Comparison of Volumetric and Material Balance Estimates of Gas-in-place Washington Cockfield "D" Sand Reservoir
Authors: Elkins Lincoln F. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
This paper was prepared for the Improved Oil Recovery Symposium of the Society of Petroleum Engineers of AIME, to be held in Tulsa, Okla., March 22–24, 1976. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
After development of this gas reservoir complete, the volumetric estimate of gas-in place was about 250 M cf. Net gas production place was about 250 M cf. Net gas production during cycling and subsequent blowdown, which reduced pressure in the reservoir from 4590 psig initially to an average pressure of 540 psig, indicates gas actually in place was about 135 M cf. An in-depth review of the reservoir has been made to determine what combination of problems in core analyses, log interpretations, and problems in core analyses, log interpretations, and geologic considerations led to this 85 percent overestimate of gas-in-place.

Introduction
The most common method of determining residual oil saturation as a target for improved recovery is to estimate the oil-in-place originally and then to subtract past oil production and reserves remaining with conventional extraction methods. Recently, other techniques such as conventional well logs in new wells drilled into watered out areas, pressure cores, loginject-log techniques, tracer studies, etc., have been used to measure more directly the residual oil saturation in specific reservoirs. Quite often residual oil saturations actually found have been much lower than those estimated by the material balance method.
Most volumetric techniques for estimating free gas-in-place initially are identical to those for determining oil-in-place initially. Early pressure-production performance of Cockfield "D" sand gas-condensate reservoir, Washington Field, St. Landry Parish, La., indicated considerably less gas-in-place than that calculated by the conventional volumetric method. In an attempt to resolve this discrepancy every well drilled after the discovery well was cored there were extensive special core analyses and many reservoir pressure surveys were made. An effort was made to match early transient pressure-production performance during cycling pressure-production performance during cycling operations with a resistance-capacitance electric analyzer. Even with all of these data, the volumetric estimates of gas-in-place remained at least 1.5 times larger than those based on the early reservoir performance. See reference 1 for more detailed discussion of reservoir performance.
After a period of temporary gas sales, the reservoir was cycled for 11 years. Then gas was sold until April 1975 when the reservoir was converted to gas storage.