Dual Compressibility Characteristics of Lignite, Subbituminous, and High-Volatile Bituminous Coals: A New Insight into Permeability

Abstract

Pore compressibility is an important physical property of coal, which controls geomechanical behavior and hydrocarbon reserve evaluation of coal seams. However, the representation of pore compressibility is usually oversimplified in coalbed methane (CBM) reservoir analysis. This study proposes the adoption of the dual compressibility concept, where both accessible porosity (connected pore) and the inaccessible portion of the coal compressibility contribute to the storage and transport capacities of coal reservoirs, provided the availability of mercury porosimetry data. We characterize dual compressibility as a function of confining pressure and identified the effect of factors, such as coal rank, moisture content, and porosity. It was found that pore compressibility reduces decreased one order of magnitude, from 10−3–10−2 to 10−4–10−3 MPa−1, with confining pressure increasing up to 413.3 MPa. The calculated compressibility of accessible pores is one-to-two orders of magnitude higher than the inaccessible portion of the coal. Using a compressibility correction on pore volume connectivity and permeability estimates showed a 23% reduction in those estimates. Results suggest that high pore compressibility affects storage and transport capacity more significantly than previously regarded. We propose a new model to estimate coal permeability as a function of confining pressure and accessible pore compressibility. The model uses integrated intrusion correction and pore compressibility values from the Katz–Thompson method. Pore volume and permeability response with pressure were evaluated according to coal rank, macro-lithotype, and seam. This work provides an improved methodology to estimate hydrocarbon reserves and CBM reservoir performance through more accurate pore volume and permeability evaluations from mercury porosimetry data.