Abstract

The injection of CO2 will give rise to a variety of coupled physical and chemical processes, which may affect the hydraulic integrity of caprocks. Likewise, when CO2 is stored as a component of EOR projects, the initial depletion of the reservoir, and all the influences associated with the EOR processes may affect the hydraulic integrity of caprocks. Therefore, it is necessary to understand how the hydraulic integrity evolves to make geological storage of greenhouse gases a feasible option. Such information must be imbedded in a performance assessment to determine the integrity of a reservoir and its caprock over an extended period of time. A preliminary methodology to evaluate the hydraulic integrity of the Weyburn project is presented.

Introduction

Bounding seals constitute the barrier against migration of CO2 in geological storage projects. Bounding seals are composed of overburden immediately above the injection horizon (typically termed caprocks) and underburden and the wellbore systems used to access the injection horizon. A substantial amount of research and investigation into the properties of seals, especially within the context of hydrocarbon migration and "traps" has been conducted over the last two decades1. The context for these investigations, however, is generally focused on geological processes giving rise to the existence of a seal or trap and the potential for hydrocarbon reservoir development. The existence of hydrocarbon reservoirs, especially gas reservoirs, is commonly offered as evidence of the suitability of a particular caprock for confining CO2 injected as part of an EOR project. For large-scale geological storage projects, however, CO2 injection alters the in situ characteristics of these caprocks, possibly degrading its sealing properties. Our knowledge of how those properties evolve under downhole injection conditions is weak. Wellbores, which provide access to a reservoir, may serve as preferential flow paths allowing upward migration of injected CO2.

Therefore, to make geological sequestration of greenhouse gases (GHG) an environmentally acceptable practice it is necessary to understand how bounding seals will react under pre- and post-operational conditions, with a focus on hydraulic integrity over both the short-term (injection stages) and long-term (hundreds of years after shutting down the injection wells).

Injection of CO2 will give rise to a variety of coupled physical and chemical processes that will affect the bounding seals and their surroundings. Among the most notable are changes to the stress field due to pore pressure and temperature changes, immiscible displacement of one of the phases by CO2, partial dissolution of CO2 into the aqueous phase, and chemical interactions between the CO2 and the aquifer and caprock. Therefore, permeability is affected by these changes, resulting in changes in the hydraulic integrity of caprocks during the lifetime of the project. It is expected that these changes will be most prominent during the short-term (injection stages). Likewise, such changes and aging will affect the integrity of wellbores and its component such as casings or seals of abandoned wells.

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