A New Method for Estimating Waterflood Oil Recovery Efficiency Using Post-Waterflood NMR and Dielectric Well Logs, Belridge Field, California

Abstract

It sounds like a contradiction -assessing waterflood oil recovery by layer in a reservoir without knowing the original saturation distribution. However, we have developed a new method for estimating layer oil recovery efficiency using nuclear magnetic resonance (NMR) and dielectric logs run in wells within a mature, onshore waterflood. We use post-waterflood NMR T2 signals to give the reservoir rock pore-body size distribution at depths along the wellbore. This depth-dependent distribution can be converted into a series of oil-water capillary-pressure curves. The capillary pressure curves can then be modeled into the original, pre-waterflood saturation height model (SHM), which gives original oil in place (OOIP). The dielectric log, also acquired postwaterflood, is used to measure the remaining oil distribution in the reservoir. The breakthrough concept is that the saturation difference between the NMR-derived saturation height model and the post-waterflood dielectric log, which corresponds to the amount of oil swept by the flood, can be measured post-waterflood "all in one go."Conventionally, pre-primary recovery well logs are needed to delineate native-reservoir saturation distributions and provide a baseline for OOIP. Although ~1500 resistivity well logs were acquired in the diatomite reservoirs of the South Belridge field in the San Joaquin Valley, California, between 1979 and 1987, our understanding of localized original saturation variations is complicated by many factors. These include primary depletion and the injection of variable salinity water over time. For these reasons, this "all in one go" alternative method was developed to estimate OOIP and waterflood sweep efficiency in the diatomite.The biggest test of this new technique came when comparing the OOIP calculated from the NMR-inferred capillary pressure model against the existing geologic model (localized averages of early well logs) used for the South Belridge field. We found that the OOIP calculated by the new technique agreed favorably with that calculated by the existing geologic model. As a result, a re-assessment of sweep efficiency in the multi-layered South Belridge waterflood is being undertaken on a layerby-layer basis. This new, post-waterflood NMR and dielectric logging assessment of sweep has potential applications to other reservoirs.