Geometry and mineral heterogeneity controls on precipitation in fractures: An X-ray micro-tomography and reactive transport modeling study

Abstract

The processes that affect reorganization of flow and transport in fractures during precipitation were investigated experimentally and numerically in order to highlight the interplay between calcite precipitation, flow and growth substrate. Calcite was precipitated from a supersaturated solution at two different flow rates (0.4 or 12 cm3·hr−1) into artificial fractures made in dolomitic limestone. Although the inlet fluid composition and temperature were identical for all the experiments, facture sealing and precipitation patterns were closely linked to the mineral substrate, fracture geometry and flow field reorganization. Calcite precipitation rate is highly variable along and between the flow paths and depends on the local saturation index of the reactive fluid. In addition, calcite precipitates preferentially on calcite comparatively to dolomite substrate. 2D reactive transport modeling accounting for the negative feedback between porosity and permeability decrease succeeded in quantitatively reproducing the experimental observations, such as the evolution of the fracture void obtained from X-ray micro-tomography (XMT), the evolution of the calcium breakthrough concentration and fracture sealing. The corresponding model was then used to evaluate the impacts of fluid flow and reactivity on randomly generated anisotropic fractures geometries. Simulation results reveal that the sealing capacity of fractures and associated reorganization of flow depends on the Damköhler number, i.e., the ratio between characteristic times for advective transport and reaction, although other factors like the fracture geometry and kinetic law play also a role at this stage. More importantly, the existence of a critical saturation index for initiating precipitation onto mineral substrate can substantially impair the prediction of the sealing capacity, and will depend on the spatial heterogeneity and connectivity of the mineral substrate. Most cases result in a strong precipitation gradient along the flow direction that will increase the permeability anisotropy. However, under certain conditions, precipitation allows the fracture to recover more isotropic properties that will lead back to a more uniform flow field and sealing. Negative feedback between precipitation process and transport during fracture closure supports a strong reorganization of flow of supersaturated fluids during geo-engineering operations.