Voidage Calculations for Water Flood Schemes

Abstract

Abstract The calculation of several distinct physical quantities are sometimeslumped, and not without confusion, under the term "voidage calculations." Eachtype of voidage calculation, such as withdrawals at constant pressure or gasfilled volume, has its place in evaluating the performance of water floods. Thebasis of each is the material balance. The physical significance of the variousvoidage concepts and the application of the material balance in the estimationof reservoir pressure are examined in this paper. It is shown that the common method of determining withdrawals, based on thesimple concept of a two-phase volume factor, while useful for demonstratingwhether or not reservoir pressure is being maintained, has limited value indetermining the overall status of a water flood. A new equation for determiningthe gas filled volume in a reservoir is presented. From it, a simple definitionof cumulative voidage is proposed. Suggestions are made regarding the use ofconventional reservoir fluid analyses. The estimation of reservoir pressure andcalculation of voidage are illustrated by examples. Introduction The word "voidage" or the phrase "voidage calculation" are rather looselydefined terms frequently used by engineers in connection with water-floodperformance. In one commonly used context, "voidage" is the volume that theproduced fluid occupies at reservoir conditions. An almost equally commonconcept of "voidage" is the amount of void or gas-filled space in a reservoir.In fact, these concepts are so intimately associated that it is sometimesincorrectly assumed that the production of one reservoir barrel of fluid, be itoil, gas or water, will result in a like increase in the void space in areservoir. Similar confusion can result from the misinterpretation of the term"net fill-up" which is in effect the opposite of "voidage." The purpose of this paper is to examine the various concepts of voidage andto describe their application to the evaluation of flood performance. Emphasisis laid on material balance principles for development of the equations. A newequation for the calculation of the gas-filled volume in a reservoir ispresented and a voidage concept based on this equation is proposed.