Integrated Petrophysical Studies for Subsurface Carbon Sequestration

Petrophysics is a core component of subsurface characterization and monitoring for carbon sequestration. The United States Environmental Protection Agency’s underground injection control rules for carbon storage (class VI wells) include subsurface characterization, CO2 plume modeling, and monitoring during pre-injection, injection, and post-injection, where petrophysicists can play important roles. Many carbon capture and storage (CCS)-related petrophysical studies are limited to porosity and permeability for storage capacity and injectivity; however, there are several scientific questions and regulations that need a thorough formation evaluation. We present three case studies to show how integrated fit-for-purpose petrophysical approaches helped address a few critical questions, such as the impact of formation water salinity, pore pressure, and fractures on the feasibility of CO2 storage. Although not addressed in this study, we recognize the importance of petrophysics to study the integrity of the confining zones, CO2 trapping mechanisms (i.e., CO2 mineralization), wellbore integrity, and monitoring. We use triple-combo logs and fluid salinity data from the Gulf Coast, Texas, for the first case study on salinity. For the second study on pore pressure, we analyze quad-combo logs and mud weight using a combination of Eaton’s and sonic overpressure indicator (SOPI) approach in the southern Gulf Coast, Texas. We integrate core, borehole image log, and shear wave imaging information in the last case study on a naturally fractured carbonate reservoir. CCS operations are required to ensure the underground sources of drinking water (USDWs) are not endangered due to brine or CO2 plume migration. Results show that a reservoir has 25 to 30% porosity, 10 to 1,000 md permeability, and a high net-to-gross thickness at a depth of 3,000 to 7,000 ft, but an updip portion of the reservoir is in the freshwater zone (salinity < 10,000 mg/L), posing risks to groundwater contamination due to updip CO2 migration and requires further assessment and strategic monitoring. The second case study shows the importance of pore pressure in assessing CO2 storage capacity and minimizing compression costs. The carbon storage window is between the supercritical CO2 depth and the overpressure boundary. As pore pressure increases, the “pressure space” available for storage below geomechanically defined fracture pressure is diminished. Our pressure map reduces uncertainties in identifying the overpressure boundary in multiple wells by a few 100 ft (containing multiple potential sandstone reservoirs) compared to the published studies. This increased storage capacity estimates of the entire zone. The last case study shows the importance of locating near-wellbore and far-wellbore fractures and their controls on CO2 storage in a fractured carbonate reservoir. Results show that fractured dolomites have higher porosity and permeability than host limestone, which is tight. Borehole shear imaging results show that high-angle fracture zones can be located up to approximately 120 ft away from the injection well, which can store CO2. Some of these fractures are closed and partially open, leading to reduced storage capacity. The study offers lessons learned from multiple case studies, showing pertinent problems where petrophysics can help facilitate successful CCS operations. It shows how existing and emerging technologies can be implemented, as well as the need to develop new concepts and tools for CCS.