4-dimensional study of dissolution precipitation creep in oedometric compaction experiments of NaCl-biotite aggregates

Abstract

Understanding the dynamic evolution of hydraulic rock properties and the associated deformation mechanisms is a crucial factor in establishing the potential of groundwater resources, geological repositories as well as fossil fuel reservoirs. Chemical compaction by dissolution precipitation creep (DPC) is a major ductile deformation mechanism in the Earth’s upper crust that affects the porosity and permeability of its host rock and therefore influences transport properties and fluid flow herein. However, investigations of the process were in the past limited to observations on natural samples, laboratory experiments and numerical modelling, either lacking a temporal or spatial component or the validation of the model by direct observations. The link between space and time only became possible recently with the implementation of X-ray computed microtomography (microCT) in the earth sciences. By means of microCT imaging time-resolved, three-dimensional data (4D datasets) can be used to investigate the dynamic spatio-temporal evolution of hydraulic properties and the effect of mechanical-chemical feedback mechanisms. This thesis investigates slow chemical compaction mechanisms and their influence on porosity in NaCl-biotite rock-analogues. Oedometric compaction experiments on samples of different compositions and structures (layered vs. homogeneous) were conducted and time-resolved (4D) microtomographic data used to capture the dynamic evolution of the transport properties with progressing compaction. Furthermore, emphasis was placed on the effect of phyllosilicates upon deformation by dissolution precipitation creep, which is generally regarded to reinforce the process. Percolation analysis in combination with advanced digital volume correlation techniques showed that the presence of phyllosilicates influences the dynamic evolution of the porosity in the samples by promoting a reduction of porosity in their vicinity to the point of disconnecting the pore volume. However, the absence of strain localisation around those phyllosilicates and a homogeneous distribution of axial shortening across the samples invites a renewed discussion of the effect of phyllosilicates on dissolution precipitation creep, with a particular emphasis on the length scales of the processes involved. The results presented in this thesis, are among the first spatial-temporal data sets resolved on the microscale that demonstrate the crucial role of dissolution precipitation creep for the dynamic evolution of fluid transport properties. In addition to that, they encourage discussing classical theoretical models of diffusive transport and give an insight into the remaining challenges of experimental dissolution precipitation creep, especially concerning the nucleation and growth of stylolites.