Fracture and Consequences of Caprock and Cement Observed by Integrated Triaxial Coreflood and X-ray Tomography

Abstract:

Mechanical damage to caprock and wellbore systems threatens the long-term integrity of CO2 storage reservoirs. While it is widely recognized that CO₂ injection can generate damage-causing stresses, there is little information on the hydrologic consequences of the resulting fractures in terms of the magnitude of potential CO₂ leakage. In this study, we perform experiments at reservoir conditions designed to fracture caprock and cement while simultaneously observing changes in permeability coupled with direct observation of fracture growth using x-ray video radiography and fracture geometry by x-ray tomography. Triaxial coreflood experiments using a direct-shear configuration were performed at confining pressures ranging from 3-22 MPa at 20 ^oC. Permeability was strongly dependent on confining pressure but also on the orientation of the fractures in relation to bedding. Samples fractured at high confining pressures had low permeability (typically <0.1 mD). Samples fractured at low confining pressures had permeabilities that ranged from 10s of mD across bedding to as high as 1 D parallel to bedding.

Video radiography was collected through observations parallel to the direct shear plane. These captured the rate of fracture growth (on the order of seconds) and were used in relation to the onset of elevated permeability to investigate fracture-permeability dynamics. X-ray tomography was conducted at pressure but under static conditions and showed that fracture apertures at high confining pressures were significantly smaller than fracture apertures recovered at atmospheric conditions. Preliminary results suggest that increasing material plasticity accompanying high confining pressures resulted in decreased permeability, smaller apertures, and more poorly connected fracture networks. Typical sequestration conditions correspond to the higher confining pressures used in this study, suggesting that the failure characteristics of caprock and cement may be capable of limiting fracture transmissivity in response to mechanical damage.