EFFECT OF EXTREMELY DEEP VADOSE ZONE ON LEAKING CO2

This study focuses on the improvement of the Monitoring-Mitigation-Verification (MMV) program for the Southwest Partnership’s San Juan Basin CO₂ Injection Site in New Mexico. The CO₂ is being injected into an unmineable coal seam for enhanced coal bed methane recovery. The location of this coal layer is thousands of feet underground. In case of a potential leak, the MMV technologies are expected to detect the leakage before any significant amount reaches the surface environment. The MMV technologies, on the other hand, need previous experience on the behavior of the leaking CO₂ for an optimum deployment in the field.

This study is looking at the possible leak scenarios at the San Juan Basin Injection Site using computer simulations. The TOUGH2 computer model is based on the limited field data currently available, and assuming the missing data from previous experience in other sequestration projects. As more field data becomes available, the simulations can be improved.

The scenario is based on the leakage of 1% of the injected CO₂ per year out of the coal bed. The leaking gas makes its way into the vadose zone through a fracture. The vadose zone is assumed to be 300 m thick with a ridge that is present in the upper 100 m.

The CO₂ plume then spreads in this vast vadose zone, but its expression in the sections that are within the range of the most MMV devices takes a long time after the leakage starts. Even then, the spatial extent of the surface expression is similar to the part of the iceberg above the water. Much of the leaking CO₂ spreads in the underground, and stays there due to negative buoyancy.

Low permeability layers deflect the CO₂ plume to spread laterally. Consequently, the surface expression of the CO₂ is delayed even more. Ideally, sequestration sites with extremely deep vadose zones need MMV technologies that can penetrate deep into the ground for early detection of a possible leakage.

The ridge structure in the upper 100 m comes as a possible location of early detection. The CO₂ plume seeps into this area before it shows itself at the surface; an effect which is enhanced by the presence of the low permeability layers. Depressions in the vicinity of such sequestration sites can be ideal locations for sensor placement to achieve early detection.