How Permeability Depends on Stress and Pore Pressure in Coalbeds: A New Model

TLDR

Permeability in naturally fractured coal formations is highly sensitive to changes in effective stress or pore pressure. The study introduces a single‑equation model to compute pore volume compressibility and permeability in coal as functions of effective stress and matrix shrinkage. The model derives PV compressibility from fundamental reservoir parameters and applies a single equation suitable for uniaxial strain conditions, with stress/permeability functions calibrated against history‑matched rates from a San Juan Basin boomer well. The model shows that permeability varies with drawdown, PV compressibility is high and variable, and permeability can rebound at low drawdown for high modulus and matrix shrinkage, with calibrated stress/permeability functions matching observed well rates.

Abstract

Summary In naturally fractured formations such as coal, permeability is sensitive to changes in stress or pore pressure (i.e., changes in effective stress). This paper presents a new theoretical model for calculating pore volume (PV) compressibility and permeability in coals as a function of effective stress and matrix shrinkage, by means of a single equation. The equation is appropriate for uniaxial strain conditions, as expected in a reservoir. The model predicts how permeability changes as pressure is decreased (i.e., drawdown). PV compressibility is derived in this theory from fundamental reservoir parameters. It is not constant, as often assumed. PV compressibility is high in coals because porosity is so small. A rebound in permeability can occur at lower drawdown pressures for the highest modulus and matrix shrinkage values. We have also history matched rates from a boomer well in the fairway of the San Juan basin by use of various stress-dependent permeability functions. The best fit stress/permeability function is then compared with the new theory.