3-d mapping of groundwater total dissolved solids at the Midway-Sunset Oil Field using borehole geophysics and historical produced water geochemistry

Abstract

Mapping the distribution of groundwater salinity can help us make better groundwater management decisions. Salinity mapping efforts using borehole geophysics and produced water geochemistry have been successful at many oil fields in California’s San Joaquin Basin. Estimates of total dissolved solids (TDS) are calculated using the resistivity-porosity (RP) method, based on Archie’s equation, which uses deep resistivity, porosity and temperature to calculate groundwater TDS from geophysical logs. These estimates can be tuned with TDS measurements from produced water geochemical records. At the Midway-Sunset oil field, there are challenges to applying the RP method. Decades of enhanced oil recovery operations have led to significant variations in the geothermal gradient, and detrital diatomite in sands causes geophysical porosity logs to overestimate total porosity. This is a problem because TDS calculations require resistivity, temperature, and porosity data. In addition, produced water samples collected near preexisting wastewater injection wells may be contaminated, and thus do not represent formation water salinity. To solve these challenges, we avoid resistivity logs likely affected by steaming using a spatiotemporal algorithm that discards resistivity logs located near preexisting injection wells. We used the same spatiotemporal algorithm to discard produced water geochemical TDS measurements located closest to preexisting injection wells. To correct for geophysical porosity tool mismeasurements, three different porosity models were explored. Selected estimates of TDS and geochemical measurements were then kriged to generate 3-d volume models of TDS. In the northern portion of the field, the TDS maps and cross sections reveal that the depth to the 10,000 parts per million (ppm) TDS boundary extends to deeper than 1500 feet below sea level, while in the southern region the boundary is much shallower, with a maximum depth of ~150 feet elevation. From our model, we conclude that stratigraphy, faults, and freshwater recharge are the most likely factors contributing to the existing salinity trend.

AAPG Datapages/Search and Discovery Article #90339 ©2019 AAPG Pacific Section Convention 2019, Long Beach, California, April 1-3, 2019