Cement Sheath Integrity During Thermal Cycling
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In the construction process of oil and gas wells, primary cementing constitutes a critical procedure of placing a cement sheath in the annulus between casing and formation, or between the casing strings. The main purpose is to provide mechanical stability to the wellbore and to ensure zonal isolation through the entire well service lifetime. Failures to achieve a proper primary cementing, and to ensure long-term sealing capabilities of the cement sheath, may severely limit the ability of a well to reach its full producing potential. Well cementing technology is a mature area, however, many operators, specifically in the North Sea, have been concerned about the ability of the cement sheath to maintain sealing integrity, given the increasing number of reported failures in mature wells. In offshore fields, these well integrity problems signify a serious concern since they imply costly repairs, limitations with respect to shortand long-term use of production and injection wells, safety and environmental issues. Therefore, recent efforts have been undertaken to achieve enhancements in the technology and standardization of logging procedures to evaluate the status of the cased-hole cement sheath barrier. Nevertheless, the verification of the cement sheath functionality still represents a technical challenge, given the uncertainty and high complexity associated with performing the task downhole. Further, it is well-know that, even in the presence of an initially good cement job, repeated temperature variations, i.e. thermal cycling, can have a detrimental impact on the integrity of the cement sheath over time. Temperature variations will cause the casing, cement and formation, to expand or contract accordingly with their distinct thermal and mechanical properties, which induce stresses that may lead to cement sheath damage. Experimental tests can be tailored for the assessment of long-term cement sheath integrity under realistic wellbore curing and operating conditions, and may establish a step forward in designing more robust and safer wells. For this reason, the main objective of this research work has been the development and application of a laboratory set-up, conceived to investigate the potential cement sheath failure mechanisms cause by time-varying thermal loads in the wellbore. A novel laboratory set-up has been designed and constructed at the Institute of Petroleum Technology Laboratory, NNTU. The focus has been to develop a testing apparatus and protocol that enables visualization of how leak paths form and propagate throughout the well annular sealant, when exposed to cyclic thermal loads and confining pressure. The set-up allowed, for the first time, studies to be performed on downscale wellbore samples constituted of casing pipe, cement and formation. The samples are intended to represent a conventional subsea well production casing section (12 1/4-in borehole and a 9 5/8-in casing) scaled-down by a factor of ~4. Only test cases with neat class Portland G cement were assessed. However, the laboratory set-up enables the use of different types of sealants. In addition, it is presented the first use of X-ray computed tomography for the monitoring of cement sheath debonding and cracking failures over time, which provides 3D information for identification of how, where and when the damage occurred. The experimental approach was successfully implemented, and comparative studies conducted on samples with different stand-off, surrounding formation and casing surface finish gave a new insight into the development of cement sheath failures. Numerical studies, based on the finite element method, were conducted to complement the analysis of the annular cement sheath failure mechanisms observed in the test samples throughout the various thermal cycling experiments. Furthermore, it was recognized that there was a lack of relevant knowledge on the overall impact of mechanical and thermal properties on the estimations of cement sheath stresses and mechanical failures. Consequently, as part of this research work, it was extensively studied the impact of variations in casing-cement-formation material properties, geometric parameters, casing stand-off and characteristic well-loading events, on the occurrence of cement sheath damage. The test cases were defined on the basis of a sensitivity screening of random input properties of the wellbore components, used in a conventional subsea production casing section. Based on the experimental and numerical work, an assessment of the influence of casing centralization, rock formation type and casing surface condition impact on cement sheath long-term integrity are discussed in this dissertation. The main contributions of this work are presented in four international papers contained in the Appendixes. They can be summarized as follows: • A unique experimental testing rig and procedure developed to evaluate the long-term integrity of well annular sealants towards time-varying thermal loads. The testing protocol was applied for the assessment of relevant cement sheath integrity issues. • An extensive numerical work that enabled mapping of the key variables that affect cement sheath failure mechanisms. To mitigate the risk of losing zonal isolation, guidelines for the assessment of long-term cement sheath integrity and selection of cement systems mechanical properties were given. The laboratory set-up will be used further by the sponsoring SFI DrillWell Research Centre, through SINTEF Petroleum Research as project manager, to investigate and compare the capabilities of more complex cement systems regularly used by the industry, or alternative annular sealants, to withstand time-varying loads in the wellbore.