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

Although it is recognized that the opening of fractures changes with changi ng fracture spacing, it is a common practice to consider only the effects o f loading and rock properties, and to assume that opening of all fractures is independent of fracture spacing. Thus, one would expect that the more fr actures contained in a rock mass, the greater the fluid flow tate. Using th e cubic law and the Finite Element Method, we have investigated the volumet ric flow rate through a number of equally-spaced fractures in a fractured r ock 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 f low rate. This value is independent of the fluid properties, the head gradi ent, the applied extension, the internal fluid pressure and the overburden stress.