THE EVOLUTION OF RESERVOIR FRACTURE PERMEABILITY DUE TO CO2 INJECTION

The effects of CO₂ injection on permeability in naturally fractured reservoirs can be predicted by identifying fracture orientations that will become critically stressed due to injection-related pore pressure changes. The fracture networks and stress fields in the potential injection zones of the Rock Springs Uplift (RSU) CO₂ sequestration site, southwestern Wyoming, have been characterized from image logs and geomechanical analyses. These analyses were performed using extensive petro-physical well logs, image logs, pressure and Micro-Frac test data, and core; these data were collected from stratigraphic test well, RSU#1. Due to failed micro-frac tests during the drilling of RSU#1, a rock mechanics approach was used to constrain the magnitudes of the horizontal stresses.

Image logs show no preferred fracture orientation, and fracture density is variable throughout each potential injection zone. In these two zones at the RSU#1 location, pore pressures are below the hydrostatic gradient, as measured from mini-DST and VIT tests. The magnitude of the vertical principal stress was calculated at specific depths by calculating the weight of overburden from density logs. The orientation of the maximum horizontal stress is 085°±15°, determined by orientation of wellbore breakouts. The magnitude of the minimum horizontal stress can be measured directly by Micro-Frac or extended leak-off tests, but these tests were not run to completion in RSU#1. Instead, horizontal stress magnitudes were constrained using methods developed by Barton, Zoback et al. (1988). Measured breakout widths from the RSU#1 were compared to predicted widths, which were calculated from laboratory rock strength measurements and other known geomechanical parameters.

Natural fractures at current subsurface conditions at the RSU are not critically stressed due to low pore pressure relative to in-situ stress. At injection pressures above formation pore pressure and below the fracture gradient, some fractures should become critically stressed. This would both increase reservoir permeability and introduce additional permeability anisotropy. This type of information can be used to optimize the CO₂ injection strategy and modeling of the CO₂ plume.