Impact of in-situ ageing on the mineralogy and mechanical properties of Portland-based cement in geothermal applications
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This study aims to review the current knowledge on wellbore cement degradation under high pressure and temperature conditions and to experimentally investigate whether the standard methodology used for cement durability assessment can adequately describe the performance of a cement formulation at low and high temperatures. During operation the wellbore undergoes many cycles of cooling down and warming up, a process that exposes the comprising parts of the wellbore to heavy loads due to thermal stresses and strains. These thermal effects can compromise the wellbore’s operational integrity by damaging the steel casing, cracking the cement or by disrupting the bond at the interfaces cement-casing or cement-formation. Such cases of annular cement failure occur frequently in geothermal wellbores that encounter high pressures and temperatures. The special cement formulation containing 40% quartz flour, that is designed for use in wellbores operating at temperatures higher than 110 °C and is recommended by the Oil and Gas industry was used in the experimental part of this study. Two series of samples were examined. The first series was exposed to 60 °C, while in confinement, and the second was exposed to hydrothermal conditions of 120 °C and 2 bars for a total of 42 days. The selected exposure temperature of 60°C corresponds to the average wellbore temperature the cement experiences after placement in-situ, for the one month, before the wellbore becomes operational again. On the other hand, the exposure temperature of 120 °C was selected in order to determine the performance of this cement formulation just above the crucial temperature of 110 °C where cement strength retrogression occurs. Mineralogy and microstructural features of the samples were examined through XRD, SEM and optical microscopy after 28 days of exposure to accurately depict the period of cement sheath in-situ ageing. Maximum compressive strength and Young’s modulus values were obtained through uniaxial compressive tests at 2, 3, 7, 14, 28 and 42 days of exposure. The results showed that the quartz flour contained in this formulation does not react at all when exposed to 60 °C for 28 days while only 50% of it react when exposed to 120 °C for the same time. The compressive strength and Young’s modulus is higher when exposed to the hydrothermal conditions, while the porosity is lower due to precipitation of hydrates in the pores at higher temperatures. Finally, the present experiments indicate that exposure to higher temperatures increases the stiffness of the hardened cement paste of this formulation. High stiffness of the sealant indicates a slower and more brittle mechanical response of the cement sheath to the expansion and contraction of the steel casing, and increases the probability of cement sheath shear failure. Further comparison of this formulation with the standard cement used in the geothermal wellbores would require determination of its porosity ad permeability. Complete evaluation of its performance under HPHT conditions requires determining its coefficients of thermal expansion, its thermal and elastic properties, its tensile and compressive strengths and its initial state of effective stress. This could be achieved by performing triaxial tests and by using the methodology applied by rocks mechanics to determine the performance of porous materials.