Fluid Transport Properties Heterogeneity Evolution During Fault Growth: Coupling Realistic Fluid Flow Simulations with Triaxial Experimental Data and Pore Pressure Heterogeneity Measurements

Abstract

Tectonic formation, fault activity, and earthquakes are influenced by the concentration of fluid within the Earth's crust. In this work, we aim to unravel the spatial and temporal evolution of fluid transport properties during fault growth to gain insights into the dynamics of fluid flow and its impact on fault development. This is done by taking advantage of a rich data set of measurements of stress, deformation, fluid flux, local pore pressure and acoustic emission locations, coupled with three-dimensional numerical simulations. We induced quasi-static failure in an initially intact sample of Westerly granite under triaxial conditions. Fault growth was monitored using acoustic emission locations. Deformation was periodically halted to conduct flow-through tests, during which pore pressure heterogeneity and flow rate were measured. The exact geometry of the tested sample was then numerically reconstructed, and three-dimensional finite element simulations of Darcy flow were employed to estimate the heterogenous fluid flow properties for all the stages of the experiment by least-square minimisation within an adjoint framework. For each stage of the test the following two models of permeability heterogeneity were considered: i) different local permeability values are inverted for a regular grid in the fault zone and for the remaining volume of the sample; ii) empirical coefficients are inverted to link the change in permeability within the sample to the acoustic emission event density. We were able to identify the stages during the faulting process where the permeability undergoes the most significant changes: in the initial stages following peak stress, the permeability of the fault zone increases, reaching up to approximately 150 times the permeability of the bulk. The subsequent significant increase (up to approximately 400 times the permeability of the bulk) occurs when the equivalent fault slip ranges between 0.6 and 0.7 millimetres. No substantial increase is observed for the remaining stages of the faulting process. We were also able to determine the extent of permeability heterogeneity along the shear fault zone, revealing variations of up to 8 times between different zones within the fault volumes. These variations are dependent on the specific stage of the faulting process.