Design Tool to Tailor Properties of Cement for P&A Applications

Abstract

Abstract As oil and gas wells approach the end of their productive lifetime, they must be permanently abandoned. In recent years, there has been a lot of emphasis on creating modeling tools for design tailoring and placement of abandonment plugs. Moreover, it is of equal significance to devise a process and design tool for achieving long-term integrity of abandonment plugs. Such a design tool will help understand and mitigate the risks of thermo-mechanical damage or flow through plugs. A sound underlying design process should demonstrate the following attributes - (i) be flexible to accommodate different load types: structural, thermal, pore fluid, and strain loads that can be rapid or gradual in nature, (ii) communicate stresses, deformation and temperature information between load stages, (iii) allow load application on the top, bottom, and radial directions of the plug, (iv) evaluate risk of mechanical failure and permeable flow, and (v) allow tailoring of cement properties to minimize the risks. As a result, this work presents a design tool that complies with all the attributes described. The design tool has been used to model a cement plug interacting with its near wellbore environment. It has the necessary geomechanical and transient features to model rapid or gradual changes in pore pressure and stress to subsequently assess the risk of fluid flow, mechanical failure in shear, tension and/or debonding. The design tool is verified by simulating the construction phases and operational loads of a representative plug job and then comparing its outcome to the expected and/or actual behavior. The effect of plug in-situ stresses and stress anisotropy of the rock are demonstrated. Further, the response of formation's permeability changes from 50 nano Darcy to 5000 μDarcy is used to highlight the impact of fluid communication between the adjacent formation and the abandonment plug. Interestingly, the verification results align with the expected behavior. Pore pressure buildup is found to increase or decrease the risk of plug failure in shear mode depending on whether the plug is cured in compression or tension, respectively. An increase in formation permeability shows a flow diversion from the plug into the rock, thus, limiting the stress build-up in the plug. For the case analyzed herein, the encountered impact of rock stress anisotropy is minimal. Ultimately, the design tool demonstrates how a typical abandonment construction and operation process can be simulated to assess the risk of failure and fluid flow through an abandonment plug. It also highlights the critical factors that govern a plug's response, which in some cases includes the role of formation and casing.