Spectral induced polarization of calcite precipitation in a 2D porous medium

International audience; Induced calcite precipitation is used in geotechnics to modify the mechanical and hydrological properties of the underground. It also is an interesting model system to investigate mixing-induced geochemical reactions occurring in a range of hydrological systems. Laboratory experiments have shown that spectral induced polarization (SIP) measurements can detect calcite precipitation. However, most previous studies have only shown qualitatively how the SIP response changes due to calcite precipitation. This study aims to quantify the relationship between SIP response and calcite precipitation. For this, a 2D cell with a synthetic porous medium was developed so as to allow visualizing the dynamics of calcite precipitation while making SIP measurements. Here, calcite precipitation was generated by injecting Na2CO3 and CaCl2 solutions through two different inlets for 40 hours. During injection, the development of precipitation in the region where the two injected fluids mix, was monitored with a camera. Calcite crystal growth was observed with a microscope at selected locations. The SIP response was measured in three regions of the cell. The imaging results showed that calcite precipitation mainly occurred in the first 15 hours, likely because formation of the precipitate at the location of the mixing front inhibited further reactions. The SIP response near the injection started to increase after 5 hours, while the SIP response far from the injection started to increase after 15 hours. Although there was almost no visual change in the precipitation after 15 hours, the SIP response continued to increase for a much longer time, up to high values of 65 to 100 mrad depending on the position in the cell. These results suggest that the mass of calcite precipitation does not necessarily control the SIP response of calcite, but that other factors such as kinetic changes in the electric double layer of the precipitates may be important. Additionally, the peak phase was higher for the region near the injection, which is in agreement with the smaller size of the crystals determined from microscope images in this region. Our results highlight that the novel setup that combines a cell containing a 2D porous medium with SIP measurements can be successfully used to infer how calcite precipitation affects the SIP response in a dynamic system.