Water drive is the mechanism wherein the displacement of the oil is accomplished by
the net encroachment of water into the oil zone from an underlined water body called
aquifer (Fig. 5.10a).
Production of oil or gas will often change the water saturation which in turn
affects the oil and gas saturation, but the amount of change varies with the reservoir
drive mechanism. In an aquifer driven reservoir on an efficient water flood, as the oil
is produced to the surface facilities via the production tubing, the water saturation
increases accordingly to fill the space previously occupied by the withdrawn oil
(Fig. 5.10b).
This mechanism is represented mathematically as
Water Drive Index ¼ Net water influx
Hydrocarbon Voidage
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Production Characteristics (Prof Onyekonwu MO, Lecture Note
on Reservoir Engineering)
• Pressure
– Pressure is maintained (remains high) when water influx is active.
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Pressure declines slowly at first but then stabilizes due to increasing influx
with increasing pressure differential, but not when water influx is moderate.
• Oil Rate
– Rate remains constant or gradually declines prior to water breakthrough
– Rate decreases as water rate increases
• Producing GOR
– GOR remains constant as long as P > PBP
– Gradually increases if P is below the saturation pressure
• Water Production
– Dry oil until water breakthrough
– Increasing water production to an appreciable amount from the flank wells; a
sharp increase due to water coning in individual wells.
• Ultimate Recovery
– The expected oil range is 35–75%
Rock Compressibility and Connate Water Expansion
Drive
As the reservoir pressure declines, the rock and fluid expand due to the expansion of
the individual rock grains and formation compaction (individual compressibility).
The compressibility of oil, rock and water is generally relatively small which makes
the pressures in the undersaturated oil reservoirs to drop rapidly to the bubble point if
there is no aquifer support. Sometimes, this drive mechanism is not considered or it
is neglected when performing material balance calculation, especially for saturated
reservoirs.
This mechanism is represented mathematically as:
formation Drive Index ¼ rock and connate water expansion
Hydrocarbon Voidage
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Gravity Drainage Reservoirs (Prof Onyekonwu MO,
Lecture Note on Reservoir Engineering)
• The mechanism of gravity drainage is operative in an oil reservoir as a result of
difference in densities of the reservoir fluids.
• Gas coming out of solution moves updip to the crestal areas while oil moves
downdip to the wells located low on the structure (Fig. 5.11).
• Reservoir must have:
– High Dip
– High Permeability
– High Kv/Kh ratio
– Homogeneity
– Low Oil Viscosity
• Production Characteristics:
– Formation of a secondary gas cap
– Low GOR from structurally low wells
– Increasing GOR from high structure wells
– Rapid pressure decline to near dead conditions (stripper wells)
– Little or no water production
• While rates are low, RE will be high (70–80% of the initial oil in place)
eventually.
• Gravity drainage is most significant in fractured tight
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Combination Drive Reservoirs
Most oil reservoirs produce under the influence of two or more reservoir drive
mechanisms, referred to collectively as a combination drive. A common example
is an oil reservoir with an initial gas cap and an active water drive as shown in the
Fig. 5.12.
5.7.7.1 Production Trends
The production trends of a combination drive reservoir reflect the characteristics of
the dominant drive mechanism. A reservoir with a small initial gas cap and a weak
water drive will behave in a way similar to a solution gas drive reservoir, with rapidly
decreasing reservoir pressure and rising GORs. Likewise, a reservoir with a large gas
cap and a strong water drive may show very little decline in reservoir pressure while
exhibiting steadily increasing GORs and WORs. Evaluation of these production
trends is the primary method a reservoir engineer has for determining the drive
mechanisms that are active in a reservoir.
Recovery
The ultimate recovery obtained from a combination drive reservoir is a function of
the drive mechanisms active in the reservoir. The recovery may be high or low
depending on whether displacement or depletion drive mechanisms dominate. Water
drive and gas cap expansion are both displacement type drive mechanisms and have
relatively high recoveries. Solution gas drive is a depletion type drive and is
relatively inefficient.
Recovery from a combination drive reservoir can often be improved by minimizing the effect of depletion drive mechanisms by substituting or augmenting more
efficient ones through production rate management or fluid injection. To do this, the
drive mechanisms active in a reservoir must be identified early in its life
5.7.7.3 Characteristics of Combination Drive Reservoirs (Prof
Onyekonwu MO, Lecture Note on Reservoir Engineering)
• Gradually increasing water-cut in structurally low wells
• Pressure decline may be rapid if no strong water influx and no gas cap expansion.
• Continuously increasing GOR in structurally high wells if the gas cap is
expanding
• Recovery > depletion Drive but may be less than in water drive or gas-cap drive.
• When an oil reservoir is associated with a gas cap above and an aquifer below, all
drive mechanisms may be operative.
• Development strategy and well rate control are very important in the economic
recovery process.
A. If oil production rate is faster than the encroachment rates of gas cap and
water advance, pressure depletion occurs in the oil zone.
B. If oil production rate is controlled to equal voidage, it is better to have water
displace oil than gas displacing oil.
– Danger: Oil migration into gas cap due to shrinkage of gas cap volume;
some oil will be left trapped as residual.
• RE is usually greater than recovery from depletion drive but less than water drive
or gas-cap drive. The expected recovery is between 25 and 40% OOIP