INFLUENCE OF SUBCRITICAL FRACTURE GROWTH ON THE CONNECTIVITY OF FRACTURE NETWORKS

A fracture simulation model which incorporates the physics of fracture growth is used to investigate how the mechanics of fracture formation affect the flow characteristics of fractured systems. Fractures are a ssumed to grow subcritically with the growth rate given by a power law function of the energy available for fracture growth. mow characteris tics are quantified in terms-of the percent of networks percolating an d the average effective conductivity as a function of the fracture den sity. For all flaw densities considered and for values of the growth r ate exponent alpha less than or equal to 1, the flow characteristics p rimarily depend on the fracture spatial density and are similar to the flow characteristics of networks generated stochastically by assuming the fractures are randomly located. For alpha much greater than 1, th e mechanical interaction of the flaws and fractures imparts an organiz ed structure to the network resulting in isolated fractures, or zones of fractures, which form extensive, connected pathways at significantl y lower fracture densities. Experimentally measured values of alpha fo r subcritical fracture growth are typically greater than one, suggesti ng that the flow characteristics of randomly located fractures may not be representative of natural fracture networks thought to have grown subcritically. An. analysis of published fracture trace maps suggests that many natural fracture networks have fracture spatial densities ne ar the percolation threshold. It is suggested that this may be due to the existence of a self-limiting mechanism in fracture network formati on.