Incipient karst formation in carbonate rocks: Influence of fracture network topology

Abstract

Karst features in limestone aquifers often exhibit highly heterogeneous patterns, which are strongly governed by structural complexities such as the spatial distribution and organization of fractures in natural carbonate rocks. However, until now, little is known about the impact of structural complexity on karstification processes. In this paper, we use numerical models to investigate how structural heterogeneity affects the generation of incipient karst in natural fractured systems. We develop a numerical model which couples the processes of fluid flow, mass transport and dissolution kinetics that govern the growth of fracture aperture, based on discrete fracture networks. We apply the model to simulate conduit evolution in a natural joint network that displays a distinctive topological pattern. A wide range of hydraulic gradient and initial fracture aperture is considered. Our results show that the network topology may generate a significant impact on dissolution pattern depending on how flow is organized within the discrete network. If a channelized structure occurs in the flow network, the dissolution pattern depends highly on the initial hydrogeological settings, while if the flow network consists of branched segments, a similar dissolution pattern emerges for a broad range of initial hydrogeological settings. When the flow direction is normal to the persisting fracture orientation, intermittent interruptions to the positive feedback process between flow and dissolution occur. Therefore, the breakthrough is delayed. We discuss the link between our results of dissolution modeling in fracture networks to previous modelling efforts in the context of single rough fractures and fracture network formed by orthogonal fracture sets.