Improved Techniques for Evaluating Carbonate Waterfloods in West Texas

Abstract

Detailed studies of three waterfloods in Permian carbonate reservoirs of west Texas resulted in new depletion plans with major operating changes, including infill drilling and pattern modifications. Close coordination of geologic and engineering work produced a consistent approach to the relationship between reservoir description and operations when calculating ultimate recovery. Introduction Detailed studies of three waterfloods in Permian carbonate reservoirs of west Texas resulted in new depletion plans with major operating changes, including infill drilling and pattern modifications. An important aspect of these studies was the close coordination of geologic and engineering work that produced a consistent approach to the relationship between reservoir description and field procedures used to improve ultimate recovery. The Fullerton, Means, and Robertson fields are located in west Texas about 50 to 75 miles northwest of Midland (Fig. 1). These reservoirs have been producing oil since the mid-1930's and later were unitized with Exxon Co., U.S.A., as operator. These three units are typical of many west Texas carbonate waterfloods in which waterflooding began in the early 1960's and progressed through several expansion phases. Although economically successful, results were often less than predicted. As problems with early waterfloods began to develop, old concepts changed and led to more detailed studies. Ghauri et al. reported several of these changing concepts in 1974. The same year, Driscoll summarized some approaches that had been used to improve waterflood recovery. To better relate reservoir description to past performance and future operations, a special study group composed of an engineer-geologist team was formed to conduct in-depth studies of these three reservoirs. Techniques used in these studies were largely refinements of those used previously in the Permian Basin area; however, some new concepts and approaches were developed. A practical requirement was having to use available data that, in some cases, were almost 40 years old and often of poor quality. Reservoir Description Geology The fields studied are located geologically in the north-eastern part of the Central Basin Platform, a shallow shelf area separating the Delaware and Midland basins during Permian time. Fig. 1 is a geological province map showing various basin and platform areas during Permian time. The Robertson and Fullerton fields produce mainly from the Clearfork formation of Permian Leonard age, while Means Field produces primarily from the San Andres formation of Permian Guadalupian age. These fields, even though varying slightly in age, geographical location, and producing depth, exhibit similar depositional and lithologic characteristics that affect waterflood performance. These reservoirs are characterized by numerous porosity stringers within a gross, vertical, carbonate section that may be several hundred feet thick. These carbonates were deposited as limestone in a shallow shelf environment and most of the limestone later became dolomitized. JPT P. 1547^