TEMPERATURE EVOLUTION IN A SHALLOW AQUIFER ABOVE A STEAMED HEAVY-OIL RESERVOIR IN THE SAN ARDO OIL FIELD, MONTEREY COUNTY, CALIFORNIA

During production of heavy oil, hot steam is often injected into reservoirs to aid in production by lowering oil viscosity. As steam passes downward through the injection well, some heat is lost via conduction through the casing and into overlying aquifers, causing a temperature increase in the aquifer. It is important to understand how lost heat affects the overlying aquifer, since calculation of groundwater total dissolved solids (TDS) from borehole-derived resistivity requires temperature as an input.

As a first step in understanding the temperature distribution and evolution of aquifers above steamed reservoirs, we analyze the temperature structure of a fresh-to-brackish aquifer overlying two steamed reservoirs in the San Ardo Oil Field in Monterey County, California. First, borehole temperature logs from about 110 observation or long-inactive wells collected by the operator during 2008 to 2019 were downloaded from the California Geologic Energy Management Division online Well Finder database. Each of these logs consists of continuous temperature data recorded from near the surface through the overlying aquifer and into the steamed reservoir. These spatially discrete temperature data were then interpolated to 3-dimensions on an annual basis using geostatistical methods (kriging) to visualize the temperature structure of the overlying aquifer.

Preliminary results show spatially heterogeneous temperatures in the overlying aquifer that evolve in response to changing location and volume of steam injection. Broad zones of the aquifer are about 20°C warmer than what would be expected from ambient conditions. Temperature increases indicate a temporal lag after steam injection begins in an area, possibly due to thermal inertia. Future work will describe the quantitative relationship between injection volumes, other operational variables, and temperatures.