Analysis of Cement Plug Bonding Surface Integrity in CO2 Geological Storage

ABSTRACT: Using depleted oil and gas reservoirs to sequester CO2 is an effective means of mitigating the greenhouse effect. The long-term integrity of the cemented surface of the cement plug is directly related to the risk of CO2 leakage. In this paper, based on the cohesive element method, a new finite element model of cement plug-reservoir-cover is established, considering the sealing location, sealing length, and nature of cementing surface of cement plug. The influence of each factor on the long-term integrity of cementing surface is quantitatively analyzed. The simulation results show that: the cement plug sealing location should be mainly in the cover area, and the minimum length should be not less than 50m; the normal strength and failure displacement of the cemented surface determine the tendency, speed, and height of fracture development, and the fracture development slows down after penetrating the layer. This study is necessary in reducing the leakage risk of CO2 geological storage. 1. INTRODUCTION Using depleted oil and gas reservoirs to sequester CO2 is a potential method to mitigate the greenhouse effect (Leung et al., 2014). Ensuring the long-term sealing of depleted oil and gas reservoirs is the focus of attention and the key to the success of CO2 geological storage (Shukla et al., 2010; Newell et al., 2013). The types of oil and gas reservoir seal failure can be divided into two categories: wellbore containment barrier seal failure and oil and gas reservoir top cover seal failure (Zhang et al., 2011). As shown in Fig. 1, the wellbore barrier failure mode can be divided into (1) Formation/cement sheath interface failure, (2) Cement sheath/casing interface failure, (3) Casing/cement plug interface failure, (4) Cement plug body failure, (5) Cement sheath body failure, and (6) Casing body failure according to the CO2 leakage path; For the open hole section, there are two failure ways (1) and (4). (Gasda et al, 2004; Bai et al, 2015; Nagelhout et al, 2010)