Experimental Investigations of Fractured Rock Permeability Hysteresis

Researchers have been investigating stress-dependent permeability in fractured porous media for decades. The phenomenon that permeability decreases with effective pressure increase is widely observed. Due to the uneven surfaces of fractures, hysteretic behavior of fractured rock permeability is also noticed in core flood experiments. Here, a series of experiments were conducted to investigate permeability hysteresis, including a high permeability rock and extremely low permeability rock both with a single fracture. Permeability is measured using a high pressure core flood apparatus with X-Ray CT scanning to measure the fracture aperture. All the results have shown clear hysteretic behavior in both permeability and fracture aperture in repeated cycles of compression and decompression. Due to closure of fracture aperture, when a fractured rock is compressed axially, the permeability will have an exponential decline with effective pressure, as expected from stress-dependent permeability theory. When the fractured rock is decompressed afterwards, permeability increases, but not along the compression pathway and never back to the original value. This large hysteresis indicates the irreversible deformation of fracture asperities. Two other compression-decompression cycles have been done. Results show that higher maximum pressure corresponds to lower permeability cycle. The reasons for this hysteretic behavior, based on our investigation, are mainly surface asperity deformation and friction between asperities. Theoretically, a certain fraction of asperities will deform under different pressures exerted to the rock fracture surfaces, which results in different hysteresis loops accordingly. We hypothesize that friction causes the difference between compressing line and decompressing line at fixed maximum effective pressure.