Abstract:

Subsurface CO2 storage has been identified as one of the key methods to reduce the emission of CO2 to the atmosphere. Remediation or mitigation of unwanted migration from potential storage sites requires novel approaches for which feasibility is yet to be demonstrated. This study focuses on a solution to mitigate CO2 migration through naturally occurring faults, using a polymer-gel to drastically reduce the permeability of the fault. The radius of influence of the polymer injection is extended by using hydraulically stimulated fractures to transport the sealant to the leaking area. Reservoirs eligible for CO2 storage generally exhibit relatively high permeability and consequentially high leak-off of frac fluids. Therefore, the extent of hydraulic fractures is expected to be limited. Faults and fractures may be surrounded by a damaged zone with a permeability that is higher than the reservoir (up to 10 times). Considering the extra permeability of the damaged zone, the surface covered by the sealant may be extended by another 20-40%. Current results shows that this technique is technically feasible (with proper choices of polymer-gel and treatment) to mitigate CO2 leakage through a leaking fault.

Introduction

In the past years, several novel approaches to reduce CO2 emissions in the atmosphere have been proposed by the scientific community. Storing CO2 in the subsurface is considered one of the most promising method to decrease the rate of increasing CO2 emissions to the atmosphere. Injecting and storing CO2 in carefully selected reservoirs has been demonstrated to be applicable and safe, but it is not possible to completely exclude the possibility of unwanted leakage and migration of the stored CO2 towards the surface. To minimize the environmental risk related to subsurface CO2 storage that is a concern to both regulators and the general public, it is necessary to be able to implement a selection of approaches that can prevent, stop or mitigate the unwelcomed flow of CO2 towards the surface. Four main CO2 leakage mechanisms have been identified (Tongwa et al., 2013): leakage via abandoned wells, via natural fractures systems, via reactivated faults or via juxtaposition of the formation where the CO2 is stored against a formation that has high permeability (and lower pore pressure). In case escaping CO2 is detected from the hosting formation, and to decide if a mitigation or remediation technique is needed, it is crucial to evaluate the environmental risks and the economic costs related to the leakage of CO2. Manceau et al., 2014, compiled a list of several mitigation and remediation technologies that can be applied in case of migration of CO2 from geological storage units. Some of these methods are already utilized for real cases of leaking CO2, while some of them are tested only at the laboratory scale.

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