In the 1970 to 1987 time period when wells were drilled into the offshore fractured reservoirs producing from the Monterey Formations, there were uncertainties about the usefulness of conventional logs such as the gamma ray, caliper, resistivity and porosity logs. Fractured cherts exhibit tight rock conditions that complicate interpretation of such logs. Availability of decades of production performance data has offered an opportunity to revisit these old logs to develop and test algorithms that can relate a set of subsurface petrophysical data reflected on such logs to actual observed performance data.
A workflow process has been developed that allows estimation of effective fracture density and spacing from the combined responses of conventional logs. These parameters were successfully correlated with actual performance data. The proposed fracture identification indices include, effective fracture density and fracture spacing. Fracture density is defined as the total number of fractures identified in the interval of interest normalized over the depth range while fracture spacing is defined as the average distance between fractures identified in the completion interval. The results of these two parameters were correlated with the production profiles of the wells analyzed in this study. The purpose for doing this was to provide an explaination for poor performing wells and identify re-drill opportunities in untapped intervals.
This workflow process was applied to the perforated intervals of eight wells producing from an offshore field in the federal waters off the California coast in the Santa Maria basin. It was observed that both the fracture density and fracture spacing results could be correlated with well production performance. The best performing wells generally had effective fracture densities greater than 0.22 fractures/ft. and spacings less than 4.30 ft. The proposed method offers opportunities for exploiting untapped intervals in the fields producing from the Monterey Formation.