3.5 Expected Influence of Reactivation on Flow‌

Many fractured rock terrains have undergone a long geological evolution during which a succession of tectonic events occurred, each with specific tectonic regimes and stress magnitudes. When compared to the formation of new fractures, smaller stresses are required for reactivating previously existing planar structures (e.g., foliation and older fractures); thus, reactivation through shear or opening is a common phenomenon. Even when the shear displacement caused by reactivation is on the order of only a few millimeters, it can cause a fracture transmissivity increase of up to three orders of magnitude, as determined through a laboratory experiment carried out by Lamontagne (2001). This takes place because natural fracture surfaces have a certain degree of roughness due to asperities on the fracture surface, and reactivation by shear causes mismatch of the irregularities that lead to an increase in the fracture openings; this is called shear dilation. The experiment demonstrated that, under a normal stress of 3 MPa (corresponding to a depth of roughly 110 m), the increase in transmissivity is as much as three orders of magnitude, no matter the orientation of shear with respect to the irregularities on the fracture plane (Figure 41). With a larger normal stress, such as 9 MPa (depth of approximately 330 m), the transmissivity increase appears to vary with the shear orientation (Figure 41). The normal stress acting on horizontal fractures is approximately equal to the weight of the rock column, and it gradually decreases toward the ground surface. Therefore, it is expected that the transmissivity increase due to reactivation is larger at shallower depths. Details of the experiment conducted by Lamontagne (2001) can be found in Box 1: Effects of Shear Displacement on the Transmissivity of a Rock Fracture.

Figure 41 – Laboratory tests were conducted on many replicas of the same rock fracture. Results indicate a net increase of intrinsic transmissivity with increased shear displacement along the fracture plane. Intrinsic transmissivity is defined in Box 1. In the experiment the fracture is subjected to shear under different normal stress (σn) values. Under 3 MPa normal stress, a shear displacement of only 2 mm is enough to cause a general increase of three orders magnitude in the transmissivity. Under 9 MPa normal stress, the increase depends on the sense of shear along the fracture plane 0°, 90°, 180° and 270°. Sense of shear is explained in Box 1, specifically in Figures Box1-1 and Box 1-7. Modified from Lamontagne (2001).

An overall conclusion is that fractures reactivated under current tectonic stresses will likely have an increased aperture; this is also discussed in Section 4.2 and Section 4.3. Another important implication of reactivation is that rocks showing anisotropy (e.g., foliation) and pre-existing discontinuities (e.g., bedding surfaces and frequent intercalation of different lithologies) will tend to have a denser and more connected fracture network. These characteristics increase the potential for transmitting water when compared to massive, less-fractured rocks such as granites. This is corroborated by studies conducted throughout many regions, as described in Section 4.1.