The effect of microporosity on transport properties in tight reservoirs

Diagenetic changes (e.g. cementation, compaction) in tight gas sandstones (TGSS) often disconnect the original, inter-granular pore space and further create microporosity within the original grains (e.g. by dissolution) or by filling the inter-granular porosity with clay. A petrophysically rigorous fundamental model of TGSS that accounts for microporosity would make the evaluation, development and stimulation of tight gas sandstone development more robust. The reduced connectivity of matrix pores has a profound effect on transport properties such as absolute and relative permeability, resistivity and capillary pressure - saturation relationships. To address this, we construct networks that incorporate both inter-granular (primary) porosity and microporosity, and use a network model to estimate flow properties. We present algorithms to geometrically match pore throat networks from two separate length scales that can be extracted directly from 3D rock images, or be constructed to match the relevant measured properties. Microporosity and its spatial distribution have a profound effect on the relative permeability curve. When inter-granular network is disconnected (but the microporous region is not), we provide capillary pressure - saturation curves in cases thus far unresolved with any other type of modeling. The results from model media reasonably match published experimental data. While we are motivated by TGSS, with suitable characterization the model is applicable to other reservoirs with dominant microporosity component (shale, carbonates).