The impact of the depth-dependence of in-situ stresses on the effectiveness of stacked caprock reservoir systems for CO2 storage

Abstract

A candidate reservoir deep enough for CO2 storage might still be unsuitable if it carries a low minimum horizontal stress (Shmin) that limits its storage capacity, due to a higher risk of fracturing. This study investigated the impact that depth variations in the magnitude of in-situ stresses of a stacked caprock reservoir system have on the geomechanical response to CO2 injection. While geological and fluid flow properties (e.g., caprock permeability) are typically considered to be the main factors preventing CO2 leakage, this work investigates the importance of considering the stress variation across formations as an impeding factor that prevents CO2 leakage. The study employed well log, core sample, and field tests data from one such system in the Appalachian basin, where the target storage formations have low Shmin, but are overlain/underlain by formations with larger Shmin. Two coupled fluid flow – geomechanical models were constructed (1: vertically heterogeneous but laterally homogeneous 2: both vertically and laterally heterogeneous) for this investigation. Modeling results suggest that variations in the in-situ Shmin along the vertical profile of the stacked system are effective in containing the risks of fracturing to only those zones with the lowest Shmin. Furthermore, results suggest that inducing fractures – typically considered undesirable for CO2 storage – may in fact enhance injectivity without compromising the integrity of the storage system. While incorporating 3-dimensional mechanical heterogeneity results in stochastic locations of induced fractures, the estimated injectivity is slightly different from the laterally homogeneous model. The variability of stresses in a stacked caprock reservoir system merits detailed investigation as an entire system (rather than an isolated zone) in order to fully characterize the geomechanical responses to injection. As demonstrated in this work, stress changes across caprock-reservoir formations in the Appalachian basin is an essential factor preventing CO2 leakage.