2.6 Expected Influence of Fracture Types on Flow‌

The volumetric flow rate produced by a set of parallel, persistent, and planar fractures is a function of their density (number of fractures per unit distance perpendicular to the fractures) and aperture, as expressed by the cubic law by Snow (1968) and shown in Figure 3. Snow analyzed laminar flow conditions in smooth parallel plate fractures, and reported that the flow rate is proportional to the cube of the fracture aperture and is linearly related to the fracture density. Thus, fracture aperture is more influential than fracture density in controlling the quantity of water flowing through a system of parallel fractures. However, the roughness of the fracture surfaces and the presence of contact areas between the two faces, which are expected for natural fractures, cause the flow to be more tortuous than assumed by the cubic law. Even so, this law is useful for estimating values of flow parameters, rate, and velocity.

The mode of fracture propagation has a direct influence on the fracture aperture. In principle, aperture would be larger for joints, intermediate for hybrid fractures, and smaller for shear fractures. However, aperture can be modified by reactivation during later tectonic events, as explained in Section 4.2. Infilling can significantly diminish the aperture of joints and hybrid fractures. Columnar cooling joints in basalts are an example of diminished aperture as many of them are infilled with minerals that precipitated from hydrothermal fluids during the cooling process within basalt floods (as discussed in Section 4.1.2).

Connectivity, another key factor for fluid flow, is enhanced where conjugate fractures are present because they are frequently connected in three-dimensional space. In addition, it is expected that flow is enhanced at the linear intersections of those fractures. A single set of joints produces little connectivity because joints are generally parallel. A perfect parallelism of persistent fractures is rarely observed thus some limited connectivity among them is usually present.