Estimating the probability of CO 2 plumes encountering faults

One of the main concerns of CO 2 storage in saline aquifers is leakage via faults. In the early stages of site selection, site‐specific fault (map) coverages are often not available for these saline aquifers. This lack of site‐specific data motivates development of a method that makes use of available regional fault data to estimate the probability of injected CO 2 or the resulting pressure front encountering a fault, which is a necessary condition for leakage of CO 2 or brine to occur via these pathways. The probability of encounter can be calculated from areal fault density statistics generated from available data, and CO 2 plume or elevated pressure area dimensions generated by numerical simulation. Given a number of assumptions, the length of the plume or elevated pressure area perpendicular to a fault times the areal density of faults with offsets greater than some threshold of interest provides the probability of the plume or a pressure front of concern encountering such a fault. Application of this result to a previously planned, large‐scale pilot injection in the southern portion of the San Joaquin Basin yielded a 3% and 7% chance of the bulk phase CO 2 plume encountering a fully and half‐seal offsetting fault, respectively. Subsequently available data provided a first test of this approach as a half‐seal offsetting fault was discovered at a distance from the injection well that implied a 20% probability of encounter for a plume sufficiently large to reach it. © 2011 Society of Chemical Industry and John Wiley & Sons, Ltd