Changing Concepts in Carbonate Waterflooding-West Texas Denver Unit Project-An Illustrative Example

Abstract

Greatly improved carbonate reservoir definition with detailed geology and reservoir simulation has led to confident, full-scale waterflooding. Projects now use closely spaced patterns and very selective means of well completion to improve flood efficiency and increase ultimate supplemental recovery. Producing 133,000 BOPD, the Denver Unit shows the benefits of improved waterflooding concepts. Introduction Carbonate reservoirs demand special attention, particularly for drive projects, because of the gross nonuniformities that normally exist. Often numerous, thin pay intervals of varying quality are distributed over thick vertical sections of several hundred feet. Differing environmental conditions at the time of deposition insure that the rock encountered in a single wellbore will have markedly varying porosity, permeability, and fluid-flow characteristics and relationships. These same variations occur between wellbores. The use of simple assumptions or averages of rock properties in the design and execution of the projects can lead to economic failures.Carbonate waterflooding in the Permian Basin has a relatively short history. The major impetus of project initiations did not occur until the late 1950's and during the 1960's. This long delay can be attributed primarily to the lack of economic incentives - specifically, low oil allowables and depressed crude oil prices. Other contributing factors included skepticism over success, difficulties in arriving at acceptable unitization agreements among the usually numerous operators, a dearth of injection water, and the sheer magnitude of the engineering task because of the large size of the fields.A narrowly focused look at current waterflooding in carbonates would likely miss the significant evolution that has taken place in the engineering design and field operation of these projects. Major changes continue to occur in our concepts of the geologic, reservoir, injection, and production aspects. Because of our greater experience there, much of the information for this paper is derived from the Permian San Andres (dolomite) Denver Unit project operated by Shell in the Wasson field of West Texas (Fig. 1). It should be emphasized that most of the material presented here is based on Shell's experience; widely divergent philosophies still exist among operators of carbonate waterfloods in the region. Geologic Concepts A significant development of recent years has been the better understanding of the geology of carbonate reservoirs. Geologists now investigate in detail, using both surface and subsurface information, the environmental conditions that controlled deposition. Terminology routinely includes such depositional terms as supratidal, intertidal, subtidal, and marine. Such investigation depends not only on re-evaluating older available data, but also on obtaining new data in the form of cores (including detailed special analyses), and open-hole and cased-hole logs. The attitude toward acquiring new data has changed from reluctance to a recognition of necessity.The Denver Unit waterflood project was begun in 1964 on the task of a gross correlation of basically two markers-the First Porosity and the Main Payin the San Andres productive interval, whose gross thickness is some 300 to 500 ft at an average well depth of 5,100 ft. JPT P. 595^