USE OF WIRELINE LOGS FOR IDENTIFICATION OF ROCK STRENGTH VARIABILITY IN CAP-ROCK LITHOLOGIES

The characterization of cap-rock, low permeability and high capillary entry pressure lithologies is important for modeling the response of cap-rocks to increased pressures due to CO₂ injection. We evaluate the use of publically available wireline log data to provide empirical estimate of rock strength in order to determine the strength of top seal over a range of scales. This method is being used to characterize cap-rock lithologies in systems proposed for CO₂ geosequestration, these data will be combined with outcrop fracture density observations, petrology, lithologic stacking patterns and mineralogy to predict the potential for bypass.

Analysis to date includes wells with monopole and dipole sonic logs for comparison of the relationships established empirically by other workers and used in this study to estimate the dynamic values for Poisson’s Ratio and Young’s Modulus from publically available vintage well log data in Utah. This study focuses specifically on the Jurassic Carmel Formation, which is a cap-rock to the underlying proposed CO₂ injection reservoir, the Navajo Sandstone. This study compliments the well data with outcrop characterization of the Carmel Formation, which we split into 3 mechanical units based on lithologic stacking patterns, fracture density, and relationships observed between the percent shale and fracture spacing ratio.

Results obtained from the well log analysis fall within the published ranges for these rock types, however the data show a variability which is being evaluated further to understand if these observations are related to geology or artifacts associated with the wireline data. In future the use of these empirical estimates will provide a lower estimate for subsurface rock strength, as well as provide a means to closely tie outcrop observations to those made from subsurface data.