Groundwater salinity and historical water injection in oil fields in San Joaquin Valley

Produced water is often injected into the subsurface for purposes of water disposal or enhanced oil recovery techniques. Produced water is often highly saline (>10,000 ppm of total dissolved solids), which can alter the quality of nearby groundwater over time. The location and volume of these injection records are available as numerical data on public databases hosted by the Division of Oil, Gas, and Geothermal Resources. However, injection data prior to 1977 are only available as monthly reports from oil and gas producers. The undigitized nature of this pre-1977 data makes it difficult to understand the water injection history in and around oil fields during this period and how it has affected groundwater quality over time.

Produced water is often injected nearby groundwater with similar quality and must be contained away from nearby usable groundwater resources. Unfortunately, high injection volumes can increase pore pressure and create new unknown water migration pathways. The complexities of the subsurface make it difficult to predict these scenarios. However, historical water quality nearby high-volume injectors provide an insight into these processes within a specific area by identifying changes in groundwater quality over time.

Changes in groundwater quality can be measured by locating high volume water injectors and creating a time series of groundwater salinity estimations using borehole geophysical logs. Oil fields in San Joaquin Valley were selected based on volumes of pre-1977 water injection, well coverage, and available data. Individual pre-1977 well records combined with post-1977 records for select oil fields provide the location, volume, and time of injection. High volume and early injectors provide a time with less anthropogenic interference and closer representation of natural groundwater conditions. Groundwater salinity is estimated by using resistivity measurements from borehole geophysical logs from nearby wells in order to reveal any significant changes after water injection occurred.