reservoir geomecanics

409 Effects of reservoir depletion
since the result might affect the prediction of reservoir recovery, production forecast
and well placement decisions.
To illustrate the impact of porosity and permeability loss during depletion on reservoir
performance, a simple 2D conceptual single-phase flow model based on Field
Zwas constructed (courtesy Inegbenose Aitokhuehi). The reservoir was assumed to
be elliptical with dimensions of 1900 m by 960 m and a thickness of 21 m, a 50 by
50 grid is generated with an average permeability of 350 mD and an initial porosity
of 30%. Three scenarios were investigated using a commercial reservoir simulator to
demonstrate the effects of compaction and permeability reduction:
1. Constant rock compressibility: The rock compressibility is estimated as an average
change in porosity associated with the expected depletion.
2. Compaction drive: Incorporating the DARS formalism, porosity change as a function
of depletion and stress reduction is estimated. The predicted change in porosity is
input as varying pore volume multipliers in the simulators. In this scenario, no
permeability change is assumed to occur during depletion.
3. Compaction drive with permeability loss: By relating the transmissibility multiplier
to the pore volume multiplier based on the two empirical bounds of permeability
changes, both permeability and porosity loss will contribute to the estimated cumulative
production of the conceptual reservoir.
Several assumptions are made to simplify and to shorten the time required for the
simulation. The initial production rate is set to be at 10 MSTB/d (thousand surface
tank barrels per day) and no water influx or injection. This single-phase simulator is
allowed to run until it reaches a minimum bottom hole pressure of 1000 psi (∼7 MPa),
an economic limit of 100 STB/doramaximum time of 8000 days (∼22 years).
Figure 12.15 illustrates the result of the idealized reservoir for the three scenarios
outlined above. Using constant compressibility throughout the entire production in
scenario 1, the conceptual reservoir will yield about 12 MMSTB cumulative oil over
2500 days (∼7 years). If depletion-induced compaction is considered (as calculated
with DARS) for a formation with properties similar to Field Z, the recovery for this
reservoir is increased significantly to about 26 MMSTB over 7500 days (∼20.5 years).
In other words, compaction drive enhanced the recovery and extended the production
life of this conceptual reservoir. When permeability loss associated with compaction is
taken into consideration, the times estimated for recovery are extended. The predicted
recoveries ranged from 16 to 25 MMSTB over 8000 days (∼22 years) depending on
whether the upper-bound or lower-bound permeability change with porosity is used
in the simulator. In the last two cases, the production life of the reservoir is extended.
As a result, incorporating both depletion-induced compaction and permeability loss
into the simulator will significantly affect the anticipated recovery and the production
lifetime of a reservoir. In terms of recovery, production-induced compaction provides an
additional driving mechanism that increases the recovery estimate; a small reduction in
permeability (lower bound) might not have as much of an impact as a large reduction in