Initial State of Stress: The Key to Achieving Long-Term Cement-Sheath Integrity

Abstract

Abstract Cement sheath design has been undergoing very deep changes for the last five years, mainly thanks to Carbon Capture and Storage (CCS). Indeed, for this application, Operators will have to demonstrate to national regulators that cement sheath can provide zonal isolation not only during the operational phases, but also during another phase: the long term storage. Stakes are as big as the market is, covering CCS plus traditional Oil and Gas market, where harmless operations to the environment know-how has become decisive to open new business opportunities. To ensure zonal isolation, cement system should be properly designed. It should be correctly placed to ensure it is hardening as per designed. Finally, its mechanical properties should be in adequacy with the successive operations the well is submitted to. Many years dominated by rules of thumb to evaluate cement sheath damage have progressively been replaced by engineered methodologies implemented in software. They can be numerical or analytical based, dealing with mechanics of solid or porous media. What should matter are not their pros or cons, but the correct assessment of their limitations to appreciate simulation results. To evaluate stresses a material can withstand before failure, constitutive laws and failure criteria are necessary but not sufficient: Initial state of stress is mandatory as it sets the initial distance from failure. After presenting the state of art and its weaknesses to evaluate cement sheath initial state of stress, this paper presents Total's methodology based on porous mechanics where initial state of effective stress results from cement hydration evolution with time under downhole conditions. The final state of stress is the consequence of the initial state plus additional stresses from operations. This methodology has been implemented in Total's dedicated software and has been successfully applied to design cement sheath on various operations worldwide: One detailed in this paper. Introduction After a deep look at well design best practices, there is a significant gap between all the achievements done for casing design and so little for cement sheath design, whereas both components are mandatory to achieve perennial primary and secondary barriers. If primary models and methodologies for cement sheath design were issued in the 90s, it is only recently that the petroleum industry's consideration towards this essential well component has considerably increased. An explanation may be the dramatic increase in deep water wells and their associated costs since the late 90s, which have pushed companies to find solutions to mitigate associated risks. One of the main risk is sustain casing pressure (SCP), possibly leading to casing collapse and heavy workover or worse, to the loss of 10 to 50 million dollars wells. Another explanation may be that new wells are more and more technical and expensive, which suggest better understanding when designing and greater practices to mitigate these risks. However, it seems that CCS, considered as one of the solution to mitigate climate change1, and presenting a potential market of 220–2,200 Gtons of CO2 cumulatively, has the most largely contributed to this cement sheath design renewal. Indeed, wells are considered as the main potential leakage pathway for storage and companies should prove that no well will leak over time, especially once abandoned, to safely deploy CCS technology. This has kicked off the interest in tools and methodologies to design cement sheaths that remain tight over the life of the well.