THE EFFECT OF CONTACT AREA ON THE PERMEABILITY OF FRACTURES

The permeability of a rock fracture is controlled primarily by the geometry of its void space. One effect of void space geometry is to cause the fluid to follow a tortuous path to flow around the asperities, which are regions where the two faces of the fracture are in contact. To examine the tortuosity induced by the contact area, we consider an idealized fracture consisting of two parallel plates propped open by isolated asperities. Boundary-element calculations, analogue electrical conductivity measurements and an effective medium approximation are used to study the permeability of fractures with circular, elliptical and irregular asperity shapes. The permeability is seen to depend not only on the amount of contact area, but also on the shape of the asperities. For circular or elliptical asperities, very accurate estimates are found by using the effective medium theory proposed by Maxwell.