What If Chalk Becomes Mechanically Weaker During Supercritical Co2 Injection in a Depleted Gas Reservoir?

Abstract

Even though most past endeavors concluded no significant weakening in chalk due to the injection of supercritical CO2, here, we comprehensively assess the mechanical response of depleted chalk reservoirs by considering multiple types of CO2-induced alteration of chalk properties reported in literature. We perform a series of hydro-mechanical simulations on the depleted Harald East gas field, the first candidate worldwide to store CO2 in chalk formations located in the Danish North Sea. Different scenarios are considered by modifying the injection schemes (intermittent vs. continuous, low vs. high injection rate) and assumptions regarding the CO2-induced alteration of the rock’s mechanical properties. The subsidence trends of the reservoir top and the distribution of stress and elasto-visco-plastic strain across the reservoir, underburden, and along the wells are compared between scenarios. Besides, the mechanical responses of subsurface chalk upon pressure depletion during gas production and repressurisation by CO2 injection are reviewed. The results highlight the effects of stress arching occurring during production as well as injection rate, the distance of porous chalk from the injector wells, timing between repressurisation and CO2 propagation in the reservoir, and strain rebound on the deformation response of the storage site. The present study also indicates an overall negligible to minor deformation of the storage site compared to during the production phase.

Keywords: CCS, chalk, deformation, weakening, North Sea