Abstract

This paper discusses the prediction performance of a two-dimensional area reservoir simulation model. Of particular significance is the strategy to optimally harness the energy of the gas caps and aquifer to achieve maximum recovery from a group of offshore oil and gas reservoirs which are hydraulically connected through a regional aquifer: One major oil reservoir was produced at reasonable rates while the rates for the other reservoirs were automatically adjusted based on a concept of "balanced water potentials" and correct gas-oil/water-oil contact advance ratios. This strategy worked reasonably well because 96 to 99% of Estimated Ultimate Recoveries (EURs) were achieved.

Introduction

The lower and edge extremities of oil and gas reservoirs are in physical and hydraulic continuity with the enormous water volume (the aquifer) that fills the formation outside the reservoirs. The water volume may be hundreds to thousands of time larger than the oil and gas volumes. As oil and gas are produced from the reservoirs, the voidages are partially replaced by the water influx from the aquifer. Localised aquifers affecting one field are almost always present while large, regional aquifers affecting groups of fields are less common.

Localised aquifers may be primarily classified according to the geometry (radial, linear, or non-symmetrical) and, secondarily, according to the external boundary condition (infinite, finite closed, or finite-outcropping). A large regional aquifer in Woodbine sand formation in the East Texas Basin have been reported to provide driving force for oil and gas recoveries in the East Texas Hawkins Van, Cayuga and several others oil fields. Another report discussed the effects of a regional aquifer (in the Ellenburger dolomite formation in Texas Central Basin Platform, West Texas) on the pressure performance of Martin, Embar, Andector, Txl and Wheeler oil fields.

For optimal oil and gas recoveries, free gas and water must breakthrough at the well-bores at about the same time, after much oil has been displaced and produced. Too early or mismatched breakthroughs will cause oil to be bypassed. thus reducing the oil recovery. To ensure timely gas and water breakthroughs, well production rates have to be properly controlled so that gas and water fronts converge at the well-bores at the rates that would result in timely breakthroughs. Often, the strength of the gas drive is quite accurately known because the size of the gas cap and quality of the reservoir sands or carbonates is known from the wells drilled in the field. However, the size and strength of the water drive are much less known initially because very few wells are drilled in the water zone. Thus, beginning with a rough estimate, the strength of water drive has to be regularly updated by analyzing the production and pressure trends in the individual wells and the reservoir as a whole.

Figure 1 shows of a group of offshore oil and gas reservoirs (arbitrarily renamed) hydraulically connected through a regional aquifer. This aquifer provides important water drive and pressure support for the hydrocarbon recovery, however, its strength varies in different areas depending on the sand thickness, quality and continuity. P. 377^

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