Drive MechanismsDrive Mechanisms

Adrian C Todd

Heriot-Watt UniversityHeriot-Watt UniversityDEPARTMENT OF PETROLEUM ENGINEERING

Reservoir Drive Mechanisms

Definition A reservoir drive mechanism is a source of

energy for driving the fluids out through the wellbore

It is not necessarily the energy lifting the fluids to the surface, although in many cases, the same energy is capable of lifting the fluids to the surface

Reservoir Drive Mechanisms

The various drive mechanisms come from the impact of a number of phenomena:

Expansion of reservoir fluids, oil, gas & water Liberation, expansion of solution gas Expansion of reservoir rock and reduction of

pore volume. Gravity forces.

Reservoir Drive Mechanisms

There are a number of drive mechanisms: Depletion drive Solution gas drive Gas Cap drive Water drive Compaction drive Gravity drainage drive Combination drive

Depletion Drive Reservoirs A depletion drive reservoir is one in which the

hydrocarbons are NOT in contact with a supporting aquifer.

A water drive reservoir is a reservoir in contact with a supporting aquifer.

Two types: due to expansion of water as reservoir is

produced due to artesian flow from an outcrop.

Water Drive Reservoirs

Water Drive Reservoirs Expansion of an active aquifer

Active aquifer

Key issue-relative size of aquifer to

hydrocarbon accumulation

1 VcV P

Water Drive Reservoirs Artesian Flow

Key issues:

Mobility of water in aquifer

Barriers to flow

Compaction Drive Not a common drive

mechanism. Characteristics can be

dramatic as a result of increase in net overburden stress as pore pressure reduced.

Nature of the rock or its consolidation determines extent of mechanism

Compaction Drive Ekofisk Field a dramatic example

Gravity Drainage Drive Due to the relative density of the fluids and high

vertical permeabilities. Fractured reservoirs

Gravity Drainage Drive Examples - Lake Maracaibo - Venezuala

Depletion Type Reservoirs

Energy comes from the expansion of fluids in the reservoir and its associated pore space

Two types: Solution gas drive Gas cap drive

Solution Gas Drive Two stages of drive: Above the bubble point. Fluid production comes from the effective compressibility of

the system: Oil Water Pore space Compressibility drive Below the bubble point

Solution Gas Drive Above bubble point: Low compressibility of oil,

connate water and pore space.

Rapid decline in pressure. Impact of water and pore

space should not be neglected

Solution Gas Drive When bubble point reached: Expanding gas provides force to drive oil. Solution gas drive Dissolved gas drive

Liberated gas can also migrate vertically and form secondary gas cap

Solution Gas Drive

Wellbore

Liberated solution gas

Secondary gas cap

Depletion Drive - Gas Cap Drive Where there is already free

gas in the reservoir. Compared to the initial

undersaturated condition for solution gas drive.

Energy from very high compressibility of gas cap.

Some energy from solution gas drive which is also present.

Oil expansion is very low

Water Drive Reservoirs

Two types Edge water drive Bottom water drive

Combination Drive

Rare for reservoirs to fit into the simple pure drive classification

Many have a combination of drives during production period.

Reservoir Performance

The production characteristics of reservoirs. Oil-gas ratio pressure decline water production can give indications of drive mechanism

Reservoir Performance-Solution Gas DriveWhen bubble point reached gas comes out of solution.

Initially no flow of gas since critical gas saturation not achieved

Gas Displacement of Oil Gas is a non-wetting

phase. Gas permeability is

zero until a ‘critical’ gas saturation is reached.

Reservoir Performance-Solution Gas Drive

Size of gas bubbles increases until

critical gas saturation reached.

The gas now has relative

permeability

Oil relative permeability decreases.

Gas Displacement of Oil

Reservoir Performance-Solution Gas Drive

Gas moves ahead of the associated oil

Depending on vertical relative permeability secondary gas cap formed

Production Phases Production build up - may exist depends on drilling strategy. Plateau phase- production maintained at design capacity.-

duration depends on economics of project. Decline phase - reservoir not able to deliver design capacity Abandonment- rate depends on size of project and op. costs.

Solution Gas DriveInitial pressure drop rapid due to low compressibility of system

Pressure continues to decline and solution gas drive becomes effective

Reduced oil production due to decreasing pressure and reducing relative permeability to oil

Gas production increases as gas comes out of solution and moves ahead of associated oil due to favourable relative permeability

Solution Gas Drive Distinctive feature of solution gas drive is the

producing gas to oil ratio- Rp

Above bubble point all gas in solution Rp =Rsi

At bubble point initial gas produced below critical gas saturation. Rp<Rsi

Gas becomes mobile and moves ahead of its oil Rp>Rsi

Maximum GOR as oil produced with a low GOR

Solution Gas Drive By definition should be no

water production. Due to rapid pressure drop

artificial lift required in early years.

Expected oil recovery, low, 5-30% STOIIP.

Well locations low to encourage vertical gas migration

Gas Cap Drive Initial condition free gas in gas cap. Gas contact will be at bubble point. Gas has considerable compressibility. To get flow gas comes out of solution at producing

interval. Some degree of solution gas drive.

Gas Cap DriveOil ProductionHas a significant decline but less than solution gas drive.Decline due to reducing pressure and solution gas drive

PressureProduction of fluids largely due to high compressibility of gas cap.Pressure declines slowly depending on gas cap size.

Gas-Oil RatioEarly stages GOR steady.Slowly impact of solution gas drive increases RpLow gas viscosity>high gas mobilityGas by-passing oil

Gas Cap Drive Water ProductionLike solution gas drive negligible water production.

Well behaviourLonger, depends on gas cap size

Recovery20-40% STOIIP

Well LocationsAway from gas oil contactNot too close to water oil contact

Water Drive Majority of water drive reservoirs energy from compressibility of aquifer. Effectiveness depends on ability of water to replace volume of oil

produced. Key issue- size and permeability of aquifer. For compressibility to be effective the relative size needs to be very

large. Challenge to reservoir engineer is to predict behaviour prior to

production. Difficult to justify exploration costs to determine the size of a water

accumulation

Water Drive - Rate Sensitivity The features of a natural water drive are strongly

influenced by the rate sensitivity of these reservoirs.

Can the water replace the rate of voidage loss due to oil production?

If not the pressure will drop and another drive mechanism will also be effective. e.g. solution gas drive,

Water Drive - Artesian flowOil flowrate is less than

potential flow of water from aquifer

Producing GOR RpRemains constant since reservoir undersaturated.Pressure at oil water contact constantPlateau phase possible.Decline due to water production

Water cut, determines when abandonment of well occurs

Water Drive - Compressibility of aquifer .Oil flowrate is less than potential flow

of water from aquiferProducing GOR Remains constant since reservoir undersaturated.

Pressure Declines as aquifer decompresses

Productivity remains high.Reduces as water production increases

Water Drive-Rate Sensitivity

Oil flowrate is more than potential flow of water from aquifer

Pressure drops below bubble point GOR increasesSolution gas drive-combination drive

Cutting back oil production enable aquifer to support production-water drive

Oil Production rate

Water Drive

Water production, Oil recovery Water production an early and characteristic feature Produced at the expense of oil Total fluid production steady Oil recovery 35-60% STOIIP. Depends on range of issues: reservoir characteristics - e.g.heterogeneity

Water Drive - History Matching Size and geometry of the aquifer and its permeability and compressibility

characteristics are required for aquifer calculations Characteristics often only determined after production started. Pressure support from aquifer calculated from pressure -production data. History matching. May require significant production of 5% STOIIP. Pressure depth surveys in open hole a valuable ‘tool’. Uncertainty over natural water drive often results in artificial water drive.

Solution Gas Drive

Characteristics Reservoir pressure Gas-Oil Ratio Production rate Water production Well behaviour Expected recovery

Trend Declines rapidly First low then rises through a maximum First high, then declines rapidly None Requires artificial lift early 5-30% STOIIP

Gas Cap Drive

Characteristics Reservoir pressure Gas-Oil Ratio Production rate Water production Well behaviour Expected recovery

Trend falls slowly and continuously Rises continuously First high, then declines gradually Absent or negligible Long flowing life depending on gas cap 20-40% STOIIP

Water Drive

Characteristics Reservoir pressure Gas-Oil Ratio Water production Well behaviour Expected recovery

Trend Remains high Steady Early and increases to large amount Flow until water production excessive up to 60% STOIIP

Recovery

GOR