Well integrity assessment for CO_2 injection: A numerical case study on thermo-mechanical behavior in downhole CO_2 environments

摘要

Purpose - In the carbon dioxide capture and storage (CCS) project, the integrity of CO_2 injection wells plays a vital role in the long-term safety of CO_2 storage. The authors aim to practically investigate possible CO_2 leakage of a CO_2 injection well section during the injection operation and shut-in by the thermomechanical FEM simulation. The application of numerical simulation to the CO_2 injection well deep underground is the first step that will help in the quantitative evaluation of the mechanical risks. Design/methodology/approach - The injection of CO_2 at a temperature different from those of the well and the surrounding geological formation is likely to cause different thermal deformations of constitutive well materials. This could lead to cement cracking and microannuli openings at the interfaces of different materials such as casing/cement and cement/rock. In this paper, the possibility and order of magnitude of cement cracking and microannuli creation in the cross section of the well are assessed from a numerical case study within a classical thermomechanical finite element model framework. Findings - The possibility of compressive failure and tensile cracking in the cement of the studied wells due to CO_2 injection is small unless a large casing eccentricity or an initial defect in the cement is present. Some microannuli openings are generated at interfaces cement/casing and/or cement/rock during the CO_2 injection because of different thermal shrinkage of each material. However, the width is not important enough to cause significant CO_2 leakage under the studied conditions. The use of "flexible" cement especially developed for oil well applications could mitigate the risk of cement cracking during CO_2 injection. Originality/value - Numerous experimental studies on the chemical deterioration of the cement under severe conditions have been carried out. On the other hand, only a few investigations have focused on the mechanical behavior under thermal/pressure changes related to CO_2 injection. In this paper, the quantitative analysis to investigate cement cracking and microannuli formation is achieved to help in the identification of possible mechanical defects to cause CO_2 leakage. In addition, the discussion about the risk of the possible casing eccentricity and the application of flexible cement in the oil and gas field to CO_2 injection well could be practically useful.