Effects of CO2 on mechanical variability and constitutive behavior of the Lower Tuscaloosa Formation, Cranfield Injection Site, USA

We characterize geomechanical constitutive behavior of reservoir sandstones at conditions simulating the Cranfield Southeast Regional Carbon Sequestration Partnership injection program. From two cores of Lower Tuscaloosa Formation, three sandstone lithofacies were identified for mechanical testing based on permeability and lithology. These include: chlorite-cemented conglomeratic sandstone (Facies A); quartz-cemented fine sandstone (Facies B); and quartz- and calcite-cemented very fine sandstone (Facies C). We performed a suite of compression tests for each lithofacies at 100 degrees C and pore pressure of 30 MPa, including hydrostatic compression and triaxial tests at several confining pressures. Plugs were saturated with supercritical CO2-saturated brine. Chemical environment affected the mechanical response of all three lithofacies, which experience initial plastic yielding at stresses far below estimated in situ stress. Measured elastic moduli degradation defines a secondary yield surface coinciding with in situ stress for Facies B and C. Facies A shows measurable volumetric creep strain and a failure envelope below estimates of in situ stress, linked to damage of chlorite cements by acidic pore solutions. The substantial weakening of a particular lithofacies by CO2 demonstrates a possible chemical-mechanical coupling during injection at Cranfield with implications for CO2 injection, reservoir permeability stimulation, and enhanced oil recovery. (C) 2016 The Author(s). Published by Elsevier Ltd.