Time weighted storage capacity for geological sequestration

Abstract

Several investigations have tabulated the storage capacity for CO2 in the regions around the world, and it is widely accepted that sufficient pore volume exists in deep subsurface formations to permit large-scale sequestration of anthropogenic CO2. Almost all of these investigations correct the bulk pore volume available for storage efficiencies, which are approximations of volumetric sweep efficiency (areal, vertical and gravity override efficiencies) and displacement sweep efficiency. Meaningful mitigation of emissions will require annual storage rates of the order of Gt CO2 within a few decades. Storage capacity estimates should therefore also incorporate the time required to place CO2 into the volume. We present an approach for weighting capacities in this fashion. We apply it to compute “time weighted storage capacity” using tabulated properties of 1200 North American oil reservoirs. The distribution of properties is presumed representative of brine-saturated structures that would be used for CO2 storage. Formation injectivity is non-uniformly distributed with formation pore volume: the set of reservoirs with above average injectivity comprises only 10% of the total pore volume. This non-uniformity is a primary reason that time weighted storage capacity for a large set of structures is significantly less than ultimate volumetric capacity. Moreover we find that the resource requirement (number of structures required to achieve a target storage rate) varies non-linearly with storage rate. Hence timeweighted capacities should be used to establish feasibility of large-scale sequestration and to optimize deployment of sequestration projects.