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Factors affecting the permeability of gas shales Pathi, Venkat Suryanarayana Murthy


The mechanical properties and matrix permeability of gas shales are the most important properties in determining their production capacity. In this research, I have investigated the matrix permeability and rock mechanical properties of Western Canadian and Woodford shales. The matrix permeability was measured using pulse decay experiment. The pulse decay experiment was employed with triaxial experiments combined with mercury porosimetry, helium pydnometry, Rock-Eval pyrolysis, SEM and X-ray diffraction analysis to measure rock strength, pore size, porosity, total organic content, fabric and composition of samples. The permeability results were correlated with effective stress, anisotropy, fabric, rock strength, porosity, pore size and total organic content. Mineralogy plays an important role in determining the permeability of Canadian and Woodford shales. Higher permeability was observed in samples with high clay content, and low permeability was observed in samples with high quartz and carbonate content. Among the clay-, silica-, and calcite-rich Canadian shales, the calcite-rich shales had a very low permeability (1O⁻⁷ md) compared to other shales. The permeability of all shales decline exponentially with increasing effective stress. Samples that were tested parallel to bedding had higher permeabilities than samples were tested normal to bedding. Among shales, the quartz-rich shales showed differences of three to four orders of magnitude for the samples tested parallel to bedding, compared to those tested normal to the bedding. The largest anisotropy was found in the clay-rich samples. Clay-rich shales also have a well developed fabric with a strongly preferred orientation, while the quartz-rich shales had random orientation of the fabric. The porosimetry results suggest that fluid flow is mostly in the meso (2-50 nm) and macro pores (>50 nm) of the Woodford shales. Samples with higher clay content (>30%) showed a higher intrusion volume in macro pores, while samples with higher quartz content showed intrusion volume in micro pores. Porosity is correlated to permeability in the Western Canadian shales and showed a linear correlation within the Woodford shales. Even though calcite-rich Canadian shales and quartz-rich Woodford shales have high TOC content, TOC was not seen to effect permeability. Triaxial compression rock testing was conducted on the Woodford shales to measure the elastic properties and strength behaviour of shale. Lithologic composition plays an important role in the strength and pore compressibility of shale. Quartz-rich or carbonate rich shales have a brittle behaviour and clay-rich shales have a ductile behaviour. Pore compressibility is greater in the clay-rich shales, and less in the quartz-rich shales.

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