The evolution of permeability with pressure and temperature in microfractured granite

Measurements of permeability at high-pressure and high-temperature are critical to model and understand the behaviour and evolution of geothermal systems. To perform such measurements and provide constraints on the permeability of crustal rocks, we designed and tested a new apparatus.

Our high-pressure, high-temperature permeameter consists of three independent parts: the permeant gas circuit, the confining fluid circuit, and the heating element. For each measurement, a cylindrical sample is placed between the up- and downstream platens, into an annular Viton jacket which is secured within the pressure vessel. A confining pressure can be applied to the sample by filling the void space between the vessel and jacket through the inlet with kerosene. The confining pressure can be increased up to 50 MPa using a high-pressure hand pump. The temperature of the system can then be increased from room-temperature to up to 150 °C using a heating mantle wrapped around the pressure vessel and connected to a control box. After the confining pressure and temperature have been applied to the system, the permeability measurement is performed by flowing nitrogen (the permeant gas) through the sample while monitoring the pressure differential between the upstream pressure transducer and atmospheric pressure downstream of the sample at different volumetric flow rates (the steady-state method), measured using the downstream flowmeter.

Using this new experimental apparatus, the permeability of Lanhélin granite (from France) samples were measured. Cylindrical samples were prepared and thermally stressed (heated to 700 °C) to ensure that their permeabilities lie in the range that can be measured in our set-up (> 10^-18 m²). Permeability measurements were then performed under confining pressures of 2, 5, 10, 15, 20, 30, 40, and 50 MPa at room temperature, 50, and 100 °C. Our results provide the evolution of the permeability of microfractured granite in various pressure and temperature conditions, which will serve to inform numerical modelling designed to explore the influence of in-situ conditions on fluid flow within a fractured geothermal reservoir.