Laboratory Testing of Long Term Fracture Permeability in Shales

摘要

The results from laboratory and theoretical work pertaining tornthe permeability of fractures in shale are presented. The mainrnobjective of the work is to improve existing models forrnhydraulic fractures in basin simulators.rnA simple theoretical model is proposed to quantify therneffect of creep on the permeability of fractures in shales. Thernmodel assumes that the aperture of the fracture changes due torncreep of the intact shale material at contact points (asperities).rnThe interaction between contact points is not taken intornaccount. The time dependent deformation of the asperities isrnbased on a model published in the literature for the behaviourrnof clays, and which is calibrated on cap rock materials testedrnat NGI. The permeability of the fracture is calculated directlyrnfrom the aperture changes, assuming that the flow along thernfracture can be represented as flow between parallel plates.rnThe results from the model show that the effect of creep on thernfracture aperture and permeability is not negligible, althoughrnclosure of the fracture due to creep is only attained afterrnextremely long times.rnSome preliminary results obtained with new laboratoryrnequipment suitable to investigate the long term permeabilityrnand hydraulic aperture of fractures in shale are presented.rnFractured samples from typical North Sea cap shale arernsubjected to constant effective normal stress. Flow through thernfracture and change of fracture aperture are measuredrncontinuously with time. The results show that the fracturernbehaviour is highly sensitive to its degree of "matedness". Thernmeasured significant variation in permeability during creeprndoes not seem compatible with the measured small reductionrnin fracture aperture. The preliminary test results demonstraternthe experimental difficulties in obtaining reliable fracturernpermeability (conductivity) with time.