Shuvajit Bhattacharya, Alex Bump, Seyyed Hosseini, Bureau of Economic Geology, The University of Texas at Austin
An accurate understanding of overpressure at a carbon capture and storage (CCS) site is critical to ensure geomechanical stability, minimize CO₂ compression cost, and produced brine management. Pressure build-up in an overpressured area due to injection may result in seal failure and restrict injection rates. The study presents the importance of overpressure and its identification and mapping in delineating the boundary of the carbon storage window in the subsurface. Our study also demonstrates how to repurpose existing and proven oil and gas technologies for CCS. The carbon storage window is between the supercritical depth and “hard” overpressure (pore pressure more than 0.7 psi/ft) boundary. We analyze >200 wells in the southern Gulf Coast (Texas) to identify and map overpressure. We use wireline logs (e.g., spontaneous potential, gamma-ray, compressional sonic, and resistivity), drilling mud weight data, bottom hole temperature, and relevant literature for this study. The storage window contains several Miocene age sandstone packages of varying quality. Across the overpressure zone, the sonic travel time increases, and resistivity decreases significantly. Based on well correlation and mapping of the study area, the depth of the hard overpressure regime is at ~9,000-10,500 ft, across a thick shale section. This situation is favorable as it allows testing multiple Miocene sandstone reservoirs for carbon storage in the same wells. However, there are indications of “soft” overpressure (pore pressure between 0.465 psi/ft and 0.7 psi/ft) at a shallower depth (7,000-8,000 ft) in some locations. Because of the sparsity and vintage of the data, along with well drillers’ tendency to drill in an overbalanced way (i.e., application of more mud weight than what is exactly needed at a shallow depth), one would want to test all sandstone reservoirs in the entire storage window. Our overpressure maps mostly match the Burke et al., 2012 (USGS Open-File Report 2013–1058) maps of regional overpressure of the onshore and offshore Gulf of Mexico, primarily built based on the mud weight data. However, our maps provide more granularity, which is needed for site-scale CCS and regulatory compliance. Our methods and maps reduce the uncertainty in picking the top depth of the overpressure zone in multiple wells by a few hundred feet, compared to the published regional studies.