The process of injection and withdrawal from tight gas reservoirs is a multiphysics and multicomponent problem. The aim of the present work is to capture the physics associated with the injection of CO 2 into tight shales, and assess and mitigate the risks associated with reservoir overpressure. The overpressure caused by CO 2 injection usually triggers the onset of formation–deformation, which inadvertently affects the state of the stress in the target geological formations and its surroundings, the monitoring of which is critical to understand the risks in conjunction with CO 2 storage. In the present work, a novel fully coupled fully implicit flow and geomechanics simulator is introduced to describe the physics in conjunction with an extended injection phase of CO 2 . The developed model solves for pressure saturation and porosity and permeability changes considering a multicomponent system while principally focusing on the adsorption and diffusion of CO 2 and stress-dependent reservoir deformation employing cell-centred finite volume method. It is envisaged that the injection of CO 2 , while with the primary purpose of storage, will parallelly enhance the recovery from shale gas due to lateral sweep effects. Based on these mechanisms, for the case study of a tight gas field, the applicability of the simulation model is tested for formations with varied rock and fluid moduli in a 20-year simulation period. © Springer Nature Singapore Pte Ltd. 2019.