Mineral surface area largely controls the interactions of minerals with fluids and microbes, and the rate of chemical reactions, including mineral dissolution, precipitation, and oxidation-reduction which involves a transfer of electrons between two species. New research from EESA explores the impact of surface roughness on mineral dissolution rates in micro-fractures within rocks deep underground.
The United States derives 80 percent of its energy from the Earth’s subsurface using technologies that depend upon scientific understanding of how fractures in subsurface rock evolve in response to variable fluids circulating deep underground. The EESA study was led by research scientist Hang Deng and carried out as part of a broader research project funded by the Exascale and EFRC projects. It helps shed light on the degree to which characteristics observed at the micro-scale such as surface roughness influence the evolution of micro-fractures caused by rock-fluid interactions.
Researchers performed pore-scale, reactive transport simulations in a series of synthetic 2D rough fractures to investigate the compound effects of surface roughness on the reaction rates in fractures. The impact of surface area on mineral dissolution is commonly accounted for by using the surface roughness factor (SRF), which is the ratio between the total surface area and the nominal or geometric surface area. Simulation results show that while reaction rates increase with SRF, the increase is not linearly proportional to that of the surface area.
For the simulations, a series of 2D rough profiles were used to represent cross-sections perpendicular to the fracture plane and can be considered as 3D fractures of unit width with parallel profiles in the direction perpendicular to the flow. All fracture cross-sections are 1,000 µm in length with an average aperture of 100 µm. The size is typical of a micro crack, a single pore, or a single grid cell ina larger fracture domain.
These results indicate that fine-scale surface roughness has the potential to extend the range of conditions in which concentration gradients can develop within single pores and fractures and impact rates observed at larger scales, such as in the field.
“Mineral reaction rates that eventually control the evolution of fractured rocks are sensitive to microscale geometric heterogeneity, such as surface roughness. The impacts of surface roughness are typically accounted for by using surface roughness factor, which does not include the interplay with flow and transport, or by using empirical parameters that correct for the interactions with flow and transport,” said research scientist and lead author of the study Hang Deng.
“This pore-scale study enables us to probe the compound effects of surface roughness on reaction rates, with detailed analysis of the contribution of surface area increase and potential flow and transport limitation. From this analysis, we were able to develop an upscaling rule to better parameterize reaction rate used for larger-scale studies.”