Effect of Repeated Temperature Cycles on Thermal Expansion Coefficient and Elastic Moduli of High Porosity Chalks
29 Pages Posted: 19 Mar 2024
Abstract
The use of chalk reservoirs for storage of fluids can have negative influence on the rocks stiffness, not least due to repeated changes in temperature. This is relevant in connection with cycles of water injection, hydrocarbon extraction, and cooling induced by CO2 storage. Detrimental effects could be anticipated because temperature cycles are known to degrade calcite cemented sedimentary rocks and marble. This study documents how temperature cycles influence elasticity and thermal expansion coefficient of two chalks with different degree of calcite cementation. The more indurated and less porous Kansas chalk, and the less indurated and more porous Belgian Mons chalk. As a basis for the study, the elastic envelope (p-q diagram) was established for the two chalks. Following temperature cycling, elastic bulk modulus measured under hydrostatic conditions was found to decrease for the soft Mons chalk, while no significant effect was found for the stiffer Kansas chalk. For both chalks, Young´s modulus measured under deviatoric stress, showed no effect of temperature cycling, but the thermal expansion coefficient of the Kansas chalk was found to decrease with increasing number of temperature cycles. A less clear, but similar trend was seen for the Mons samples. The thermal-elastic coupling coefficient, needed to evaluate effective stresses during cooling, is measured before and after the repeated temperature changes.
Keywords: Thermal-elasticity, softening of elastic constants, failure due to cooling, experiments, chalk, CO2 injection
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