Effects of internal structure and local stresses on fracture propagation, deflection,and arrest in fault zones

摘要

The way that faults transport crustal fluids is important in many fields of earth sciences such as petroleum geology, geothermal research, volcanology, seismology, and hydrogeology. For understanding the permeability evolution and maintenance in a fault zone, its internal structure and associated local stresses and mechanical properties must be known. This follows because the permeability is primarily related to fracture propagation and their linking up into interconnected clusters in the fault zone. Here we show that a fault zone can be regarded as an elastic inclusion with mechanical properties that differ from those of the host rock. As a consequence, the fault zone modifies the associated regional stress field and develops its own local stress field which normally differs significantly, both as regard magnitude and orientation of the principal stresses, from the regional field. The local stress field, together with fault-rock heterogeneities and interfaces (discontinuities; fractures, contacts), determine fracture propagation, deflection (along discontinuities/interfaces), and arrest in the fault zone and, thereby, its permeability development. We provide new data on the internal structure of fault zones, in particular the fracture frequency in the damage zone as a function of distance from the fault core. New numerical models show that the local stress field inside a fault zone, modelled as an inclusion, differ significantly from those of the host rock, both as regards the magnitude and the directions of the principal stresses. Also, when the mechanical layering of the damage zone, due to variation in its fracture frequency, is considered, the numerical models show abrupt changes in local stresses not only between the core and the damage zone but also within the damage zone itself. Abrupt changes in local stresses within the fault zone generate barriers to fracture propagation and contribute to fracture deflection and/or arrest. Also, analytical solutions of the effects of material toughness (the critical energy release rate) of layers and their interfaces show that propagating fractures commonly become deflected into, and often arrested at, the interfaces. Generally, fractures propagating from a compliant (soft) layer towards a stiffer one tend to become deflected and arrested at the contact between the layers, whereas fractures propagating from a stiff layer towards a softer one tend to penetrate the contact. Thus, it is normally easier for fractures to propagate from the host rock into the damage zone than vice versa. Similarly, it is easier for fractures to propagate from the outer, stiffer parts of the damage zone to the inner, softer parts, and from the stiff host rock to the outer damage zone, than in the opposite directions. These conclusions contribute to increased understanding as to how fractures propagate and become arrested within fault zones, and how the fault zone thickness is confined at any particular time during its evolution.