Experimental and Model Study on the Time-Dependent Permeability of Rock Fractures Induced by Mechanical Creep

Fracture permeability is one of the critical factors affecting thermal production in hot dry rock reservoirs. Mechanical creep can cause temporal reduction of fracture permeability. However, the study solely on mechanical creep is limited, particularly under high confining stress. In addition, a physics-based stress- and time-dependent permeability model is essential for predicting the in situ geothermal production. This work aims to study the mechanical creep on the time-dependent fracture permeability. Long-term flow tests through single fractured granite samples under constant loading (20, 35, and 50 MPa, respectively) and stepwise increased loading (20 → 35 → 50 MPa) were conducted. The influence of the loading stress on the creep rate and the influence of the time on the permeability damage were quantitatively investigated. Based on the experimental data, a permeability model considering both stress and time effects was established based on viscous–elastic mechanics. According to the study, we obtained the following conclusions: (1) A higher constant confining stress can result in larger creep deformation, a larger damage ratio of hydraulic aperture (eh), and a longer duration of rapid reduction of eh. (2) The previously accumulated creep deformation can affect the subsequent time effect on the temporal evolution of eh when the loading stress changes, causing eh rapid reduction stage to weaken or disappear. (3) The transient creep behavior of eh can be described by the Kelvin creep model, and the maximum damage caused by the creep deformation is almost linearly proportional to the loading stress. The increase in stress caused by the bridging effect between adjacent contact asperities can dramatically reduce the creep rate. (4) The established permeability model can effectively predict the permeability with change in both stress and time considering the effect of accumulated creep deformation on the subsequent creep deformation, and it can be easily implemented in numerical simulation.