The results from laboratory and theoretical work pertaining to the permeability of fractures in shale are presented. The main objective of the work is to improve existing models for hydraulic fractures in basin simulators.
A simple theoretical model is proposed to quantify the effect of creep on the permeability of fractures in shales. The model assumes that the aperture of the fracture changes due to creep of the intact shale material at contact points (asperities). The interaction between contact points is not taken into account. The time dependent deformation of the asperities is based on a model published in the literature for the behaviour of clays, and which is calibrated on cap rock materials tested at NGI. The permeability of the fracture is calculated directly from the aperture changes, assuming that the flow along the fracture can be represented as flow between parallel plates. The results from the model show that the effect of creep on the fracture aperture and permeability is not negligible, although closure of the fracture due to creep is only attained after extremely long times.
Some preliminary results obtained with new laboratory equipment suitable to investigate the long term permeability and hydraulic aperture of fractures in shale are presented. Fractured samples from typical North Sea cap shale are subjected to constant effective normal stress. Flow through the fracture and change of fracture aperture are measured continuously with time. The results show that the fracture behaviour is highly sensitive to its degree of "matedness". The measured significant variation in permeability during creep does not seem compatible with the measured small reduction in fracture aperture. The preliminary test results demonstrate the experimental difficulties in obtaining reliable fracture permeability (conductivity) with time.