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

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℃, 40℃ and 60℃, 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 （△T/ 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 disso- lution 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℃ （during the 25℃-4℃ interval） and 42.6% when the temperature increases by 20℃ （during the 40℃-60℃ interval）. It can be concluded that the permeability decreases to a greater extent for larger increases in temperature.