2.7 Highlights on Fracture Types and Groundwater Flow with Opportunities to Exercise Knowledge Gained by Reading Sections 1 and 2‌

The most important implications of fracture types (i.e., extension and shear) and of structures formed at different crustal levels for groundwater flow in a fracture network are:

• Ductile structures such as foliation, formed at greater depths, do not produce porosity, however because they are commonly reactivated as fractures during brittle deformation, they influence the fracture network configuration. Thus, rocks with foliation, veins, and other previous anisotropies, will tend to have a denser fracture network and a higher transmissivity.
• The cubic law (Snow, 1968) demonstrates that fracture aperture is more influential than fracture density in controlling the quantity of water flowing in a fracture system. Thus, joints can carry more flow than shear fractures.
• 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, the flow can be enhanced at the linear intersections of those fractures.

Exercises 1 through 7 elucidate the constraints on the formation of joints and faults and their association in time and space. They describe plausible scenarios in terms of depth of deformation, mechanical parameters of different rock types, principal stress magnitude, tectonic regimes and fracture types that are formed. With these concrete geological scenarios, we have the opportunity to understand that different types of rock have different mechanical characteristics, such as failure envelopes, cohesion and elastic properties. These factors, together with stress magnitude, control whether extension or shear fractures (parallel or conjugate patterns) are formed, and how fracture geometry, such as spacing can vary from one rock to another. Thus, it becomes evident that stress magnitude and types of rock have influence on the fracture geometry, such as aperture, orientation, and density, and consequently on the fracture network configuration and connectivity, as well as preferential groundwater flow pathways.

Mohr diagrams synthesize the relationships between stress magnitude, mechanical properties of different rock types and types of fractures. Learning the theory underpinning a Mohr diagram, as developed in sections 2.4 and 2.5, is not a simple task. Thus, exercises 2 through 7 offer opportunities of learning and realizing the applications of the Mohr diagram on real problems.

Exercise 1 offers an opportunity to consider the depth and timing at which ductile and brittle structures develop and to explore their ability to carry water.

Exercises 2 through 7 address how stress magnitude and mechanical properties of rocks control the fracture types that are formed as deduced from Mohr diagram construction as well as from tectonic regimes.

Exercise 2 offers an opportunity to explore the ease of brittle deformation at a given depth, under non-tectonic conditions, for different rock types (dolomite and a mudstone).

Exercise 3 offers an opportunity to trace failure envelopes using laboratory data from compressional triaxial tests and to deduce what rock is prone to have more closely spaced fractures and a more connected fracture network.

Exercise 4 offers an opportunity to determine what tensile strength is necessary to form joints (fractures with larger aperture) from failure envelopes of two rock types.

Exercise 5 offers an opportunity to consider how fractures can be formed under non-tectonic conditions by the changes of fluid pressure. It also shows that depending on rock type the hydraulic fractures can be extension (joints) or shear types.

Exercise 6 offers an opportunity to consider: what fracture types will be formed under given stress states represented in the Mohr circle; what tectonic regimes would occur; and what fracture orientations would form for given orientations of the principal stresses.

Exercise 7 offers an opportunity to consider what would be the minimum principal stress for the formation of normal faults and the maximum principal stress for the formation of thrust faults at the same given depth. This exercise also provides field data on in-situ stress measurements and explores the types of faults that are prone to forming at shallow depths, as well as the consequences for the orientation of more transmissive fractures close to the Earth’s surface.