Characteristics Of A San Andres Reservoir
Authors: Carlson T.C. , Sipes L.D. 
Published: December 2007

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Abstract
The relative commercial importance of the San Andres formation in the Permian Basin of West Texas is emphasized, and the general characteristics of the oil reservoirs are discussed. The rock properties and fluid distribution in a typical San Andres reservoir located on the North Basin Platform are investigated. In this reservoir, 80 per cent of the wells have been cored through at least part of the producing interval. The characteristics of the reservoir rock and the distribution of its contained fluids are determined with the assistance of special laboratory measurements. The vertical and areal definition of a 90-ft transition zone and "tilted" water table is determined and shown by production tests to be statistically reliable. The performance of a pilot water injection project in this reservoir is discussed. The injection rate and pressure response time are compared with values calculated from the reservoir rock and fluid properties.

Introduction
The San Andres formation of Permian age accounts for nearly 20 per cent of the current oil production from the Permian Basin of West Texas. In the older San Andres fields, relatively little quantitative data are available concerning the reservoir rock properties. However, in the Reeves (San Andres) field, 80 percent of the wells were cored in at least part of the producing interval; and these data, along with special laboratory measurements, permitted an unusually complete investigation of this typical San Andres reservoir. Laboratory measurements of rock and fluid properties were used to describe the reservoir, to define the fluid distribution, and to predict the performance of both well completions and water injection operations. The results of these calculations compared favorably with actual performance of both initial well completions and a pilot water injection project.
In view of the importance of the San Andres as an oil producing formation, its huge secondary recovery potential and the limited data available in the older fields, this discussion of the Reeves field was prepared to further disseminate the available information.

THE SAN ANDRES FORMATION
The San Andres formation is an oil and gas-bearing member of the Guadalupe Series of Permian age. It occurs in the Permian Basin geologic province of West Texas and Southeast New Mexico, and it is by far the most important oil-producing formation in the Permian Basin. It has been estimated that the San Andres formation accounts for nearly 20 percent of the Basin's current oil-producing rate. What is of even greater significance is the estimate that approximately 20 billion bbl of oil will remain in these reservoirs after being exploited by both primary and conventional secondary recovery mechanisms.
San Andres oil reservoirs in the West Texas Southeastern New Mexico area occur mostly on the periphery of the Midland Basin, and to a lesser degree, on the eastern side of the Delaware Basin, which are members of the greater Permian Basin. The general orientation of these reservoirs is presented on Fig. 1. The long axis of the basin with which they are associated. In most cases, the trap is formed by a combination of structure and lithology changes within a formation made up of massive limestone and dolomite with varying amounts of evaporite.
The San Andres reservoirs range in depth from 1,500 to 6,000 ft, and the producing thickness ranges from 3 to 200 ft. The average porosity ranges from 7 to 16 percent, and the average permeability ranges from 1.0 to about 50 md, the typical reservoir occurring in the lower portion of the range. Fractures and tilted water-oil contacts are noted in some San Andres reservoirs, and extensive water-oil transition zones are common.