Pore-Space Statistics and Capillary Pressure Curves From Volume-Controlled Porosimetry

Abstract

Summary Far more information about pore spaces of reservoir rock samples can be obtained from volume-controlled mercury porosimetry than from conventional pressure-controlled mercury porosimetry. Yuan and Swanson demonstrated this in landmark experiments with their apparatus for pore examination (APEX). With a mechanism-based, computer-facilitated model of pore-level displacement, we discover how much useful information about a porous medium can be extracted from volume-controlled mercury porosimetry. APEX mercury injection is quasistatic; so is our simulation. Displacements under this condition consist of smooth, reversible changes punctuated by jumps in capillary pressure, Pc, the sequence of which follows from the structure of the porous medium and the saturation history. Thus, careful examination of Pc fluctuations provides detailed information about pore-size distributions. The results account quantitatively for the APEX mercury Pc curves precisely measured by Yuan and Swanson. In our work, sample size—an aspect not reported previously—is a main factor in APEX response. Using Monte Carlo simulation, we find the optimum-sized specimen for examining a pore space with particular properties and investigate withdrawal experiments, withdrawal after partial reinjection, and full scanning loops.