustment of the choke to re-adjust gas flow rate to the datum setting. The range of datum flow rate settings is limited to a maximum permitted value.

b) Mode 7 The flow rate of output from the production tube through the line 16 is sensed by the venturi type transducer 104. Differential pressure is sensed at the throat of the venturi and at the full bore inlet by the differential pressure transducer 105. The output signal from the transducer 105, representing flow rate from the production tube, is communicated to the comparator box 45 through the line 106. In mode 7 a signal through the communication line 44 adjustably sets a datum for output flow rate, the second datum. The signal from the transducer 105 is compared with the second datum in the comparator box 45, any error causing output through line 109 which changes the datum setting for gas-lift gas flow (the first datum) in comparator box 46.

Thus, the choke 12 is adjusted to achieve the gas flow required to achieve the datum set for flow rate from the production tube as described above; an increased of gas-lift gas flow increasing production tube flow and decrease of gas-lift flow having the opposite effect.

c) Mode 8 The flow rate of output from the production tube is sensed by the venturi type transducer 104 as in Mode 7 above, and the differential pressure signal from transducer 105 is communicated to comparator box 45 through the line 106.

The output signal from the transducer 102 representing the gas-lift gas flow is also taken to box 45 through the line 103 and the branch 107, where the gas flow rate is related to the flow rate from the production tube by the control system to determine the proportion of gas-lift gas in the output flow from the production tube. The control box 45 compares this ratio with a datum (the third datum) adjustably settable through the communication line 44 and through the line 109, and any error adjusts the gas-lift gas flow datum (the first datum) in the control box 46 to increase or decrease gas-lift gas flow to achieve the requisite ratio setting as described above.

d) Mode 9 Signals are taken to the comparator box 45 through the lines 106 and 107 as described with regard to Mode 8 above, and the ratio of gas-lift gas to flow from the production tube is determined by the control. In the case of Mode 9, however, the last three comparator readings are memorised together with any resulting adjustments. These values are taken into account in subsequent adjustments to increase or decrease gas-lift gas flow by altering the datum setting in comparator box 46, the first datum, in order to optimise conditions, for example to minimise the ratio of gas-lift gas to production tube output, so achieving flow from the reservoir with a minimum ratio of gas-lift gas.

An interval timer in the comparator box 45 makes comparisons with resulting adjustments at intervals as determined by a setting through the communication line, say only once every minute, to enable the new conditions and pressures resulting from a correction to have settled down to steady values before the next reading, comparisons and adjustments, if required, are made. The mode is selected according to signals from the communication line 44 which includes instructions regarding time intervals, datum settings, and whether the control is to operate on datum settings or is to adopt maximising, or optimising, procedures.

Thus the control described achieves the following objectives: The rate of flow of gas into the annulus for gas-lift is accurately controlled according to a first datum setting and is not affected by pressure and flow changes in the production tube.

The flow rate from the reservoir may be controlled according to a second datum setting in a control which acts on the first datum setting to increase or decrease gas-lift flow rate to achieve the required flow rate from the reservoir.

The proportion of gas-lift gas in the output from the production tube may be controlled by a third datum setting in a control which acts on the first datum setting to increase or decrease gas-lift flow rate to achieve the desired ratio of gas-lift flow rate to production tube flow rate.

The proportion of gas-lift gas in the output from the production tube may be controlled to a minimum by optimising procedure effective on the first datum setting to increase or decrease gas-lift flow rate.

Figure 3 The down-hole completion of Fig. 3 is similar to that of Fig. 2 except as described below.

Five modes of control are described, any one of which may be used to control gas-lift: (a) Mode 10 As described above under Mode 6 in relation to Fig.2.

(b) Mode 11 The flow rate form the reservoir into the production tube 4 is sensed by the venturi type down-hole transducer 204 instead of at the well head. Pressure is sensed at the throat of the venturi and at the full bore inlet, to act on the differential pressure transducer 205. The output signal from the transducer 205, representing flow rate from the reservoir; is communicated to the comparator box 45 through the line 206. In this mode, a signal through the communication line 44 adjustably sets a datum for flow rate from the reservoir, the second datum.The signal from the transducer 205 is compared with the second datum in the comparator box 45, any error causing output through line 109 which changes the datum setting for gas-lift gas flow (the first datum) in comparator box 46 so that the choke is adjusted to achieve the gas flow required to achieve the datum set for flow rate from the reservoir into the production tube as described above. A decrease of gas-lift gas flow decreases production tube flow rate and an increase of gas-lift flow has the opposite effect, up to the point at which excess gas in the flow lines 16 and 25 has the effect of restricting flow from the production tube 4.

The control system may be manual in that the operator may read flow rate from the reservoir from the transducer 205 and adjust it as required by resetting of the first datum.

(c) Mode 12 Previous readings from the transducer 205 and 102 and adjustments made are stored in memory. Readings from transducer 205 is taken and compared.

The stored readings, and appropriate adjustments automatically made to the first datum setting, are taken into account, and should flow from the reservoir not be at a maximum the control decides upon and takes appropriate action by adjustment of datum setting for gas-lift flow.

For example, if the reading from transducer 205 is greater than the previous reading, the memory is searched to see how the gas-lift supply was adjusted to obtain this increase - increased or decreased - and a small adjustment in the same direction is taken. If, on the other hand, the reading from the transducer 205 is less than previously, the memory is searched to see what action was last taken to cause this reduction, and a small amount of opposite adjustment is made.

(d) Mode 1 3 The flow rate of output from the reservoir is sensed by the venturi type transducer 204 as in Mode 9 above and the differential pressure signal from transducer 205 is communicated to comparator box 45 through the line 206.

The output signal from the transducer 102 representing the gas-lift gas flow is also taken to box 45 though the line 103 and the branch 107, where the gas flow rate is related to the flow rate from the reservoir by the control system to determine the proportion of gas-lift gas in the output flow from the production tube. The control box 45 compares this ratio with a datum (the third datum) adjustably settable through the communication line 44 and through the line 109 and any error adjusts the gas-lift gas flow datum (the first datum) in the control box 46 to increase or decrease gas-lift gas flow and thus achieve the requisite ratio setting as described above.

(e) Mode 14 Signals are taken to the comparator box 45 though the lines 206 and 107 as described with regard to Mode 11 above, and the ratio of gas-lift gas to flow from the production tube is determined by the control. The control retains in memory the last three comparator readings, and the resulting adjustments made, and decides whether to increase or decrease gas-lift gas flow by altering the datum setting in comparator box 46, the first datum, in order to optimise conditions, for example to minimise the ratio of gas-lift gas to production tube output, so achieving flow from the reservoir with a minimum ratio of gas-lift gas.

An interval timer in the comparator box 45 makes comparisons with resulting adjustments at intervals as determined by a setting though the communication line, say only once every minute, to enable the new conditions and pressures resulting from a correction to have settled down to steady values before the next reading, comparisons and adjustments, if required, are made. The mode selected is according to signal from the communication line 44 which includes instructions regarding time intervals, datum settings, and whether the control is to operate on datum settings or is to adopt maximising, or optimising, procedures.

The flow rate from the reservoir may be controlled according to a second datum setting in a control which acts on the first datum setting to increase or decrease gas-lift flow rate to achieve the required flow rate from the reservoir, or to maximise flow rate from the reservoir. This procedure may be extended to maximise flow rate from the reservoir.

The proportion of gas-lift gas in the output from the production tube may be controlled by a third datum setting in a control which acts on the first datum setting to increase or decrease gas-lift flow rate to achieve the desired ratio of gas-lift rate to production tube flow rate.

Figure 4 The arrangements illustrated in Fig. 4 are similar to those of Fig.2 except as explained below.

For effective management of gas-lift wells feeding into a common manifold it is necessary to have information on the flow of product from each reservoir individually. Current practice in this regard uses individual test lines from each well feeding to a test separator at the platform, such that the output from each well is charted against gas-lift gas supply to the well, in accordance with practice when individual risers from each well are taken to the platform. This procedure however is not precise as subsequent back pressures on the output from a well due to several wells feeding to a common manifold, including fluctuating interaction, alter the output charted for any given gas-lift gas supply.

For good management of the well it is desired: (a) To be able to adjust the flow rate, preferably the mass flow rate, of gas for gas-lift, but for the flow rate to be substantially constant when set according to the setting, and for this flow rate to be maintained.

(b) That the flow rate, preferably the mass flow rate, from the production tube be measured and the flow rate (mass flow rate) of the gas-lift gas be subtracted to determine product delivery from the reservoir.

(c) That the product delivery from the reservoir be adjusted by regulation of the gas-lift gas flow rate to obtain conditions required, eg to maximise product delivery in relation to gas-lift gas flow or according to best utilisation of available gas supply for the cluster of wells.

(d) That the measuring and monitoring means used be simple and cost effective, preferably not involving the complex techniques of multi-phase flow-meters as capacitance measurements and gamma ray absorption.

(e) That control of well output by choking between production tube and manifold be rendered unnecessary so avoiding the situation of encouraging flow by gas-lift only to do the opposite for control purposes - ie that the product delivery from the reservoir be adjusted solely by regulation of gas-lift gas flow rate as in (c) above.

(f) That information on gas-lift gas flow rate, preferably mass flow rate, and product delivery rate be available to operators to enable them to control the well by manual adjustment of settings or to place the well in automatic control according to their desired parameters.

(g) That in regard to automatic control some elements which may require maintenance or change, as software, be located at the operator's position, eg at the platform, with control loop closed by the signalling line, as by fibre optic or multiplex transmissions.

(h) That the information on product delivery be capable of analysis in terms of mass flow of liquid product from the reservoir, and mass flow of natural gas released from the reservoir with the liquid product.

In the installation of Fig. 4, the flow of gas to the annulus is again sensed by a venturi 101. The pressure differential between entry and throat is measured by the differential pressure transducer 102 to determine gas velocity in the throat of the venturi 101. The output from 102 passes by the line 110 to the computer box 111 which also receives information on pressure applicable at the throat through the line 112, and on temperature applicable at the throat through the line 113 to convert the velocity signal received through line 110 to mass flow rate. The calculated mass flow rate is signalled through the line 103 and 107 to the control boxes 46 and 45 which are described later.

Similarly output from the connecting bore 10 of the production tube to the conduit 16 passes to the manifold 17 through the venturi type flow measurement transducer 104. The pressure differential is measured by the differential pressure transducer 105 the output from which passes by the line 114 to the computer box 115 which also receives information on pressure applicable at the throat to the venturi 104 through the line 116 and of temperature applicable at the throat through the line 177 to convert the signal received through the line 114 into a mass flow signal through the line 106 to the control box 45 which is described later.

The control box 45 can transmit values received through the lines 106 and 107 to the remote control area (eg the platform) through the communicating line 44 for assessment by operators on any adjustments required, or, in automatic mode, for automatic action following comparison with datums set for required performance.

The operation of the choke valve 12 in conjunction with the transducer arrangement 101 and 102 and the control box 46 is as follows: A datum for mass flow of gas-lift gas is set in the control box 46. The valve 12 is partially open to permit gas flow from the conduit 11 to the conduit 13.

This flow causes a pressure drop in the venturi between inlet and throat according to the velocity of the flow. The differential pressure is measured by the differential pressure transducer 102. From this mass flow rate is obtained as follows: The density of the gas at the throat is computed according to gas laws and the gas constant R, in that density = mass/volume - P/RT where P is absolute pressure R is gas constant T is absolute temperature This is calculated in the computer box 111, which is provided with data as to the cross sectional area of the throat of the venturi 101. Mass flow rate is thus calculated as velocity multiplied by throat area multiplied by density, and the result is communicated by the line 103 to the comparator box 46.

The comparator box 46 compares the mass flow rate information from the line 103 with datum setting for required mass flow rate and any error is signalled through the line 108 to the control box 41 to adjust the opening of the choke valve 12 to obtain and hold the desired rate of mass flow, regardless of input gas pressures or pressure in the annulus 6. Thus the rate of mass flow into the annulus is maintained constant at the desired value and is not affected by changes of pressure, as for example by changes of pressure in the production tube, and that rate of mass flow is adjustable by reset of datum either from manual or from automatic command into the comparator box 46.

The output from the production tube comprises liquid product from the reservoir, and gas product from the reservoir in known proportions so that the density of fluid from the reservoir is known. The gas-lift gas supplied is determined by the transducer 101 as described above and the density of the gas at the temperature of the flow through the transducer 104 is calculated from gas laws. The combined output passes through the venturi type transducer 104 from which differential pressure is measured by the differential pressure transducer 105.

The flow measurement through the transducers 104/105 is integrated over time intervals to avoid errors which may be caused by the flow through it not being consistently homogeneous.

The output signal from the transducer 105 passes to the computer box 115 together with signals of pressure and temperature as described above to signal the line 106 to the comparator box 45.

The comparator box 45 also receives, through the line 107, information on the mass flow rate of gas-lift gas supply which it uses, in conjunction with gas density at the temperature of the flow through the transducer 104 and with the known density of product from the reservoir at the temperature of flow through the transducer 104, from which the mass flow rate of gas through the transducer 101 is subtracted to give mass flow rate from the reservoir.

This mass flow rate from the reservoir is signalled through the line 44 to the operator at the platform who may decide to adjust output by signalling change of gas-lift gas mass flow rate by change of datum in comparator box 46.

Alternatively, in automatic mode, a datum setting, adjustable from the platform, of desired mass flow rate of product may be set in the comparator box 45 and compared with the actual mass flow rate of product signalled through the line 106, any errpr causing signal through the line 109 to change the datum setting for gas-lift gas mass flow rate to cause increase or decrease of product mass flow rate to achieve product flow according to its datum setting.

It will be appreciated that due to the capacitance of gas in the annul us 6 and due to the volume of fluid in the production tube 4 and the rates of fluid flow some time elapses between a change of gas-lift gas supply by regulation of the choke valve 12 and the time when steady-state conditions are reached of the new rate of flow into the conduit 16. Accordingly, in automatic mode, a time delay is provided between the time when a change is made to the first datum setting (for gas-lift gas mass flow rate) and the time when any error remaining on the second datum setting (for desired mass flow rate of reservoir product) can cause another adjustment of first datum setting.

While for convenience of illustration the comparator box 45 is shown within the same control box 42 as the comparator box 46, the comparator box 45 may more conveniently be situated at the platform, for example to facilitate any change of software that may be required with communications as necessary through the communications line 44.

Where it is preferred to have information on the liquid content from the reservoir separated from the content of gas release from the reservoir this may be easily calculated from knowledge of the GOR (gas/oil ratio) of the well as the density of the liquid (oil) is known, and the density of the gas is calculated from the gas equation (144 pv = mRT) where p and T are known to the comparator box 115 (through lines 116 and 117 respectively).

Thus the control achieves the following objectives: (a) For a given datum setting gas-lift supply to the production tube is maintained constant to achieve stability of control.

(b) The mass flow rate of gas supplied for gas-lift is measured.

(c) The resulting mass flow rate from the production tube is also measured and the mass flow rate from the reservoir calculated by substraction of gas supplied for gas-lift.

(d) Accordingly, the output from the well is controlled by control of gas-lift supply which may be adjusted by manual control of datum setting or may be automatically controlled by automatic adjustment of datum setting.

(e) No choke control from production tube to manifold is required additionally.

(f) Simple transducers are used not requiring expensive techniques as capacitance measurements or gamma ray absorption.

(g) Full information of well flow conditions, including temperatures, and pressures are available to operators for the management of wells.

(h) Where preferred, control elements, as software, may be located where accessible, eg at the platform, for ease of change and maintenance.

In a preferred embodiment all signals from the transducers are taken back to the position of operator control, as a platform, for processing.

Claims

1. A gas-lift well having a production tube which travels alongside an annulus or similar passage through which gas is fed to the production tube, in which the supply of gas to the annulus is automatically regulated via a feedback loop.

2. A gas-lift well according to Claim 1, in which the feedback loop is arranged to regulate the supply of gas depending upon sensed annulus gas pressure.

3. A gas-lift well according to Claim 1, in which the feedback loop is arranged to regulate the supply of gas depending upon differential annulus/production tube pressure.

4. A gas-lift well according to Claim 1, in which the feedback loop is arranged to regulate the supply of gas depending upon liquid content of the production tube flow.

5. A gas-lift well according to Claim 1, in which the feedback loop is arranged to regulate the supply of gas depending upon the liquid/gas ratio of the production tube flow.

6. A gas-lift well according to Claim 1, in which the feedback loop is arranged to regulate the supply of gas depending upon the pressure of the production tube flow.

7. A gas-lift well according to Claim 1, in which the feedback loop is arranged to regulate the supply of gas depending upon flow rate of the production tube flow.

8. A gas-lift well according to Claim 7, in which the supply of gas is regulated depending upon mass flow rate of the production tube flow, calculated from sensed pressure drop and temperature of the gas.

9. A gas-lift well according to Claim 7 or 8, in which the feedback loop is arranged to regulate the supply of gas depending upon the flow rate of the product in the production tube, prior to addition of gas-lift gas.

10. A gas-lift well according to any preceding claim, in which the supply of gas to the annulus is automatically regulated to bring the flow rate of gas to the annulus towards a target datum setting.

11. A gas-lift well according to Claim 10, in which the supply of gas is automatically regulated depending upon the mass flow rate of gas to the annulus, calculated using sensed pressure drop and temperature of the gas.

12. A gas-lift well according to Claim 10 or 11, in which the target datum setting is adjustable from a remote location.

13. A gas-lift well according to Claim 10, 11 or 12, in which the datum setting for the gas flow rate is limited to a preset maximum.

14. A gas-lift well accordingto any of Claims 10 to 13, in which the feedback loop is arranged to vary the datum setting for the gas flow rate.

15. A gas-lift well substantially as described with reference to any of Figures 1 to 4 of the drawings.