Mitigation Strategies for the Risk of CO2 Migration Through Wellbores

Storing carbon dioxide (CO2) underground is considered the most effective way for long-term safe and low-cost CO2 sequestration. This recent application requires long-term wellbore integrity. A leaking wellbore annulus can be a pathway for CO2migration into unplanned zones (other formations, adjacent reservoir zones, and other areas) leading to economic loss, reduction of CO2storage efficiency, and potential compromise of the field for storage. This CO2 leakage through the annulus may occur much more rapidly than geologic leakage through the formation rock. The possibility of such leaks raises considerable concern about the long-term wellbore isolation and the durability of hydrated cement that is used to isolate the annulus across the producing/injection intervals in CO2-related wells. With the lack of industry standard practices dealing with wellbore isolation for the time scale of geological storage, a methodology to mitigate the associated risks is required. This requirement led to the need and development of a laboratory qualification of resistant cements and the long-term modeling of cement-sheath integrity.This article presents the results of a comprehensive study on the degradation of cement in simulating the interaction of the set cement with injected supercritical CO2under downhole conditions. The methodology and the equipment are described for testing conventional Portland cement and measuring the evolution of its alteration process with time under CO2 conditions. Experimental details and analytical methods are discussed. Data relating cement-strength loss and CO2 penetration in Portland cement are presented. The evolution of cement chemistry and porosity with time is highlighted by scanning electron microscopy analyses, back-scattered electron images, and Hg-porosimetry measurements. A first fluid-flow-geochemistry modeling for Portland cement is proposed. The results are compared to equivalent studies on a new CO2-resistant material; the comparison shows significant promise for this new material. This CO2-resistant material will enable the hydrocarbon production industry to store the burnt residue over the long term in a safer and more responsible manner.