Subsurface CO2 storage in geologic traps–Procedures for evaluating trap adequacy

Abstract

Abstract The oil and gas industry has a long history of evaluating the adequacy of subsurface traps for containing buoyant fluids and has developed methods for weighing the geologic probability of trap leakage when limited data are available. We have adapted these methods for evaluating the adequacy of sub-surface saline formation traps for CO 2 storage. A principal difference in the evaluation for gas and oil exploration versus CO 2 storage is that the probability of leakage is viewed differently. The oil and gas industry tolerates substantial leak probabilities in exploration if the potential gas or oil accumulation is sufficiently large, but the tolerance for CO 2 leakage (i.e. the probability of injected CO 2 returning to the surface) will likely be much lower, and that low tolerance has the potential to limit the amount of CO 2 injected into a subsurface trap. We consider three types of saline formation traps: • Depleted gas or oil fields • Drilled trap structures that failed to discover commercial gas or oil volumes • Undrilled trap structures Each of these types has some probability of CO 2 leakage, but the geologic probability is smaller and better constrained in depleted gas or oil fields and greater and more uncertain in untested structures. However, each of these trap types still possess some geologic probability of leakage. We evaluate the overall probability by considering for each trap element (e.g., capillary seal capacity) the likelihood that each element is adequately developed and the quality and quantity of data available to make that judgment. By following procedures applied to gas and oil trap evaluation, we arrive at probability distributions for a range of CO 2 fill limits. What this analysis reveals, however, is that if we fill traps with CO 2 to levels with minimal probability of CO 2 leakage, the volume of CO 2 that could be stored in subsurface traps is small. Moreover, all traps will, on average, have huge remaining pore volumes that could accommodate additional CO 2 storage and could go unfilled (i.e. pore volumes with a higher probability of leakage). We propose an evaluation method based on a series of redundant traps in a fill-leak sequence. The purpose of secondary and tertiary traps arranged in series is that they allow for filling the primary trap to its uncertain fill limit while minimizing the chance of leakage to the surface.