Dynamic Evolution of Permeability in Response to Chemo‐Mechanical Compaction

Abstract

Pressure-solution creep is an important fluid-mediated deformation mechanism, causing chemo-mechanical transformations and porosity and permeability changes in rocks. The presence of phyllosilicates, in particular, has previously been hypothesized to further reduce porosity and pore connectivity. Nevertheless, a full characterization of the spatiotemporal evolution of permeability during this process has yet to be reported. A pure NaCl aggregate and a mixture of NaCl and biotite were deformed through pressure-solution creep while monitoring their microstructural evolution through computed X-ray microtomography. The evolution of permeability and fluid velocity of the samples were computed by using the pore geometries from the X-ray microtomography as input for the Lattice-Boltzmann modeling.The results indicate that, as deformation proceeds, porosity and permeability decrease in both samples. In the salt-biotite sample pressure solution creep causes the formation of a compaction band perpendicular to the direction of loading, forming a barrier for permeability. Along the other two directions, pore connectivity and permeability are retained in the marginal salt layers, making the sample strongly anisotropic. The presence of biotite controls the way pore coordination number evolves and hence the connectivity of the pathways. Biotite flakes create an enhanced porosity decrease leading to compaction and reduction of pore connectivity. This reduction in porosity affects local stresses and local contact areas, reducing over time the driving force. According to a texture-porosity process, the reduction in porosity causes salt ions to dissolve in the marginal salt and precipitate within the biotite-bearing layer, where the bulk volume of salt grains increases over time.

Four-dimensional X-ray micro-tomography imaging of dynamic processes in geosciences

“…Rock deformation experiments that use 4D imaging have opened a new era in rock physics and geomechanics to improve our knowledge of fluid-mediated and stress-controlled deformation mechanisms [Renard et al, 2004, Viggiani et al, 2004, Gratier et al, 2015, Macente et al, 2019. With the development of triaxial deformation apparatuses installed on synchrotron beamlines, it is now possible to analyze several important geomechanical and geophysical processes.…”

Micro- and nano-porosity of the active Alpine Fault zone, New Zealand

“…In the present study, we have chosen a simple segmentation technique by applying a threshold to the grayscale images to separate the void space from the solid. This technique has been used in many studies in the last 2 decades to characterize porosity in rocks, including some very recent studies in rock physics (Macente et al, 2019;Renard et al, 2019). The segmented porosity volume depends strongly on the choice of the threshold and some studies have demonstrated that the final porosity estimated by different segmentation methods can vary by 20 % (Andrä et al, 2013).…”