MR11B-1878 presented at 2010 Fall Meeting, AGU, San Francisco, Calif.
The characterization of rock strength for cap rock, low-permeability and high capillary entry pressure, lithologies is important for modeling their response to increased fluid pressures in CO2 sequestration schemes and other geo-engineering operations such as waste disposal and recovery of fluids from plays with insufficient permeability. We investigate Mesozoic fine-grained sequences to evaluate the nature of sedimentological and structural processes that may control the nature of brittle deformation observed in these sequences. To characterize these fine-grained sequences we use a combination of data obtained from outcrop, including fracture orientation and density, lithologic stacking patterns, grain-size distributions and mineralogy to predict the potential for fluid bypass. In addition we also use publically available wireline log data to estimate the rock strength of these units in the subsurface. Outcrop observations of the mixed silisiclastic carbonate sequence of the Carmel Formation shows vertically continuous fractures crossing lithologic boundaries and analysis of scan-line and fracture orientation data indicate three discontinuity sets. Sets one and two are a conjugate pair of non-mineralized discontinuities which include faults and fractures, with a mean orientations of 323°/78° and 100°/76° respectively and discontinuity spacing ranging between 0.2-0.4 per meter. Set three is a near vertical, mineralized fracture set which is cross-cut by fractures of set one and two, with a mean orientation 31°/80 and a fracture spacing of 0.2 fractures per meter. Use of publically available wireline log data allows us to estimate rock strength over larger scales than those provided by the local outcrops. The wells selected for analysis are located within a 15-km radius from the outcrop location. Gamma ray, sonic, and density logs from 7 wells were digitized from scanned originals. The inverse of the sonic log data was taken to obtain a velocity value (Vp), no dipole sonic logs are available therefore shear velocity was calculated using empirical relationships established by other workers. Having both compression and shear velocity as well as bulk density values we are able to empirically determine Poisson’s Ratio and Young’s Modulus. Poisson’s Ratio values for these wells ranges between 0.18-0.48, and Young’s Modulus results range from 7GPa-50 GPa. Most of the results obtained by this data analysis fall within the published ranges for these rock types, but the data show variability, which must be further evaluated to understand if these are related to geology or artifacts associated with the wireline.
Petrie, Elizabeth S. and Evans, James P., "Use of wireline logs to estimate strength of cap-rock lithologies" (2010). Graduate Student Posters. Paper 9.