Getting more for less: The unusual efficiency of fluid flow in fractures

Although it is recognized that the opening of fractures changes with changing fracture spacing, it is a common practice to consider only the effects of loading and rock properties, and to assume that opening of all fractures is independent of fracture spacing. Thus, one would expect that the more fractures contained in a rock mass, the greater the fluid flow tate. Using the cubic law and the Finite Element Method, we have investigated the volumetric flow rate through a number of equally-spaced fractures in a fractured rock layer as a function of the ratio of fracture spacing to layer thickness, under a remote extension. Results show that there is an optimum value for the ratio of fracture spacing to layer thickness that yields the maximum flow rate. This value is independent of the fluid properties, the head gradient, the applied extension, the internal fluid pressure and the overburden stress.