(559c) A Comparison of the Effectiveness of Three Types of Borehole Seismic Technologies for Monitoring CO2 Injected into Carbonate Pinnacle Reefs

Battelle manages the Midwest Regional Carbon Sequestration Program (MRCSP) which is a multi-year program funded by the U.S. DOE Regional Carbon Sequestration Program (RCSP) and private partners that is aimed at developing information, technologies and capabilities related to carbon capture and storage (CCS) and carbon capture, utilization and storage (CCUS). The MRCSP program is in its third of three phases, the goal of which is to inject and monitor one million metric tons of carbon dioxide (CO₂) into a series of pinnacle reef carbonate reservoirs in northern Michigan to determine the efficacy of this geologic system for CCS/CCUS. Thousands of pinnacle reef carbonate reservoirs are present in Michigan, most are oil-bearing or gas-bearing. While the individual reefs are relatively small structures, collectively the reefs have an enormous capacity for storing CO₂. The pinnacle reefs are attractive low-risk CO₂ storage reservoirs because they are encased by a thick sequence of low permeability carbonate, salt, and shale that serves to prevent the CO₂ from migrating upward due to its buoyancy. Despite these desirable characteristics, monitoring the position and quantity of the injected CO₂ may be necessary to demonstrate that the CO₂ is remaining within the intended storage formation. Seismic monitoring has been shown to be effective for delineating CO₂ in clastic rocks (sandstone) having high porosity; however, there are few examples where seismic monitoring has been conducted in carbonates. This paper will present results of three types of seismic monitoring, all borehole techniques, that are being deployed in the third phase of MRCSP and discuss challenges associated with each type due to the natural geologic system and other anthropogenic factors such as oil- and gas-well construction practices.

Three types of borehole seismic monitoring were conducted. The objective of each monitoring study was to evaluate the effectiveness of the respective seismic technology for detecting and delineating the horizontal and vertical distribution of (free-phase) CO₂ in a deep carbonate reservoir (approximately 4,000 to 5,000 ft below surface). In one study, time-lapse Vertical Seismic Profile (VSP) seismic surveys were performed in one reef (Reef A), including one baseline VSP in 2013 before starting CO₂ injection and one repeat VSP in 2016 after injecting approximately 200,000 metric tons of CO₂. Time-lapse Distributed Acoustic Sensing (DAS) VSP seismic surveys were performed in a second reef (Reef B), including one baseline DAS VSP before starting CO₂ injection and one DAS VSP after injecting approximately 87,000 metric tons of CO₂. In the third study, a single cross-well seismic survey was implemented in Reef B, also after 87,000 metric tons of CO₂ had been injected, to attempt to discern the horizontal and vertical distribution of CO₂ based on the difference in the response of compressional (P) wave and shear (S) wave velocities. Data acquisition has been completed for all three studies and data processing is either completed or nearly complete.