Effect of repeated temperature cycles on thermal expansion coefficient and elastic moduli of high porosity chalks

This paper describes the impact of repeated temperature cycles on elastic mechanical parameters of chalk. The experiments were performed at both hydrostatic and deviatoric stress geometries at an axial stress 70% of the stress at failure. Thus, shear failure tests are reported. Geomechanical stability during cooling was evaluated at constant axial stress, while the side stress was reduced to counteract thermal contraction, ensuring uniaxial strain conditions. The study was motivated by the fact that chalk reservoirs are exposed to cooling due to intermittent water injection during hydrocarbon extraction and are future prospects for CO 2 storage. Detrimental effects on rock-mechanical stiffness and strengths were 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. Following temperature cycling, elastic bulk modulus measured under hydrostatic conditions decreased for the softer 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. At hydrostatic stresses, the thermal expansion coefficient of the Kansas chalk was found to decrease with increasing number of temperature cycles, while a less clear, but similar trend was seen for the Mons chalk samples. The thermal-elastic coupling coefficient, needed to evaluate effective stresses during cooling, was measured before and after the repeated temperature changes. • Evaluate the impact of temperature cycles on elastic parameters. • Complete failure envelope. • Unique triaxial tests with constant overburden and reduced side stress. • The thermal-elastic parameters affect geomechanical stability assessments during cooling.