Studies On Stability Of White Oil-Based Water-in-Oil Emulsions

Oil-based drilling fluids are water-in-oil(W/O)emulsions,which 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 white oil and gas-to-liquid as the base oil phase have attracted attention because of the low risk of environmental pollution.Emulsions are thermodynamically unstable,and phase separation or demulsification easily occurs at high temperature,which limits their application in drilling engineering.The development of high-temperature resistant oil-based drilling fluids has become one of the hot topics in the field of oilfield chemical engineering.Studying the stability of W/O emulsions,especially high-temperature stability,is of great significance to the development and application of high-temperature oil-based drilling fluids.In this thesis,a white oil-based emulsion was selected as the model to study the high-temperature stability of W/O emulsions.Especially,the effect of high-temperature aging treatment(including aging temperature and time,simulating the drilling cycle process)and organoclays was mainly investigated.The influence mechanism is discussed based on surface/interfacial activity and interfacial/bulk phase rheological measurements.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 thesis are as follows:(1)Interfacial activity of emulsifiers used in oil-based drilling fluidThree emulsifiers(RHJ-1,RHJ-2 and RHJ-3)were selected as models,and their surface/interface activities and the viscoelasticity of their adsorption films at the white oil/water interface were determined.The effect of temperature on the properties was investigated.The results show that the three emulsifiers studied do not change the surface tensions of water and white oil,but all can significantly reduce the white oil/water interfacial tension(by?30 mN/m),showing obvious interfacial activity.The strength trend of the interfacial activity is:RHJ-1>RHJ-2>RHJ-3.The change in temperature in the range of 20?80 0C almost does not affect the interfacial activity of the three emulsifiers.RHJ-1 and RHJ-2 can form viscoelastic films at the white oil/water interface,with a dominant elastic behavior.The interfacial film strength(dilatational modulus)of RHJ-2 is higher than that of RHJ-1.The combination of RHJ-1 and RHJ-2 can form a mixed interfacial film.Probably,high temperature(120?180?)may lead to the chemical reaction between the emulsifier molecules,which can enhance the strength of the mixed film.These results deepen the understanding of the interfacial activity of emulsifiers.(2)Stability of water-in-white oil emulsionA model W/O emulsion was prepared using white oil as the oil phase,25 wt%CaC12 aqueous solution as the dispersed 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.The results show that after aging at high temperature,the emulsion only separates oil but not water when standing,indicating that the aqueous phase exists in a good emulsified state.High temperature(66?232 0C)aging can enhance the stability of the emulsion system(or reduce the release rate of oil phase).It was found that the emulsion can be stable for more than 16 d after aging at 180?232 ?(with a static stability rate of>90%).Aging time(16?72 h)has little effect on the stability.No three-dimensional network structures are detected in the emulsion system,so its stability is mainly determined by the viscoelasticity of the interfacial films of emulsifiers.The emulsion stability has a good positive correlation with the interfacial film strength(the amplitude of the dilatational modulus)of the emulsifiers.These results provide important information for the development of high temperature resistant oil-based drilling fluids.(3)Effect of organoclays on the stability of white oil-based emulsionUsing two organoclays(OC-HT and OC-LT)as models,the effect of organoclays(including their combination)on the high-temperature stability of the emulsion was studied,and the influence mechanism was discussed.The results show that the influence of organoclays on the emulsion stability is related to the aging temperature.When the aging temperature is 66?180 0C,the organoclays can significantly enhance the stability of the emulsion,while when the aging temperature is too high(such as 232?),organoclays reduce the emulsion stability.With an increase in the aging temperature(66?180?)or in the aging time(16?72 h),the stability of the organoclay@emulsion system tends to decrease.Organoclays can enhance the yield value,viscosity,and viscoelasticity of the emulsion,indicating that the organoclays can form three-dimensional network structures in the system,which can inhibit the high temperature release of the oil phase,thereby can enhance the emulsion stability.As the aging temperature increases(in 66-180?)or the aging time prolongs(in 16-72 h),the strength of the three-dimensional network structures tends to be weaken,probably arising from the desorption of the surface modifier from organoclays.It is found that a positive correlation exists between the network structure strength and the emulsion stability.In particular,after aging at 200? for the OC-LT@emulsion and OC-HT@emulsion and at 180? for the OC-LT/OC-HT@emulsion,the systems still exhibit excellent stability,which is of great significance to their practical application.