INTEGRATED SUBSURFACE CHARACTERIZATION OF SHEAR ZONES IN SALT DOMES: IMPLICATIONS FOR FUTURE UNDERGROUND ENERGY STORAGE

Storage caverns in salt domes account for 7% of the United States underground natural gas storage capacity. Originally designed for storing large volumes of natural gas and unrefined petroleum products, cavern designs are being repurposed for sustainable energy resources. In this study, we characterize a salt dome in the US Gulf Coast that is actively being used for natural gas storage, using a combination of geological and geophysical data to identify potentially hazardous structures that can impact future storage projects. We installed a temporary array of nodal seismometers with station spacings of 0.2-1.9 km to record seismicity over a 12-month period. A machine learning model was designed for detecting microearthquakes in high industrial noise settings while excluding signals from local anthropogenic sources. 152 microearthquakes with magnitudes between -3 to 2 M_L were detected at depths between 0.2 to 4 km within the salt. Events are preferentially clustered around a previously identified internal shear zone on the eastern side of the dome and a newly identified shear zone on the western side of the dome. Internal shear zones occur where the salt quality is generally degraded. A well integrity failure in 2021 that occurred during our recording, and prior documented incidents show that caverns leached within proximity of these shear zones experience frequent instability incidents, suggesting that shear zones directly impact cavern deformation. The effects on seven caverns were assessed using time-lapse 3D sonar surveys showing changes in cavern shape. Since shear zones are not readily mappable with reflection seismic surveys, our approach provides an improved methodology for mapping the structure of shear zones away from sparse well/cavern control. Our workflow can be applied at different stages of cavern field development ranging from site selection to developed facilities expansion and is useful for optimizing cavern placement for storing sustainable energy resources.