Laboratory Measurements of Matrix Permeability and Slippage Enhanced Permeability in Gas Shales
We are conducting laboratory experiments on gas shales samples examining the effects of confining stress, pore pressure and pore fluid type on permeability. Experiments were carried out on intact core plugs from the Eagleford, Haynesville, Marcellus, Montney and Barnett shale reservoirs. We developed a methodology to separate the decrease of permeability with increasing effective stress (the difference between hydrostatic confining pressure and pore pressure) and the increase of permeability at very low pore pressure due to molecular slippage effects. These effects are also known as Knudsen diffusion or Klinkenberg effects. In addition, by isolating the Klinkenberg effect we are able to estimate the effective size of the flow paths within each sample. Our measurements show that the permeability of the rock is significantly enhanced at low pore pressures (<1000 psi) due to slippage effects. Preliminary results suggest the effective flow paths of the samples investigated are on the order of tens of nanometers to about 100 nanometers in a high permeability sample. These results are in close agreements with pore size estimates for SEM images. From the magnitude of the Klinkenberg gas slippage effect, an effective Knudsen diffusivity was also calculated. These estimates can be used in reservoir simulation to more accurately predict the long-time production behavior of these shales. Finally, the relative contribution of Knudsen Diffusion to total flow is calculated. We show that the contribution is likely to be negligible at initial reservoir pressures, but becomes increasingly more important as flowing pressure declines.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013