CA1201789A - Metering separator for determining the liquid mass flow rate in a gas-liquid oilfield production stream - Google Patents

Abstract

"METERING SEPARATOR FOR DETERMINING THE LIQUID MASS
FLOW RATE IN A GAS-LIQUID OILFIELD PRODUCTION STREAM"
ABSTRACT OF THE DISCLOSURE
The separator accepts both the casing annulus and tubing production streams issuing from an in situ combustion project pro-duction well. Most of the gas in the flow is first separated from the liquid and solids, by centrifugal action. This gas is removed from the separator. The remaining mixture is accumulated until a pre-determined weight is in hand, at which point the accumulated batch is dumped. The time taken to accumulate the batch is noted. Com-putational elements then determine the mass flow rate of the dumped mixture.
The separator comprises an upright cylindrical vessel having tangential inlet means at its upper end and a dump outlet at its base. The gas-liquid-solids production mixture enters the vessel chamber through the inlet means and is spun, to form a central gas vortex and an outer, downwardly descending liquid-solids mixture. The separated gas leaves the chamber through an outlet line at the upper end of the vessel. The liquid-solids mixture accumulates as a batch in a collection zone in the lower end of the vessel chamber. Elements are provided for maintaining a generally constant backpressure in the vessel, to provide the drive needed for discharging the batch of liquid-solids mixture therefrom. A pair of pressure sensors are positioned in the chamber, one at the base of the chamber and the other part way up its length. These pressure sensors are connected with a unit adapted to measure the pressure differential and emit a signal indicative thereof. When the pressure differential signals reach a pre-determined value, a dump valve in the outlet line is opened by elements responsive to the signals. When the pressure differential signals reach a pre-determined minimum value, the dump valve is closed. A computer is utilized to calculate the liquid mass flow rate from the weight and time measurements.

Description

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1 BAGKGROUND OF TWE I~VENTION

2 (a) Field of the Invention

3 The present invention relates to a metering separator

4 adapted to first separate off the gas from a stream containing both gas and liquid~ and then meter the liquid to provide a mass flow rate 6 thereof.
7 More particularlyg the assembly comprises: means for 8 separating and removing the greatest part of the gas from the stream;
9 means for accumulating the de-gassified liquid; means For monitoring the increasing weight of the accumulating batch; means for dumping 11 the batch when its weight reaches a pre-determined value; means for 12 monitoring the time taken to accumulate the batch; and means for 13 computing the mass flow rate of the liquid from the weight and time 14 information so derived and displaying the results.

(b) Prior Art 16 The present invention has been developed in connec-tion 17 with monitoring the production from wells in an in sitll combustion 18 project. While the invention is expected to find application as a 19 meter outside this particular type of operation, the problems associated with metering combustion project streams will be discussed below, to ~1 illuminate the qualities sought in the development of the invention.
22 Combustion project production wells produce streams 23 containing a mixture of gases, oil, water and solids. The proportions 24 of these components vary constantly and over quite wide ranges.
The wells frequently are designed to produce from both the 26 casing annulus and the tubing. The annulus stream is usually mainly 27 gas, but can contain substantial amounts of liquid. The tubing flow 28 is usually liquid but can contain substantial amounts of gas. It is 29 desirable to meter the total combined gas content of both streams and the total combined liquid content of them.

1 As the annulus stream is usually mainly gas~ it will 2 create operating difficulties if introduced into most separators.
3 Thus this stream is commonly routed directly to the flare line ands 4 in many cases3 no measurement of its quantity or rate is made.
The "liquid fraction" in the two streams commonly comprises 6 viscous emulsions9 which comprise oil, water and gases~ Volume 7 measurement is therefore ineffective, because of the unknown quantity 8 of contained gas. In addi-tion~ flow of the viscous liquid fraction 9 through pipelines and meters is complex~ laminar flow and globular flow cause differential flow velocities of the various cornponents 11 (free water, free gas and emulsion)~ so that one velocity measure-12 ment of the fluids in a line or meter is meaningless, even if the 13 density of the mixture could be determined.
14 It is desirable to meter this gas content in the production.
Therefore it is a preferred object of this invention to provide a 16 separator adapted to accept and meter both the casing and tubing flows 17 at the same time.
18 One prior art device which has found commercial application 19 in connection with metering the tubing stream of an in situ combustion production well can be re-ferred to as a pivoting bucket meter. This 21 meter involves a V-shaped container having two side-by-side, open-22 topped compartments. The container is pivotally mounted at its base, 23 so that each compartment can tip back and forth between -fill and 24 discharge positions. In the fill position, one of the compartments is positioned beneath the outlet of the production flowline. In the 26 discharge position, the filled compartment is tipped to dump its con-27 tents. While one compartment is filling, the other is discharging.
28 The container is counterbalanced in such a way that it requires the 29 accumulation of a certain weight of fluid in the compartment before the container will pivot.

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1 In the use of the pivoting bucket meter, the number of 2 dumps~ occurring in a certain time period~ are counted. The mass 3 flow rate of the liquid can be approximated by calculations based 4 on the weight and time data so obtained.
The pivoting bucket meter has been associated with certain 6 problems, when used in connection with the production from an in situ 7 combustion project. These problems mainly arise from the relatively 8 high concentrations of gas in the production stream. When the stream 9 is de-pressurized, by discharging into the open-topped compartment~
much foam is generated. The possibility then exists that the pro-11 duction will overflow the compartment, without enough weight having 12 been accumulated to pivot the compartment to the discharge mode.
13 This of course deleteriously affects the metering operation and 14 creates an undesirable spill.
The patent prior art discloses the concept of first 16 separating the gas from the production stream and then metering the 17 residùe liquid. This is, for example, disclosed in United States 18 patent No. 2,936,622, issued to Glasgow. The Glasgow reference is 19 of interest because it teaches centrifuging the production stream in an upper chamber, to separate gas from the liquid. The gas is vented 21 through a top outlet~ The residue liquid then passes through a 22 transFer line into a second chamber positioned beneath the first.
23 Here the liquid is accumulated until it contacts and raises a float.
24 The movement of the float initiates the closing of a valve in the transfer line and the opening of a dump valve in an outlet line from 26 the second chamber. When the liquid has substantially drained from 27 the second chamber, a second float at the base of the chamber is 28 lowered and causes reversal of the valves.

1 In summary, the Glasgow unit couples gas separation with 2 float-controlled volume metering.
3 Separators which use floats as the controlling means have 4 been found wanting when used to meter in situ combustion project streams.This is because the gas in the emulsion is difficult to remove completely 6 and foam is still present in the metering chamber. This foam will 7 activate the float prematurely and result in an inaccurate reading of 8 the true liquid volume being passed.
9 A third approach to metering this type of production involved simply producing it into a storage tank and timing the 11 accumulation. The production then is held in the tank long enough 12 to allow the bulk of the gas to break out and be vented~ The residue 13 is then measured and centriFuging of samples will give a breakdown 14 of the oil9 water and solids. With this information, the mass flow rate of the oil can be calculated.
16 However, while accurate, this type of metering yields 17 data that may be several days old. It is preferable that the 18 information be as current as possible, for purposes of analyzing well 19 pumping problems and understanding what may be taking place in the ~sub-surface reservoir.
21 There is thus still a need for a device capable of 22 accurately monitoring the liquid content mass flow rate of a gassy 23 oil stream, such as the combined casing and tubing production of 2~ an in situ combustion project production well.

SUMMARY OF THE INVENTION
26 The present invention is based on the concept of uniting, 27 in one vessel,means for conducting the steps of:

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1 (1) separating by~centrifuyal action substantially all 2 of the gas in the stream from the liquid, with 3 concomitant remo~al of the gas from the vessel as 4 an overhead stream;
(2) accumulatiny the de-gassified 1iquid in a collection 6 zone positioned beneath the gas separation ~one;
7 (3) monitoring (that is, repeatedly making a measure of) 8 the increasing weight of the accumulating batch of g liquid~ until it reaches a pre-determined value~ then lo opening an outlet in the base of the vessel to dump 1l the batch from the chamber, and monitoring the de-12 creasing weight of ,that part of the draining batch which remains in the chamber, until it reaches a 1~ pre-determined minimum value, and then closing the outlet, at which time the procedure is repeated;
16 (4) maintaining a generally constant backpressure in 17 the chamber throughout and using this pressure to 18 quickly dump the batch;
19 (5) noting the time taken to accumulate each batch;
(6) and computing the approximate mass fl,ow rate o-F, 21 the liquid using the weight and time information so 22 acquired~
23 The separator incorporating this combination of features 24 has been shown to be capable o-F handling simu'ltaneously both the annulus and tubing streams.
26 In the fl,oat-operated rneasuring systems~ the upper ~loat 27 must be positioned at the l.evel point reached by the upper surface of 28 a batch of pure liquid having the desired weightu In contradistinction~
29 in the applicant's separator, wherein differentia'l pressure sensors are preferably used, the upper sensor may be positioned at a point we'l'l 31 above where the liquid column would reasonably be expected to reach, 7~

1 as this sensor functions only to register the gas backpressure in the 2 chamber. Thus, the varying height of the batch columns does not 3 cause premature and misleading triggering of batch dumping.

4 DESC~IPTION OF THE DRAWING
Figure 1 is a perspective view showing the separator, 6 including its inlet and outlet lines and controls, the sidewall of 7 the separator vessel being partly broken away to show the vessel 8 internals; and 9 Figure 2 is a schematic showing the controls and micro-processor.

jj DESCRIPTION OF THE PREFERRED EMBODIMENT
12 Having reference to Figure 1, the separator 1 comprises 13 a vertical, cylindrical vessel 2 closed at its top and bottom ends 1~ and forming an internal chamber 3.
Mounted within the chamber 3 adjacent its upper end is 16 an involute inlet assembly 4 of known design. This assembly 4 com-17 prises a vertical, spirally extending wall 5 which cooperates with 1~ transverse upper and lower walls 6,7 and the vessel side wall 8 to 1~ form an involute passageway 9 of expand;ng section. The passageway g terminates in an outlet 10, which communicates with the lower end 21 of the chamber 3, referred to hereinafter as the liquid collec~ion 22 zone 11. A production inlet pipe 12 extends through the vessel side 23 wall 8 and discharges tangentially against the inner end of the spiral 24 wall 5.
Thus the production stream can be introduced by the inlet 26 pipe 12 to contact the spiral wall 5. The stream follows the wall 5, 27 which induces it to spin. As the "diameter" of the spiral wall 5 28 at its inner end is relatively small, the stream is spun at high 29 velocity at the inner end of the passageway 9. This causes most of the contained gas to break out and form a gas vortex 13, which moves 31 upwardly into the upper end 1~ of the chamber 3. The remaining 1 components of the stream (referred to hereinafter as "the liquid") 2 continue through the passageway 9 at a diminishing velocity, as 3 they spread out through its expanding cross-sectional area. The 4 liquid stream exits from the passageway 9 through the outlet 10 and ~ollows a downwardly-descend;ng spiral path along the inner surface 6 of the vessel side wall. It accumulates as a batch 15 in the liquid 7 collection zone 11.
8 An outlet line 16 extends from the upper end of the vessel 9 2 and communicates with the upper end of the chamber 3r The separated gas exits from the vessel 2 through this line.
11 A known orifice gas flow meter 17 is mounted in the 12 outlet flowline 16. This meter 17 is operative to generate electrical 13 signals indicative of the flow rate of the gas passing through the 14 line 16. These signals are transmitted through the cable 1~ to the microprocessor 19.
16 A backpressure valve 20 (Type 4195B~ available from 17 Fisher Controls) is also mounted in the gas outlet flowline 16. This 18 valve 20 is operative to maintain a substantially constant pressure of 19 20-S0 psi in the chamber 3~ The backpressure is required to ensure rapid discharge of each ba-~ch 15 of accumulated liquid.
21 A dump line 21 extends from the lower end of the vessel 22 2 and communicates with the chamber 3. The batches 15 of accumulated 23 liquid are removed from the separator through this dump line 21.
24 A solenoid-controlled pneumatic dump valve 22 (Type 1052, available from Fisher Control), is mounted in the line 21. This 26 valve 22 is operative, when opened, to permit liquid accumulated in the 27 collection zone 11 to discharge~ When the batch 15 of liquid is 2~3 being accumulated, this valve 22 is closed. The valve 22 is connec-ted 29 by the tubing 23 to the solenoid 24. The solenoid 24 is, in turn, con-nected by cable 25 to the microprocessor 19. As described below, the 31 microprocessor 19 is operable to actuate the solenoid 24, to open and 32 close the dump valve 22.

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Upper and lower pressure sensors 25, 26 are mounted in the 2 vessel side wall 8 and extend into the chamber 3, at points A and B
3 respectively. The lower sensor 26 is positioned in the liquid collection 4 zone 11, close to the base of the vessel 2 . The upper sensor 25 i s positioned in a gas disengagement zone 27 extending between the liquid 6 collection zone 11 and the base of the inlet assembly 4. Thus the upper 7 sensor 25 is well above the upper end of the liquid collection zone 11 8 and is exposed only to the gas pressure within the chamber 3. The g bottom sensor 26 is exposed to the combined gas pressure and the head of liquid accumulated above it.
11 The sensors 25, 26 (Type 351, available from ITT Barton 12 Instruments) are operative to transmit signals, indicative of ,the13 pressure at points A and B respectively, through the cables 28, 2914 to the differential pressure unit 30 (Model 224, available fr,om ITT
Barton Instruments). The differential pressure unit 30 is operative 16 to generate a signal indicative of the pressure differential existing 17 between points A and B, which is transmitted through cable 31 to the 18 microprocessor 19.
19 With reference to the schematic diagram of Figure 2, the microprocessor 19 comprises, in essence, a computer which is 21 operative to receive information from differential pressure cell 30 22 and the gas flowmeter 17 via cables' 31 and 18. An internal clock 23 (not shown) within microprocessor l9 records time. The microprocessor 24 is pre-programmed with data pertaining to separator vessel cross-sectional area and gas flowmeter characteristics.
26 As the differential pressure signal from the differential27 pressure cell 30 increases from its lowest value, microprocessor 19 28 measures the fill time. When the signal From differential pressure29 cell 30 reaches a pre-determined high value, the timing of the fill cycle stops electronically in the microprocessor 19, at which time 31 solenoid 24 is signalled to commence the dump cycle.

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The microprocessor 19, knowing the change in differential 2 pressure, the crass-sectional area of the vessel~ the mass of fluid, 3 and the time increment for the fill cycle, computes the mass flowrate 4 of fluid into the vessel. The microprocessor performs the mass divided by time calculation to give the mass ~lowrate. As the 6 separator performs the dump cycle during part of the fill cycle9 the 7 time increment for dumping is accounted for and computed accordingly~
8 When the dump cycle has proceeded so that the differentialg pressure cell 30 feeds a signal to the microprocessor 19 equivalent to the pre-determined low value, a signal is transmitted by the cable 11 25 to solenoid 24, which rapidly closes outlet valve 22.
12 The gas flowrate from gas outlet line 16 is similarly 13 computed in the microprocessor 19 and integrated over a period of 14 time to give a total gas flowrate.
The microprocessor 19 may be provided with digital dis-16 plays 34 and 35, to provide on-site readings of instantaneous flowrate17 and total fluid produced over a given time period.
18 A float assembly 32 (Type 244V-Model 2900~ available from 19 Fisher Controls) is mounted in the vessel wall 8 and extends into the gas disengagement zone 27. This float assembly 32 -Functions as an 21 overflow shut off. In the event that the dumping system is not working, 22 the float assembly 32 is used, in conventional manner~ to activate a 23 valve (not shown) in the production line from the well.
24 As an alternative to the pressure sensor system which has been tested and described, it is contemplated that one could mount 26 the vessel on a support assembly and use load cells associated there-27 with to monitor the increasing weight of the unit~ Flexible lines 28 would have to be used to feed and dump such a unit.
29 The scope of the invention is defined in the following claims.

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A metering separator for metering the liquid content of a production well stream containing liquid and gas, comprising:
a closed vessel forming a single internal chamber having an upper gas separation zone and a lower liquid collection zone;
means, associated with the vessel, for introducing the stream into the upper zone and inducing it to spin to form a central gas vortex and an outer, downwardly descending, liquid-containing layer and thereby separating the greatest part of the gas from the liquid-containing fraction, which latter fraction accumulates as a batch in the liquid collection zone;
outlet means, associated with the lower end of the vessel and communicating with the chamber, for removing collected liquid therefrom;
means, associated with the outlet means, for controlling liquid removal therethrough;
gas outlet means, associated with the upper end of the vessel and communicating with the chamber, for removing separated gas therefrom;
means, associated with the gas outlet means, for maintaining backpressure within the chamber;
means, associated with the vessel, for monitoring the weight of the head of liquid in the collection zone and generating signals indicative of said weight;
control means, associated with the monitoring means and the liquid removal controlling means, for receiving said signals and opening said controlling means, when the signals indicate that a batch of a first pre-determined weight has been accumulated, and closing said controlling means, when the signals indicate that the liquid still remaining in the collection zone has a weight equal to a second pre-determined value;

means for monitoring the times taken to accumulate and discharge each batch;
and means, associated with said weight and time monitoring means, for computing and displaying an indication of the mass flow rate of liquid passing through the vessel.

2. A metering separator for metering the liquid content of a production well stream containing liquid and gas, comprising:
a closed, generally cylindrical vessel forming a single internal chamber having an upper gas separation zone and a lower liquid collection zone;
means, associated with the upper end of the vessel, for introducing the stream into the upper zone and inducing it to spin to form a central gas vortex and an outer, downwardly-descending, liquid-containing layer which accumulates as a batch in the liquid collection zone;
outlet means, associated with the lower end of the vessel and communicating with the chamber, for removing collected liquid therefrom;
dump valve means, associated with the outlet means, for controlling liquid removal therethrough;
gas outlet means, associated with the upper end of the vessel and communicating with the chamber, for removing separated gas therefrom;
means, associated with the gas outlet means, for maintaining backpressure within the chamber;
first and second pressure sensors mounted in vertically spaced apart relationship in the vessel wall, the upper sensor being operative to measure only the gas backpressure in the chamber, the lower sensor being operative to measure the combined head of the gas back-pressure and the head of liquid extending above said sensor, said sensors being further operative to transmit pressure signals indicative of said measurements;
first means, operatively connected to the sensors, for establishing a measure of the differential pressure and emitting a signal indicative thereof;
second means, operatively connected to the first means and the dump valve means, for monitoring said differential pressure measurements and opening said dump valve means, when the signals indicate that a batch of a first pre-determined weight has been accumulated, and closing said dump valve means, when the signals indicate that the liquid still remaining in the collection zone has a weight equal to a second pre-determined value;
means for monitoring the times taken to accumulate and discharge each batch;
and means, associated with said weight and time monitoring means, for computing and displaying an indication of the mass flow rate of liquid passing through the vessel.

3. A method for metering, in a generally cylindrical vessel defining a single closed chamber, the liquid production of a production well producing a flow of gas and liquid, comprising:
introducing the flow tangentially into the upper end of the chamber to cause it to spin, whereby the gas forms a central vortex and the liquid forms an outer downwardly descending layer;
venting the separated gas from the chamber, while maintaining back pressure therein;
collecting the liquid as a batch in the lower end of the chamber;
sensing the pressure differential existing between a point adjacent the base of the chamber where a head, equivalent to the back-pressure plus the head of the batch, exists, and a point spaced thereabove, where the head is only equivalent to that of the backpressure;

dumping substantially all of the batch of collected liquid from the chamber when the pressure differential indicates that a desired weight of liquid has accumulated;
monitoring the times taken to accumulate and dump the said weight;
and computing and displaying an indication of the mass flow rate of the liquid passing through the vessel.