Abstract
In mature oil fields of Kern County, California, enhanced oil recovery techniques are often a useful aid for the production of oil. Waterflood is one type of enhanced oil recovery technique that involves injecting large volumes of water into reservoirs to increase pore pressure and displace hydrocarbons toward oil production wells. While injected fluids are intended to be contained in the injection zone, there is some concern that they may migrate or displace deep saline groundwater into shallower usable groundwater resources. While the most direct way of assessing changes in salinity in overlying aquifers is groundwater geochemistry, these are rather scarce in many oil fields of interest. To supplement these measurements, groundwater total dissolved solids (TDS), was calculated from borehole resistivity logs from oil and gas wells using standard petrophysical equations. These calculations, collected in a small area and over time, can provide insight into changing salinity conditions in formations above and adjacent to injection zones. This method is applied to the North Coles Levee oil field, which had high-volume water injection beginning in 1970 for a waterflood project into the deep Stevens reservoirs at the same time that source water was extracted from the overlying Tulare aquifer. Borehole resistivity was collected from clean, wet sands from overlying formations in an area with containing both waterflood injection and water-source production wells. These results indicate an increase in TDS in the shallow Tulare formation of ~ 3,000 ppm and decrease in TDS in the Etchegoin Formation by ~ 2,000 ppm. This suggests that the increase in salinity could be due to upward flow of groundwater caused by a combination of water injection in deeper reservoirs increasing pressure and water extraction in shallower aquifers decreasing pressure. While it is not clear what the mechanisms are, the water-source wells may have lowered shallow hydraulic head enough to cause upward vertical pressure gradients. Saline water may then have taken advantage of potential conduits such as wells with old spud or abandonment dates and faults.