Summary
The gravity-drainage and oil-reinfiltration processes that occur in the
gas-cap zone of naturally fractured reservoirs (NFRs) are studied through
single porosity refined grid simulations. A stack of initially oil-saturated
matrix blocks in the presence of connate water surrounded by gas-saturated
fractures is considered; gas is provided at the top of the stack at a constant
pressure under gravity-capillary dominated flow conditions. An in-house
reservoir simulator, SIMPUMA-FRAC, and two other commercial simulators were
used to run the numerical experiments; the three simulators gave basically the
same results.
Gravity-drainage and oil-reinfiltration rates, along with average fluid
saturations, were computed in the stack of matrix blocks through time.
Pseudofunctions for oil reinfiltration and gravity drainage were developed and
considered in a revised formulation of the dual-porosity flow equations used in
the fractured reservoir simulation.
The modified dual-porosity equations were implemented in SIMPUMA-FRAC
(Galindo-Nava 1998; Galindo-Nava et al. 1998), and solutions were verified with
good results against those obtained from the equivalent single porosity refined
grid simulations. The same simulations--considering gravity drainage and oil
reinfiltration processes--were attempted to run in the two other commercial
simulators, in their dual-porosity mode and using available options. Results
obtained were different among them and significantly different from those
obtained from SIMPUMA-FRAC.
Introduction
One of the most important aspects in the numerical simulation of fractured
reservoirs is the description of the processes that occur during the
rock-matrix/fracture fluid exchange and the connection with the fractured
network. This description was initially done in a simplified manner and
therefore incomplete (Gilman and Kazemi 1988; Saidi and Sakthikumar 1993).
Experiments and theoretical and numerical studies (Saidi and Sakthikumar
1993; Horie et al. 1998; Tan and Firoozabadi 1990; Coats 1989) have allowed to
understand that there are mechanisms and processes, such as oil
reinfiltritation or oil imbibition and capillary continuity between matrix
blocks, that were not taken into account with sufficient detail in the original
dual-porosity formulations to model them properly and that modify significantly
the oil-production forecast and the ultimate recovery in an NFR.
The main idea of this paper is to study in further detail the oil
reinfiltration process that occurs in the gas invaded zone (gas cap zone) in an
NFR and to evaluate its modeling to implement it in a dual-porosity numerical
simulator.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
22 May 2007
- Meeting paper published:
27 June 2007
- Revised manuscript received:
16 September 2008
- Manuscript approved:
26 September 2008
- Published online:
1 June 2009
- Version of record:
1 June 2009
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