Dr. Qinhong (Max) Hu
The University of Texas at Arlington
Microscopic pore structure characteristics of porous media – 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, but it experiences a fast 1st-year decline (64%, from the exhaust of matrix gas in the closest vicinity of the fracture network) and low recovery rates (~15% ?) of the estimated gas-in-place. Gas recovery in this tight formation is apparently limited by diffusive transport from the matrix storage in intraparticle organic nanopores to the stimulated fracture network.
This presentation discusses various approaches to investigating pore structure of Barnett Shale. These approaches include imbibition and tracer diffusion, porosimetry (MIP, N2 and vapor adsorption/desorption isotherms, NMR cyroporometry), and imaging (Wood's metal impregnation, FIB/SEM). Our results show that the Barnett Shale pores have a 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 (on the order of 10,000) in the Barnett Shale. The presentation will end with the discussion of ongoing laboratory study on methane transport (diffusion and sorption/desorption) in the Barnett Shale, and perspectives on gas production decline behavior in producing wells.