Inverted U-shaped permeability enhancement due to thermally induced desorption determined from strain-based analysis of experiments on shale at constant pore pressure

We explore the impact of thermally induced desorption on permeability evolution in shale at constant pore pressure. Permeability loss due to thermal expansion of mineral aggregates competes with permeability enhancement due to thermal desorption and shrinkage with increasing temperature. In experiments using core plugs of Marcellus shale, permeability increases 20% from 296 to 322 K for a pre-fractured sample and 11% from 304 to 334 K for an intact sample. Dynamic bulk modulus decreases from 14.6 to 11.8 GPa when permeability increases from 2.8 to 3.1·10⁻²¹ m², suggesting that pore volume is expanding due to desorption. We develop a model for thermal-sorptive permeability enhancement that accounts for pore evolution due to overprinted but competitive thermal and sorptive strains. A scaling factor between 0 and 1 is included to account for the volumetric boundary condition ranging from free boundary expansion to fixed bulk volume. The change in fracture aperture is directly impacted by fracture density. While permeability evolution in shales is generally characterized by a “U” shaped behavior with increasing pore pressure, our model shows that thermally induced permeability evolution at constant pore pressure is characterized as an inverted “U”. This is due to permeability enhancement at temperatures close to the reference temperature and permeability loss at higher temperatures. We attribute this to larger changes in adsorbed volume at lower temperatures competing with linear thermal strain that then outpaces desorption at higher temperatures. Both thermal and sorptive strains are modulated by the mineral distribution within the shale. Discretized images of mineral distribution suggest that there may be a larger local permeability enhancement than predicted by bulk strain measurements alone, due to the concentration of porosity near sorptive components. Our results include a novel analysis of permeability evolution due to desorption at constant pore pressure.