Well Integrity Analysis During CO2 Injection in a Depleted Chalk Field

Abstract

ABSTRACT: The present study is part of the Bifrost project and aims at assessing the feasibility of storing CO2 in chalk reservoirs by using the depleted Harald East gas field located in the northern part of the Danish North Sea as a showcase. Chalk is a singular rock with a high compressibility and the resulting pronounced compaction during hydrocarbon exploitation and possibly during CO2 storage raises questions on the integrity of cement surrounding the legacy, monitoring, and injector wells. In this context, the study carries out a well stability analysis of a well present in the field. The strain and stress distribution computed by a full-field coupled thermo-hydro-mechanical model is used as input data in a finite element model, which captures the properties and mechanical behavior of a near wellbore area (formation, cement, and casing) located in the seal interval of the field. The results show negligible deformation in the cement during hydrocarbon production and CO2 storage, thereby suggesting low risk of leakage along the studied well. 1. INTRODUCTION Complying with the ambitious objectives of the intergovernmental panel on climate change in terms of amount of greenhouse gas emission reduction requires the quick implementation of multiple carbon capture storage (CCS) projects worldwide (IPCC, 2005). In this context, the re-use of pre-existing oil and gas infrastructures and fields that were thoroughly studied for decades represents a time- and cost-efficient opportunity associated with a relatively low risk of leakage due to a proof of containment at least at pre-production time (Schmelz et al., 2020). In the Danish North Sea, over 800 Mt of CO2 storage capacity was estimated within mature chalk fields (Bonto et al., 2021). Assessing the possibility of storing CO2 in chalk deposits is currently based on laboratory studies such as batch-reactor and flow-through experiments combined with triaxial tests. Exposures of the rock to CO2 in supercritical and gas phase under temperature and pressure conditions relevant for the subsurface of the North Sea indicate no to minor mechanical alteration of the elastic, shear, plastic, and time-dependent properties (Alam et al., 2014; Ditlevsen & Christensen, 2010; Liteanu et al., 2013; Schroeder et al., 2001).