4D porosity evolution during pressure-solution of NaCl in the presence of phyllosilicates

Abstract

Pressure-solution creep is one of the most common crustal deformation mechanisms, inducing changes in the porosity and permeability of rocks. For a variety of rock types undergoing pressure solution, it has been shown that the presence of phyllosilicates may significantly enhance the rate of the pressure-solution process. In this experimental investigation, we present 4dimensional (three dimensions + time) X-ray microtomographic data that contrast deformation by pressure-solution of a pure NaCl aggregate with that of a mixture of NaCl and biotite. The results show that for mixed samples (NaCl+biotite), phyllosilicates induce a marked reduction in porosity and pore connectivity and contribute to an increase in the local strain rates by an order of magnitude over pure NaCl samples. At the same time, phyllosilicates do not induce strain localization in the sample. We discuss various possible explanations for these observations including a possible positive feedback between the porosity distribution and pressure solution. Our study yields novel insights into the local effects of phyllosilicates during pressure-solution creep and provides full 4-dimensional imaging and characterization of the coupled evolution of porosity and pore connectivity over previously unprecedented experimental time scales.

Dynamic Evolution of Permeability in Response to Chemo‐Mechanical Compaction

“…The starting points for this study are the samples from the study by Macente et al (2018), which used NaCl samples with two different compositions deformed by PSC. NaCl was chosen as rock-analogue geomaterial because previous compaction experiments have shown that it deforms quickly at room temperature and relatively low loads, thus making it an ideal candidate to study pressure-solution processes in the laboratory (Gratier, 1993;Renard et al, 2001;Renard et al, 2004;Schutjens & Spiers, 1999;Spiers et al, 1990).…”

“…We also sought the relationships between permeability and porosity, on one hand, as well as the coordination number between the pores and the main flow stream and pore surface area, on the other, to explore the functional relationships between changes in bulk properties upon PSC and the associated pore-system attributes. For this purpose, the segmented pore geometry, derived from Macente et al (2018), was skeletonized using a medial-axis thinning algorithm (see supporting information and Figure S2). For each pore, the coordination number (from 1 to 8) was calculated by identifying how many edges are connected to one node.…”

“…Specifically, the presence of phyllosilicate minerals in deformational bands contributes to lowering the local permeability as compared to the host rock (Fossen et al, 2007;Heap et al, 2014;Renard et al, 2004). Macente et al (2018) studied the evolution of porosity and grain microstructures upon induced-PSC in samples imaged through X-ray computed microtomography (μCT) and analyzed using digital volume correlation. Two different sample compositions were tested: a pure NaCl sample (hereafter called "salt") and a mixture of NaCl and biotite grains (hereafter called "salt biotite").…”

“…Specific chemical processes responsible for enhancing PSC at pore-and grain-scale require close examination through dynamic imaging, which enables the direct monitoring of the ongoing processes and microstructure evolution. Two samples, a pure salt and a mixture of salt and biotite, were previously saturated with brine solution and left to compact for~3 months to study the evolution of PSC and its effects on porosity and local strains (Macente et al, 2018). In this study, we performed Lattice-Boltzmann modeling (LBM) simulations on the acquired time-lapse μCT data sets of the sample volumes to calculate the effect of PSC on permeability.…”

Neutron Imaging of Cadmium Sorption and Transport in Porous Rocks

“…Recent advances in tomographic measurements, specifically, X-rays, neutrons and magnetic resonance imaging techniques, allow a more detailed observation of the rock internal structure and its evolution through time (Carlson, 2006). Additionally, 3D image analysis, such as Digital Volume Correlation (DVC), provides local strain tensors showing localization and strain distribution within the specimens (Viggiani et al, 2007;Tudisco et al, 2015Tudisco et al, , 2019Macente et al, 2018). Among the different approaches to obtain 3D tomograms, X-ray imaging is traditionally preferred over neutron as it allows higher spatial and temporal resolution (Pacureanu et al, 2012;Cnudde and Boone, 2013;Olbinado and Rack, 2019).…”