It goes without saying that water scarcity is a widespread and increasingly pressing global challenge. One of the methods which can mitigate water shortage is to increase freshwater production via desalination of saline waters. Seawater and saline aquifer sources represent 97.5% of all water on Earth. Hence, treating even a small portion of saline water could significantly reduce water shortage. Although reverse osmosis is one of the state-of-the-art pressure-driven membrane desalination technologies, it is incapable of desalinating high-salinity streams due to the very high osmotic pressure to overcome. Membrane distillation (MD) is one of the emerging methods, which has attracted much attention for desalinating highly saline brines. MD is a thermally driven process in which only vapor molecules pass through the pores of a microporous hydrophobic membrane. This process, however, has not been fully commercialized due to a number of challenges, including “pore wetting”. Pore wetting refers to the presence of liquid, instead of just water vapor, inside the membrane pores, which may cause a decrease in MD flux and/or deterioration of distillate quality. Herein, a comprehensive review on pore wetting is presented, and then this phenomenon is investigated from four aspects. In the first phase of this project, a theoretical model is presented according to which the pore size distribution of membrane, a parameter affecting pore wetting risk, is estimated by employing only a few experimental data points in accordance with the wet/dry method, reducing the number of data required to be recorded largely. In the next phase, an equation is presented for the estimation of liquid entry pressure (LEP), a membrane parameter closely related to pore wetting, using computational fluid dynamics (CFD) tools and genetic programming (GP) as an intelligent technique. This equation can estimate LEP in closer agreement to experimental values in comparison to the Young-Laplace equation. In the third phase, movement of liquid-gas interface inside the membrane pore is tracked using a well-founded model, and consequently, the pressure and velocity at the interface and the required time for replacement are studied. Finally, in the last phase, a model is developed for pore wetting in vacuum MD, considering heat and mass balances at the vapor-liquid interface. This model assumes that heat only enters the pore inlet and is removed due to liquid vaporization at the vapor-liquid interface, with heat transfer through the pore wall neglected. This model shows that partial pore wetting is possible since the vapor-liquid interface might remain within the pore at the steady-state condition. Further, this model can predict the decrease in temperature from the pore inlet to the vapor-liquid interface, a phenomenon that has been reported in the literature without any proof.

The study of multiphase flow in porous media is highly relevant to many problems of great scientific importance, such as CO2 storage and enhanced oil recovery. Even though significant progress has been made in these areas, many challenges still remain. For instance, the leakage of stored CO2 may occur due to the capillary trapping failure of cap rock. Approximately 70% of oil cannot be easily recovered from underground, because the oil is held in tight porous rocks. Although CO2 storage and enhanced oil recovery are engineering processes at a geological scale, they are predominantly controlled by the transport and displacement of CO2 and reservoir fluids in aquifers and reservoirs, which are further controlled by wetting and fluid properties at pore scale. This work focuses on experimental investigations of pore-scale wetting and displacement of fluids and CO2 in porous core samples. Pore wetting, which has been measured based on contact angle, is a principal control on multiphase flow through porous media. However, contact angle measurement on other than flat surfaces still remains a challenge. In order to indicate the wetting in a small pore, a new pore contact angle measurement technique is developed in this study to directly measure the contact angles of fluids and CO2 in micron-sized pores. The equilibrium and dynamic contact angles of various liquids are directly measured in single glass capillaries, by studying the effects of surface tension, viscosity and chemical structure. The pore contact angles are compared with the contact angles on a planar substrate. The pore contact angle of a confined liquid in a glass capillary differs from the contact angle measured on a flat glass surface in an open space. Surface tension is not the only dominant factor affecting contact angle. The static contact angle in a glass pore also varies with liquid chemical structure. Viscosity and surface tension can significantly affect the dynamic pore contact angle. A new empirical correlation is developed based on our experimental data to describe dynamic pore wetting. The CO2-fluid contact angle in porous media is an important factor affecting the feasibility of long-term permanent CO2 storage. It determines CO2 flow and distribution in reservoirs or aquifers, and thus ultimately finally the storage capacity. CO2-fluid contact angles were measured in small water-wet pores and oil-wet pores, investigating the effect of CO2 phase (gas/liquid/supercritical). The CO2 phase significantly affects the CO2-fluid contact angle in an oil-wet pore. Supercritical CO2-fluid contact angles are larger than gas CO2-fluid contact angles, but are smaller than liquid CO2-fluid contact angles. However, this significant CO2 phase effect on contact angle was not observed in a water-wet pore. Another key issue considered in this study is two-phase flow displacement in porous media. This strongly relates to the important macroscopic parameters for multiphase flow transport in porous media, such as capillary pressure and relative permeability. Here CO2-water displacements are studied by conducting CO2 flooding experiments in a sandstone core sample, considering the effects of CO2 phase, pressure and CO2 injection rate. The capillary pressure-saturation curve, water production behaviour and relative permeability are investigated for gas CO2-water, liquid CO2-water and supercritical CO2-water displacements in porous media. The pressure-dependant drainage capillary pressures are obtained as a result of CO2-water interfacial tension. Various water production behaviours are obtained for gas CO2-water and liquid CO2-water displacements, mainly due to the effect of CO2 dissolution. The significant irregular capillary pressure-saturation curves and water production behaviors can be observed for the supercritical CO2-water displacements. The water and CO2 relative permeabilities for CO2-water displacements in a porous media are then predicted.

Avec une population toujours croissante et la pénurie de plus en plus importante des ressources en eau douce, l’humanité s’est tournée vers les océans pour ses besoins en eau potable. Afin de faire face aux limites des procédés conventionnels de dessalement d’eau de mer, la distillation membranaire (DM) connaît un intérêt croissant. Même si l’intérêt envers la DM pour le dessalement d’eau de mer est apparu au cours des dernières décennies, aujourd’hui le risque de mouillage des membranes est l’un des obstacles majeurs à son développement industriel. Dans le cadre du projet ANR « WETMEM», l’objectif de cette thèse est de développer des outils de compréhension des mécanismes de mouillage en distillation membranaire sous vide. Plusieurs membranes, fabriquées par des partenaires du projet WETMEM, et commerciales ont été étudiées afin de comprendre l’influence des propriétés des membranes sur les indicateurs de mouillabilité. De plus, une définition et une classification des mécanismes de mouillage ont été proposées. Par la suite, deux indicateurs de mouillage ont été développés à l'aide de la microscopie électronique à balayage et de la spectroscopie de dispersion de rayons X selon une méthode appelée « Détection d'intrusion de traceur dissous ». Une preuve de concept a été fournie, dans laquelle différents mécanismes de mouillage ont pu être visualisés et interprétés. Ces indicateurs ex situ ont alors été utilisés avec des indicateurs de mouillabilité (Angle de contact, Pression d’intrusion de liquide) afin de comprendre l’influence de la température (35-50 ° C), de la salinité (22-310 g / L de NaCl sol.) et du débit (400 - 4000 Re) sur le mouillage et la mouillabilité d'une membrane de PVDF en distillation membranaire sous vide. Il a alors été constaté que la salinité a l’impact le plus important sur le mouillage par rapport aux autres paramètres de fonctionnement. En outre, un outil optique in-situ non invasif a été développé. Il permet de visualiser le mouillage in-situ en distillation membranaire. La progression du mouillage in situ a été observée à différentes échelles et pour différentes solutions salines et eaux de mer. / With an ever-increasing population and the growing disparity in potable water resource, humanity has turned its attention to the oceans for its potable water needs. To overcome the current limitations in current desalination technologies, membrane distillation (MD) is actively being developed. The interest of MD for seawater desalination was established in the last decades but today the risk of membrane wetting is one of the major barrier for industrial implementation of MD. Under the framework of the ANR project “WETMEM”, the issue of this thesis was to develop tools for better understanding wetting mechanisms in vacuum membrane distillation. Several fabricated (WETMEM partners) and commercial membranes were studied to understand the influences of membrane properties on wettability. Therefore, a definition and classification on wetting were formulated. After that two wetting indicators were developed using scanning electron microscopy and X-ray dispersion spectroscopy under a method called “Detection of Dissolved Tracer Intrusion”. A proof of concept was provided with various wetting mechanisms visualized and interpreted. These ex-situ indicators were used with wettability tools (Contact Angle, Liquid Entry Pressure) to understand the influence of temperature (35-50°C), salinity (22-310 g/L NaCl sol.) and flow rate (400 – 4000 Re) on wetting and wettability of a PVDF membrane under vacuum membrane distillation. Indeed, it was found that salinity has a greater impact on wetting than the other operating parameters. Additionally, a proof of concept was provided for non-invasive in-situ optical method for visualizing wetting in membrane distillation. Progression of in-situ wetting visualization was validated at different scales for various saline solutions and seawaters.