The Hydraulic Fracturing Test Site 1 (HFTS-1) was a field study performed in the Wolfcamp Formation in the West Texas Permian (Midland) Basin, USA, with a focus on improving the efficiency of hydraulic fracturing. Investigating site-specific rock-fluid geochemical interactions during hydraulic fracturing is an important step to understanding the impact on formation shale porosity, permeability, and long-term shale gas production. During field operations in this region, hydraulic fracturing fluid (HFF) injection usually starts with a concentrated acid spearhead for rapid rock dissolution, followed by the injection of near-neutral pH slickwater containing chemicals and proppants. A multistep sequential injection approach was used to investigate different stages of rock-fluid interactions. The carbonate content in the host rock is important when acid spearhead is considered, as carbonate mineral dissolution is rapid and can result in porosity and permeability changes in the shale matrix. In this study, we designed flow-through experiments using fractured carbonate-rich and clay-rich Wolfcamp shale cores with (1) a short-time acid soaking step and (2) a long-term slickwater flow-through step to simulate the injection method used at HFTS-1. The fluid chemistry was analyzed. A thorough mineralogical progression [e.g., Calcium (Ca) dissolution and iron (Fe) redox progression] in the cores during HFF injection was also characterized and imaged by synchrotron microprobe. Reactive transport modeling was performed based on the experimental setup. The results showed that the acid spearhead is a crucial step in creating a reaction front by mineral dissolution, especially in carbonate-rich shales. A slight layer of ferrihydrite precipitated during the slickwater flow-through period. This study provides insights into potential geochemical impact due to hydraulic fracturing operations in the Permian Basin.

Mineral precipitation is ubiquitous in natural and engineered environments, such as carbon mineralization, contaminant remediation, and oil recovery in unconventional reservoirs. The precipitation process continuously alters the medium permeability, thereby influencing fluid transport and subsequent reaction kinetics. The diversity of preferential precipitation zones controls flow and transport efficiency as well as the capacity of mineral sequestration and immobilization. Taking barite precipitation as an example, previous studies have examined this process in porous and/or fractured media, but pore-scale mechanisms under varying flowing and geochemical conditions remain unexplored. In this study, we conducted real-rock microfluidic experiments to investigate the precipitation dynamics within a fractured porous system. Direct observations of the evolution of the porous structure and flow channel and quantifications of barite precipitation dynamics using X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), revealed two distinct precipitation regimes: precipitation on the fracture surface (regime I) and precipitation in the alteration zone (regime II). Through theoretical analysis of the rate of advection and nucleation, we defined a dimensionless number Da above which regime I occurs and regime II prevails otherwise. At the large Da number, when the precipitation rate is large compared with the flow rate, precipitation on the fracture surface is favored. As the precipitation regimes are expected to impact differently the permeability of the fractured porous media, the mass transfer across matrix and fractures, and the spatial distributions of coprecipitated contaminants, our work sheds light on accurately modeling reactive transport in fractured porous media across diverse applications.

Fluid–rock interactions involving chemical dissolution, mechanical erosion, and multiphase flow are central to a wide range of geological and engineering processes, yet they remain poorly understood due to the lack of integrated in situ observation tools. Existing methods often compromise between spatial resolution and temporal dynamics. Here, we develop a real-rock microfluidic platform that enables simultaneous visualization and quantification of erosion dynamics in multiphase reactive systems. The platform integrates fluorescence microscopy, micro-particle image velocimetry, and ion chromatography to monitor the coupled evolution of solid–liquid–gas interfaces and flow velocity fields at micrometer-scale resolution. Microfluidic chips fabricated directly from limestone preserve natural mineral heterogeneity, and the platform enables direct observation of rock surface evolution and multiphase flow behavior. This facilitates decoupled analysis of chemical dissolution and mechanical erosion—two processes often difficult to isolate in traditional systems. Using this system, we investigate erosion during acid–rock interactions and identify a transition between two regimes—transport-limited and reaction-limited—controlled by CO2 bubble mobility. In the transport-limited regime, immobile bubbles confine flow to thin films, enhancing dissolution and particle detachment. In the reaction-limited regime, surface-adhered bubbles shield reactive areas and reduce shear stress, suppressing erosion. We derive scaling laws that distinguish chemical and mechanical erosion rates and validate a theoretical model for the critical Péclet number marking the regime transition. This study advances understanding of erosion under multiphase flow and introduces a versatile experimental framework for probing pore-scale reactive transport. The platform can be extended to other rock types and fluids, offering a powerful tool for studying geochemical, physical, and biological processes in complex subsurface environments.