Geochemical Investigation for CAES Project in a Depleted Gas Reservoir

 San Joaquin County, CA

Compressed Air Energy Storage (CAES) in depleted gas fields is being investigated as a potential means to help store energy during times of excess electricity generation from wind and solar energy projects. Although depleted gas fields have been found to offer advantages for energy storage in California’s power distribution grid, they also present unique challenges. Among these is the possibility of geochemical reactions when reservoir materials and contents are exposed to air for the first time in millions of years.

Geochemical Investigation 
As part of a pilot study funded by the U.S. Department of Energy, the California Public Utilities Commission and the California Energy Commission, JJ&A investigated the nature, extent and potential effects of geochemical changes during air injection and flow testing in a gas reservoir in San Joaquin County.

The project included a review of available literature, evaluation of log and core test data from a selected gas field in San Joaquin Valley that is about 1 mile deep, testing of a core sample in a controlled pressure chamber to assess changes and reaction products when the core is exposed to air, and testing of gas samples, produced water samples and solids samples collected during performance of an in situ air injection and withdrawal test. X-Ray Diffraction, thin section microscopy and scanning electron microscopy performed on core samples identified low concentrations of pyrite, siderite and amphibolite, as well as iron-bearing clays that could potentially react with oxygen to produce iron oxides and other reaction products. During core chamber testing, a small amount of oxygen in the core chamber was found to be consumed and carbon dioxide was generated. Stable isotope testing indicated the carbon dioxide was generated from carbonate in the core rather than from oxidation of organics.

During full scale air injection and withdrawal testing, temperatures increased significantly within the reservoir. At the same time, testing indicated that oxygen was being consumed and carbon dioxide was being produced, while the pH of produced water was decreasing, and sulfate, iron, potassium, and sodium concentrations were increasing. Solids ejected during withdrawal testing consisted mainly of iron oxide phases including magnetite and goethite. The primary reactions interpreted from these observations were pyrite and siderite oxidation producing iron oxide, sulfuric acid, carbon dioxide and heat, acid reaction with siderite, and hydrolysis of silicates. Evidence of significant microbial activity was not detected.

JJ&A provided recommendations to manage oxygen depletion and corrosivity effects, including further investigation, heat flow modeling and monitoring, and reservoir development and operating procedures. Significant porosity and permeability changes were not expected to occur.