Studies On Stability Of Gas-to-Liquid Based Water-in-Oil Emulsions

Oil-based drilling fluids are widely used in drilling engineering due to their excellent properties of lubrication,temperature resistance,shale suppression,and reservoir protection.In recent years,oil-based drilling fluids developed with gas-to-liquid(GTL)and white oil as the base oil phase have attracted much attention due to the reduced risk of environmental pollution.Oil-based drilling fluids are essentially a water-in-oil(W/O)emulsion,and their good stability is the key to ensure the safety of drilling engineering.High temperatures usually cause the decrease in the stability of emulsions or the phase separation of emulsions,which limits the practical application of emulsions.Research and development of high-temperature oil-based drilling fluids has become one of the hot topics in the field of oil chemical engineering recently.In this paper,a GTL-based W/O emulsion was selected as the model to study the stability of W/O emulsions.The effects of the high-temperature aging treatment(including aging temperature and time,simulating the drilling cycle process)and organoclays on the stability of the W/O emulsion was mainly investigated through the surface/interfacial tension and interface/bulk phase rheology measurements.The influence mechanism was discussed.This work provides useful information for the development and application of high-temperature resistant oil-based drilling fluids.The main research contents and conclusions of this paper are as follows:(1)Interfacial activity of emulsifiers used in oil-based drilling fluidTwo kinds of emulsifiers(RHJ-1 and RHJ-2)were selected as models,and the surface/interfacial activity and the viscoelasticity of interfacial films were determined using the surface/interfacial tension tester and the interfacial rheometer.The results show that the two emulsifiers all do not change the surface tensions of water and GTL,but both of them can significantly reduce the GTL/water interfacial tension(by about 30-40 mN/m),showing obvious interfacial activity.The interface activity of RHJ-1 is significantly higher than that of RHJ-2.The change of temperature in the range of 18-80? does not affect the interfacial activity of the emulsifiers.Adding Ca(OH)2 to the water phase can improve the interfacial activity of emulsifiers,which may be caused by the calcification reaction of emulsifiers.RHJ-1 and RHJ-2 can form viscoelastic interfacial films at the GTL/water interface,with a dominant elastic behavior.The strength(expansion modulus)of RHJ-2 interfacial film is significantly higher than that of RHJ-1 interfacial film.The mixture of RHJ-1 and RHJ-2 can form a mixed interfacial film,and its strength lies between the strengths of RHJ-1 film and RHJ-2 film.These results deepen the understanding of the interfacial activity of emulsifiers.(2)Stability of GTL-based water-in-oil emulsionsA model W/O emulsion was prepared using GTL as the oil phase,25 wt%CaCl2 aqueous solution as the water phase,a mixture of RHJ-1 and RHJ-2(with a mass ratio of 2/1)as the emulsifier,and adding Ca(OH)2 in the system.The effect of the high-temperature aging treatment(including aging temperature and time)on the stability of the model emulsion was investigated.The influence mechanism was discussed based on the dispersion state observations and the interface/bulk phase rheological measurements.The results show that after high-temperature aging,only oil but no water is separated from the emulsion when it is standing,indicating that the aqueous phase exists in a good emulsifying state.High-temperature aging can enhance the stability of the emulsion(i.e.,reduce the release rate of oil phase),probably arising from the calcification reaction of emulsifiers.With an increase in the aging temperature from 66? to 232?,the stability of the system increases first and then decreases,showing the optimal aging temperature of 180?210?.The combination of RHJ-1 and RHJ-2 is beneficial to the high-temperature stability of the emulsion.The emulsion is a Newtonian fluid without three-dimensional network structures forming,and thus its stability is mainly determined by the viscoelasticity of the emulsifier interfacial films.It was found that the high-temperature aging can enhance the elasticity of the interfacial films,so can enhance the stability of the emulsion.Probably,the emulsifier molecules can react with each other at high temperature to form products with higher emulsification capacity.These results provide important information for the development of high-temperature oil-based drilling fluids.(3)Effect of organoclays on the stability of GTL-based emulsionUsing two organoclays(OC-HT and OC-LT)as models,the influence of organoclays on the high-temperature stability of water-in-GTL emulsion was investigated.The results show that organoclays can significantly enhance the stability of the emulsion.Compared with a single organoclay,the combination of OC-LT and OC-HT can result in a higher stability for the emulsion.The aging temperature(66?232?)has a significant effect on the stability of the organoclay@emulsion system.With the temperature increases,the stability of the system increases first and then decreases.The optimal aging temperature is 180?200 0C.The aging time(16?72 h)has no obvious effect on the stability of the system.Organoclays can significantly enhance the viscosity,yield stress,and viscoelasticity of the emulsion,indicating that three-dimensional network structures form in the system,which can effectively inhibit the high-temperature release of the oil phase.Therefore,organoclays can significantly enhance the stability of the emulsion.The stabilizing effect of organoclays for emulsions is of great significance to the performance control of oil-based drilling fluids.