Heterogeneous rocks with broad pore size distributions, such as carbonates and clay-rich sandstones, often display widely varying behavior. The interaction of micro- and macro-porosity in these complex rocks makes them difficult both to understand and to characterize. Pore scale imaging and modeling has the potential to greatly improve our understanding and predictive capabilities of these complex rocks. However, a single imaging technique can rarely capture the relevant length scales in desired detail due to restrictions of image size vs. resolution. Similarly, direct simulations of transport properties in heterogeneous rocks with broad pore size distributions are prohibitively expensive computationally.
In the present work we present a multi-scale imaging and modeling workflow to compute transport properties of rocks with wide pore size distributions. A dry/wet micro-CT imaging sequence is used to spatially map the porosity distribution and connectivity of macro- (i.e. resolved) and micro-porous (i.e. sub-resolution) regions. The registered 3D porosity map is transformed into a multi-scale pore network model where macro-porosity regions are modeled in a conventional pore network manner while micro-porous network elements are treated as a continuous porous medium. The necessary input parameters to characterize the micro-porous elements are computed from high resolution BSE images of different micro-porosity regions.
The multi-scale workflow was applied to two samples of tight sandstones. Computed petrophysical properties as well as capillary pressure and relative permeability relationships are compared with available experimental data. The agreement between computed and measured results is encouraging. The results clearly demonstrate that the distribution and connectivity between macro- and micro-porosity regions control the observed transport properties.