Where does the natural energy of a reservoir come from?

Liberation and expansion of solution gas Influx of aquifer water Contraction of reservoir rock skeleton Expansion of original reservoir fluids

- Free gas, if present- Interstitial water- Oil, if present

• Gravitational forces

Oil Reservoir Drive Mechanisms• Solution gas drive

• Gas-cap drive

• Water drive

• Gravity-drainage drive

• Combination drive

Gas Reservoir Drive Mechanisms• Volumetric reservoir (gas expansion drive)

• Water drive

Ultimate oil and gas recoveries vary depending on the drive mechanism. For oil; water drive is most effective. Typical primary recoveries are in the 25-40% range (maximum 75%).

For gas; gravity drainage, water drive and depletion drive can provide > 80% recovery.

Solution gas DriveThe principle of solution gas drive or depletion drive is the expansion of dissolved gas and liquid oil in response to a pressure drop. The change in fluid volume results in production.

Above the bubble point, only liquid oil expansion occurs. Below the bubble point, both liquid oil expansion and gas expansion contribute to volume change.

The Upper Cretaceous Cardium sand reservoir is an example of a solution gas drive reservoir.

Solution gas drive mechanism

Oil

A. Original Conditions

B-C. Partially depleted

Oil producing wells

Oil producing wells Liberated

solution

gas

CP

LIQUID

GAS

P

T

20%40%60%80%

X

Pi

PbA

B

C

Oil recovery in solution gas drive reservoirs

Bubblepoint

pressure

Initial reservoir

pressure

0 5 10 15 Oil recovery, % of OOIP

Re

ser

voir

pre

ss

ure

, psi

g

Dissolved gas reservoirs typically recover between 5 and 25% OIIP and 60 to 80% GIIP.

SOLUTION GASDRIVE HISTORY

watercut

GOR (R)pressure

time

Rsi

OIL PRODUCTON

• Rapid and continuous pressure drop, rate of decline falls at bubble point pressure.• R (producing gas oil ratio) low until p = pb, then increases to maximum and

declines.• Absent or minimal water influx (watercut).• Gravity drainage is a special case in steeply dipping reservoirs where gas drives

out more oil.• Well production declines rapidly; early pumping often required.

Pb

Gas Cap Drive Mechanism

Cross Section

Oil producing well

Oilzone

OilzoneGas cap

The principle of gas cap drive is the expansion of free gas in response to a pressure drop. The change in fluid volume results in production.

Gas cap expansion maintains the pressure in the oil leg.

Gas cap drive reservoirs typically recover 20 to 40% OIIP, sometimes as high as 60%.

The Lower Mississippian Turner Valley carbonate was a gas cap drive reservoir.

GAS CAP DRIVEHISTORY

watercut

GOR (R)

pressure

time

Rsi

OIL PRODUCTON

• pressure drops continuously, but slowly.• R (producing gas oil ratio) increases continuously.• water influx (watercut) absent or minimal• gas cap cannot be allowed to shrink or oil encroachment will occur

resulting in reduced recovery.• oil leg wells can eventually produce gas.• Wells have long flowing life (depending on the size of the gas cap).

Natural Water Drive MechanismThe principle of natural water drive is that an aquifer provides the energy for hydrocarbon production. Both water expansion, as a result of pressure reduction, and inflow are involved.

Natural water drive is associated with high recovery rates; oil from 35-75% OIIP; gas from 60-80% GIIP.

It is not uncommon for flow from the surface to supply the energy for natural water drive.

When a pressure drop occurs, both the oil and water liquid phases expand resulting in production. Additionally, water inflow radially and vertically displaces the oil towards the producers.

Cross-section view

Plane view

Water

Hydrocarbon

The Upper Devonian Leduc pools are driven by inflow from the Cooking Lake Aquifer.

Different Water Drive Mechansims

Oil producing well

Water Water

Cross Section

Oil Zone

Both bottom water drive, where the water leg underlies the entire reservoir, and edge water drive, where only part of the areal extent is contacted by water, are recognized.

Oil producing well

Cross Section

Oil Zone

Water

Edge Water Drive Bottom Water Drive

NATURAL WATERDRIVE HISTORY watercut

GOR (R)

pressure

time

Rsi

OIL PRODUCTON

• Pressure remains high; small drop.• R (producing gas oil ratio) remains low.• Water influx starts early and increases to appreciable levels.• Residual oil may be trapped behind the advancing water.• Wells flow freely until water production (watercut) becomes excessive.

COMPACTION DRIVE

In compaction drive, the energy for oil production is provided by the collapse of the porous medium skeleton and expansion of the pore fluids when the reservoir pressure drops. The increase in the "grain pressure" or effective stress causes pore “collapse” and “compaction” (consolidation) of the reservoir.

This drive mechanism is common in highly compressible, unconsolidated reservoirs such as those found in California, Venezuela, and the heavy oil deposits of western Canada. Also in high-porosity chalks (e.g., North Sea).

The Lower Cretaceous Mannville (Clearwater) sands in the Cold Lake district provide an example of compaction drive.

Gravity-drainage Drive Mechanism

Within reservoirs with high dip angles and having high perms (// dip), gas tends to migrate updip while oil migrates downdip towards the well. This mechanism traps gas energy in the reservoir naturally. This mechanism contains a high recovery efficiency similar to water drive

Oil

Oil

Oil

Point A

Point B

Point C

Gas

Gas

Gas

Formation of a Secondary Gas Cap during gas solution liberation

The example shows a combination of natural water influx and gas cap drive.

In many of the western Canadian heavy oil deposits, solution gas drive and compaction drive act in combination, for example the Lower Cretaceous Mannville (Waseca) sand in the Lloydminster district.

COMBINATION DRIVE

In combination-drive reservoirs, at least two of the basic drive mechanisms are active in expelling oil:

1. Solution gas exsolution2. Gas cap expansion3. Natural water influx4. Pore “collapse”

RESERVOIR PERFORMANCE DATA (1)

Pressure trends in reservoirs under various drive mechanisms are distinctive.

100

0 10 20 30 40 50

% OIIP Produced

P%

WATER DRIVE

GAS CAP DRIVESOLUTION

GAS DRIVE

80

60

40

20

0

RESERVOIR PERFORMANCE DATA (2)

Producing GOR is also strongly diagnostic of drive mechanism.

0 10 20 30 40 50

%OIIP Produced

GOR %

SOLUTIONGAS DRIVE

GAS CAP DRIVE

WATER DRIVE

100

80

60

40

20

0

Recovery Factors for Oil Reservoirs

Average Oil RecoveryFactors,

% of OOIPDrive Mechanism

Range AverageSolution-gas drive 5 - 30 15Gas-cap drive 15 - 50 30Water drive 30 - 60 40Gravity-drainagedrive

16 - 85 50

Recovery factor is defined as the fraction (or percentage) of the volume of hydrocarbon produced (recovered) from the amount of volume initially in place.

Recovery Factors for Gas Reservoirs

Average Gas RecoveryFactors,

% of OGIPDrive Mechanism

Range AverageVolumetric reservoir(Gas expansion drive)

70 - 90 80

Water drive 35 - 65 50

Estimating Oil Recovery Factors

• Solution-gas drive - API study

• Water drive - API study

• Water drive - Guthrie-Greenberger study

1741.0

3722.0

0979.01611.01

8.41a

bwi

obob

wiR p

pS

k

B

SE

2159.0

1903.0

0770.00422.01

9.54a

iwi

oi

w

oi

wiR p

pS

k

B

SE

114.00003.0538.1log136.0256.0log272.0 1010 hSkE owiR

These correlations work best for sandstone reservoirs.

NomenclatureER = Oil recovery efficiency (recovery factor), [% (for API

study); fraction (for G-G study)] = Reservoir porosity, fractionSwi = Interstitial water saturation, fractionBob = Formation volume factor of oil at bubblepoint, RB/STBk = Reservoir permeability, [darcy (for API study);

md (For G-G study)]ob = Oil viscosity at bubblepoint pressure, cppb = Bubblepoint pressure of oil, psigpa = Abandonment reservoir pressure, psig

Suitable Characteristics for Oil Recovery

Solution-gas drive oil reservoirs Low oil density Low oil viscosity High oil bubblepoint pressure

• Gas-cap drive oil reservoirs Favorable oil properties Relatively large ratio of gas cap to

oil zone High reservoir dip angle Thick oil column

Water drive oil reservoirs– Large aquifer– Low oil viscosity– High relative oil permeability– Little reservoir heterogeneity

and stratification

Gravity drainage oil reservoirs

– High reservoir dip angle– Favorable permeability

distribution– Large fluid density difference– Large segregation area– Low withdrawal

Suitable Characteristics for Gas Recovery

Volumetric gas reservoir (gas expansion drive)– Low abandonment

pressure

Water-drive gas reservoir– Small aquifer

– Small degree of reservoir heterogeneity and stratification