The impacts of effective stress and CO2 sorption on the matrix permeability of shale reservoir rocks

•Effective stress greatly affects fluid flow paths in relatively permeable samples.•Nanoscale porosity is sensitive to CO2 sorption in ultra-low permeability samples.•Increased surface area from fracking with CO2 may be counteracted by sorption effects.

We assess the impacts of effective stress and CO2 sorption on the bedding-parallel matrix permeability of the Utica shale through pressure pulse-decay experiments. We first measure permeability using argon at relatively high (14.6 MPa) and low (2.8 MPa) effective stresses to assess both pressure dependence and recoverability. We subsequently measure permeability using supercritical CO2 and again using argon to assess changes due to CO2 sorption. We find that injection of both argon and supercritical CO2 reduces matrix permeability in distinct fashion. Samples with permeability higher than 10−20 m2 experience a large permeability reduction after treatment with argon, but a minor change after treatment with supercritical CO2. However, samples with permeability lower than this threshold undergo a slight change after treatment with argon, but a dramatic reduction after treatment with supercritical CO2. These results indicate that effective stress plays an important role in the evolution of relatively permeable facies, while CO2 sorption dominates the change of ultra-low permeability facies. The permeability reduction due to CO2 sorption varies inversely with initial permeability, which suggests that increased surface area from hydraulic stimulation with CO2 may be counteracted by sorption effects in ultra-low permeability facies. We develop a conceptual model to explain how CO2 sorption induces porosity reduction and volumetric expansion to constrict fluid flow pathways in shale reservoir rocks.