Thermal Pressurisation Coefficient During Heating and Rapid Hydrostatic Loading of Porous Media

Abstract

The thermo-poro-elastic response of porous media to changes in effective pressure and temperature is of major interest in various fields of reservoir usage, e.g. energy storage, geothermal energy and shale gas extraction. The change of the pore pressure of an undrained rock specimen during heating can be expressed by the thermal pressurisation coefficient, which relates the induced pore pressure change to the applied temperature change. Various studies investigated the poro-elastic and thermo-elastic behaviour of reservoir rocks, separately. But in geothermal reservoirs and in reservoirs used for energy storage temperature and pressure will change simultaneously due to the injection of fluid. The reservoir rock consists of the solid framework and the fluid inside the pore space. Due to the different thermal expansion of these components, a temperature change causes a decrease or increase the reservoir pressure. Such a thermal pressurisation is more pronounced for undrained conditions and for high effective pressures. Within this study, non-isothermal hydrostatic compression tests were performed on jacketed cylindrical rock specimens of 10 cm length and 5 cm diameter. A major focus of this study was the pore pressure build-up within the specimen, which was placed inside a high pressure and temperature test assembly. In order to measure the pore pressure build-up within the specimen during undrained experiments, a novel fibre optic pressure and temperature sensor was installed within the pressure chamber. In order to quantify the magnitude of thermal pressurisation, we induced temperature change of 1 K due to rapid hydrostatic loading (10 MPa / 15 min) followed by a 45 min equilibration period of an undrained sample. Here the thermal pressurisation coefficient increases with an increase in effective pressure. The magnitude of the thermal pressurisation coefficient was measured to be more than 2 MPa/K even for moderate effective pressures less than 10 MPa.