Neutron Imaging of Cadmium Sorption and Transport in Porous Rocks

Abstract

Understanding fluid flow in rocks is crucial to quantify many natural processes such as ground water flow and naturally triggered seismicity, as well as engineering questions such as displacement of contaminants, the eligibility of subsurface waste storage, geothermal energy usage, oil and gas recovery and artificially induced seismicity. Two key parameters that control the variability of fluid flow and the movement of dissolved chemical species are (i) the local hydraulic conductivity, and (ii) the local sorption properties of the dissolved chemical species by the solid matrix. These parameters can be constrained through tomography imaging of rock samples subjected to fluid injection under constrained flow rate and pressure. The neutron imaging technique is ideal to explore fluid localization in porous materials due to the high but variable sensitivity of neutrons to the different hydrogen isotopes. However, until recently, this technique was underused in geology because of its large acquisition time. With the improved acquisition times of newly setup neutron beamlines, it has become easier to study fluid flow. In the current set of experiments, we demonstrate the feasibility of in-situ 2D and 3D time-lapse neutron imaging of fluid and pollutant percolation in rocks, in particular that of cadmium salt. Cadmium is a hazardous compound that is found in many electronic devices, including batteries and is a common contaminant in soil and groundwater. It also exhibits higher contrast in neutron attenuation with respect to heavy water, and is therefore an ideal tracer. Time-lapse 2D radiographies and 3D neutron tomographies of the samples were acquired on two neutron beamlines (ILL, France and SINQ, Switzerland). We performed two sets of experiments, imbibition and injection experiments, where we imaged in-situ flow properties, such as local permeability and interactions between cadmium and the solid rock matrix. Our results indicate that even within these cm-scale porous rocks, cadmium transport follows preferential pathways, and locally interacts within the limestone samples. Our results demonstrate that the use of neutron imaging provides additional insights on subsurface transport of pollutants.

4D Neutron Imaging of Solute Transport and Fluid Flow in Sandstone Before and After Mineral Precipitation

“…The Institut Laue‐Langevin in Grenoble, France is a neutron source where high‐speed neutron tomographies with a rapid acquisition time of around 1 minute have been obtained to study water invasion into air‐filled samples of rock (Tudisco et al., 2019). The neutron source SINQ at the Paul Scherrer Institute in Switzerland is another facility where we show in the present study the capability of in situ analysis of the local changes in fluid flow properties of rocks, expanding the possibilities for studying fluid dynamics in rock samples (Cordonnier et al., 2019). In the present study, we quantify the spatial fluctuations of fluid velocity before and after calcium carbonate precipitation in porous sandstone.…”

“…The experiment BS0 is a test to demonstrate the use of three‐dimensional time‐resolved neutron imaging in investigating fluid mixing phenomena within porous rock. We used two miscible fluids, a solution of deuterium oxide heavy water (D 2 O) and a solution of heavy water with dissolved cadmium (CdCl 2 ), an element that provides good contrast for neutron imaging and that was used in a previous study (Cordonnier et al., 2019). After fully flushing the sample with heavy water, we injected heavy water from one inlet at the top of the core sample and 1 M of CdCl 2 (aq) (dissolved in heavy water) from the other inlet at the top.…”

“…To effectively detect and monitor D 2 O injection, the core sample initially had to be saturated with a mixture of D 2 O and H 2 O, ensuring an optimal signal-to-noise ratio. Following a series of calibration tests, similar to the procedure followed by Cordonnier et al (2019), we chose a combination of 50% D 2 O and 50% H 2 O as a reasonable mixture, which is neither too transparent and nor too opaque to neutrons. The flow rates were calculated to ensure that at least one pore volume was injected in the sample over a duration sufficient to acquire the desired number of complete scans.…”

Dynamic neutron imaging of solute transport and fluid flow in sandstone before and after mineral precipitation

“…The experiment BS0 is a test to demonstrate the feasibility of three-dimensional time-lapse neutron imaging to study fluid mixing in porous rock. We used two miscible fluids, a solution of deuterium oxide heavy water (D 2 O) and a solution of heavy water with dissolved cadmium (CdCl 2 ), an element that provides good contrast for neutron imaging and that was used in a previous study (Cordonnier et al, 2019). After fully flushing the sample with heavy water, we injected heavy water from one inlet at the top of the core sample and 1 M of CdCl 2 (aq) (dissolved in heavy water) from the other inlet at the top.…”

Review of Microfluidic Devices and Imaging Techniques for Fluid Flow Study in Porous Geomaterials

“…They found that after 15 PV water injection, the CO 2 saturation was decreased from 30% to 10%. Moreover, 2D and 3D neutron imaging were used by Cordonnier et al [ 321 ] to study cadmium (a common contaminant in soil and groundwater) sorption and transport in porous media. They performed a series of flow-through experiments on different sandstone and limestone core samples and imaged in-situ flow properties.…”