Hydro-mechanical coupling in porous rocks: hidden dependences to the microstructure?

Abstract

SUMMARY While hydro-mechanical coupling in rocks is generally well understood, exotic rock poroelastic responses—such as unexpected dependence to fluid diffusion time and low skeleton moduli—have been reported. Hydro-mechanical coupling, or poroelasticity, explains how fluid-saturated rocks respond to either confining or fluid pressure variations. This coupling is usually inferred from the apparent mechanical and hydraulic properties: mechanical properties determine the strain level experienced by the rock when submitted to pressures at a timescale when fluid pressure is equilibrated, which is in turn ruled by hydraulic properties. However, the coupling between properties might not always be straightforward, particularly for rocks in which two distinct families of pore types coexist: spherical pores and cracks. Comparing it with reported laboratory data sets on pressure-dependent hydraulic and elastic properties in sandstones of different porosity confirms that the well-known simple concepts of networks in parallel apply and yield opposite dependencies for the two properties on the two pore families. Because hydro-mechanical coupling implies that the two properties—that depend differently on the pore families—will be interdependent, we further apply the same concept of parallel network. It yields that, although under apparent drained conditions, typical poroelasticity experiments could underestimate the rock compressibility ${C_{\mathrm{ bp}}}$, measured as a response to fluid pressure variation, and underestimate the related skeleton (or unjacketed) bulk modulus ${K_\mathrm{ s}} = \ 1/{C_\mathrm{ s}}$.