The effect of deformation on permeability development in anhydrite and implications for caprock integrity during geological storage of CO2

Geological storage of CO2 in depleted oil and gas reservoirs is one of the most promising options to reduce atmospheric CO2 concentrations. Of great importance to CO2 mitigation strategies is maintaining caprock integrity. Worldwide many current injection sites and potential storage sites are overlain by anhydrite-bearing seal formations. However, little is known about the magnitude of the permeability change accompanying dilatation and failure of anhydrite under reservoir conditions. To this extent, we have performed triaxial compression experiments together with argon gas permeability measurements on Zechstein anhydrite, which caps many potential CO2 storage sites in the Netherlands. Our experiments were performed at room temperature at confining pressures of 3.5-25 MPa. We observed a transition from brittle to semi-brittle behaviour over the experimental range, and peak strength could be described by a Mogi-type failure envelope. Dynamic permeability measurements showed a change from 'impermeable' (< 10-21 m2) to permeable (10-16 to 10-19 m2) as a result of mechanical damage. The onset of measurable permeability was associated with an increase in the rate of dilatation at low pressures (3.5-5 MPa), and with the turning point from compaction to dilatation in the volumetric versus axial strain curve at higher pressures (10-25 MPa). Sample permeability was largely controlled by the permeability of the shear faults developed. Static, postfailure permeability decreased with increasing effective mean stress. Our results demonstrated that caprock integrity will not be compromised by mechanical damage and permeability development. Geofluids (2010) 10, 369-387.