Experimental observations of heterogeneous strains inside a dual porosity sample under the influence of gas-sorption: A case study of fractured coal

Abstract A “permeability equilibration time” is typically assumed in interpreting permeability measurements – indicating that equilibration has been reached and both sorption-induced changes in deformation and their impact on permeability evolution have ceased. However, for extremely low matrix permeability (tight) dual porosity rocks, this “permeability equilibration time” may easily exceed the time interval between two consecutive permeability measurements – invalidating the interpretation of a steady permeability if the non-steady state conditions are not correctly accommodated. This is especially important where pressure diffusion from fracture to matrix results in a non-monotonic and non-asymptotic approach to a steady permeability, but instead contains multiple stages, plateaus and permeability reversals. We validated this hypothesis through experiments and analysis. Experiments measured the non-monotonic and scale-dependent deformations of fracture and matrix and linked these directly to the dynamic evolution of reservoir permeability. These laboratory strain measurements were integrated with numerical analyses to explore how mass and stresses transferred between matrix and fracture and were coupled under conditions of constant confining pressure. Strain gauges were distributed to directly measure stress transfer between matrix and fracture and interrogated deformation at different scales and at different proximities to control fractures. The prismatic sample of coal was tested under freely expanding boundary conditions. Optical microscopy and X-ray CT imaging were used to define the fracture distribution throughout the sample with mercury intrusion (capillary) porosimetry (MICP) constraining the pore size distribution and enabling independent estimation of matrix permeability. A numerical model was built and verified by matching measured strains and then applying this to model the evolution coal permeability from initial to ultimate equilibrium. Both the experimental and numerical results show that the final equilibrium state (pressure, stress and mass contents) for the matrix system extends to months rather than hours and suggests that some current permeability data may therefore reflect a non-equilibrium permeability state. Results also show that during this non-equilibrium condition, the swelling of the matrix near the fracture will cause not only compaction and narrowing of the fracture, but also shrinkage of the matrix that is distant from the fracture under constant confining pressure condition. Both experimental and numerical results demonstrate that the evolution of non-equilibrium strain/permeability is determined by the matrix-fracture interactions, including sorption-induced swelling/shrinking, through transient stresses in matrix and fractures. And that these non-equilibrium stress transfers determine the dynamic permeability evolution during gas extraction (e.g., CH4) or injection (e.g., CO2) at reservoir scale for tight dual porosity rocks (e.g., coal and shale).