Fault Permeability and Strength Evolution Related to Fracturing and Healing Episodic Processes (Years to Millennia): the Role of Pressure Solution

Abstract

It is well known that fluids flow through faults and fractures but it is also demonstrated that fault zones act as impermeable barriers. Consequently, one must consider that faults are successively open and closed paths for fluids. On the human-activity time scale (years to millennia), studies of the seismic cycle offer the possibility of making a model of such evolution. According to this model, seismic (or hydraulic) fracturing opens fluid paths almost instantaneously through the faults with associated weakening and post-fracturing creep processes. Fault healing processes then progressively close such fluid paths, associated with fault strengthening and fluid pressure recovery. Such transient behaviors have major consequences in the studies of: the evolution of permeability along faults with application tooil-field reservoir exploitation and fluid and waste storage; the evolution of fluid fluxes along faults with application to mass balance and climate evolution on the scale of the earth; the timing of earthquakes and the probability of their occurrence. The aim is to understand and evaluate the kinetics of the processes and the specific characteristic times of the fracturing and healing cycles. Results from laboratory experiments and natural fault studies are presented that show how pressure solution processes can explain both creep and sealing processes and the way they are associated in nature. The various fault-healing processes are discussed with their various characteristics in times from weeks to millennia. It is shown how they can be integrated into creep and sealing laws. Laboratory experiments give the values of some parameters of the laws (kinetics, thermodynamic). Other parameters must always be evaluated from the study of natural structures (geometry of path transfer, pressure and temperature conditions, nature of minerals and fluids). Consequently, the duration of the fracturing and sealing cycle is related to some extent to the geological context of a faulted area. Finally, as the mechanisms of permeability and strength evolution interact and occur on various scales of time and space, they must be integrated into numerical models, which are briefly discussed.