Casing design for geothermal wells at super critical conditions
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In this thesis, mechanisms governing the mechanical response of casing systems in high-temperature geothermal wells have been studied. To increase the efficiency of geothermal energy production it is desirable to produce from high enthalpy fluids at super-critical state. This involves high down-hole temperatures and the casing system might experience temperature changes in excess of 500°C. Casing systems subjected to such extreme thermal loading are susceptible to failure modes that will compromise the operation of the well. The work in this thesis was carried out as a part of SINTEF's contribution to the Hot-CaSe project, which is initiated by Equinor and their GeoMagma project The goals of this thesis are; 1) to gain more understanding of the mechanisms and phenomena governing the response of high-temperature geothermal wells, 2) to evaluate the feasibility of casing concepts developed in the Hot-CaSe project, and 3) to evaluate the prospect of using the general purpose finite element software Abaqus to simulate full-scale casing systems. To achieve these goals, several analytical and numerical models were developed. An analytic derivation was used to evaluate the stress state in the casing under thermal loading and its relation to the apparent radial stiffness of the surrounding formation. A large-scale (1000 meters) model was developed and used to evaluate the possibility of a globally sliding casing, with special attention to the effect of contact properties. Two short section models were developed to evaluate local sliding, and a corrugated casing structure. A disc section model was used to validate the analytic solution and to further study the stress state under transient thermal conditions. It was revealed that the axial stress is dominating the system response when the casing is not sliding. Further, it was shown that due to the radial thermal expansion, even small values of friction in contact between casing and cement will lead to friction locking for long sections, when the entire surface is in contact. Limiting contact by not cementing the casing, or introducing a micro-annulus between casing and cement might remedy this. Using a corrugated casing will not reduce the peak stress for wavelength between 2 and 12 meters and 15mm amplitude.