Sipes Jr., L. D., Member AIME

Abstract

In the past seven or eight years large gas fields have been discovered in the Delaware Basin of West Texas. The principal producing formation is the Ellenburger. Because marketing facilities have lagged development and relatively small volumes of gas have been produced from the reservoirs, decisions on produced from the reservoirs, decisions on large investments are being made on volumetric estimates of gas in place. How accurate are these estimates?

A volumetric estimate of initial gas in place for the Ellenburger reservoir in the Coyanosa field has been made. Performance data are now available from which an initial gas in place can be determined. A comparison of the place can be determined. A comparison of the volumetric and performance derived values is illuminating and provides guidelines for determining volumetric gas in place in other Delaware Basin Ellenburger reservoirs.

Introduction

Large gas reserves have been discovered in the Delaware and Val Verde Basins of West Texas in the past few years. The principal accumulations have been in the Ellenburger formation of Ordivician age. Wells are deep and expensive, necessitating wide spacing to be economic. Tens of millions of dollars have been spent and are being spent to explore for and develop these deep reserves, often found below 20,000 feet. Figure 1 illustrates the location of the reservoirs which have been discovered in this trend.

At the outset of the Deep Delaware Basin boom, the Permian Basin was not a major gas supply area. In 1960, only three interstate transmission lines were taking gas from the area. No large intrastate markets were supplied from the area. Much of produced gas was from oil wells. With the discovery of large gas reservoirs, several interstate and intrastate lines sought to purchase gas and justify the necessary facilities to transport the gas to ready markets. This process takes time, particularly when Federal Power Commission particularly when Federal Power Commission certification must be obtained. As a result, marketing lagged development rather badly at times. Takes from completed wells were low or non-existent although the purchasing companies were pushing for certification and construction of facilities. Many interstate purchasers eased their producers' burdens by making prepayments under take or pay contracts or by making production loans. production loans. Because takes were low, little reliable performance data were available from which performance data were available from which reserves could be determined during the first few years. Management decisions, consequently, had to be made on volumetric estimates of reserves. Data are now available on a limited number of reservoirs to permit definition of reserves by both the permit definition of reserves by both the volumetric and material balance techniques for comparison.

A volumetric analysis of the Coyanosa Ellenburger reservoir has yielded an initial gas in place of 595 BCF. Material balance work defines the initial gas in place at 616 BCF estimate. The volumetric method is thus substantiated. Procedures and guidelines established should be useful in analyzing Delaware Basin Ellenburger reservoirs where material balance data are not yet reliable.

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