Acid Erosion of Carbonate Fractures and Accessibility of Arsenic-Bearing Minerals: In Operando Synchrotron-Based Microfluidic Experiment

A radiochemical lab-on-a-chip paired with computer vision to unlock the crystallization kinetics of (Ba,Ra)SO4

(Ra,Ba)SO4 solid solutions are commonly encountered as problematic scales in subsurface energy-related applications, e.g., geothermal systems, hydraulic fracturing, conventional oil and gas, etc. Despite its relevance, its crystallization kinetics were never determined because of radium (226), high radioactivity (3.7 × 1010 Bq g-1), and utilization in contemporary research, therefore constrained to trace amounts (< 10-8 M) with the composition of BaxRa1-xSO4 commonly restricted to x > 0.99. What if lab-on-a-chip technology could create new opportunities, enabling the study of highly radioactive radium beyond traces to access new information? In this work, we developed a lab-on-a-chip experiment paired with computer vision to evaluate the crystal growth rate of (Ba,Ra)SO4 solid solutions. The computer vision algorithm enhances experimental throughput, yielding robust statistical insights and further advancing the efficiency of such experiments. The 3D analysis results of the precipitated crystals using confocal Raman spectroscopy suggested that {210} faces grew twice as fast as {001} faces, mirroring a common observation reported for pure barite. The crystal growth rate of (Ba0.5Ra0.5)SO4 follows a second-order reaction with a kinetic constant equal to (1.23 ± 0.09) × 10-10 mol m-2 s-1.

Environmental impact of solution pH on the formation and migration of iron colloids in deep subsurface energy systems

Deep subsurface stimulation processes often promote fluid-rock interactions that can lead to the formation of small colloidal particles that are suspected to migrate through the rock matrix, partially or fully clog pores and microfractures, and promote the mobilization of contaminants. Thus, the goal of this work is to understand the geochemical changes of the host rock in response to reservoir stimulation that promote the formation and migration of colloids. Two different carbonate-rich shales were exposed to different solution pHs (pH = 2 and 7). Iron and other mineral transformations at the shale-fluid interface were first characterized by synchrotron-based XRF mapping. Then, colloids that were able to migrate from the shale into the bulk fluid were characterized by synchrotron-based extended X-ray absorption structure (EXAFS), scanning electron microscopy (SEM), and single-particle inductively coupled plasma time-of-flight mass spectrometry (sp-icpTOF-MS). When exposed to the pH = 2 solution, extensive mineral dissolution and secondary precipitation was observed; iron-(oxyhydr)oxide colloids colocated with silicates were observed by SEM at the fluid-shale interfaces, and the mobilization of chromium and nickel with these iron colloids into the bulk fluid was detected by sp-icpTOF-MS. Iron EXAFS spectra of the solution at the shale-fluid interface suggests the rapid (within minutes) formation of ferrihydrite-like nanoparticles. Thus, we demonstrate that the pH neutralization promotes the mobilization of existing silicate minerals and the rapid formation of new iron colloids. These Fe colloids have the potential to migrate through the shale matrix and mobilize other heavy metals (such as Cr and Ni, in this study) and impacting groundwater quality, as well produced waters from these hydraulic fracturing operations.

Micro X-ray fluorescence reveals pore space details and spatially-resolved porosity of rock-based microfluidic devices

Characterization of microscopic details of the fabric of mudstones and shales (i.e., structure and composition) is important to understand their storage and transport properties. Current characterization methods struggle to probe reliably multiple scales of interest (e.g., pore and fracture) and measure properties at the finest resolution under representative in situ conditions. Micro X-ray fluorescence (μXRF) is a high-performance imaging technique that produces elemental images at sub-10 μm spatial resolution and could offer insight into a diversity of shale properties, such as mineral composition, porosity, and in situ pressure gradients. This study designed and carried out a porosity mapping protocol using model and real-rock microfluidic devices and contrast fluids. Etched silicon micromodels with real-rock pore network patterns served as ideal models to establish a proof of concept. Measurements were performed on a novel μXRF microscope not powered by synchrotron radiation. We registered the μXRF datasets with the binary rock masks used for micromodel fabrication and applied segmentation algorithms to compare porosities. We assessed expected advantages and limitations through a sensitivity analysis and beam study. μXRF is an important new imaging technique for microfluidic applications.

Probing multiscale dissolution dynamics in natural rocks through microfluidics and compositional analysis

Mineral dissolution significantly impacts many geological systems. Carbon released by diagenesis, carbon sequestration, and acid injection are examples where geochemical reactions, fluid flow, and solute transport are strongly coupled. The complexity in these systems involves interplay between various mechanisms that operate at timescales ranging from microseconds to years. Current experimental techniques characterize dissolution processes using static images that are acquired with long measurement times and/or low spatial resolution. These limitations prevent direct observation of how dissolution reactions progress within an intact rock with spatially heterogeneous mineralogy and morphology. We utilize microfluidic cells embedded with thin rock samples to visualize dissolution with significant temporal resolution (100 ms) in a large observation window (3 × 3 mm). We injected acidic fluid into eight shale samples ranging from 8 to 86 wt % carbonate. The pre- and postreaction microstructures are characterized at the scale of pores (0.1 to 1 µm) and fractures (1 to 1,000 µm). We observe that nonreactive particle exposure, fracture morphology, and loss of rock strength are strongly dependent on both the relative volume of reactive grains and their distribution. Time-resolved images of the rock unveil the spatiotemporal dynamics of dissolution, including two-phase flow effects in real time and illustrate the changes in the fracture interface across the range of compositions. Moreover, the dynamical data provide an approach for characterizing reactivity parameters of natural heterogeneous samples when porous media effects are not negligible. The platform and workflow provide real-time characterization of geochemical reactions and inform various subsurface engineering processes.

Microfluidics as an Emerging Platform for Exploring Soil Environmental Processes: A Critical Review

Investigating environmental processes, especially those occurring in soils, calls for innovative and multidisciplinary technologies that can provide insights at the microscale. The heterogeneity, opacity, and dynamics make the soil a "black box" where interactions and processes are elusive. Recently, microfluidics has emerged as a powerful research platform and experimental tool which can create artificial soil micromodels, enabling exploring soil processes on a chip. Micro/nanofabricated microfluidic devices can mimic some of the key features of soil with highly controlled physical and chemical microenvironments at the scale of pores, aggregates, and microbes. The combination of various techniques makes microfluidics an integrated approach for observation, reaction, analysis, and characterization. In this review, we systematically summarize the emerging applications of microfluidic soil platforms, from investigating soil interfacial processes and soil microbial processes to soil analysis and high-throughput screening. We highlight how innovative microfluidic devices are used to provide new insights into soil processes, mechanisms, and effects at the microscale, which contribute to an integrated interrogation of the soil systems across different scales. Critical discussions of the practical limitations of microfluidic soil platforms and perspectives of future research directions are summarized. We envisage that microfluidics will represent the technological advances toward microscopic, controllable, and in situ soil research.