Transmissivity of rock fractures: Normal loading and shear reactivation

Enhanced Geothermal Systems represent a major field of study in the context of renewable energy resources. To create extractable energy from those reservoirs, a high enough fluid flow rate for production needs to be achieved. This fluid flow rate is directly related to the permeability of the fracture system in the reservoir. Hence, by increasing the reservoir’s permeability the fluid flow increases as well. For this purpose, shear stimulation has proven itself effective but remains a challenging field of study. Many aspects of this mechanism are still unknown and have to be explored further. This study focuses on the fracture transmissivity evolution of low porosity rocks (i.e. granite and marble) with smooth fracture surfaces subjected to progressing shear displacement until several millimetres of offset. The aim of this study was to record fracture transmissivities continuously throughout the experiment without stopping the mechanical loading processes and thereby using the oscillatory pore pressure method. In this context, a new experimental procedure was created making a direct shear configuration using a standard triaxial apparatus possible. Both samples were exposed to a constant confining pressure of 25 MPa which can be directly reflected as normal stress on the fracture. The granite fracture transmissivities were in the range of 6.8317·10−20 to 3.4429·10−19 m3. They first followed a decreasing trend until reaching the minimum value. Then a strong peak could be observed after which transmissivities were increasing again until the end of the experiment. Numerous peaks and deviations from the linear trend could also be observed. The marble fracture transmissivities were in the range of 1.4604 · 10−20 to 2.1110 · 10−20 m3 and were generally lower than the granite’s. Also, the transmissivity evolution followed a different path, continiously and slowly decreasing until the final position. The curve was generally more flattened and did not show any strong peaks. These results showed that rock fractures behave in different manners according to their lithology and more precisely, to the material’s hardness. Rock fractures often form wear products which can either obstruct the flow paths or enhance them as a function of the wear mechanism and wear volume. The granite sample encountered the debris formation mechanism that would transition from mild to severe wear. This transition could be observed in its fracture transmissivity passing from a decreasing to an increasing trend. On the other hand, the marble sample was subjected to a continual asperity smoothing mechanism, thereby slowly closing up the fracture and decreasing its fracture transmissivity. Within the frame of EGS, the results of this work can have interesting implications for production rates. Results showed that not only loading conditions can affect the fluid flow through rock fractures. Rock lithologies and ranges of shear displacement also play an important role and should be explored further.