Analysis of Gasfloods Using Semilog Graphs of Free-Gas Gas/Oil Ratio vs. Cumulative Oil Production

Abstract

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Abstract This work describes the use of semilog graphs of the free-gas gas/oil ratio (GOR) vs. cumulative oil production to analyze the performance of fields with large-scale gasflood operations. After gas breakthrough, and if field operations are reasonably constant, linear trends are found on these graphs and these trends can be extrapolated to project future oil recovery. This technique is similar to the use of semilog graphs of water/oil ratio (WOR) vs. cumulative oil production to analyze waterflood performance. The conceptual and theoretical bases for this approach are described, along with assumptions required to develop this technique from basic flow equations. Analysis of miscible-gasflood historical data from the Hassi Messaoud (HMD) (Algeria) and University Block 31 (west Texas, USA) oil fields is presented by use of this technique. As was the case with waterflood analysis using log-WOR-vs.-cumulative-oil-production graphs, for gasflood analysis this technique is shown to apply somewhat more generally than the theory might suggest. This technique is shown to be applicable at the individual-production-well level, pattern level, or for a reservoir as a whole. Introduction For the past 60 years, a technique used to analyze waterflood performance has been to prepare semilog graphs of the producing WOR vs. cumulative oil production. These graphs have been used because, in most cases, a linear trend is observed from which future production performance can be projected. The purpose of this paper is to discuss the application of a similar technique to fields in which there have been long-term, large-scale gasflooding operations. The process of gas displacing oil is similar to that of water displacing viscous oil, given that the viscosity ratio is approximately the same. There are differences, and those will be discussed. The primary difference is that in most oil fields, the oil contains considerable dissolved gas, which must be taken into account when analyzing gasflood performance. Other differences are considered that generally appear either to be insignificant or to counteract each other.