Evolution of limestone fracture permeability under coupled thermal, hydrological, mechanical, and chemical conditions

Abstract

The effect of temperature on the rock fracture permeability is a very important factor in the prediction of the permeability of enhanced geothermal systems and in reservoir engineering. In this study, the flow-through experiments were conducted on a single limestone fracture at different temperatures of 25°C, 40° C and 60° C, and with differential pressures of 0.3 MPa and 0.4 MPa. The experimental results suggest a complex temporal evolution of the fracture aperture. The aperture increases considerably with increasing temperature and reduces gradually to a steady value at a stable temperature. The results of three short-term experiments (QT-1, QT-2, QT-3) indicate an exponential relationship between the permeability and the temperature change ratio (ΔT/T), which provides a further evidence that the rising temperature increases the aperture. It is shown that the changing temperature has its influence on two possible accounts: the chemical dissolution and the pressure dissolution. These two processes have opposite impacts on the fracture permeability. The chemical dissolution increases the permeability with a rising temperature while the pressure dissolution reduces the permeability with a stable temperature. These make a very complex picture of the permeability evolution. Our results show that the fracture permeability reduces 39.2% when the temperature increases by 15°C (during the 25°C–40°C interval) and 42.6% when the temperature increases by 20°C (during the 40°C–60°C interval). It can be concluded that the permeability decreases to a greater extent for larger increases in temperature.