Water Drive Reservoir

1. WaterDriveReservoir and Water/Aquifer Influx:
PME 3110
Reservoir Engineering
Lecture#11

2. Introduction
Nearly all hydrocarbon reservoirs are surrounded by
water-bearing rocks called aquifers.
Aquifers may be substantially larger than the oil or gas reservoirs they adjoin as
to appear infinite in size
Aquifers may be so small in size as to be negligible in their effect on reservoir
performance
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3. As reservoir fluids are produced, a pressure differential develops
between the surrounding aquifer and the reservoir. The aquifer reacts
by encroaching across the original hydrocarbon-water contact.
Many gas and oil reservoirs produced by a mechanism termed water
drive. (natural water drive)
water drive is dependent on the size of aquifer and the pressure drop
from the aquifer to the reservoir.
During production aquifer response comes in a form of waterinflux,
commonly called water encroachment
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4. Classification of AQUIFERS
Degree of pressure maintenance
Flow regimes
Outer boundary conditions
Flow geometries
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5. Degree of Pressure Maintenance
Active water drive: The term active water drive refers to the
water encroachment mechanism in which the rate of water influx
equals the reservoir total production rate. during any long period,
the production rate and reservoir pressure remain reasonably
constant
Partial water drive: a relatively small. aquifer can
guarantee only limited pressure maintenance
Limited water drive: The term limited water drive refers to
the water encroachment mechanism in which the rate of water
influx is less than the reservoir total production rate.
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9. OUTER BOUNDARY CONDITIONS
Infinite system indicates that the effect of the pressure
changes at the oil/aquifer boundary can never be felt at the outer
boundary.
Finite system indicates that the aquifer outer limit is affectedby
the influx into the oil zone and that the pressure at this outer limit
changes with time.
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10. FLOW REGIMES
 Steady-state
 Semisteady-state
 Unsteady-state
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11. FLOW GEOMETRIES
Edge-waterdrive
Bottom-waterdrive
Linear-waterdrive
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12. Recognition of Natural Water Influx
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Normally very little information is obtained during the exploration and
development period of a reservoir concerning the presence or
characteristics of an aquifer that could provide a source of water influx
during the depletion period.
Natural water drive may be assumed by analogy with nearby producing
reservoirs, but early reservoir performance trends can provide clues.
A comparatively low, and decreasing, rate of reservoir pressure decline
with increasing cumulative withdrawals is indicative of fluid influx.
If the reservoir pressure is below the oil saturation pressure, a low rate
of increase in produced GOR is also indicative of fluid influx.

13. Recognition of Natural Water Influx (Cont’d)
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Early water production from edge wells is indicative of water
encroachment.
Such observations must be tempered by the possibility that the early
water production is due to formation fractures, thin high-permeability
streaks, or to coning in connection with a limited aquifer.
The water production may be due to casing leaks. Calculation of
increasing original oil-in-place from successive reservoir pressure
surveys by using the material balance and assuming no water influx is
also indicative of fluid influx.

14. Pot aquifer
Schilthuis’ steady-state
Hurst’s modified steady-state
The van Everdingen-Hurst unsteady-
state Edge-water drive
Bottom-water drive
The Carter-Tracy unsteady-state
Fetkovich’s method unsteady-
state Radial aquifer
Linear aquifer
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WATER INFLUX MODELS

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The Pot Aquifer Model

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23. References
Advanced Reservoir Engineering
by Paul McKinney, Tarek Ahmed
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