Gas Occurrence In The Devonian Shale
Authors: Smith E.C. , Cremean S.P. , Kozair G. 
Published: December 2007

Estimates of recoverable reserves in the eastern Devonian shales range from a few trillion cubic feet to several hundred trillion cubic feet. The most pessimistic estimates assume that all recoverable gas occurs within natural fracture porosity. More optimistic estimates assume a porosity. More optimistic estimates assume a substantial gas contribution from the shale matrix close to fractures. This paper presents the available evidence for both views including long-term shale production characteristics, laboratory measurements on shale cores, and numerical modeling studies. The weight of all available evidence favors the position that the matrix provides a major contribution of the recoverable gas from the Devonian shales, with a smaller amount derived from the fracture void volume.

Introduction

For several years, the Columbia Gas System Service Corporation has shown an increasing interest in promoting Devonian shale research and development in the Appalachian Basin. A systematic exploitation program would hinge upon the drilling of a large program would hinge upon the drilling of a large number of test wells in key parts of the Basin. This, in turn, requires prior demonstration of the basin's potential gas supply. Our research continues to support estimates of 200 to 900 trillion cubic feet of potential gas supplies based on production records, core gas content measurements, production records, core gas content measurements, well logs and well cuttings descriptions.

Several reports published by the Office of Technology Assessment, Federal Power Commission, Federal Energy Regulatory Commission, TRW and Lewin and Associates on the Appalachian Devonian shale provide other resource estimates using different provide other resource estimates using different assumptions. One very comprehensive report by the National Petroleum Council should be issued in the near future.

Although we know that the shale matrix contains a large volume of mobile gas, the question remains what fraction can be produced economically within a reasonable time frame. Since the shale has a very low permeability, economic production requires a fracture network to provide sufficient flow, except where the section contains permeable siltstones or sandstones. Gas production rates will depend upon the size, spacing and geometry of these fractures.

The extent of these controls is the subject of a lively debate which focuses on the dynamics of shale gas production. Two basic schools of thought exist:the bulk of the recoverable gas derives from the matrix, but fractures are necessary to provide rapid flow to the well, and b) the bulk of provide rapid flow to the well, andthe bulk of the recoverable gas is contained within natural fractures in the shale section and the contribution from the matrix over the life of the well is unimportant.

This debate on fracture vs. matrix production began at least 45 years ago. The proceedings of a 1935 Devonian shale symposium mentioned both concepts. Each concept claims the support of production data and history matching of production production data and history matching of production decline curves. Both sides can account for the total gas production (using appropriate assumptions and input data). Both have the support of numerical models.

Columbia's interest in these production models stems from their influence on reserve estimations. We must resolve whether matrix gas can be recovered before using the matrix gas content as a basis for supply projections. This paper reviews the assumptions, input data, and calculations used in each approach. Although we may not definitely prove which is correct until after a widespread drilling program, we can compare the likelihood of each program, we can compare the likelihood of each concept. Comparing the two arguments may help us narrow the range of values for the assumed input data and gain some insight into fracture size and spacing. These factors should prove useful in determining recovery factors, well spacing and fracture designs.

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