Search Results (3)

Accurate characterization of fracture roughness is critical for modeling groundwater flow and solute transport in fractured rock aquifers, where subsurface heterogeneity significantly impacts contaminant migration and water resource management. This study investigates fracture roughness characterization by integrating the Weierstrass–Mandelbrot approach with 3D-printed experimental validation and numerical simulation verification. Specifically, all the related parameters including fractal dimensions (D), frequency density (λ), segmentation accuracy (s), and summation number (n), which control the generation of fracture roughness, along with investigation scales (rs), were initially considered, and their corresponding impacts on the fracture roughness characteristics were examined. The results revealed that D is the primary factor controlling fracture roughness characteristics, while λ shows secondary importance when exceeding 1.3. The roughness remains stable when s ≤ 3 mm, n > 200, and rs ≥ 240 × 240 mm². Two multivariate regression models were established to describe the relationship between fracture roughness and influencing factors. The proposed methodology significantly enhances the precision of groundwater flow and solute transport simulations in fractured media through advanced high-fidelity fracture characterization, offering substantial improvements in groundwater resource management and contaminant remediation strategies. Full article

The pore structure and connectivity of coal are the primary factors influencing the permeability of coal reservoirs. However, clay and carbonate minerals are commonly found filling the pores and fractures within coal. To address the impact of these minerals on fracturing effectiveness, acidic fracturing technology has been introduced. This technique has proven to be an effective measure for enhancing the extraction rate of low-permeability coal seams with high mineral content. In this study, coal samples were treated with a 3% HCl solution, and the changes in the pore structure of the coal before and after acidification were analyzed through low-temperature nitrogen adsorption and X-ray diffraction (XRD) testing. The results were as follows: After acidification, the specific surface area, total pore volume, pore volume in different stages, and average pore size of the coal samples all significantly increased. Specifically, the BET specific surface area grew by an average of 4.8 times and the total pore volume expanded by an average of 7.7 times, with the pore volumes in the pore size ranges of <10 nm and 10–60 nm increasing by an average of 10.1 times and 7.7 times. The smoothness of the pore surface and connectivity of the pore structure in the coal samples improved, as indicated by decreased fractal dimensions D₁ (reflecting pore surface roughness) and D₂ (representing pore size distribution uniformity). The acidification mechanism was mainly attributed to the dissolution of carbonate minerals in the coal, which led to the removal of obstructive minerals such as ankerite and calcite that had accumulated in the coal pores. This resulted in the formation of new micropores and microfractures, achieving pore volume enhancement and pore expansion. Full article

The primary aim of this study is to explore how varying flow rates impact the heat transfer in single fractures, taking into account the effects of surface roughness, aperture and external temperature of the rock. Utilizing COMSOL Multiphysics, the fluid flow and heat transfer through 3D fracture models characterized by different roughness and apertures were simulated with volumetric flow rates ranging from 1 × 10⁻⁶ m³/s to 1 × 10⁻⁵ m³/s. The combined effects of these factors on key metrics, including outlet temperature, thermal breakthrough time, energy extraction efficiency, and heat transfer coefficients were systematically analyzed. The results indicate that water flow rate dominantly influences heat transfer, followed by fracture surface morphology and rock external temperature. Higher flow rates enhance both heat transfer and total heat extraction, while also increasing temperature non-uniformity, which improves overall heat extraction efficiency. Surface roughness significantly affects temperature distribution, leading to heterogeneous thermal profiles, especially in narrower fractures. Additionally, higher external temperatures and flow rates facilitate faster thermal breakthroughs by reducing thermal resistance. The interplay between surface roughness and thermal breakthrough time is intricate, with increased roughness prolonging breakthrough times in smaller apertures but potentially reducing them in larger ones. At smaller apertures, increasing the JRC from 2.29 to 17.33 results in a 1.01 to 1.20 times increase in thermal breakthrough time, whereas at larger apertures, thermal breakthrough time decreases by a factor of 1.01 to 1.29. This highlights the importance of carefully selecting fluid parameter in the design of geothermal projects to optimize heat extraction efficiency. Full article