This report is one of a series of reports documenting the methods and findings of a multi-year, multi-disciplinary project conducted by the Pacific Earthquake Engineering Research Center (PEER) with the Lawrence Berkeley National Laboratory (LBNL), and funded by the California Energy Commission (CEC). The overall project is titled “Performance-based Earthquake Engineering Assessment Tool for Natural Gas Storage and Pipeline Systems” henceforth referred to as the “OpenSRA Project.” The overall goal of the OpenSRA project is to create an open-source research-based seismic risk assessment tool for natural gas infrastructure that can be used by utility stakeholders to better understand state-wide risks, prioritize mitigation, plan new gas infrastructure, and help focus post-earthquake repair work.
The project team includes researchers from LBNL, UC Berkeley, UC San Diego, University of Nevada Reno, the NHERI SimCenter at UC Berkeley, and Slate Geotechnical Consultants and its subcontractors Lettis Consultants International (LCI) and Thomas O’Rourke. Focused research to advance the seismic risk assessment tool was conducted by Task Groups, each addressing a particular area of study and expertise, and collaborating with the other Task Groups.
This report is the product of Task Group C: Performance of natural gas storage well casings and caprock. The scope of this report is focused on the fragility of gas storage wells when subjected to fault displacement. Subsurface well failures have led to severe environmental, health, and safety problems, such as oil spills and gas leaks. Past studies have shown that wells can fail when plastic shear strain is accumulated, however, few studies have investigated the effects of geometric and mechanical properties of a well-formation system on the probability of failure. Such an analysis allows assessment of the seismic fragilities of gas storage wells. In this study, numerical simulations were carried out on three different well configurations (that are representative of the gas storage wells in California), modelling their behavior during shearing caused by fault displacement. The objectives of this research were (1) to explore key parameters of the well-formation system affecting the development of well damage, and (2) to estimate the ranges of critical fault displacements at which casing and tubing fail when the values of the key parameters were statistically varied. Results show that the fault angle and fault core width (i.e., the thickness of the shearing layer) are the key parameters for all the examined well configurations, as they accounted for approximately 70% to 90% of the variability of the plastic shear strain development. In the cases where the annulus between the casing and formation was not cemented, the fault angle alone accounted for over 95% of the variability. The critical fault displacements ranged between 0.8 and 3 cm (casing with cemented annulus), 11 and 29 cm (casing with uncemented annulus), 10 and 28 cm (tubing, cemented casing-formation annulus), and 17 and 49 cm (tubing, uncemented annulus). Hence, variations in the geometric and mechanical properties of the well-formation system were found to affect the critical fault displacement by up to about a factor of three in each of the three cases.
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