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Multiscale transport of adsorbing and nonadsorbing gas through shale as a function of effective stress

Recent studies indicate the existence of micro-cracks in low-permeability gas shale. The micro-cracks percolate through the organic matter and inorganic matrix creating relatively conductive conduits. This work studies dual continuum behavior as a function of net effective stress in intact field cores using non-adsorptive and adsorptive gases. Helium and carbon dioxide are used in separate tests to study the fraction of gas residing in the micropores and macropores of a Haynesville sample (roughly 7% porosity, 2.5 μd).
Tests are conducted using pressure pulse decay and an analysis of the entire pressure history that permits delineation of the fraction of the porosity contributing to advective versus diffusive transport. The gas permeability is estimated numerically by matching the pressure response obtained from the model. The pressure history-match provides information about the gas permeability, porosity, and volumetric diffusion parameter (history-match parameter) as a function of stress. Sorption isotherms (Gibbs excess and absolute) are also obtained.
One sample with delineated cracks showed a gas fraction of 48% in the macropores while a homogenous sample with no apparent cracks showed a gas fraction of 90% in the macropores. This observation suggests that inclusion of Knudsen diffusion transport is important to describe samples with relatively large micropore volumes while Darcy/slip flow can adequately describe transport in samples with large macropore volumes. Because helium is non-adsorptive, diffusion is analyzed independent of sorption effects in our transient tests. Carbon dioxide, on the other hand, is strongly adsorptive. Carbon dioxide highlights the impact of sorption on gas transport at the micro- and macro-scales. Study results measure dual continuum characteristics at core-level, define gas slippage and Knudsen diffusion at both scales, and extract the fraction of pore space undergoing advective transport as stress on the sample increases.

Author(s):

Khalid Alnoaimi    
Stanford University
United States

Anthony Kovscek    
Stanford University
United States