MULTIPLE APPROACHES TO CHARACTERIZING PORE STRUCTURE IN BARNETT SHALE

Microscopic pore characteristics of porous media (e.g., pore size, pore-size distribution, and pore connectivity) control macroscopic fluid flow and chemical transport, such as the hydrocarbon recovery in stimulated fractured shales. This work discusses various approaches to investigating pore structure (both geometry and topology) of the Barnett Shale in north Texas. Approaches include imbibition, tracer diffusion, porosimetry (mercury intrusion porosimetry, vapor transport and capillary condensation, nuclear magnetic resonance cyroporometry), and imaging (nano-tomography, Wood’s metal impregnation, focused ion beam/SEM). Results show that the Barnett Shale pores are predominantly in the nm size range, with a measured median pore-throat diameter of about 6 nm. The nm-sized pore size, along with the low pore connectivity, leads to extremely low diffusion in shale matrix as measured using liquid diffusion and laser ablation-ICP-MS. 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. Our evolving complementary approaches, with their advantages and disadvantages, provide a rich toolbox for tackling the pore structure characteristics in tight shale and other natural rocks.