Mitigation Strategies for the Risk of CO2 Migration Through Wellbores

Abstract

TX 75083-3836, U.S.A., fax 1.972.952.9435. AbstractStoring carbon dioxide (CO 2 ) underground is considered the most effective way for long-term safe and low-cost CO 2 sequestration. This recent application requires long-term wellbore integrity. A leaking wellbore annulus can be a pathway for CO 2 migration into unplanned zones (other formations, adjacent reservoir zones, and other areas) leading to economic loss, reduction of CO 2 storage efficiency, and potential compromise of the field for storage. This CO 2 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 CO 2 -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 CO 2 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 CO 2 conditions. Experimental details and analytical methods are discussed. Data relating cement-strength loss and CO 2 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 CO 2 -resistant material; the comparison shows significant promise for this new material. This CO 2 -resistant material will enable the hydrocarbon production industry to store the burnt residue over the long term in a safer and more responsible manner.

Durability of Hardened Portland Cement Paste used for Oilwell Cementing

“…But a severe depletion of calcium on the outer layer is generally observed. Recently, it was shown that the formation of a calcium carbonate-rich layer may retard the leaching phenomenon but does not stop it [2,5].…”

“…Corrosion of steel induced by acid gas-containing brines is well documented while data on degradation by wet CO 2 or wet H 2 S is more limited [1]. For cementitious materials, there is an abundant literature dealing with deterioration of cement pastes caused by a CO 2 environment [2][3][4][5]. Published data on degradation mechanisms of cement-based materials exposed to H 2 S environments are more scarce [6,7].…”

“…Due to the presence of corrosive gas, special attention has to be paid to the design of well materials (casing, cementing materials). Corrosion of steel induced by acid gas-containing brines is well documented while data on degradation by wet CO 2 or wet H 2 S is more limited [1]. For cementitious materials, there is an abundant literature dealing with deterioration of cement pastes caused by a CO 2 environment [2][3][4][5].…”

Well Technologies for CO2 Geological Storage: CO2-Resistant Cement

“…1). The pressurization and depressurization phases of the CO 2 experiments follow a well-defined procedure described by Barlet-Gouédard et al (2006).…”

Formation of Hydroxyapatite in Portland Cement Paste

“…A number of studies have shown that wellbore cement is susceptible to CO 2 attack, leading to rapid degradation [6,7], whereas other studies indicate that CO 2 penetration and reaction with cement is limited [5,8], and the long-term performance of wellbore cement in a CO 2 -enhanced oil recovery field was good, providing an effective barrier to significant fluid flow for decades [9,10]. Despite numerous previous studies reporting the geochemical and mineralogical alteration of well cement by CO 2 under geologic sequestration conditions [5,8,11], the rate of Portland cement degradation and long-term integrity of wellbore cement material is still in question and under debate.…”