An Analysis of the Influence of Fracture Geometry on Mass Transport in Fractured Media

Abstract

A stochastic modeling technique has been developed to investigate mass transport within a network of discrete fractures. The fracture network is composed of two orthogonal fracture sets, with fracture length, location, and aperture characterized by appropriate probability distributions. Emphasis is placed on understanding how fracture geometry influences mass transport within a network of discontinuous fractures. The network is aligned in such a manner that one fracture set (set one) forms the dominant pathway for transport. The primary role of the second fracture set is to provide connecting pathways between the discontinuous fractures of set one. Results suggest that mass transport can be described in terms of the directness of the connection between the upstream and downstream boundaries. Variations in fracture geometry which have the effect of increasing the probability that a relatively indirect or circuitous pathway exists through the network generally lead to an increase in both the mean and the standard deviation in the arrival time of various breakthrough fractions. Examples include a reduction in the number of fractures forming the set aligned with the direction of the hydraulic gradient or a decrease in the average length of fractures forming that set. Variations in fracture geometry which have the effect of increasing the probability that a relatively direct pathway will exist through the network have the opposite effect. Transport is sensitive to the variability in fracture aperture. The connectivity of the fracture network is also shown to influence the magnitude of dispersive effects.