Abstract

This study investigates the effects of the oil producing rate, absolute permeability and well completion interval on oil recovery in a solution gas drive reservoir. The investigation was made using a radial, two-dimensional, three-phase, numerical, reservoir model. Three-phase drainage and imbibition relative permeabilities were considered in the model.

Ultimate oil recoveries for the three rate cases were 22.8, 20.6 and 17.6 percent of the stock-tank oil initially in place. It is concluded that in the radial system, as in the linear system., oil recoveries are greater at highest producing rates. At low rates, gas segregates to the top of the reservoir and is produced rapidly, depleting reservoir energy. Recoveries were higher for lower absolute permeabilities. One anisotropic case was considered with a 500 md horizontal permeability and a 25 md vertical permeability. This case achieved the highest ultimate recovery of all the cases considered.

Three cases were compared to investigate the effects of selective completion interval Recoveries were slightly higher forlower completion intervals, but not significantly so.

Recoveries in all cases except the anisotropic permeability case are lower than predicted using a steady-state type depletion calculation.

Introduction

This study investigates the effects of oil producing rate, absolute permeability and well completion interval on the recovery performance of solution gas drive reservoirs. Reasons for observed performance are analyzed in view of pressure and saturation conditions existing in the reservoir during depletion.

Since the solution gas drive process is a very basic recovery process. it has been studied extensively. Early studies by Miller et al.1 and Loper and Calhoun2, using modified steady-state calculation procedures. concluded that solution gas drive recovery was independent of oil producing rate or pressure drawdown.

The advent of digital computers made it possible to use finite difference schernes to solve the non-linear partial differential equations that describe unsteady- state reservoir flow. Early models were necessarily somewhat simplified. Heuer et al.3 used a two dimensional two-phase model to investigatethe effect of producing rate on solution gas drive recovery. The model did not include capillarity or gravity. They concluded that ultimate recovery was independent of producing rate. Levine and Prats4 used a similar model to investigate the effects of absolute permeability on recovery. They found that ultimate recoveries were greater for higher absolute permeabilities. but they attributed this to cutting off production at an arbitrary economic producing rate rather than to some change in the production mechanism.

Ridings et al.5 used a two-phase. one-dimensional model that included gravity and capillarity to investigate the effects of oil rate on recovery. They concluded that ultimate recovery is not dependent on producing rate.

Morse and Morse: and Whiting used two-phase linear and two-dimensional models to investigate the solution gas drive process. Their models included capillary and gravity effects and accounted for both drainage and imbibition relative permeabilities. The conclusions they drew did not. agree with those drawn from the studies that used more simplified models.

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