Constant vs aperture-dependent fracture cementation rates: investigation of fluid flow behavior in discrete fracture network models.

Abstract:

Progressive cementation and sealing of fractures significantly control fault mechanics and fluid flow. Cementation of fractures is typically assumed to occur at a constant rate, leading to fracture aperture distributions that decrease homogeneously through time. A recent study, however, showed that cementation rates in individual fractures may vary as a function of the initial fracture aperture when mineral growth is facilitated by large-scale fluid advection. In this study, we use discrete fracture network models to examine how fracture sealing under constant versus aperture-dependent cementation rates influences the partitioning of fluid flow through time. The discrete fracture network modeling software “dfnWorks”, was used to generate randomized fracture networks parameterized with fracture orientation data compiled from field studies. Single phase flow simulations were then performed for each network over five timesteps. At each timestep, individual fracture apertures were updated according to the end-member models of sealing rate. Results show that when fracture cementation proceeds at a constant rate for all fractures, the number of fractures contributing to overall fluid flow decreases with time and the flow becomes increasingly channelized into fractures with larger apertures. The opposite behavior is observed when an aperture-dependent cementation rate is implemented. The number of fractures participating in fluid flow increases with time, and flow becomes homogeneously distributed throughout the network. These results are observed regardless of variations in volumetric fracture intensity, initial aperture distribution, fluid flow direction, and the randomness of fracture position. Thus, the documented variations in fluid flow distribution appear to be an intrinsic feature of fracture networks when subjected to varying mechanisms of progressive closure. These results have implications for the spatial distribution of fluid flow in fractured geothermal and carbon storage reservoir when mineral precipitation occurs.