Effects of Particle Size Reduction on the Pore Structure and Accessibility in Natural Porous Materials

Abstract

Unconventional gas reservoirs comprise natural porous materials, such as shales, with low permeability, which can limit gas recovery. Pore structure evaluation in shales often relies on gas adsorption (GA) and mercury intrusion porosimetry (MIP), and the assessment of the isolated porosity and pore accessibility is frequently overlooked. In this study, we examined the effect of the sample particle size on GA, MIP, and helium pycnometry (HeP) for shales. For comparison, similar assessments were made for natural porous zeolite with interconnected micro-, meso-, and macropores. GA studies revealed that, even with a particle size reduction to 212 μm, isolated pores remained inaccessible. Size reduction in materials with interconnected pores, however, showed no impact on pore structure evaluation using GA. In MIP studies on shales, a secondary effect from interparticle intrusion for pore diameters of <1 μm was observed for particle sizes below 212 μm. Analysis of the trends in gas adsorption isotherm hysteresis with particle size reduction provided insight into the pore structure accessibility. An increase in hysteresis was observed when isolated pores were accessed, while it decreased with a reduction in the particle size for materials with interconnected pore structures.