Abstract
Groundwater is a vital water source for California's vast population and agricultural industry. However, recent drought conditions have increased the demand for usable groundwater, making careful management necessary for its preservation. In California's oil and gas fields, there is concern that with oil and gas production occurring at relatively shallow depths there may be an increased risk of operations causing unintended contamination of nearby usable groundwater. The United States Environmental Protection Agency defines non-exempt groundwater with less than 10,000 milligrams per liter (mg/L) of total dissolved solids (TDS) as an Underground Source of Drinking Water (USDW). It is thus a legally protected source of usable groundwater. It is essential to know the distribution of usable groundwater to help preserve this resource and establish a water quality baseline. To do this, we constructed maps and cross-sections that show the distribution of usable groundwater using TDS concentrations throughout the Cat Canyon Oil Field. To map the distribution of groundwater TDS at the Cat Canyon Oil Field in three- dimensions (3D), TDS is estimated using geophysical measurements taken from oil and gas borehole geophysical logs using Archie’s equation. These estimates, along with published produced-water and groundwater geochemical data, are then interpolated from discrete points into a 3D volume using kriging methods. The geochemical data are further used as validation data for the TDS model.
However, mapping TDS in the Cat Canyon Oil Field presents unique challenges due to the effects of thermally enhanced oil recovery operations (TEOR) in the East, Central, and Sisquoc production areas which have disturbed natural temperature conditions. Because temperature impact TDS calculations, two separate TDS models were built to address these challenges. The first model uses all available geophysical data, while the second model calculated TDS using a series of data subsets filtered based on the distance between thermally stimulated wells and wells with geophysical measurements. The objective of the dual TDS model approach was to analyze the effects of TEOR operations and build a TDS model with data not impacted by TEOR operations.
Based on the lowest mean absolute percentage error (MAPE) of 38.74% from all the TDS models, the optimal radius from which to exclude wells likely impacted by TEOR operations was found to be 250 meters. The resulting subset of data and the TDS model were selected as the model iteration that best represented natural (pre-steam) conditions.
Results from TDS maps and cross-sections reveal that TDS varies in concentration with depth across the field. In the upper 600 meters of the study volume, TDS decreases from south to north in the oil field. At -400 meters elevation, the TDS distribution changes, where in the bottom 400 meters of the modeled volume, TDS concentrations grade from highs in the north to low in the south. The depth of the boundary of usable groundwater (10,000 mg/L TDS) was found to be variable across the field. The boundary deepens from the Careaga Formation in the Sisquoc Area into the Siquoc Formation in the East and Central Areas. A low permeability layer at the top of the reservoir formations may act as a seal to vertical movement between aquifer and reservoir formations, while the northeast dip direction of the overlying aquifer formations directs groundwater flow from the southeast to the northwest.