Damage zones commonly exist alongside crustal faults, where a combination of micro- and macrocracks form intricate pathways for fluid flow. The permeability of these zones can exhibit rapid changes as the crack network evolves over time and space. Accurate measurements of permeability under high pressure and high temperature conditions are imperative for modeling and comprehending the behavior and evolution of geothermal systems within crustal fault zones. To facilitate such measurements and provide valuable insights into the permeability of crustal rocks, we have devised and tested a new experimental apparatus.
Our high-pressure, high-temperature permeameter comprises three independent components: the permeant circuit, the confining pressure circuit, and the heating elements. A confining pressure of up to 50 MPa can be applied (using silicon oil as the confining fluid), to which a temperature of up to 150°C can be superimposed. Once the desired confining pressure and temperature are attained, we conduct permeability measurements by flowing nitrogen (as the permeant gas) through the sample, simultaneously monitoring the pressure differential between the upstream pressure transducer and the atmospheric pressure downstream of the sample. Permeability measurements are conducted using either the steady-state method, with varying flow rates monitored by the downstream flowmeter, or the pulse decay method, by analyzing the pressure decay following a sudden pressure pulse.
Using our experimental apparatus, we determined the permeability of microcracked Lanhélin granite samples as well as that of samples with both microcracks and a single cross-cutting macrofracture. Permeability measurements were performed under confining pressures ranging from 2 to 50 MPa at room temperature, 75 °C, and 150 °C. At room temperature, the permeability of microcracked granite decreases by over two orders of magnitude between 2 and 50 MPa of confining pressure, following an exponential trend. This trend remains constant across all temperatures investigated. Moreover, at a fixed confining pressure, increasing temperature results in a decrease in permeability. In granite samples that contain both microcracks and a cross-cutting fracture, permeability is more than four orders of magnitude higher than in a microcracked granite without macrofracture. The influence of temperature and pressure on permeability is also significantly reduced in granite samples with both microcracks and a cross-cutting fracture, in contrast to microcracked granite alone. These findings hold significant value in informing numerical models that seek to explore the impact of in-situ conditions on fluid flow within fractured geothermal reservoirs.
