Abstract
Enhanced oil recovery in the form of steam injection is used to allow the production of heavy oil from the Lombardi and Aurignac Sands in Monterey County, California. This steam injection unevenly heats the shallow aquifer overlying the oil reservoir, modifying the natural thermal gradient. This makes mapping groundwater total dissolved solids (TDS) with borehole geophysics and Archie's equation difficult as resistivity is temperature dependent, requiring an understanding of a heterogeneous 3-dimensional temperature field. In order to map TDS in the shallow aquifer above the oil reservoir in this field, continuous temperature measurements from temperature observation wells were used to map the temperature for two time periods. For the early-steam time period (the 1960s, at the beginning of steam injection) a geothermal gradient was fit to the minimum of the continuous temperature data, under the principal that the lowest temperature from the earliest available data should approximate the initial geothermal gradient. For the late-steam time period (post 2000, after decades of steam injection) the temperature in the subsurface was mapped in a 3-dimensional kriged volume derived from available continuous temperature data.
Using the temperature data, resistivity measured from clean sands, and available porosity data, TDS was calculated using Archie's equation for the early-steam and late-steam periods, then interpolated into a 3-d volume. However, because of limited resistivity data and relatively high uncertainty, it could not be determined if TDS changed between the two time periods.