Formation Of High Saline O/W Nanoemulsions By The Phase Inversion Temperature Method For Oil-based Mud Removal

Nanoemulsions are a class of emulsions with droplet size in the range between 20 and 500 nm. In contrast with microemulsions, which are thermodynamically stable and require a high concentration of surfactants, nanoemulsions are kinetically stable and possess moderate surfactant concentrations. All of these desirable advantages make nanoemulsions very attractive for practical applications in pharmaceuticals, cosmetics, foods, drilling fluids applications and many other fields.According to the literature, nanoemulsions can be prepared by both high-energy and low-energy emulsification methods. High-energy emulsification methods require intense mechanical energy generated by high-pressure homogenizers or ultrasound generators. In contrast, there is an increasing interest in low-energy emulsification methods as they make use of the internal physicochemical energy stored in the components. Nanoemulsions formation by low-energy methods can be achieved by phase inversion temperature (PIT) method, phase inversion composition (PIC) method, microemulsion dilution method and spontaneous emulsification and so on.Formation of nanoemulsions by the PIT method with nonionic surfactants of the ethoxylated type has been studied systematically, but there are still some deficiencies. For example, after the emulsification at the PIT the samples must be quickly cooled with gentle stir to obtain fine nanoemulsions, which limits the wide applications of the PIT method. Therefore, investigating the effect of cooling methods on the properties of nanoemulsions is helpful to optimize the preparation of nanoemulsions and broaden the application of mixed surfactant systems in industrial processes. Detergency of oil-based mud from the wellbore is a key element of excellent cement bond between casing and formation. When using surfactant solutions to remove oil-based mud, the whole system is heated by the high temperature in the downhole, and then the dispersion is cooled when it returned to the ground. This process is very similar to the formation mechanism of nanoemulsions by the PIT method. Therefore, we use the formation mechanism of nanoemulsions by the PIT method to simulate the removal of oil-based mud. In addition, high salinity of Ca2+ is common in natural subsurface reservoirs in oil recovery, which can tune the PIT of the system and interact strongly with the ionic surfactants, leading to the destabilization of the emulsions.Based on the above background in nanoemulsion research, aiming at the shortcomings of preparation of nanoemulsions by the PIT method and good detergency of oil-based mud, we select nonionic and anionic surfactant mixtures AES-C12EO4 and AES-C12EO4-C12EO23 systems to obtain desirable performance. The present dissertation includes two topics.Firstly, O/W nanoemulsions in brine/AES-C12EO4/paraffin oil system have been prepared by the PIT method. The influence of surfactant mixing ratio, inorganic salts and cooling methods on the nanoemulsion properties including the clearing boundary temperature (TCB), PIT, electrophoretic properties and long-term stability were investigated by means of conductivity, turbidity, zeta potential and dynamic light scattering. It was found that the PIT and TCB of the systems increase with increasing anionic surfactant concentrations and decreasing inorganic salt concentrations. The differences of the appearance and droplet size of nanoemulsions prepared with diverse cooling methods decrease with increasing salt concentrations and internal phase concentrations. The main instability mechanism of the high salinity nanoemulsions stabilized by anionic-nonionic mixed surfactants is Ostwald ripening. These findings are helpful to optimize the preparation of nanoemulsions by the PIT methods in industrial processes.Secondly, to simulate the detergency of oil-based mud, O/W nanoemulsions in CaCl2/AES/C12EO4/C12EO23/paraffin oil system have been formed by the PIT method. We study the influence factors of cleaning efficient, such as the surfactant components, CaCl2 concentration and the emulsification temperature. In addition, we investigate the stability of the nanoemulsions to prove that the oil-based mud residue could be dispersed stably by the detergency systems.