Subsurface fracture network characterization for a CO2 basalt mineralization target: new insight on fluid flow from wellbore logs for the Jizan Group

Permanent CO2 storage in altered basalts, which may have lost most of their primary porosity and permeability, is facilitated by subsurface fractures, governing the fluid flow inside the storage reservoir. A successful pilot test demonstrated the in situ CO2 mineralization potential of the highly fractured basaltic rocks of the Jizan Group in SW Saudi Arabia. We characterize the subsurface fracture networks utilizing image logs acquired from the wells drilled at the pilot site. The hydraulic connectivity and circulation pathways between the injector and producer wells are inferred by combining the fracture distribution, mechanical aperture at the wellbore walls, fracture persistence between the two wells and data acquired from flowmeter logs. Five fracture sets were identified with the most abundant one striking NW–SE and dipping 50–60° towards the SW and NE. Furthermore, an abundance of stress-induced wellbore breakouts reveals a NW–SE direction of the present-day maximum horizontal stress axis. This orientation is consistent with the regional tectonics and it is sub-parallel to the mechanically open fractures, a favourable orientation for fluid flow. Outcrop studies and subsurface data reveal primary rock layers tilting towards the SW and an extensive presence of NE-dipping faults and NW–SE-striking fractures. These observations are consistent with the Jizan Group fracture network characteristics in outcrops. Although, on the surface, the fracture sets that strike in different directions suggest and/or display high interconnectivity, flowmeter logs reveal that despite the basalts being pervasively fractured, fluid flow in the subsurface seems to be controlled predominantly by three fracture zones with wide mechanical apertures. Despite that, our findings align with the positive results from the pilot test, confirming that the subsurface at the site possesses sufficient permeability to transmit injected H2O–CO2 fluid along preferential flow paths, primarily through fractures with wide mechanical apertures. This suggests that the Jizan Group is a good target for in situ CO2 mineralization in basalt, owing to its extensive fracture network and the presence of highly permeable fluid flow zones, which enable carbonated waters to flow and react with a significant volume of rock.