Some Uses and Limitations of Model Studies in Cycling

Abstract

Abstract The use of model studies for the development of invasion patterns for cyclingis illustrated by model studies obtained with a recently developed apparatus inthe solution of actual cycling problems. Introduction Depletion of a gas reservoir by cycling under pressure maintenance may bedivided into three general phases. In the first phase, injection of dry gas isused to move all possible wet gas to producing wells prior to breakthrough ofappreciable quantities of dry gas into producing wells. In the second phase, injection moves additional wet gas to producing wells while moderate quantitiesof dry gas are being produced. The third phase consists of production withoutinjection to deplete the commercial gas reserve which remains after the cyclingoperation. Thus, in the first phase, a portion of the reservoir is invaded bydry gas, and the dry gas usually displaces something less than 100 pct of thewet gas from the invaded portions of the reservoir. The second phase enlargesthe invaded portion of the reservoir and, at the same time, increases recoveryfrom that zone invaded by dry gas in the first phase of the cycling operation.The result is an early yield of a substantial quantity of the liquefiablehydrocarbons in the reservoir with a minimum of the retrograde losses whichoccur in some reservoirs. The overall recovery from such an operation isusually high. The summation of the paths taken by dry gas in a cycling operation can betermed an invasion pattern. The use of electrical models to predict invasionpatterns has been the subject of several contributions to the literature, theresults of such model studies as applied to actual cases has been discussed byKelton and Miller and Lents. Recently, an improved apparatus for conducting potentiometric model studies hasbeen developed, as described by Lee. S The purpose of this paper is to presentmodel studies or invasion patterns obtained with this instrument in thesolution of actual cycling problems. The percentage of displacement of wet gas within the invaded portion of thereservoir in cycling, increase in recoveries of wet gas by continuing cyclingafter dry gas breakthrough, recoveries of wet and dry gas by production aftercycling, or economic factors in cycling are beyond the scope of this paper.Discussions which follow will be limited entirely to the development ofsuitable invasion patterns for cycling under pressure maintenance and the useof model studies therein. T.P. 2230