The Effects of Rock Mineralogy on Matrix Permeability in the Utica Shale

Abstract:

We conducted pulse-decay permeability measurements on four horizontally oriented Utica Shale samples to examine the effects of rock mineralogy on transport mechanisms using both a non-adsorbing gas (Argon) and an adsorbing gas (CO₂). The mineralogy of the shale samples varied from clay-rich to calcite-rich. We conducted the experiments at a temperature of 38.5°C, confining pressures ranging from 4.83 to 22.75 MPa, and pore fluid pressures ranging from 1.38 to 8.96 MPa. We measured the permeability at a range of confining pressures and pore pressures in order to independently test the effects of effective stress, confining pressure and pore pressure on permeability.

Our results show that shale mineralogy did not have an impact on permeability. The permeability of clay-rich samples varied between 0.26 and 1.10 microdarcy. The permeability of calcite-rich samples varied between 0.18 and 2.05 microdarcy. Additionally, we found that the shale mineralogy affected the stress-dependent permeability. The magnitude of permeability reduction as a function of effective stress was dependent on the overall rock mineralogy. When the effective stress increased from 3.45 MPa to 13.79 MPa, the permeability of the clay-rich and calcite-rich samples decreased by 85% and 48%, respectively. Based on Klinkenberg analysis, we found that the mean effective pore radius for the clay-rich sample decreased from 27 nm at 3.45 MPa effective stress to 15 nm at 13.79 MPa effective stress (44% reduction). The mean effective pore radius for the calcite-rich sample decreased from 49 nm to 38 nm (22% reduction). These findings suggest that variations in rock mineralogy lead to different responses in mechanical deformation as the effective stress increases with depletion. Finally, our CO₂ permeability measurements show that the CO₂ permeability for the clay-rich sample decreased by 40% compared to Argon permeability. The CO₂ permeability for the calcite-rich sample did not shift and was equivalent to the prior Argon permeability. As the measurements with both gases were performed at the same mean free path to achieve the same impact of slippage flow on permeability, the observed difference in permeability was due to the adsorption of CO₂. These findings suggest that rock mineralogy affects how CO₂-shale interactions lead to permeability change.