Influence of Fluid‐Assisted Healing on Fault Permeability Structure

Abstract

Microcracks in fault damage zones can heal under thermally controlled processes. If a flow communication exists between a fluid source and the fault damage zone, warm fluids can migrate into it, change its thermal conditions, and assist healing. The crack life span depends on the local temperature and is, thus, modified by the infiltration of warm fluids. The features of the initial fault architecture govern how the fluids will propagate within the fault damage zones. This affects the rate of healing and the rate of permeability reduction. The region infiltrated by fluids will then show a significant decrease in its permeability, as seen in many field examples like the Alpine Fault. Conventionally, the damage zones immediately near the fault core have a high permeability that decays as we go further away. However, the region adjacent to the fault core, in the Alpine Fault, New Zealand, has the lowest permeability in the current interseismic period. As shown by our simulations, this can be due to healing and sealing, favored by the localized high geothermal gradients (confirmed by the drilling data) and the upward fluid migration through the fault relay structure, which accelerated mass diffusion and minerals precipitation.

Effect of Permeability Evolution in Fault Damage Zones on Earthquake Recurrence

“…Nevertheless, the system will eventually reach a stable state where the pore pressure will stay fixed or oscillate in a narrow range. Furthermore, the temperatures in the fault zones are often controlled by a variety of factors, such as shear heating during coseismic ruptures (Rice, 2006), heat transferred by fluid infiltration (Engvik et al., 2009; Yehya & Rice, 2020), etc. While some may have a long‐term effect on the healing rate of off‐fault cracks and should be incorporated into the simulation framework, some just have a short‐term effect (such as shear heating) which has a limited influence on the permeability evolution during long‐term earthquake cycles, and may be considered when focusing on assessing the short‐term permeability changes.…”

“…This could be either by surface diffusion or by dissolution and precipitation under elevated temperatures. Fluid assisted healing is thermally activated and enhanced by local changes in temperature induced by warm fluid migration (Yehya & Rice, 2020). Hence, the crack lifetime can be expressed as (Smith & Evans, 1984):…”

“…This could be either by surface diffusion or by dissolution and precipitation under elevated temperatures. Fluid assisted healing is thermally activated and enhanced by local changes in temperature induced by warm fluid migration (Yehya & Rice, 2020). Hence, the crack lifetime can be expressed as (Smith & Evans, 1984): where is the crack lifetime, is the local temperature, is the activation energy for surface diffusion, is the gas constant, and are, respectively, the reference crack lifetime and the reference temperature obtained from experiments (Smith & Evans, 1984).…”

“…The temporal and spatial evolution of permeability (deformation/healing dependent) is important to better assess the effects of fluid injection or pressure perturbation on nearby faults (Yehya et al., 2018). In some cases, healing of micro‐cracks in damage zones can create a barrier to fluid flow to delay pore pressure diffusion and the pressurization of the fault core (Yehya & Rice, 2020). On the other hand, the enhancement of permeability by deformation under high pore pressure from fluid injection rapidly facilitates the pressurization of the fault core.…”

Controls on fracture openness and reactivation in Forsmark, Sweden

“…The state of fracture networks in crystalline bedrock generally results from a long deformation history, where the rock permeability and mechanical properties evolved due to successions of fracture failure and sealing processes 1 – 4 . Because fractures are the main conduits for flow and transport, understanding how these past events have shaped the current open fraction of the fracture network (called “openness” hereafter) is key to understanding geological reservoirs in the context of industrial applications, such as geothermal energy, carbon sequestration, or energy and waste storage projects.…”