Characteristics of Mechanical Wellbore Failure and Damage: Insights of Discrete Element Modelling and Application to CO₂ Storage

Abstract

Wellbore zonal isolation is particularly important for subsurface storage of CO₂ , where well integrity must be ensured for very long time spans. In this study, three dimensional discrete element models of wellbore systems have been used to simulate failure and damage of wellbore cement and surrounding rock. The models allow simulation of wellbore failure and damage in wellbore systems with different geometries and with perfect (idealized) or imperfect plug-casing-cement and cement sheath-rock interfaces. The aim is to determine critical stress conditions for mechanical wellbore failure and associated damage. Comparison of model simulations with conventional geomechanical analysis is used to determine limits in injection pressures. If these limits are exceeded, wellbore failure and upward fluid migration through wellbore cement may occur due to alignment of fractures and the formation of connected fracture networks. Imperfect cementation or a cement sheath that is not evenly distributed around the wellbore enhances local damage or failure. Although the analysis relies on upper bounds on the changes in stresses around wellbores caused by an increase in pressure, it shows that loss of well integrity during injection of CO₂ or other fluids may occur, in particular by axial loading due to reservoir compaction.

1. INTRODUCTION

Wells that penetrate subsurface CO₂ storage complexes may act as potential pathways for leakage of CO₂ to the surface [e.g., 1-3] . For storage of CO₂ to be meaningful, well integrity has to be ensured for large injection volumes and over much longer time scales (100’s to 1000’s of years) than in the case of hydrocarbon production (10’s of years of depletion). Ensuring well integrity not only during injection period but also in the post-operational period over such long timescales represents a challenge for Carbon Capture and Storage (CCS) projects that is not encountered before in the E&P industry. The long term-ability of wells to inhibit CO₂ migration has been identified as a significant potential risk for the long-term security of geological storage facilities [e.g., 2]. The risk of leakage through "old" abandoned wells requires particular attention. Regulations for well abandonment were less comprehensive than they are today. In addition, future use of abandoned reservoirs for CO₂ storage was not taken into account when old wells were completed and abandoned. The risks of leakage through abandoned wells may be a showstopper for a CCS project as it was in the case of CO₂ storage in the depleted De Lier field in the North Sea, offshore of the Netherlands [4]. The project was discontinued after a feasibility assessment of CCS found that there was a high potential for leakage through old wells. Repair of these wells was uneconomic and in some cases, technically impossible.