A question in the minds of many is the potential use of saline aquifers in California for storing compressed air and for CO2 storage. This paper is the result of an extensive study on the geological properties of subsurface saline water containing geologic layers located below the freshwater limits in the San Joaquin Valley (SJV) of California. There are many thousands of pass-through wells drilled for hydrocarbon extraction in the area that can provide subsurface information on the saline aquifers. We discuss some of the saline aquifer properties and geologic aspects associated with the subsurface storage of compressed air and or carbon dioxide.

The raw database to generate the information included archives of CalGEM with respect to the existing and previously drilled oil and gas wells in the SJV Basin as well as separate studies by the USGS, Kang (2016), and Gillespi (2017). We mapped these aquifers across the valley and estimated ranges of pore volumes, the deliverability and the injectivity range for storage purposes. We also studied the sealing characteristics of these sands with respect to over and under burden and the geologic faulting in the San Joaquin Basin. We studied the drilling reports of many key wells and identified the lithologies of interest and examined relevant petrophysical properties. Estimates of capacity and deliverability were generated for these intervals. The legal ownership issues of operating these saline aquifers as storage were not part of this study.

Our critical observations include aspects of salinity, petrophysical properties, and the areal extent. Knowing the salt content of in-situ water is essential for site selection and the economics of repurposing idle wells to connect to these aquifers. The base of drinkable water (USDWs) (<10,000 mg/L) deepens northwest to southeast over the San Joaquin Basin, likely due to significant freshwater recharge from the Sierra Nevada Mountains. In the northwestern portion of Kern County, a number of wells intersect waters with 3000 and 10,000 ppm above 2000 ft TVD, within the Tulare Formation. At North Belridge field, a salinity reversal is apparent below 6900 ft., and salinities for zones below 7200 ft. range from 10,000 to 32,000 ppm (Gillespi, 2019). From the maps and correlative sections that relate to the areal extent of the target saltwater sands, we estimated the range of storage volumes, injectivity, and deliverability capacities for various wet sands.

The information generated and included in the paper is a reference point for the operators in the SJV, CA. It can help with the site selection for potentially converting some or all existing idle wells that are on the verge of abandonment and repurposing the wells for energy storage and for subsurface CO2 and other waste disposal purposes using the shallow saline aquifers.

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