THE APPLICATIONS AND PERFORMANCEOF JET PUMPS IN OIL AND GAS PRODUCTION

HANDLING MULTIPHASE FLOWM. M. (SACHA) SARSHAR

Dr. N. A. BEGCALTEC, U.K

ABSTRACT

Boosting production and total recovery from many oil and gas fields requires theuse of a boosting system. Use of jet pumps is a cost-effective way to achieve thisobjective. Energy from a high pressure source or high pressure wells is used asthe motive flow. When multiphase flow, a mixture of gas, oil and/or water, isinvolved, the system requires a special design.

This paper describes the principle of operation, the performance, the range ofapplications of the system and how multiphase flow is handled using WELLCOMtechnology. The benefits of the system and economic advantages are alsohighlighted.

Key words: Jet pumps, eductors, ejectors, WELLCOM, multiphase pumping, boosting systems

1. INTRODUCTION

Production and total recovery from many oil and gas fields are restricted because of low reservoirpressure. Transport of produced fluids from the reservoir to production and process facilitiesrequires sufficient energy to cope with the loss of pressure along the wellbore, risers andtransportation pipelines as the case may be. An optimum operation pressure is also required atprocess plants to maintain optimum production yield and to minimise additional boostingrequirements for the oil phase and the separated gas phase which are generally transported bypipeline for export purposes. Such constraints on production pressure impose higher backpressures on wells and restrict their production rate. Under higher flowrates, the productionpressure is often too low to meet the above said requirements. Therefore in order to maintainor increase production and recovery, often a boosting system is required to compensate for thesaid losses during the field life.

Boosting systems can be installed at various points along the production path, namely: downholeat reservoir level, along the wellbore, at wellhead - subsea or topsides, at manifolds and alongthe transportation pipeline. There are also a variety of options and types of boosting systemswhich can provide the added energy. Their selection and suitability is, however, governed by avariety of technical and economic factors.

The use of jet pumps is a cost-effective way to boost production from many oil and gas fields. This paper outlines the range of their applications and focuses on cases where production of bothgas and liquid phases is involved. As separation of gas and liquid phases is necessary in manyapplications involving two-phase flow, the use of compact separators is discussed as a preferredway to achieve the said duty.

2. JET PUMP TECHNOLOGY

Jet pumps, also known as ejectors or eductors, enable energy from a high pressure (HP) sourceto be used to boost the pressure of low pressure (LP) fluids. The unit shown in Figure 1 consistsof a nozzle, a suction chamber, a mixing tube and a diffuser. High pressure fluid passes throughthe nozzle where part of its potential (pressure) energy is converted into kinetic energy (velocity). As a result, the pressure downstream of the nozzle drops. At this point, low pressure fluids areintroduced through the suction (LP) chamber. The mixture passes through a mixing tube wheretransfer of momentum and energy takes place between HP and LP fluids. The velocity of thefluids is then reduced gradually as they pass through the diffuser where further pressure recoveryis achieved.

The jet pump therefore offers two main benefits:

(a) It allows the LP fluids to flow at a lower pressure without being exposed directly to thepressure of the HP line or the downstream back pressure.

(b) The pressure of the LP fluids is increased through the jet pump, which helps in theirtransportation by pipelines or enables the required downstream operating pressure to bemet.

In both oil and gas production, the jet pump thus enables the LP wells to operate at a lowerpressure. Under the alternative conventional operation, the HP flow is choked down and, whenmanifolded with the LP flow, the LP wells will be exposed directly to the pipeline pressurewhich restricts their production.

Both motive and suction flows could be a gas or liquid phase or the mixture of the two. Theperformance of jet pumps is dependent on whether the motive or suction flow is gas, liquid ora mixture of the two phases.

Jet pumps are not new and their invention dates back to 1852 by James Thompson in England. The theory was further developed by Jim Rankin in 1870. The standard reference work wasdone by a number of researchers including Lorenz (1910) and Gosline and O�Brien (1933). In1930, six patents were granted for their application downhole. Downhole application of jetpumps has, however, become extensive, mainly since the 1970s (Ref. 1).

Most of the above applications have been related to single phase flow where both motive andsuction fluids were gas, steam or a liquid phase. Limited work has been carried out since the1950s investigating the design and performance of jet pumps handling two phase (gas and liquid)flow. Whilst the performance and design of jet pumps is reasonably well understood in singlephase operations, their performance and design under two phase operations is less understood. Also in most previous industrial applications, the jet pumps were used mainly to combine fluidsat different pressures and maximum boosting of the LP pressure was not as important as it is inthe oil and gas industry applications. This short-coming led to CALTEC carrying out anextensive programme of development work during 1993 - 98, focusing on applications relatedto the oil and gas production where both gas and liquid phases were involved and maximumboost was desired for the LP pressure. This work led also to the development of acomprehensive multiphase model and the WELLCOM� system which is described in this paper.

3. PERFORMANCE AND APPLICATIONS OF JET PUMPS IN GASPRODUCTION APPLICATIONS

In gas production applications, if no liquid phase is involved, the performance of the jet pump

is dictated primarily by two factors, namely HP to LP pressure ratioPP

LP

HP and LP to HP mass flow

ratio MM

HP

LP . These terms have been defined in the Nomenclature, section 11. The performance

of the jet pump in terms of pressure boost can be defined by the ratio of the discharge pressure

to LP pressure P

PLP

discharge . Figure 2 shows typical performance curves for a jet pump operating

under different HP to LP pressure ratios and flow ratios. The performance of the jet pump in thisapplication deteriorates if a liquid phase is also produced with the LP gas. This effect is,however, minimal if the liquid phase is below 1% or 2% by volume. Figure 3 shows the effectof liquid on a given gas-gas jet pump design (solid lines).

The main reason for the reduction in the boost achieved for the LP pressure is the additional

energy required in boosting the pressure of the liquid phase which has a significantly higher masscompared with gas. Incurring additional losses through the jet pump, because of the highermixture density, is a further reason for the drop in performance. Further increase in the liquid

phase reduces the P

PLP

discharge pressure ratio further until the jet pump becomes ineffective as a

booster and becomes mainly a commingler of the HP and LP fluids without generating any boost. If the liquid flowrate in the LP gas is known, the jet pump can be designed to optimise itsperformance as dotted lines in Figure 3 show. The solid lines show the performance of the jetpump originally designed for handling the LP gas alone. When the liquid phase in the LP flowis at levels which affect the performance of the jet pump significantly, separation of the liquidphase becomes necessary. This can be achieved simply and economically if compact separators,such as I-SEP described in section 6, are used. The excessive liquid phase in the LP gas couldalso lead to choking of the flow through the jet pump, if the unit is not designed correctly, andcould lead to severe restrictions in the LP flow.

Presence of liquid beyond 1% or 2% by volume of the HP gas (the motive flow) also affects theperformance of the jet pump, leading also to the restriction of flow through the nozzle. The useof a compact separator described in section 6 is therefore advisable in such cases.

Jet pumps have been used extensively onshore and offshore in gas production applications whereboth the HP and LP gas are relatively dry (Ref. 2 & 3).

DOWNHOLE APPLICATIONS

In downhole applications if the produced gas contains liquids (water or condensate), lifting ofthe produced liquid becomes a problem for low pressure and low producing wells. Low velocityof gas in this case could lead to the liquid phase dropping out and causing slug and churn flowthrough the wellbore, thus restricting production and also creating undesirable flow conditionsfor the downstream separation system. Injection of gas downhole through the tubing or coiledtubing increases the velocity of gas through the wellbore and helps in lifting the produced liquidphase more effectively. A gas-driven jet pump can also be used downhole in this application. Although its pressure boosting capability could be severely restricted because of presence of theliquid phase in well fluids, its ability to generate a good homogeneous gas-liquid mixture withsmall droplets of the liquid phase helps significantly in lifting the produced liquids through thewellbore effectively.

In cases where the produced fluid is water, it is feasible to use a downhole separator (such as I-SEP), and use a jet pump downhole to inject the produced water into a disposal zone in theformation (see Figure 4). Water can be used as the motive fluid in this case. Disposal ofproduced water in this way not only helps to increase production, but also eliminates theadditional cost of separation, treatment and disposal of produced water offshore or onshore. Thisconcept is relatively new, but all the components of the system are available and reliable becauseof their simplicity.

A further use of jet pumps downhole in gas production is for cases where, because of lowreservoir pressure, the produced liquids accumulate within the wellbore creating further back

pressure on the production zone and leading to the complete stoppage of production. In thiscase, a jet pump can be lowered by coil tubing and, using HP gas as the motive flow, theproduced and accumulated liquids can be lifted in a controlled way and the well can be broughtback to production cost-effectively.

4. PERFORMANCE AND APPLICATIONS OF JET PUMPS IN OIL PRODUCTION

The downhole application of jet pumps in oil production has been mainly in cases when little orno gas is present at production zone. In this case, a liquid phase (oil or water) can be used as themotive flow. In some applications involving the production of heavy oil, diluents such as naphtacan also be added to reduce viscosity (Ref. 4). If the production pressure is below the bubblepoint of the reservoir fluids, gas will also be produced. In this case, use of liquids as the motiveflow may not be so effective, while gas as the motive flow will help to reduce the hydrostatichead of fluids in the wellbore and reduce the back pressure at production zone effectively.

Beyond the wellhead, production of oil involves generally a mixture of gas and liquid phases. If the motive flow is supplied from high pressure oil wells, separation of gas and liquid phaseis required so that the liquid phase (oil and/or water) is used as the motive flow. TheWELLCOM system (Ref. 5), used for such applications, consists of three main components asshown in Figure 5:

� A compact separator� A specially designed jet pump� A commingling spool.

The compact separator removes gas from the liquid phase and enables only the liquid phase tobe the motive fluid passing through the nozzle of the jet pump. The specially designed jet pumptakes the entire multiphase flow from the low pressure well and, in a similar way as describedin section 2, transfer of energy between HP and LP fluids takes place within the jet pump. Theseparated HP gas is then combined with the flow from the outlet of the jet pump which is at alower pressure, using the commingler, without restricting this flow. All components arerelatively simple, reliable, with no moving parts, and require no active control. The stepped typevalve on the gas outlet of the compact separator is used primarily to optimise the performanceof the compact separator, and its setting needs to be adjusted only if there is a significant changein the flowrate, the pressure or gas volume fraction (GVF) of the motive flow during the fieldlife.

The performance of jet pumps in oil production application is governed by HP to LP pressureratio, LP to HP volumetric flow ratio (Mv) and the GVF of the LP fluids. The performance ofsuch jet pumps is often defined in terms of pressure ratio �N� and efficiency η (seeNomenclature section 11 for definitions).

Figures 6 and 7 show the performance of the jet pump in multiphase application for a given HPto LP pressure ratio and for different GVF values. The GVF of the low pressure flow plays adominant part in the level of boost achieved as shown in Figure 8. When the GVF of low

pressure fluids is high, a significant portion of the HP energy is spent in increasing the pressureof LP gas. It is for this reason that in applications where the GVF of the LP fluids is high, thedual WELLCOM system shown in Figure 9 becomes a more effective way to boost the LPpressure.

This system allows gas and liquid phases to be separated for both HP and LP fluids. Two jetpumps are used in this case - a gas-gas unit where HP gas boosts the pressure of the LP gas anda liquid-liquid jet pump where the HP liquid phase boosts the pressure of the LP liquids. Inorder for this system to be effective there should be a good pressure ratio and flow ratio for theHP and LP gas so that the boost in the pressure of LP gas is close to that achieved by the jetpump handling the liquid phases. Table 1 shows the comparison of a scenario where the DualWELLCOM system provides a significantly higher level of boost for the LP fluids comparedwith a single standard unit.

The importance and effect of liquid present in the gas phase, or gas in a liquid phase, is shownclearly by looking at the speed of sound through a pure gas or liquid phase and phases containinga small proportion of one phase in the other. The steep slopes of the curve shown in Figure 10at either end of the gas volume fraction axis shows the sensitivity and rapid change of the sonicvelocity in this range. For the jet pump, this could have a dramatic role in its performance if theeffect of this phenomenon is not correctly incorporated in its design and the prediction of itsperformance. This is particularly relevant to gas production applications involving the presenceof a liquid phase in the LP flow which could lead to choking of the flow through the jet pump.

5. THE ROLE OF COMPACT SEPARATOR

The need to separate gas from the liquid phase has been outlined in this paper when jet pumpsare used. Conventional gravity separators can be used to achieve the desired duties. Gravityseparators are, however, bulky and in most offshore or subsea applications are best avoidedbecause of the added size, weight and cost, in comparison with compact separators. A newgeneration of compact separators is becoming available which provide the specified duties in amuch simpler way, and require only a fraction of the space occupied by conventional separators. An example of such compact units is I-SEP compact separator (Ref. 6). It is worth noting thatseparators in such applications are required to operate under the following conditions:

(a) High operating pressure dictated by the pressure of HP fluids, as in most cases when HPwells are used to provide the motive flow, the wellhead choke should be fully open tobenefit from the wellhead pressure in its entirety.

(b) Flow fluctuations and slugging generated upstream along the wellbore, the pipeline or theriser as a result of the flow regime or the configuration of the piping system.

(c) Should require minimum or no active control to improve their reliability.(d) In gas production applications, high gas volume fractions in excess of 98% could be

involved.

6. I-SEP COMPACT SEPARATOR

I-SEP is a dual involute compact separator which generates cyclonic, high “g” forces to effect

separation. Figure 11 shows the key components of the system. The first involute generates thehigh spin and high �g� forces as the fluid mixture enters the unit. The mixture then passesthrough a short separation chamber where the spinning fluids with different densities separate. The denser phase is forced to gather along the wall of the chamber, while at the centre core thelighter phase is collected. As the fluids reach the second involute, the denser phase (liquid orsand) is captured by the specially designed involute and the lighter phase is captured within thecentre core, using a vortex finder.

This unit is very compact and has been also miniaturised for downhole applications. Aminiaturised version of I-SEP has been successfully used outside the oil industry applications indomestic bagless vacuum cleaners. Hoover�s vortex 2000 vacuum cleaners hold two compactI-SEP units operating in series and achieve in excess of 99.9% dust separation efficiency.

APPLICATIONS OF I-SEP

In oil and gas applications this unit can be used for a variety of duties including:

� Gas-liquid separation for a wide range of GVF values� Knock-out liquid from wet gas, e.g. at the inlet of gas compressors� Sand separation from gas-sand or liquid-sand mixtures� Debottlenecking where bulk of the gas can be separated upstream of an existing gravity

separator (see Figure 12)� Inside conventional separators at entry point to help with the coalescence of droplets in each

phase and the separation of gas and liquid phases. It is also an effective device to reduce thehigh velocities associated with the flow at entry to separators which is traditionally achievedfar less effectively by using conventional inlet diverters

� Knock-out the liquid carry-over in the separated gas phase from the outlet of conventionalseparators (see Figure 13)

� Applications on floating platforms, as this separator is not sensitive to the motion of thevessel.

This device has also potential in oil-water separation onshore, topsides or downhole, althoughits full range of operation and the associated separation efficiencies in this application are yet tobe fully investigated and demonstrated.

Typical I-SEP units range between 2" to 10" nominal size, which refers to their line size at entrypoint. Typical footprint ranges from 300 x 300 mm to 900 x 900 mm respectively with a similarrange for their height. In applications where high flowrates above the capacity of one unit areinvolved, two or more units can operate in parallel. In downhole application, severalminiaturised units can be stacked up forming a slim-line separation string which can fit inside7" and 9” tubing or casing.

THE PERFORMANCE OF I-SEP

The performance of practically all compact separators is affected by the fluctuations in thepressure and the upstream flow regime. The result is the carry-over of one phase into another.

I-SEP is no exception and, depending on the severity of the flow fluctuations at entry to the unit, some gas will be carried over in the separated liquid phase, and some liquid will be carried overin the gas phase. The amount of carry-over varies generally between a few percent to as high as10% or more of one phase into the other. In cases where flow fluctuations are minimal, as in thecase of some high GVF values in excess of 95% and at high velocities, the carry-over is expectedto be no more than 2% or 3%. It is also possible to achieve a high level of purity in a separatedphase (whichever is preferred) beyond 98% to 99%. This is achieved by design or by the controlof the back pressure on the relevant outlet line. So, in applications where - for instance - thepurity of the liquid phase is desired, this can be achieved by the said method and by toleratingfurther carry-over of liquid in the separated gas phase as a result of imposing an additional backpressure on the liquid outlet line.

Further development of I-SEP has led to the inclusion of an add-on patented feature to improveits efficiency significantly under flow fluctuation conditions. The add-on feature removessuccessfully all the liquid likely to be carried-over in the separated gas phase and enables bothphases to achieve a purity of 99% or better. I-SEP works well in a wide range of gas volumefractions; typical range is 50% to 99%. It also has a good turn-down capability. In most casesits performance is within the accepted range with turn-down ratios of up to 5 to 1. The separatoroperates without the need for any level control or active pressure control. It is, however, requiredto be able to adjust the back pressure on each outlet line to optimise its performance when thereis a major change in the flowrate or the GVF of the mixture during its field life.

Pressure loss through the system is moderate and ranges, in general, between 0.2 to 2 bardepending on the flowrate, the pressure and the mixture density as dictated by the GVF. At highGVF values beyond 90%, the pressure loss is expected to be below 1 bar in general.

7. ECONOMICS

Jet pumps are low cost items in any application, compared with alternative boosting systems. Thefield applications so far have proved this point as, in practically all cases, the recovery period forthe capital has been a matter of a few weeks to a few months. Figure 14 and Table 2 showtypical examples of the increase in production and capital recovery period. Figure 14 shows anumber of scenarios in oil production where added revenue ranging between �5 to �10.4 millionhas been made with oil price taken at $10/bbl. The short payback period confirms that even ifthe duration of using the system is short because of rapid decline in the pressure of HP or LPwells, or for any other reasons, still the use of the jet pump is justified and makes an economicsense. For this reason, even the deferment of using alternative costly boosting systems such ascompressors or multiphase pumps, leads to cost saving.

8. BENEFITS

The main benefit of the jet pump in both oil and gas production is the increase in the productionfrom LP wells with the added possibility of increasing the total recovery from the field. Other

benefits include one or more of the following:

� Increase in the pressure for transportation of produced fluids by pipeline� Could eliminate the need for separate LP pipeline or LP process train in some applications� Prevents or defers the cost of alternative boosting systems� In gas production, it could prevent flaring of LP gas with added environmental benefits� Minimum to no maintenance is required� High level of reliability� Requires no active control� Could be used as a low-cost system for well start-up or well kick-off operations� Modifications during the service life is at low-cost to optimise the design and performance

as production conditions change.

9. CONCLUSIONS

Use of jet pumps is a cost-effective way to boost the production from many oil and gas fields.

The level of gain depends on the amount of energy available from a high pressure source and theproductivity characterics of LP wells.

In applications where multiphase flow (gas and liquid) is involved, the system requires a specialdesign. Full understanding of the performance of jet pumps handling multiphase flow isrequired to achieve maximum efficiency from the system.

There has been a number of units in operations successfully worldwide, although still their usehas not extended sufficiently - partly because of lack of awareness of their potential in boostingproduction.

Because of their relative low-cost, their use even over a short period of one year could well bejustified as the period for the recovery of the capital is short, often a matter of a few weeks ormonths.

10. NOMENCLATURE

GVF = Q + Q

Q

lg

g

Mv = Q

Q + Q

l(HP)

g(LP)l(LP)

N = P - P

P - PdischargeHP

LPdischarge

Pressure ratio (inlet) = PP

LP

HP

Mass flow ratio = MM

HP

LP

Efficiency of the Liquid-Multiphase jet pump:

Terms:

GVF Gas Volume Fraction at opening pressure and temperatureM Mass flowrateMv Volumetric flow ratioN Pressure ratioPHP HP pressure at jet pump inletPLP LP pressure at jet pump inletPdischarge Discharge pressure at jet pump outletQ Volumetric flow at operating pressure and temperatureη Jet pump efficiency

Subscript:

l Liquidg gasHP High pressure (or motive)LP Low pressure (or suction)

Q )P - P(P

P Q P + Q )P - P( =

l(HP)dischargeHP

LP

dischargeg(LP)LPl(LP)LPdischarge �

�

���

�ln�

11. REFERENCES

1. H BradleyPetroleum Engineering HandbookSociety of Petroleum Engineers, Texas, USA, 1987.

2. K Ashton, A J Green, A ReadGas production improvements using ejectors.SPE paper 26684, Offshore Europe, Aberdeen, pp 523 - 531, September 1993.

3. M Villa, G De Ghetto, F Paone - Agip SPAUse of ejectors for boosting low pressure oil and gas wells.Offshore Mediterranian Conference, OMC 97, Italy, pp 717 - 731.

4. N Conti, P Giunta, R PozziEnhanced oil productivity by diluent lift. Offshore Mediterranean Conference and Exhibition, Ravenna, Italy, pp 54 - 73, March1993.

5. M M (Sacha) Sarshar (CALTEC), M Villa, F Paone (Agip SPA), W L Loh, N A Beg(CALTEC)Field testing and applications of WELLCOM system.8th International Conference, Multiphase 97, Cannes, France, pp 293 - 318.

6. M M (Sacha) Sarshar (CALTEC)Design of separators for floating installations to reduce the impact of motion.2nd annual industry forum - Production Separation Systems, 24 - 25 February 1999,Aberdeen, arranged by IIR.

TABLE 1: COMPARING THE PERFORMANCE OF SINGLE AND DUALWELLCOM SYSTEMS

Case High Pressure (HP) Low Pressure (LP) DischargePressure

Pressurebarg

Liquidflowrate

m3/h

Gasflowrate

sm3/d

GVF(%) Pressure

barg

Liquidflowrate

m3/h

Gasflowrate

sm3/d

GVF(%)

barg

A 70 100 423069 75 30 40 221662 90 34

B-1 70 0 423069 100 30 0 221662 100 40

B-2 70 100 0 0 30 40 0 0 45

Case A:Single WELLCOM system Case B-1 & B-2: Dual WELLCOM system gas & liquidjet pumps

TABLE 2: WELLCOM ECONOMICS - GAS PRODUCTION

Increase in Production Gas Price

(1) 0.75 x 106 scf per day(2) 2 x 106 scf per day(3) 15 x 106 scf per dayPayback period(1) 40 - 50 days(2) 20 - 25 days(3) 6 - 8 days

�1/1000scf

capital �30k - �37.5kcapital �40k - �50k capital �80k - �120k