Mitigation strategies for the risk of CO2 migration through wellbores
Authors: Barlet-Gouedard Veronique , Rimmele Gaetan , Goffe Bruno , Porcherie Olivier 
Published: December 2007

Abstract
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 CO2 migration into unplanned zones (other formations, adjacent reservoir zones, and other areas) leading to economic loss, reduction of CO2 storage 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 CO2 under 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.

Introduction
Storing carbon dioxide (CO2) underground is considered the most effective way for long-term safe and low-cost CO2 sequestration[1,2]. There are three main types of geological reservoirs[3] with capacity sufficient to store captured CO2: depleted oil and gas reservoirs, deep saline aquifer reservoirs, unminable coal beds. The reservoirs need to be at a depth greater than 800 m so that the CO2 is in a supercritical state at a temperature and a pressure above its critical point (31.6°C, 7.3 MPa). These pressure and temperature conditions allow storing CO2 in a relatively small volume. The ideal storage site would involve high pressure but at the lowest possible temperature to be in the most dense properties of the supercritical CO2. For these reasons, regions with low geothermal gradients are preferable.
Piping CO2 emissions for underground injection is not a novel concept and is already often used for the purposes of enhanced oil and gas recovery[4,5,6]. However, this application of CO2 injection is not intended for long-term storage, which is a more recent concept that needs a long-term wellbore integrity strategy to be developed. Indeed, the major risk associated by the public with CO2 injection is a well failure, which may result in escape of CO2 that will migrate upwards. The likelihood of a sudden escape of all CO2 stored in an underground reservoir is extremely small. The main risks are: CO2 and CH4 leakage, seismicity and ground movement (subsidence or uplift).
Failure of the cement, in the injection interval or beyond it, may create preferential channels for CO2 migration back to the surface. This may occur on a much faster time scale than geological leakage.
It is hence important to explain that wellbore integrity will ensure that CO2 stays underground for several hundred years and beyond.