Porosity and Permeability of Eastern Devonian Gas Shale

Abstract

SPE Member Abstract High-precision core analysis has been performed on eight samples of Devonian gas shale from the Appalachian Basin. Seven of the core samples consist of the Upper Devonian age Huron Member of the Ohio Shale, six of which came from wells in the Ohio River valley, and the seventh from a well in east-central Kentucky. The eighth core sample consists of Middle Devonian age Marcellus Shale obtained from a well in Morgantown, West Virginia. The core analysis was originally intended to supply accurate input data for Devonian shale numerical reservoir simulation. Unexpectedly, the results have also shown that there are a number of previously unknown factors which influence or control gas production from organic-rich shales of the Appalachian Basin. The presence of petroleum as a mobile liquid phase in the pores of all seven Huron Shale samples effectively limits the gas porosity of this formation to less than 0.2 percent, and permeability of the rock matrix to gas is less than 0.1 microdarcy at reservoir stress. The Marcellus Shale core, on the other hand, was free of a mobile liquid phase and had a measured gas porosity of approximately 10% under stress with a fairly strong "adsorption" component. Permeability to gas (K ) was highly stress-dependent, ranging from about 20 microdarcies at a net stress of 3000 psi down to about 5 microdarcies at a net stress of 6000 psi. The conclusion reached from this study is that Devonian shale in the Appalachian Basin is a considerably more complex natural gas resource than previously thought. Production potential varies widely with geographic location and stratigraphy, just as it does with other gas and oil resources. Introduction High precision core analysis at the Institute of Gas Technology is performed in a device known as the Computer Operated Rock Analysis Laboratory (CORAL). The CORAL is capable of measuring actual gas flow rates through rock as low as 10(-6) cm3/second to accuracy of a few percent, and can measure gas permeabilities with a resolution of +/- 0.2 nanodarcy. Other rock properties measured by the CORAL include gas porosity under stress with a resolution of about +/-2% of the measured value, and pore volume compressibility. A description of the engineering and operational design of the CORAL has been presented by Randolph in SPE/DOE Paper 11765. Although the CORAL was originally designed to perform high-precision core analysis measurements on western tight gas sandstones, it soon became apparent that the accuracy and high resolution of this equipment would also have applications to other tight gas formations, such as Devonian shale. In the past, there have been several situations where Devonian shale permeabilities were reported from runs in equipment designed for tight sands. In both cases reported, the porosity and permeability values measured were near the resolution limits of the equipment and a substantial degree of uncertainty remains concerning the accuracy of the results. It should be emphasized that porosity and permeability are not single numbers to be measured and reported for each sample analyzed in the laboratory. Rather, these are coefficients that appear in the differential equations used to calculate fluid content and movement in porous media. For most high porosity, high permeability formations, adequate descriptions of well and reservoir performance can be achieved by assuming that these coefficients are constants. This is not a valid assumption for tight sands or for Devonian shale where the small pore sizes affect fluid flow through these rocks on a molecular scale. P. 75^