Integrated Laboratory and Numerical Investigation of Leakage Pathways Development in CO2 Injection Wells

Abstract

The integrity of the cement sheath is the crucial part to maintain zonal isolation and to prevent the unwanted leakage of injection fluid for CO2 injection wells. During the lifespan of an injection well, loads arising from multiple processes can affect the state of stress in the wellbore system and, under severe conditions, may induce failure in the cement sheath. Different types of failure, including debonding (micro-annuli), radial cracking, and shear failure, can occur throughout the life of the well and cause severe leakage problems and dramatically reduces the injection efficiency. The MT-URL Well-Sealing experiment is designed to test the efficacy of novel sealants in a well with induced defects via pressure and thermal cycling (pressure and heating tests) as a proxy field scaled well that has undergone damage during CO2 injection. The MT-URL well is highly instrumented and has proven can closely mimic complex cased and cemented interval delaminations (debonding) at the casing-cement and cement-formation boundaries as well as fracturing within the cement sheath. The MT-URL is located in NE Switzerland near the village of St. Ursanne (Fig. 1). This study uses the integrated staged finite element and the laboratory bench-scaled measurement approach to simulate loads arising during the lifespan of the Mt. Terri CO2 injection well and to predict the initiation, evolution, and severity of cement failure under downhole conditions. Particular emphasis is placed on the cement hardening process due to its significant influence on the failure occurrence in the cement. To obtain the temporal variation of cement material properties for subsequent numerical simulation, an experimental procedure, including the measurement of elastic parameters and bleeding test for cement fluid loss, is performed under downhole conditions of the Mt. Terri CO2 injection well. The staged finite element approach used to simulate the cement hardening process in this study includes major processes that have significant mechanical influences, such as the development of elastic properties, bulk shrinkage, reduction of pore pressure, and development of strength. The resulting state of stress is analyzed to assess the onset and evolution of cement failure in the composite wellbore system under downhole conditions. The knowledge from this study can raise awareness of the necessity of predicting and evaluating cement failure and wellbore integrity under downhole conditions. The enhanced understanding of the cement hardening process and cement failure occurrence can provide insights and guidance to future construction of CO2 injection wells and optimize the operation process of CO2 injection.