--> --> Abstract: Pore Structure Inhibits Gas Diffusion in the Barnett Shale, by Qinhong Hu, Zhiye Gao, Sheng Peng, and Robert Ewing; #90152 (2012)

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Pore Structure Inhibits Gas Diffusion in the Barnett Shale.

Qinhong Hu, Zhiye Gao, Sheng Peng, and Robert Ewing
Department of Earth and Environmental Science, University of Texas at Arlington and Iowa State University

The Barnett Shale is a profitable gas field, but at current recovery rates only 10-15% of the estimated gas-in-place will be extracted. Gas recovery in this tight formation is limited by diffusive transport from the matrix storage to the stimulated fracture network. But despite the central role of diffusion, there are no systematic studies examining the measurements and effects of pore structure on diffusion of the Barnett Shale. We present results of a study of pore structure (pore connectivity, tortuosity, pore-size distribution) in the Barnett Shale. Pore-sze distribution was measured by both mercury intrusion porosimetry (MIP) and vapor absorption porosimetry. The pores are predominantly in the nm size range (with a measured medium pore diameter of 6.5 nm), but pore size is not the major contributor to low gas recovery. The low gas diffusion appears to be caused by low pore connectivity in the Barnett Shale. This was established by imbibition tests, a relatively easy screening technique for determining whether a rock sample has low connectivity. Where gravity effects are negligible, water imbibition into a hydrophilic porous medium with well-connected pore spaces leads to mass uptake proportional to time0.5. With sparsely-connected pores, an imbibition exponent of 0.26 is obtained, as we’ve consistently observed for the shale samples. We also directly measured chemical diffusion in the Barnett shale using a suite of tracers, followed by chemical mapping using laser ablation-ICP-MS. Tortuosity calculated from both mercury intrusion porosimetry and saturated diffusion tests is quite low, as expected from the low pore connectivity.

 

AAPG Search and Discovery Article #90152©2012 AAPG Southwest Section Meeting, Fort Worth, Texas, 19-22 May 2012