Abstract
The oil recovery process is controlled by the rates of gas injection and oil
production, relative permeabilities, reservoir heterogeneities and the balance
among viscous, capillary and gravity forces. Crestal gas injection in
horizontal, vertical or reef type oil reservoirs recovers significant volumes
of the residual oil due to the gas-oil gravity drainage mechanism, indicating
the significance of gravity forces. This study investigates the effects of the
parameters that control the process (e.g., rate of the gas injection and oil
production) and reservoir heterogeneities on the overall performance of
immiscible gravity drainage enhanced oil recovery (EOR). Reservoir simulation
studies are conducted to map effective combinations of these parameters with
respect to the oil recovery performance.
Introduction
Gravity forces play an important role at nearly every stage of the producing
life of the reservoir, whether it is primary depletion, secondary water or gas
injection schemes or tertiary enhanced or improved oil recovery methods(1).
They can be advantageously used to maximize oil recovery from the oil bearing
zone under investigation through gravity drainage mechanism. Several cases
reported in the literature suggest that it could deliver as high as 87 to 95%
incremental oil recoveries in contrast to other gas injection EOR methods.
Gas-Oil Gravity Drainage Process
Gravity drainage is a process in which gravity acts as a main driving force
and where gas replaces voidage volume(2). It is commonly implemented in either
of the dipping or pinnacle reef type reservoirs.
CO2-assisted gravity drainage EOR process is a top-down process
in which gas is injected in the gas cap through vertical wells at a rate lower
than the critical rate (Figure 1). Critical rate is the rate at which injection
gas fingers through oil zone (viscous instabilities) leading to its premature
breakthrough at the production wells. Injected gas segregates and creates a
gas-oil interface. Controlled oil production is started through horizontal
wells placed at the bottom of the oil zone such that the voidage created by oil
withdrawal (in addition to minor dissolved volumes) is replaced by the
equivalent CO2 injection volume. When this happens, pressure
differential across the gas cap and oil zone [that is gas-oil contact (GOC)]
stay at or close to zero implying that the pressure in the gas zone behind the
CO2 floodfront would be constant(3). This helps to maintain the
reservoir pressure nearly constant.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
4 June 2008
- Meeting paper published:
17 June 2008
- Revised manuscript received:
22 December 2009
- Manuscript approved:
6 January 2010
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