Understanding the leakage of greenhouse gasses from seep geologic formations during geologic carbon storage and hydraulic fracturing: intermediate scale testing challenges

The carbon storage and energy development activities in deep geologic zones that potentially affect water quality in shallow aquifers are of central importance in the energy-water nexus. The extraction of natural gas involves hydraulically fracturing deep shale formations. The storing of carbon dioxide in deep geologic formations is pursued to mitigate global warming. Both these activities have the potential to contaminate the shallow aquifers used for potable water and return the greenhouse gases to the atmosphere. In the case of carbon storage, both during and post-injection phases, it is possible for the CO2 and formation brine to leak through natural faults, pressure-induced fractures, or failed well casings. Two scientific challenges have to be addressed to safely store the carbon in the deep formation while protecting the shallow aquifers. The first, characterizing the affected geologic formations, and the second is monitoring the leakage. Monitoring involves determining leakage locations and tracking of the gas and brine plume through the geologic formation between the deep confining layer used for storage and the shallow aquifer. Challenges to the characterization derive from the limitations and sparsity of observational data in deep formations.  Effective monitoring poses both scientific and engineering challenges as the leakage locations not known, and the resulting pathways cannot be predicted easily. This paper presents two studies where intermediate-scale testing systems were used to understand the processes that occur during the leakage of stored supercritical CO2.  The focus of the first study was to better understand the process of CO2 gas exsolution after a leak from the deep confining formation. The second study addresses the issue of monitoring brine leakage from the confined formation where supercritical CO2 is stored. The improved understanding of these leakage processes will help to develop assessment and monitoring systems for storage permeance and protecting shallow sources of potable groundwater. It is not feasible to conduct experiments in the field to obtain both the fundamental process understanding and test and validate developed modeling and monitoring tools due to lack of control of boundary and initial conditions and expense in fully characterizing deep formations. Tests in intermediate scale synthetic aquifers where highly controlled experiments can be conducted to obtain accurate data provide an alternative to overcome this challenge. However, designing test systems and performing tests under ambient laboratory conditions different types of challenges. This paper will present some of the challenges and how they were overcome. The results on new process insights, how the data was used to assess the natural capacity of the aquifer attenuation of leaking gas, and validating inversion methods for site characterization and leakage detection will be presented. Even though this study focused on CO2 leakage, the results will be of value in problems of natural has leakage during hydraulic fracturing in alternate energy development.