The Influence of Gas Saturation on Waterflood Performance - Variations Caused by Performance - Variations Caused by Changes in Flooding Rate

Abstract

It has been recognized that the presence of a gas saturation prior to waterflooding can have an important influence on oil recovery. The published results on the subject are derived from laboratory experiments on essentially one-dimensional horizontal systems flooded with low pressure differentials. In field applications, high pressure gradients could cause important effects not noted in the laboratory studies such as the disappearance of part or all of the gas by solution in the oil bank. Also, it has been realized that gravity forces make it impossible to initiate and maintain a uniform gas saturation from top to bottom of the production section.

By the use of numerical models a study has been made of the effects of flooding rate on performance of waterfloods in reservoirs having performance of waterfloods in reservoirs having gas saturation. Flooding rates over a very wide range have been simulated. At very high rates the gas is put into solution in the oil bank ahead of the water so no gas saturation effect is noted. The simulated performance at very low rates in the absence of gravity check very well with the published laboratory data. Introduction of gravity causes markedly different performance from that of the one-dimensional performance from that of the one-dimensional horizontal systems because of the nonuniform vertical distribution of initial gas saturation and gravity effects on the injected water distribution.

Introduction

At the present time, a large fraction of the oil originally in place is left unrecovered in oil reservoirs after depletion by the best methods available. The need for added recovery is of equal importance with the search for new oil-producing structures. Waterflooding is, and has been in the past, the most universally used method for increasing oil recovery beyond the levels provided by natural depletion. Improvements of waterflooding should lead to economic recovery of additional oil in substantial quantities. One of the methods that has been proposed for increasing recovery of oil by water proposed for increasing recovery of oil by water displacement involves creating a gas phase ahead of the waterflood front.

As early as 1922, Russell found experimentally that flooding of oil reservoirs by a mixture of water and gas yielded considerably more oil than water or gas drive alone. In the year 1932, producers in the Bradford field injected their surplus produced gas into oil reservoirs to conserve gas. The operators noted that the injection of gas prior to waterflooding resulted in improved oil recoveries.

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