PhD Thesis Defence Presentations - Andreas Tzachristas

Mineral scaling in porous media that take place during multiphase flow is a major problem for a broad spectrum of industrial and environmental applications, such as CO₂ capture and storage (Carbon Capture Utilization and Storage (CCUS)), enhanced oil recovery (EOR), utilization of geothermal energy, membrane filtration processes etc. In the case of oil production or gas storage, the insoluble salt scale deposits reduce local rock permeability or may cause tube clogging. During past decades, sparingly soluble salt precipitation mechanisms have been investigated in relation with several parameters such as pH value, temperature, ionic composition, the presence of additives in the supersaturated fluids, the presence of seeds etc. However, to reduce the complexity of the system, even though most of the natural and engineered porous media used in the applications are of mixed or fractional wettability, the parameter of surface wettability has been overlooked up to date. Wettability is defined as the property that expresses the chemical affinity of a solid with a fluid or a fluid system and it is usually expressed by the contact angle of a fluid drop on a surface in the case of one fluid or the drop of the heavier fluid on a surface placed at the bottom of a cell containing the less heavy fluid, in the case of a fluid system. In the present dissertation, the effect of wettability on the precipitation of calcium carbonate from the microscale to the bed - scale was investigated experimentally, under quasi-static flow conditions. In the first part, the innovative microfluidic technology was used to obtain a more detailed and representative view of the basic physicochemical properties of fluids and the basic mechanisms which take place during the precipitation of salts for various wettability conditions and for various fluid systems. Microfluidics enable the visualization of very small quantities and give the opportunity to access the initiation of a phenomenon. Spontaneous CaCO₃ precipitation experiments were performed by mixing under flow conditions solutions supersaturated solutions of CaCl₂ and NaHCO₃, in hydrophilic and neutral-wet microchannels at various values of supersaturation ratio: SR = 10.5, 30.2, 61.7. A constant mean flow rate equal to 0.50 μL/min was used. The experiments were conducted in the absence of organic phases, in the presence of water-oil (dodecane) interfaces, in the presence of organic phase miscible with aqueous phase (ethylene glycol) at different concentrations (10-20%, v/v) and in the presence of a surfactant docusate sodium (Aerosol OT) in the absence and in the presence of water-oil interfaces. The time of observation of the first crystallite and its growth as well as all crystallites precipitated along the microchip were recorded during the experiments. Next, the polymorphs formed were identified by Raman spectroscopy. In the case of absence of organic phases, in the neutral wet microchannels, the first crystals were detected faster and mainly near the walls of the microchannels, due to the lower surface energy caused by the larger contact angle of the aqueous phase with the hydrophobic solid surface. In addition, in the neutral-wet microchannels mainly aragonite was formed while in the hydrophilic only calcite was formed. In the presence of water – oil interfaces, nucleation was accelerated. Moreover, in the case of the hydrophobic microchannel the residual oil saturation was higher resulting in a larger number of nuclei. The presence of ethylene glycol accelerated the precipitation of calcium carbonate compared to the corresponding in the absence of ethylene glycol, in both types of wettability, due to the reduced surface tension which resulted in reduction of Gibbs free energy required for the initiation of nucleation. However, differentiations were obtained considering the time of observation of first crystal and the crystal growth rate for hydrophilic and the hydrophobic microchips.  The presence of surfactant in the hydrophilic microchannels at the lowest supersaturation ratio value (SR = 10.5) accelerated nucleation but reduced the crystal growth rate as the presence of AOT favored secondary nucleation because the hydrophilic part was redistributed to hydrophilic surface. At the highest value of the supersaturation ratio (SR = 30.2), the surfactant accelerated secondary nucleation and secondary crystal growth. In addition, the higher value of supersaturation ratio in combination with the surfactant resulted in crystallization of thermodynamically unstable forms of calcium carbonate (vaterite). In the neutral-wet microchannel, the presence of the surfactant at the lowest supersaturation ratio value (SR = 10.5) inhibited nucleation and crystal growth along the entire length of the microchannel, while at the highest supersaturation ratio value (SR = 30.2) the crystal growth was unaffected.  Finally, the presence of surfactant of water- oil interfaces at the lowest supersaturation ratio (SR = 10.5) in both types of wettability resulted in complete inhibition of nucleation and crystal growth along the entire length of the microchannel, while the stronger inhibition was observed in the neutral- wet microchip due to the higher chemical affinity of the hydrophobic part of the surfactant with the water- oil interfaces. In the second part of this dissertation, calcium carbonate precipitation experiments were performed in beds of different wettability (homogeneously hydrophilic, homogeneously hydrophobic, and fractionally wet) at a constant total flow rate 2 mL/h, at initial supersaturation values SR_i = 537, 222, 86 in the absence of organic phases, in the presence of water-oil interfaces, in the presence of organic phase miscible with aqueous phase (ethylene glycol). During the experiments, samples were collected at the outlet of the bed to measure the total calcium concentration and the pH value. At the end of each experiment, the bed was opened and samples from the interior of the packing material were collected for X-ray diffraction (XRD) analysis and Scanning Electron Microscopy (SEM). It should be noted that in these experiments due to high supersaturation values, the effect of wettability is low concerning the initiation of crystallization, however the effect of wettability is significant concerning the pathway of supersaturation solution. In absence of organic phases in the hydrophilic conditions the precipitation took place along the entire length of the bed, while in the hydrophilic conditions the precipitation took place mainly at the bed entrance. High initial supersaturation ratio (SR_i = 537) resulted in less stable polymorphs precipitation (aragonite, vaterite, amorphous calcium carbonate), apart from to the most thermodynamically stable calcite. Hydrophilic conditions resulted mainly in calcite precipitation. In the presence of water – oil interfaces in all three different wettability conditions, nucleation and crystal growth were accelerated. However, hydrophobic conditions, favored intensive precipitation due to higher oil residual saturation. Hydrophobic conditions favored the precipitation of larger number of smaller crystals due to the higher number of nucleation’s sites. In presence of water- oil interfaces, independently of wettability conditions calcite was favored due to the reduction of the Gibbs free energy required for crystallization. In the presence of ethylene glycol (10 % v/v), calcium carbonate precipitation accelerated, and the beds were clogged earlier. Finally, fractionally wet beds favored the formation of deposits on the hydrophilic surface due to hydrophilic pathways along the beds.