Pore structure and chemical diffusion in the Barnett Shale

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 effects of pore structure on diffusivity of the Barnett Shale. We present preliminary results of a new study of pore structure (pore connectivity, tortuosity, pore-size distribution) in the Barnett Shale. 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 diffusivity appears to be caused by low pore connectivity in the Barnett Shale. Chemical diffusion in sparsely-connected pore spaces is not well described by classical Fickian behavior; anomalous behavior is suggested by percolation theory, and confirmed by results of our imbibition tests. The imbibition test is 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. Tortuosity calculated from both mercury intrusion porosimetry and saturated diffusion tests is quite high, as expected from the low pore connectivity.