Multiphase flow processes taking place in oil production operations, CO2 and/or natural gas storage in underground reservoirs, in geothermal energy production, membrane operations etc., are accompanied with undesirable scale deposits formation either on rocks or on the walls of pores or metallic equipment, rendering the corresponding operations significantly less efficient. Scale formation depends not only on the morphology and structure but also on the wetting properties of the substrates on which scale is deposited or formed through nucleation and crystal growth. In the present work, in situ nucleation and growth of calcium carbonate is investigated in a microfluidics cell both in the absence of organic phase. Nucleation and crystal growth were directly observed in both hydrophobic and hydrophilic microchannels. Results are compared with work done in batch reactors, which showed that the presence of organic, water immiscible phase accelerated the formation of calcium carbonate, while depending on the solution supersaturation different metastable calcium carbonate phases were stabilized. The confinement of the supersaturated solutions in the microchannels revealed that the mechanisms changed from bulk to surface diffusion, depending on the flow rates of the supersaturated solutions in the microchannels.
Sparingly soluble salt deposits are often encountered in industrial applications, in the aquatic environment and in various biological processes. Oil and gas production industry is faced with the efficient confrontation of scale deposits tenaciously adhering on process equipment parts resulting to serious operational problems.1,2,3 Similar problems due to the formation of various insoluble salts on membranes are encountered in water desalination,4,5 in heat exchangers in geothermal energy production plants and in CO2 storage operations. 6,7 In oil and gas industry calcium carbonate and calcium sulfate deposits are commonly encountered,8 while other salts form to a much lower extent. The cause of the formation of the salt deposits, as a rule is due to increase of the respective supersaturations because of temperature and pressure changes, which in turn affect the solubility of the respective salts. Among others, wettability and surface structure have been reported to affect the propensity of surfaces to form calcium carbonate scale deposits. Wettability of materials by fluids, is a property strongly dependent on the physicochemical affinity of the respective material with a fluid or with a system of fluids. A drop of a liquid may either spread freely or shrink on a solid surface. In the former case the solid is termed as liquid-wet while in the latter as non-wetting. In oil and gas production industry, the respective reservoirs consist of heterogenous formations of minerals with different extent of wetting from water. The presence of oil inside their pores affects fluids flow and may change pore surface wettability to more oil-wet.10 According to recently published work involving mineralization of calcium carbonate in two phase systems consisting of water supersaturated with respect to calcium carbonate and an immiscible with water organic phase, showed that the precipitated calcium carbonate includes all calcium carbonate polymoprphs: vaterite, aragonite and the thermodynamically most stable calcite.[11] In the present work, we have investigated the precipitation of calcium carbonate from supersaturated solutions using a microfluidic approach aiming at further understanding the formation of this salt in the pore's environment. The microfluidics approach offers a unique opportunity to monitor the process at different solution conditions, as it offers the possibility to visualize the nucleation and growth of solids and to vary the wettability of the substrates on which the formation of sparingly soluble salts takes place. 12 More specifically, the precipitation of calcium carbonate in supersaturated solutions (saturation ratio values, SR, 40.5 and 61.7) was investigated in a microfluidic reactor system with the possibility of using either hydrophobic or hydrophilic micro-channel. The formation of the mineral phase was monitored with a video-camera. The time of the observation of the first crystal past mixing (and therefore of the establishment of supersaturation) and the crystal growth were recorded. Raman spectroscopy analysis was used for the mineralogical characterization of the crystals. To our knowledge, it is the first time that the effect of wettability is investigated in microchips.