Reaction-induced porosity fingering: Replacement dynamic and porosity evolution in the KBr-KCl system

Abstract

In this contribution, we use X-ray computed micro-tomography (X-CT) to observe and quantify dynamic pattern and porosity formation in a fluid-mediated replacement reaction. The evolution of connected porosity distribution helps to understand how fluid can migrate through a transforming rock, for example during dolomitization, a phenomenon extensively reported in sedimentary basins. Two types of experiment were carried out, in both cases a single crystal of KBr was immersed in a static bath of saturated aqueous KCl at room temperature and atmospheric pressure, and in both cases the replacement process was monitored in 3D using X-CT. In the first type of experiment a crystal of KBr was taken out, scanned, and returned to the solution in cycles (discontinuous replacement). In the second type of experiment, 3 samples of KBr were continuously reacted for 15, 55 mins and 5.5 hours respectively, with the latter being replaced completely (continuous replacement). X-CT of KBr-KCl replacement offers new insights into dynamic porosity development and transport mechanisms during replacement. As the reaction progresses the sample composition changes from KBr to KCl via a K(Br,Cl) solid solution series which generates porosity in the form of fingers that account for a final molar volume reduction of 37% when pure KCl is formed. These fingers form during an initial and transient advection regime followed by a diffusion dominated system, which is reflected by the reaction propagation, front morphology, and mass evolution. The porosity develops as fingers perpendicular to the sample walls, which allow a faster transport of reactant than in the rest of the crystal, before fingers coarsen and connect laterally. In the continuous experiment, finger coarsening has a dynamic behaviour consistent with fingering processes observed in nature. In the discontinuous experiment, which can be compared to rock weathering or to replacement driven by intermittent fluid contact, the pore structure changes from well-organized parallel fingers to a complex 3D connected network, shedding light on the alteration of reservoir properties during weathering.

Formation of Strontianite and Witherite Cohesive Layers on Calcite Surfaces for Building Stone Conservation

“…The coexistence of several epitactic relationships within Volmer–Weber epitactic layers determines that these layers accumulate small amounts of porosity as differently oriented 3D nuclei coalesce and leave micropores trapped between them. , Epitactic overgrowth porosity plays a key role in guaranteeing the progress of solvent-mediated mineral replacement reactions. After the surfaces of the parent crystals become totally carpeted by a secondary phase epitactic overgrowth, the existence of a network of interconnected pores within the epitactic layer provides a path for the continuous communication between the primary–secondary phase interface and the bulk solution. , When the replacement reaction involves a negative molar volume change, the preservation of the external shape of the primary phase requires that the molar volume loss is balanced by the generation of an equal volume of transitional porosity. , This porosity adds up to the intrinsic characteristic of Volmer–Weber epitactic layers. The permeability of the resulting porosity network depends on a variety of features, including the total porosity volume, its size, morphology, density and distribution, its interconnectivity, and so forth. ,− Volmer–Weber layers that contain both intrinsic microporosity and porosity, generated as a result of the pseudomorphic mineral replacement reaction fail to effectively armor the underlaying substrate.…”

“…After the surfaces of the parent crystals become totally carpeted by a secondary phase epitactic overgrowth, the existence of a network of interconnected pores within the epitactic layer provides a path for the continuous communication between the primary–secondary phase interface and the bulk solution. , When the replacement reaction involves a negative molar volume change, the preservation of the external shape of the primary phase requires that the molar volume loss is balanced by the generation of an equal volume of transitional porosity. , This porosity adds up to the intrinsic characteristic of Volmer–Weber epitactic layers. The permeability of the resulting porosity network depends on a variety of features, including the total porosity volume, its size, morphology, density and distribution, its interconnectivity, and so forth. ,− Volmer–Weber layers that contain both intrinsic microporosity and porosity, generated as a result of the pseudomorphic mineral replacement reaction fail to effectively armor the underlaying substrate. Thus, the formation of such Volmer–Weber epitactic layers may significantly slowdown the kinetics of mineral replacement reactions but rarely preclude their progress. ,,,, Both strontianite and witherite have larger molar volumes than calcite ( V Str = 39.01 cm 3 /mol, V W = 45.81 cm 3 /mol, V Cal = 36.94 cm 3 /mol).…”

“…47,48 When the replacement reaction involves a negative molar volume change, the preservation of the external shape of the primary phase requires that the molar volume loss is balanced by the generation of an equal volume of transitional porosity. 30,49 This porosity adds up to the intrinsic characteristic of Volmer−Weber epitactic layers. The permeability of the resulting porosity network depends on a variety of features, including the total porosity volume, its size, morphology, density and distribution, its interconnectivity, and so forth.…”

Relative rates of fluid advection, elemental diffusion and replacement govern reaction front patterns

“…Our numerical simulations illustrate different scenarios that produce rough, irregular and boundaries in the aggregate are marked by reaction products, a pattern that is very similar to replacement reactions in fossils, sedimentary basins and metamorphic terrains. Advection is the main driving force for fluid infiltration into the system and for the development of roughness due to more permeable grain boundaries and advection fingering (Jonas et al, 2014;Kar et al, 2015;Plümper et al, 2017;Beaudoin et al, 2018). Advection, however, is not always enough to produce very rough fronts.…”

“…For example, they can be coupled in the sense that reactions may increase permeability causing a reactive infiltration instability (e.g. Chadam et al, 1986) where fluid-flow and hence further reaction is localized -Karst systems (Szymczak and Ladd, 2009), replacement of relatively dense crystals through reaction-induced porosity development (Putnis and Putnis, 2007;Beaudoin et al, 2018) and infiltration of fluids and reactions into otherwise dry, impermeable systems (Jamtveit et al, 2000). Reactions may decrease permeability and arrest the reaction front propagation (Ruiz-Agudo et al, 2016).…”