PORE CONNECTIVITY IN NATURAL ROCK: EVIDENCE AND IMPLICATIONS

Imbibition into porous media (soils and rocks), if gravity effects are negligible, normally leads to mass uptake proportional to time^0.5. The proportionality factor of 0.5 – the square-root-of-time scaling – is generally accepted as a mathematical fact. But this 0.5 power applies only to rocks with well-connected pore spaces. Rock with poorly-connected pores exhibits anomalous behavior and an exponent of 0.26 at some scales, and classical behavior at other scales. Pore connectivity in natural rock affects many fluid flow and transport behaviors (e.g., imbibition, diffusion).

This work integrates newly developed experimental (laser ablation coupled with ICP-MS) and pore-scale network modeling methods to probe pore connectivity in a suite of sedimentary, igneous, and metamorphic rocks. Experiments produced imbibition exponents from 0.2 to 0.5. Some rocks showed initial imbibition proceeding with an exponent of 0.26, then later switched to “normal” (0.5) behavior. The imbibition slope of the same rock was found to be related to the sample shape: tall thin samples were more likely to exhibit the 0.26 exponent, and to cross over to 0.5-type behavior later, while short, squat samples were less likely to display the 0.26-type behavior at all.

Preliminary results confirm that the pore space is sparsely connected in many natural rocks. This has significant implications for gas recovery in the Barnett Shale. It is proposed that study of pore connectivity in shale reservoirs is very needed to resolve questions central to fracture-matrix interactions and their impact on increased gas production.