Analysis of Reservoir Fluid Migration in the Process of CO2 Sequestration in a Partially Depleted Gas Reservoir

This paper addresses problems of reservoir fluid migrations in the process of CO2 sequestration in a partially depleted petroleum reservoir. A detailed analysis of the migrations is required to obtain fundamental characteristics of a sequestration structure, including estimation of its sequestration capacity and leakage risks. The paper presents a general discussion of the relevant mechanisms and their contributions to the analysed issues. The proposed approach to solve the problems relies on the usage of numerical structure modelling and simulations of the sequestration processes on numerical models of the structure. It is applied to a selected geological structure comprising a partially depleted gas reservoir. The modelling includes key types of reservoir fluid migrations: viscous multiphase transport and convection transport. It also takes into account other phenomena that affect fluid migrations including injected gas solubility in the formation water and gas trapping by capillary forces. Correspondingly, the leakage risks are associated with distinct leakage pathways (beyond the structural trap, to the caprock, via activated fractures). All these cases are separately modelled and their detailed characteristics are presented and discussed. The final results of the fluid migrations and their consequences for the leakage events are discussed and some generalized conclusions are drawn from the approach employed in the study.