Deposition of mineral colloids on rough rock surfaces

Deposition of colloids on mineral and rock surfaces is an important mechanism to alter surface reactivity and to govern contaminant migration. Particle retention in aquifers occurs predominantly under electrostatically unfavorable condi- tions owing to the prevailing negative charge of both mineral colloids and rock surfaces. Mineral and rock surfaces show often an irregular surface topography and roughness variations over several orders of magnitude. This complicates the colloid- surface attachment predictability and results in poor understanding towards retention efficiency. Here we study the impact of submicron-scale morphology on the interaction between rock surfaces and mineral colloids. Colloid retention experiments using micrite surfaces were performed under electrostatically unfavorable conditions. Re- sults showed a positive and linear correlation between adsorbed particle density and surface roughness (RMS roughness <100 nm). The existence of a minimum roughness range, required for initial colloid deposition was detected. Deposition occurred mainly at micrite grain boundaries that acted as surface steps. Linear deposition kinetics was found at constant flow rates. The site-specific impact of surface roughness was studied using nanostructured silicon wafer surfaces as a well-defined analog material. Experimental results from deposition on such surfaces suggest that the surface step density on collector surfaces is a critical parameter for quantitative prediction of colloid deposition. The importance of colloidal retention is highlighted for the diagenetic evolution of rocks, especially due to inhibition mechanisms that has consequences for cement mineral distribution and concentration as well as resulting reservoir quality. For further quantitative prediction and modeling of retention on rock surfaces, we suggest the application of an energy potential function that includes beside DLVO contribu- tion the impact of particle kinetic energy (via fluid-flow velocity) as well as the impact of reactive site density (via surface roughness parameter data).