Numerical Simulation of Oil Production With Simultaneous Ground Subsidence
Authors: Finol A. , Ali S.M. Farouq 
Published: December 2007

Abstract
A two-phase, two-dimensional black oil simulator was developed for simulating reservoir production behavior with simultaneously occurring reservoir formation compaction and ground subsidence at the surface.The flow equations were solved by both alternating direction implicit procedure and strongly implicit procedure. Reservoir compaction was described on the basis of the experimental data reported. The magnitude of areal subsidence at the surface was calculated using reservoir compaction, utilizing the recently developed theory of poroelasticity. poroelasticity. Computer runs were used to generate a variety of data, such as reservoir Pressure variation with oil production, for different reservoir compaction production, for different reservoir compaction coefficients. It was found that the average reservoir pressure increased with the Compaction coefficient pressure increased with the Compaction coefficient for a given cumulative oil production.The model was used for generating the reservoir formation profiles, as well as the ground subsidence bowls for a variety of conditions. It was found that the subsidence behavior strongly depends on the depth of burial. For example, with an increase in the depth, the reservoir bottom surface may actually uplift, while the top surface subsides.The model was also used for studying the effect of subsidence on pressure buildup behavior. The calculated reservoir pressure was higher in a compacting than in a noncompacting reservoir, taking into account the variation of permeability with compaction.Another phase studied was the effect of rebound on reservoir performance when gas is injected into the formation. Even though rebound is small in practice (on the order of 10 percent of subsidence), practice (on the order of 10 percent of subsidence), the effect was clearly evident in the reservoir pressure-production behavior. However, when there pressure-production behavior. However, when there was no rebound, gas injection simply inhibited compaction.Finally, the model was used for simulating the reported oil production and subsidence history of one of the Bolivar Coast oil fields in the Western Venezuela. Fair agreement was obtained between the observed and the predicted behavior.

Introduction
The phenomenon of ground subsidence associated with production of oil or gas from underground hydrocarbon reservoirs is not common; however, it does present environmental problems in a few oil-producing areas around the world. Notable examples are the Wilmington oil field, below Long Beach, Calif. where almost 30 ft of subsidence have been recorded, and the oil fields near and under Lake Maracaibo in Venezuela, where the surface has subsided as much as 10 ft. Other cases have been reported in Harris County, Tex., in the Niigata district of Japan, and in the Po Delta in Italy.Numerous causes may give rise to ground subsidence, either natural or as a result of man's activities. However, as far as the problem at hand is concerned, the observed land subsidence is considered to be a result of reservoir compaction, resulting from pore pressure decline in reservoirs that meet certain specific geometrical and structural conditions. The changes in the petrophysical properties of reservoir rocks caused by compaction properties of reservoir rocks caused by compaction have been studied to some extent, as well as the influence of such changes on the fluid production behavior of the reservoir. However, very little has been accomplished in relating the compaction of the underground reservoir with the subsidence occurring at the surface. Among the few studies conducted on this problem, the most realistic are those that consider subsidence above a disk-shaped reservoir, in which a uniform pressure reduction has occurred. These studies do not simulate the fluid production behavior of the compacting reservoir as such; this is considered to be known and is used to determine the compaction of the reservoir and the accompanying subsidence.
SPEJ
P. 411