LOW PORE CONNECTIVITY IN BARNETT SHALE

Microscopic characteristics of porous materials – pore shape, pore-size distribution, pore connectivity –control fluid flow and chemical transport, and therefore affect both exploration and production of hydrocarbons. The Barnett Shale is a profitable gas field in north Texas, but current recovery rates are only 10-15% of the estimated gas-in-place. Gas recovery in this tight formation is probably limited by its diffusive transport from the matrix storage of intraparticle organic nanopores to the stimulated fracture network.

This presentation discusses various approaches to investigating pore structure of tight Barnett Shale. These approaches include imbibition and tracer diffusion, porosimetry, and imaging. Preliminary results show that the Barnett Shale pores are predominantly in the nm size range, with a measured median pore-throat diameter of 6.5 nm. But small pore size is not the major contributor to low gas recovery; rather, the low gas diffusivity appears to be caused by low pore connectivity. 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 imbibition tests. Additionally, tortuosity calculated from both saturated diffusion and MIP tests is quite high in the Barnett Shale, as would be expected from low pore connectivity. Our evolving complementary approaches, with their several advantages and disadvantages, provide a rich toolbox for tackling the pore structure characteristics in the shales.