Laboratory assessment of healing of fractures in rock salt

Tension-induced fractures and fractures formed by saw-cut surfaces and polished surfaces were prepared in salt specimens. A series of gas flow testing was performed to monitor the changes of the fracture permeability under quasi-static loading (0.7–20 MPa) for up to 120 h. Healing tests under static loading were carried out under both dry and saturated conditions. The results suggest that the primary factors governing the healing of salt fractures are the origin and purity of the fractures, and the magnitude and duration of the fracture pressurization. Inclusions or impurities significantly reduce healing effectiveness. The hydraulic conductivity of fractures in pure salt can be reduced permanently by more than 4 orders of magnitude with applied stresses of 20 MPa for a relatively short period. In most cases the reduction of salt fracture permeability is due to fracture closure, which does not always lead to fracture healing. The closure involves visco-plastic deformation of the asperities on both sides of the salt fracture, while the healing is related to the covalent bonding between the two surfaces. Fracture roughness and brine saturation appeared to have an insignificant impact on the healing process.