Drive Mecanisms

CONTENTS

1 DEFINITION

2 NATURAL DRIVE MECHANISM TYPE2.1 Depletion Drive Reservoirs2.2 Water Drive2.3 Compaction Drive2.4 Gravity Drainage2.5 Depletion Type Reservoirs2.5.1 Solution Gas Drive2.5.2 Gas Cap Drive2.6 Water Drive Reservoirs2.7 Combination Drives

3 RESERVOIR PERFORMANCE OF DIFFERENTDRIVE SYSTEMS3.1 Solution Gas Drive3.1.1 Solution Gas Drive, Oil Production3.1.2 Solution Gas Drive, Gas / Oil Ratio3.1.3 Pressure3.1.4 Water Production, Well Behaviour, Expected

Oil Recovery and Well Location3.2 Gas Cap Drive3.2.1 Oil Production3.2.2 Pressure3.2.3 Gas / Oil Ratio3.2.4 Water Production, Well Behaviour, Expected

Oil Recovery and Well Locations3.3 Water Drive3.3.1 Rate Sensitity3.3.2 Water Production, Oil Recovery3.3.3 History Matching Aquifer Characteristics3.3.4 Well Locations

4 SUMMARY4.1 Pressure and Recovery4.2 Gas / Oil Ratio

1111Drive Mechanisms

2

LEARNING OBJECTIVES

Having worked through this chapter the Student will be able to:

• Define reservoir drive mechanism.

• Describe briefly with the aid of sketches a depletion drive reservoir.

• Describe briefly with the aid of sketches a water drive reservoir.

• Describe briefly with the aid a sketches a gravity drainage.

• Describe briefly with the aid of sketches solution gas drive distinguishingbehaviour both above and below the bubble point.

• Describe briefly with the aid of sketches gas cap drive .

• Describe briefly with the aid of sketches the reservoir performance characteristicsof a solution gas drive reservoir.

• Describe briefly with the aid of sketches the reservoir performance characteristicsof a gas drive reservoir.

• Describe briefly with the aid of sketches the reservoir performance characteristicsof water drive reservoir.

• Describe briefly with the aid of sketches the rate sensitivity aspect of waterdrive reservoir.

• Summarise the characteristics of solution gas drive, gas cap drive and waterdrive reservoirs.

Institute of Petroleum Engineering, Heriot-Watt University 3


RESERVOIR DRIVE MECHANISMS

In the previous chapters we have considered the physical properties of the porousmedia, the rock, within which the reservoir fluids are contained and the properties andbehaviour of the fluids. In this chapter we shall examine the various methods used tocalculate the performance of different reservoir types, we will introduce the variousdrive mechanisms responsible for production of fluids from a hydrocarbon reservoir.

In this qualitative description of the way in which reservoirs produce their fluids wewill see how the various basic concepts come together to give understanding to thevarious driving forces responsible for fluid production. One of the main preoccupa-tion’s of reservoir engineers is to determine the predominant drive mechanism, fordependant on the drive mechanism different recoveries of oil can be achieved.

As well as presenting natural drive mechanisms we will also review various artificialdrive mechanisms.

1 DEFINITION

A reservoir drive mechanism is a source of energy for driving the fluids out throughthe wellbore. It is not necessarily the energy lifting the fluids to the surface, althoughin many cases, the same energy is capable of lifting the fluids to the surface.

2 NATURAL DRIVE MECHANISM TYPES

There are a number of drive mechanisms, but the two main drive mechanisms aredepletion drive and water drive. Other drive mechanisms to be considered arecompaction drive and gravity drive. These drive mechanisms are natural driveenergies and are not to be confused with artificial drive energies such as gas injectionand water injection.

2.1 Depletion Drive ReservoirsA depletion type reservoir is a reservoir in which the hydrocarbons contained are NOTin contact with a large body of permeable water bearing sand. In a depletion typereservoir the reservoir is virtually totally enclosed by porous media and the onlyenergy comes from the reservoir system itself. Figures 1 and 2 illustrate the types ofaccumulations which can give rise to depletion drive characteristics.

In figure 1 the hydrocarbons are enclosed in isolated sand lenses which have beengenerated by a particular depositional environment. Over geological time the hydro-carbons have found their way into the porous media. The surrounding rocks may havepermeability but it is so low as to prevent energy transfer from other sources.

In figure 2 is illustrated another depletion type reservoir where a mature reservoir hasbeen subjected to faulting, resulting in the isolation of a part of the reservoir from therest of the accumulation. In a total field system, such a situation can give rise to partsof the reservoir having different drive mechanism characteristics.

4

GasOilWater

GasOilWater

Figure 1

Depletion reservoir:

No aquifer. Isolated sand

lenses

Figure 2

Depletion reservoir:

Aquifer limited by faults



2.2 Water Drive

GasOilWater

A water drive reservoir is one in which the hydrocarbons are in contact with a largevolume of water bearing sand. There are two types of water drive reservoirs. Thereare those where the driving energy comes primarily from the expansion of water asthe reservoir is produced, as shown in figure 3 The key issue here is the relative sizeand mobility of the water of the supporting aquifer relative to the size of thehydrocarbon accumulation.

Water drive may also be a result of artesian flow from an outcrop of the reservoirformation, figure 4. In this situation either surface water or seawater feeds into theoutcrop and replenishes the water as it moves into the reservoir to replace the oil. Thekey issues here are the mobility of the water in the aquifer and barriers to flow fromthe outcrop to the reservoir. It is not often encountered, and the water drive arisingfrom the compressibility of an aquifer, figure 3, is the more common.

Outcropof sand

Oil well

Water flow

Figure 3

Water drive:

Active aquifer

Figure 4

Reservoir having artesian

water drive.

6

2.3 Compaction DriveFigure 5 illustrates another drive mechanism, compaction drive. Although not acommon drive energy, the characteristics of its occurrence can be dramatic. Compactiondrive occurs when the hydrocarbon formation is compacted as a result of the increasein the net overburden stress as the reservoir pore pressure is reduced during production.The nature of the rock or its degree of consolidation can give rise to the mechanism.For example a shallow sand deposit which has not reached its minimum porosity leveldue to consolidation can consolidate further as the net overburden stresses increase asfluids are withdrawn. The impact of the further consolidation can give rise tosubsidence at the surface. This phenomena of compaction with increasing netoverburden stress is not restricted to unconsolidated sands, since chalk also demonstratesthis phenomena. One of the spectacular occurrences of compaction drive is thatassociated with the Ekofisk Field, in the Norwegian sector of the North Sea. This isa very undersaturated chalk reservoir. The field was developed on the basis of usingdepletion drive down to near the bubble point and then to inject sea water to maintainpressure above the bubble point. During this period of considerable pressure decline,the net overburden stress was increasing, causing the formation to compact to anextent that subsidence occurred at the seabed. In an offshore environment suchuniform subsidence can go undetected, as was the case for Ekofisk. The magnitude ofthe subsidence has been such that major jacking up of the structures has been required.

Oil

New landsurface

Old landsurface

2.4 Gravity DrainageGravitational segregation or gravity drainage can be considered as a drive mecha-nism. Figure 6 illustrates a situation where the natural density segregation of thephases can be responsible for moving the fluids to the well bore. Gravity drainage iswhere the relative density forces associated with the fluids cause the fluids, the oil, todrain down towards the production well. The tendency for the gas to migrate up andthe oil to drain down clearly will be influenced by the rate of flow of the fluids asindicated by their relative permeabilities. Gravity drainage is generally associated

Figure 5

Compaction drive



with the later stages of drive for reservoirs where other drive mechanisms have beenthe more dominant energy in earlier years. Gravity drainage can be significant andeffective in steeply dipping reservoirs which are fractured.

Of the drive mechanisms mentioned the major drive mechanisms are depletion drive,which are further classified into solution gas drive and gas cap drive and water drive.Gravity Drive typically is active during the final stages of a depletion reservoir.

Inactive aquifer

Closed in

InitialGOC

PresentGOC

ΟWC

Z

1000

GasOilWater

2.5. Depletion Type ReservoirsIn depletion drive reservoirs the energy comes from the expansion of the fluids in thereservoir and its associated pore space. There are two types of depletion drivereservoirs, solution gas drive reservoirs and gas cap drive reservoirs. In solution gasdrive reservoirs there are two stages of drive mechanism where different energies areresponsible for fluid production.

2.5.1. Solution Gas DriveIn solution gas drive reservoirs the initial condition is where the reservoir isundersaturated, i.e. above the bubble point. Production of fluids down to the bubblepoint is as a result of the effective compressibility of the system. When consideringpressure volume phase behaviour, in the chapter on phase behaviour, we observed asmall increase in volume of the oil for large reductions in pressure, for oil in theundersaturated state. Associated connate water also has a compressibility as has thepore space within which the fluids are contained. This combined compressibilityprovides the drive mechanism for depletion drive above the bubble point. Perhaps thispart of the depletion drive should be called compressibility drive. The lowcompressibility causes rapid pressure decline in this period and resulting lowrecovery. Of the three compressibilities, although it is the oil compressibility whichis the larger, the impact of the other compressibility components, the water and thepores, should not be neglected.

As pressure is reduced, oil expands due to compressibility and eventually gas comesout of solution from the oil as the bubble point pressure of the fluid is reached. Theexpanding gas provides the force to drive the oil hence the term solution gas drive.It is sometimes called dissolved gas drive (Figure 7). Gas has a high compressibility

Figure 6

Gravity drive

8

compared to liquid and therefore the pressure decline is reduced. Solution gas driveonly occurs once the bubble point pressure has been reached.

Initially no gas capand Oil above Pb

2.5.2. Gas Cap DriveAnother kind of depletion type is where there is already free gas in the reservoir,accumulated at the top of the reservoir in the form of a gas cap (Figure 8), as comparedto the undersaturated initial condition for the previous solution gas drive reservoir.This gas cap drive reservoir, as it is termed, receives its energy from the highcompressibility of the gas cap. Since there is a gas cap then the bottom hole pressurewill not be too far away from the bubble point pressure and therefore solution gas drivecould also be occurring. The gas cap provides the major source of energy but thereis also the expansion of oil and its dissolved gas and the gas coming out of solution.The oil expansion term is very low and is within the errors in calculating the two mainenergy sources. The two significant sources of driving energy are ;

(1) Gas cap expansion

(2) Expansion of gas coming out of solution

Gas cap

Gas cap expansionSolution gas liberation

With production -

Oil may be above Pb

Gas cap present initiallyOil at interface is at Pb

Oil

Figure 8

Gas cap drive reservoir

Figure 7

Solution gas drive reservoir



2.6 Water Drive ReservoirsWater drive reservoirs are also of two types. There is an edge water drive reservoir.The reservoir is thin enough so that the water is in contact with the hydrocarbons atthe edge of the reservoir (Figure 9). The other type of water drive reservoir is thebottom-water-drive reservoir; where the reservoir is so thick or the accumulation sothin that the hydrocarbons are completely underlain by water (Figure 10).

Edge water

Bottom water

Water coning

2.7 Combination Drives‘Pure’ types of reservoirs are those reservoirs where only one drive system operates,for example, depletion drive only - no water drive or water drive only - no gas drive.

It is rare for reservoirs to fit conveniently into this simple characterisation. In manyof them a combination of drive mechanisms can be activate during the production offluids. Such reservoirs are called combination drives (Figure 11). In the case in figure11, which is not unusual, we have a gas cap with the oil accumulation underlain bywater providing potential water drive. So both free gas and water are in contact withthe oil. In such a reservoir some of the energy will come from the expansion of thegas and some from the energy within the massive supporting aquifer and its associatedcompressibility.

Figure 10

Bottom water drive

reservoir

Figure 9

Edge water drive reservoir

10

Original condition

Gas Cap

Oil zoneWater Water

Gas Cap

Oil zoneWater Water

50 % Depleted

Sometimes it may be only water drive in the above situations. If the hydrocarbons aretaken out at a rate such that for every volume of oil removed water readily moves into replace the oil, then the reservoir is driven completely by water. On the other handthere may be only depletion drive. If the water does not move in to replace the oil, thenonly the gas cap would expand to provide the drive.

3 RESERVOIR PERFORMANCE OF DIFFERENT DRIVE SYSTEMS

Having considered the basic aspects of the drive types we will now examine theirrespective characteristics in relation to production, recovery and pressure declineissues.

The performance of different types of reservoirs in relation to the daily production,gas/oil ratio and water production can give some indication of the type of drivemechanism operative in the reservoir.

3.1 Solution Gas DriveIn the first part of solution gas drive, in what we termed compressibility drive, withinthe reservoir no production of gas occurs and the fluid moves as a result ofdecompression of the three components oil, water and pore space. The pressurereduction is rapid in relation to volumes produced. The gas to oil ratio produced at thesurface is constant since the reservoir at this stage is above its bubble point pressure.

Once the bubble point is reached gas comes out of solution. Initially the gas bubblesare small and isolated. The size and number of the bubbles increase until they reacha critical saturation when they form a continuous phase and become mobile. At thisstage the gas has relative permeability. The impact of the first bubbles of gas on theoil is very significant. The relative permeability to the oil is reduced by the presenceof the non wetting gas. (See gas-oil relative permeabilties in chapter 7. Figure 44) Asthe increase in saturation of gas increases at the expense of oil saturation, the relative

Figure 11

Combination water and gas

- cap drive



permeabilties move in the same directions giving rise to reduced well productivity tooil and increased productivity to gas, figure 12. That is the oil relative permeabilitydecreases and the gas relative permeability increases. The gas although providing thedisplacing medium is effectively leaking out of the system. Not only does the gasprogress to the wellbore, depending on vertical permeability characteristics it willmove vertically and may form a secondary gas cap. If this occurs it can contribute tothe drive energy. Well location and rate of production can be used to encourage gasto migrate to form such a gas cap as against being lost through production from thewellbore.

Vertical gasmigration

Gas relative permeability

Oil relative permeability

Rs< Rsi Rs

< RsiRs< Rsi

We will now review the various production profiles, specific to the drive mechanismsbut before doing so we will review the various phases of production.

Time

Pro

duct

ion

Production build up

Plateau phase

Decline phase

Abandonment0

Production Phases (figure 13)The first phase, production build up, which may exist or not depending on the drillingstrategy is the increased production as wells are brought on stream. Clearly, as in somecases, wells might be predrilled through a template and then all brought on streamtogether when connected to production facilities, such a build up of production will,therefore, not occur.

The next stage represents the period when the productivity of the production facilityis at its design capacity and the wells are throttled back to limit their productivity. Thisperiod is called the plateau phase when production is maintained at the designcapacity of the facilities. Typical production rates for the plateau period cannot bepresented since it depends on the techno-economics of the field. Clearly for a fieldwith a very large front loaded capital investment there is an incentive to have a highproduction rate during the plateau phase , say 20% of the STOIIP, whereas for a lower

Figure 12

Schematic of solution gas

drive.

Figure 13

Phases in production.

12

cost onshore field 5% might be acceptable. Governments will also impose theirconsiderations on this aspect as well.

A time will come when the reservoir is no longer able to deliver fluids to match thefacilities capacity and the field goes into the decline phase. This phase can be delayedby methods to increase production. Such methods could include artificial lift, wherethe effort required to lift the fluids from the reservoir is carried out by a downholepump or by using gas lift to reduce the density of the fluid system in the well.

There comes a time when the productivity of the reservoir is no longer able to generaterevenues to cover the costs of running the field, This abandonment time again isinfluenced by the size and nature of the operation. Clearly a single, stripper well,carrying very little operational costs, can be allowed to produce down to very lowrates. A well, as part of a very high cost offshore environment however, could beabandoned at a relatively high rate when perhaps the water proportion becomes toohigh or the productivity in relation to all production is not sufficient to meet theassociated well and production costs.

We will now review the performance characteristics of the various mechanisms inlight of the forgoing production phases.

3.1.1 Solution Gas Drive, Oil Production ( Figure 14 )After a well is drilled and production starts for a solution gas drive reservoir, thepressure drops in the vicinity of the well. The initially pressure drop is rapid as flowresults from the low compressibility of the system above the bubble point. Pressurecontinues to decline and solution gas drive becomes effective as gas comes out ofsolution. Mobility of gas occurs and the reduced mobility to oil and resultingdecreasing oil relative permeability further causes the pressure to decline andproductivity to oil flow decrease. Initially when all wells are on stream the oilproduction is high but the production rapidly declines and there is a short plateau anddecline phase until an economic limit is reached.

Time-Year

ReservoirPressure

ReservoirPressure

OilProd

OilProd

G.O.R

G.O.R

Figure 14

Production for solution gas

drive



A good analogy for this type of reservoir is the champagne bottle opened by achampion to spray the contents over enthusiastic supporters - a short lived highproduction scenario followed by rapid decline!

3.1.2 Solution Gas Drive, Gas/Oil RatioThe distinctive characteristic of the solution gas drive mechanism is related to theproducing gas to oil ratio. When the reservoir is first produced the GOR beingproduced may be low corresponding to the R

Si value of the reservoir liquid. If the

reservoir is highly undersaturated there will be a period when a constant producingGOR occurs 1-2 in figure 15.

When the bubble point is reached in the near well vicinity, the initial gas which comesout of solution is immobile and therefore oil entering the wellbore is short of theprevious level of solution gas. Theoretically at the surface the producing GOR levelis less than the original GOR 2-3 in figure 15.

As the pressure further reduces the released gas becomes mobile and moves at avelocity greater than its associated oil due to the relative permeability effects. Oilenters the well bore, with its below bubble point solution GOR value, but also gasenters the well bore from oil which has not yet arrived. The net effect is that at thesurface the producing GOR increases rapidly as free gas within the reservoir, whichhas come out of solution, moves ahead of the oil 3-4 in figure 15.

As the pressure continues to decline the productivity of the well continues to declinefrom the combined impact of reducing relative permeability and drop in bottom holepressure. The production GOR goes though a maximum as oil eventually is producedinto the well bore with a low solution GOR and the associated gas which has come outof solution has progressed much faster to the well and contributed to earlier gasproduction 4-5 in figure 15.

Pressure

Pro

duci

ng G

OR

.

Pb

GOR constantabove bubblepoint pressure

Rsi1 2

3

4

5

When the pressure drops below the bubble point throughout the reservoir a secondarygas cap may be produced and some wells have the potential of becoming gasproducers.

3.1.3 PressureAt first the pressure is high but as production continues the pressure makes a rapiddecline.

Figure 15

Producing GOR for

solution gas drive reservoir

14

3.1.4 Water Production, Well Behaviour , Expected Oil Recovery and Well LocationSince by definition there is little water present in the reservoir there should be no waterproduction to speak of. Because of the rapid pressure drop artificial lift will berequired at an early stage in the life of the reservoir. The expected oil recovery fromthese types of reservoirs is low and could be between 5 and 30% of the original oil-in-place. Abandonment of the reservoir will depend on the level of the GOR and thelack of reservoir pressure to enable production. Well locations for this drive mecha-nism are chosen to encourage vertical migration of the gas, therefore the wellsproducing zones are located structurally low, but not too close to any water contactwhich might generate water through water coning. Figure 16

Secondarygas cap

Oil water contact

3.2 Gas Cap DriveWhereas for a solution gas drive reservoir where we have a reservoir initially in anundersaturated state, for a gas cap drive reservoir, figure 7, the initial condition is areservoir with a gas cap. Since the gas oil contact will be at the bubble point pressurethe pressures within the oil accumulation will not be higher than this only so far asrelates to the density gradient of the fluid. It is the gas cap, with its considerablecompressibility, which provides the drive energy for such fields, hence the name. Toget flow in the wells it is likely that gas will come out of solution in the near well borevicinity and therefore some degree of solution gas drive will also take place. A goodanalogy for this type of reservoir is the plastic chemical dispenser fitted with a pumpto maintain gas pressure above the dispensed liquid.

Original condition

Gas Cap

Oil zoneWater Water

Gas Cap

Oil zoneWater Water

50 % Depleted

Figure 17

Gas-cap drive

Figure 16

Well location for solution

gas drive reservoir.



3.2.1 Oil ProductionThe producing characteristics for a gas cap drive reservoir are illustrated in Figure 18.Although the production may be high as in the solution gas drive, the oil productionstill has a significant decline but not as rapid as for solution gas drive. This declinein oil production is due to the reducing pressure in the reservoir but also from theimpact of solution gas drive on the relative permeability around the well bore. If thewell is allowed to produce at too fast a rate, the very favourable mobility character-istics of the gas, arising from its low viscosity compared to the oil, are such thatpreferential flow can cause gas breakthrough into the wells and the well is then lostto oil production. Indeed it is this condition which will determine well abandonment.

3.2.2 PressureWith an associated gas cap a loss of volume of fluids from the reservoir is associatedwith a relatively low drop in pressure because of the high compressibility of the gas.In solution gas drive much of the driving gas is produced, but with a gas cap the fluidremains till later in the life of the reservoir. The pressure drop for a gas cap systemtherefore declines slowly over the years. The decline will depend on the relative sizeof the gas cap to the oil accumulation. A small gas cap would be 10% of the oil volumewhereas a large gas cap would be 50% of the volume.

Time-Year

Pre

ssur

e

Oil

Pro

d (1

000)

OilProdRate

G.O

.R

Pressure

G.O.R

00 0

1 2 3 4 5 6 7

BSW %20

10

Gas Breakthrough

5

10

250

500

2500

5000

3.2.3 Gas/Oil RatioDuring the early stages of replacement of oil by gas a 100% replacement takes place.Later on gas by-passes oil and a reduced displacement efficiency. In the early stagesthe GOR remains relatively steady increasing slowly as the impact of solution gasdrive generates gas from oil still to reach the well bore. The increasing mobility ofthe gas is such that there is an increasing GOR both from dissolved gas and by-passgas and eventually the well goes to gas as the gas cap breaks through.

Figure 18

Reservoir performance gas

- cap drive.

16

3.2.4 Water Production, Well Behaviour, Expected Oil Recovery and WellLocationsLike solution gas drive there should be negligible water production. The life of thereservoir is largely a function of the size of gas cap but it is likely to be a long flowinglife. The expected oil recovery for such a system is of the order of 20 to 40% of theoriginal oil-in-place. The well locations, similar to solution gas drive, are such thatthe production interval for the wells should be situated away from the gas oil contactbut not too close to the water oil contact to risk water coning.

3.3 Water DriveThe majority of water drive reservoirs predominantly get their drive energy from thecompressibility of the aquifer system. The effectiveness of water drive depends on theability of the aquifer to replace the volume of the produced oil. The key issues witha water drive reservoir are therefore the size of the aquifer and permeability. This isbecause the only way for a low compressibility system to be effective is for its relativesize to the oil accumulation to be large, and the permeability of the aquifer to waterto enable flow though the aquifer and into the oil zone. These key issues set aconsiderable challenge to the reservoir engineer since to predict water drive behaviour,requires such information, which in pre production periods can only be obtained fromexploration activity to determine the extent and properties of the aquifer. It is difficultto obtain justification to expend such exploration costs in determining the size of awater accumulation!

3.3.1. Rate Sensitivity.The characteristic features of natural water drive reservoirs are strongly influenced bythe rate sensitivity of these reservoirs. If oil production from the formation is greaterthan the replacement flow of the aquifer then the reservoir pressure will drop andanother drive mechanism will contribute to flow, for example solution gas drive.

Three sketches below illustrate the various types of production profiles for differentaquifer types and the influence of rate sensitivity. In figure 19 we have the artesiantype aquifer where there is communication to surface water though an outcrop. In thiscase if oil is produced at a rate less than the aquifer can move water into the oil zone,then the reservoir pressure, as measured at the original oil water contact, remainsconstant. The producing gas-oil ratio also remains constant since the reservoir isundersaturated. These reservoirs will enable a plateau phase, however as in all waterdrive reservoirs the decline of the reservoirs is not due to productivity loss throughpressure decline but the production of water. The encroaching aquifer with perhaps itsfavourable mobility will preferentially move through the oil zone and if there are highpermeability layers will move through these. Eventually the water-cut, the proportionof water to total production becomes too high and the well is abandoned to oilproduction.



Pi

Rsi

Reservoir pressure

Oil production rate

Water production

Time

ProductionGOR

Outcropof sand

Oil well

Water flow

Figure 20 illustrates a more typical water drive reservoir where the drive energy comesfrom the compressibility of the aquifer system. In this case if the oil withdrawal rateis less then the rate of water encroachment from the aquifer then the reservoir pressurewill slowly decline, reflecting the decompression of the total system , the oil reservoirand the aquifer. Clearly this pressure decline is related to the size of the aquifer. Thelarger the aquifer the slower the pressure decline. As with all water drive reservoirsproductivity of the wells remains high resulting from the maintained pressure,however the productivity of the well to oil reduces as water breakthrough occurs. Soa characteristic of water drive reservoirs is the increasing water production alongsidedecreasing oil production.

Pi

Rsi

Reservoirpressure

Oil production rate

ProductionGOR

Water production

Time

Figure 19

Producing characteristics

for artesian water drive.

Figure 20

Producing characteristics

for water drive (confined

aquifer).

18

Figure 21 illustrates the rate sensitive aspect of water drive reservoirs. If the oilwithdrawal rate is higher than the water influx rate from the aquifer then the oilreservoir pressure will drop at a rate greater than would be the case with aquifersupport alone, as the compressibility of the oil reservoirs supports the flow. If thispressure drops below the bubble point then solution gas drive will occur, as evidencedby an increase in the gas-oil ratio. Cutting back oil production to a rate to less than thewater encroachment rate restores the system to water drive, with the gas-oil ratio goingback to its undersaturated level.

When two drive mechanisms function as above then we have what is termedcombination drive ( water drive and solution gas drive).

Water drive reservoirs have good pressure support. The decline in oil production isrelated to increasing water production as against pressure decline.

200010000

1000 5000

0 0 0

500

250

GOR

69 70 71 72 73 74 750

Reservoir pressure

Oil production

Water

GOR

Bsw

Ps

PROD

Wat

er p

rodu

ctio

n

25

50

BSW

Pro

duci

ng g

as /

oil

ra

tio

Res

ervo

ir pr

essu

re

psi

Oil

prod

uctio

n ra

te

B

/d

3.3.2 Water Production, Oil RecoveryBecause there is a large aquifer associated with the oil reservoir unlike depletion drivesystems, water production starts early and increases to appreciable amounts. Thiswater production is produced at the expense of oil and continues to increase until theoil/water ratio is uneconomical. Total fluid production remains reasonably steady.The expected oil recovery from a water drive reservoir is likely to be from 35 to 60%of the original oil-in-place. Clearly these recovery factors depend on a range of relatedaspects , including reservoir characteristics for example the heterogeneity as demon-strated by large permeability variations in the formation.

3.3.3. History Matching Aquifer Characteristics.Predicting the behaviour of water drive reservoirs in particular the rate of waterencroachment is not straightforward. The topic is covered in a later chapter, but asignificant perspective as mentioned previously is that data is required of the aquiferto carry out the calculations. In particular the size and geometry of the aquifer and itspermeability and compressibility characteristics. Since such information is generally

Figure 21

Reservoir performance -

Water drive.



not available during the exploration and development phase, the characteristics of theaquifer are only determined once production has been operational and the supportfrom the aquifer can be calculated from production and pressure data. (HistoryMatching). Getting such information may require producing a significant proportionof the formation say 5% of the STOIIP. RFT surveys have provided a very effectiveway of determining the aquifer strength as well as the communicating layers of theformation. Pressure depth surveys taken in an open hole development well afterproduction has started will give indications of pressure support in the formation

Because water drive, through pressure maintenance provides the most optimisticrecoveries, artificial water drive is often part of the development strategy because ofthe uncertainties of the pressure support from the associated aquifer. In the North Seafor example many reservoirs have associated aquifers. The risk of not knowing eitherthe extent or activity of the aquifers is such that many operators are using artificialwater drive systems to maintain pressure so that solution gas drive does not occur withthe consequent loss of oil production.

3.3.4. Well LocationsWell locations for water drive reservoirs are such that they should be located high inthe structure to delay water breakthrough.

4 SUMMARY

The following summaries and tables give the main features associated with the variousdrive mechanisms.

4.1 Pressure and Recovery

Water-drive -pressure declines slowly and abandonment occurs when the water cutis too-high at around 50% of recovery, but depends on local factors.

Gas-cap drive - the pressure shows a marked decline and economic pressures arereached around 20% of the original pressure when about 30% of the oil is recovered.

Solution- gas drive - the pressure drops more sharply and at 10% of the pressurereaches, an uneconomical level of recovery at about 10% of the oil-in-place.

4.2 Gas/Oil Ratio

Water drive - the curve for a water drive system shows a gas/oil ratio that remainsconstant. Variations from this indicate support from solution gas drive or other drivemechanisms

Gas-cap drive - for this drive the gas/oil ratio increases slowly and continuously.

Solution- gas drive - the curve for a solution gas drive reservoir shows that the gas/oil ratio increases sharply at first then later declines.

20

SOLUTION GAS DRIVE RESERVOIRS

Characteristics Trend1. Reservoir Pressure Declines rapidly and continuously2. Gas/Oil Ratio First low then rises to a maximum and then

drops3. Production Rate First high, then decreases rapidly and continues

to decline4. Water Production None5. Well Behaviour Requires artificial lift at early stages6. Expected Oil Recovery 5-30% of original oil-in-place

GAS CAP DRIVE RESERVOIRS

Characteristics Trend1. Reservoir Pressure Falls slowly and continuously2. Gas/oil ratio Rises continuously3. Production Rate First high, then declines gradually4. Water Production Absent or negligible5. Well Behaviour Cap Long flowing life depending on size of gas cap6. Expected Oil Recovery 20 to 40% of original oil-in-place

WATER DRIVE RESERVOIRS

Characteristics Trend1. Reservoir Pressure Remains high2. Gas/Oil Ratio Remains steady3. Water Production Starts early and increases to appreciable

amounts4. Well Behaviour Flow until water production gets excessive5. Expected Oil Recovery up to 60% original oil-in-place.

Figures 22 and 23 give the pressure and gas-oil ratio trends for various drivemechanism types



100

80

60

40

20

00 20 40 60 80 100

Water drive

Gas cap drive

Dissolvedgas drive

Res

ervo

ir pr

essu

re -

per

cent

of o

rigin

al

Oil produced - percent of original oil in place

Reservoir pressure trends for reservoirs under various drives.

5

4

3

2

1

00 20 40 60 80 100

Water drive

Gas cap driveDissolvedgas drive

GO

R M

CF

/BB

L

Oil produced - percent of original oil in place

Reservoir gas - oil ratio trends for reservoirs under various drives.

Figure 22

Figure 23

Drive Mecanisms

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