Microfluidic Study of Fracture Dissolution in Carbonate-Rich Caprocks Subjected to CO₂-Charged Brine

Abstract

Investigating fracture evolutions triggered by chemical interactions in caprocks of CO2 storage sites is of great importance when caprock integrity is concerned. Mineral heterogeneity is one of the factors affecting fracture evolution. We present results from flow-through experiments deploying a unique high pressure geo-material microfluidic cell to monitor the fracture evolution of four carbonate-rich caprocks: (1) a homogeneous carbonate-rich sample, (2) a heterogeneous carbonate-rich sample, (3) a heterogeneous carbonate-rich shale sample, and (4) a heterogeneous carbonate-rich organic shale sample, representing different levels of mineral heterogeneity. The results show rather smooth fracture wall dissolution for the homogeneous rock sample. For the heterogeneous sample without shale, however, an altered layer is formed around the fracture that leads to an increase in the fracture roughness. Chemical analyses of effluent solutions demonstrate a decrease in the bulk dissolution rate of calcite over time at a constant flow rate. For the two carbonate-rich shale samples, visual observations using optical microscopy showed little changes in fracture dissolution, although analysis of effluent chemistry confirmed calcite dissolution.

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

“…The microfluidic cell was flushed with DI water first. The acidic fluid was injected from bottom to top of the channel and was maintained at 0.2 mL/min (linear average velocity of ∼7 × 10 –4 m/s) with a syringe pump (Teledyne ISCO) for 48 h. This flow rate is within the range used in previous experimental investigations and could be expected for fracture flow in the subsurface. , …”

“…The acidic fluid was injected from bottom to top of the channel and was maintained at 0.2 mL/min (linear average velocity of ∼7 × 10 −4 m/s) with a syringe pump (Teledyne ISCO) for 48 h. This flow rate is within the range used in previous experimental investigations and could be expected for fracture flow in the subsurface. 14,18 A total of 15 overlapping mosaic 2D images, with a spatial resolution of 1.3 μm, of the windowed portion of the channel were collected every 15 min (Figure 1b) and the full-field images at each time point were stitched together. Data processing included the flat-field correction and the calculation of the transmittance (see details in the Supporting Information).…”

“…In single-mineral rocks, the fracture alteration is related to the relative magnitudes of mass transfer and reaction rate . In multimineral rocks in which fast-reacting minerals are distributed within a relatively insoluble matrix, geochemical reactions leave behind a porous altered layer. − Fracture flow experiments coupled with X-ray computed tomography (xCT) have provided physical insights regarding altered layer development and enabled quantification of altered layer thickness and the modeling of its evolution. ,, This alteration can expose slow-reacting minerals while imposing a diffusion limitation on the fast-reacting mineral(s). , The altered layer typically has weakened mechanical properties that may result in particle detachment and affect fracture morphology and stiffness under stress. ,,, …”

“…In this study, we used a novel approach that couples a microfluidic device with real-time synchrotron-based observations of fracture surface evolution. Microfluidic devices coupled with high-resolution cameras and a confocal microscope, initially developed and widely used for fluid mechanics and biology, , have been adapted to study flow and reactions in geological materials. , Recent developments, including coating the base materials with minerals , or fabricating the microfluidic device using real rock samples, , have made experiments involving complex chemical systems possible. Microfluidic experiments have also become an integral component in recent development and benchmarking of high-fidelity models .…”

Probabilistic nucleation governs time, amount, and location of mineral precipitation and geometry evolution in the porous medium

“…For example, during geological carbon storage, CO 2 pressure increases far above average. It acidifies the reservoirs with carbonic acid, leading to the accelerated dissolution of primary minerals, and precipitation of stable carbonate minerals 12 , 14 – 17 . Subsurface CO 2 storage may also cause rapid salt crystal accumulations in the near-wellbore region, prompting pore-space clogging, reduced injectivity, and pressure build-up 4 , 5 , 9 , 18 , 19 .…”