Investigation On Coalescence Characteristics Of Water In Oil Emulsion Under Electric Field

. The electric dehydration of water in crude oil is a common process in petroleum industry,however, traditional electrical dehydrator can not work for the added oilfield auxiliarychemical. The change of oil-water physical properties also brings challenges to theinvestigation on electrostatic coalescence mechanism. In this paper the influence of differentfactors on coalescence characteristics have been investigated by theoretical and experimentalmethods. The electrostatic coalescer with multi layers used in lab, the compact electrostaticcoalescer skid mounted system used in oil field and electrostatic coalescer with four channelsworking in oil field also have been designed and manufactured. It is very significant toimprove the theory of electrostatic coalescence in emulsion and practical application in oilfield. The main contents and conclusions of this paper are following.The rapid evaluation experimental system used in lab was designed. The experimentalresults indicated that the Rosin-Rammler and Log-normal distributions could satisfactorilypredict the particle diameter distribution of water droplets in W/O emulsion. The best electricfield strength to enhance droplet coalescence is related to frequency.High frequency and highwater content will result in low best electric field strength. There are two best frequencies inlow and high frequency interval. Under pulsed DC and pulsed AC electric field, the dropletdiameter is highest when the duty ratio is50%. Coalescence effect in sine AC and triangularwave electric field is bad compared with pulsed DC, pulsed AC and square wave electric field.When water content is10%, the increase of diameter is concentrated from10to20μm, whilewhich is concentrated from30to50μm under water content of20%and30%.The optical frequency and duty ratio is obtained by theoretical and empirical formula,and the result is tested by experiment. The result is suitable to low frequency, and coalescenceeffect will increase under high frequency. In electrostatic coalecer, the wave forms of squarewave, pulsed DC and pulsed AC will be disturbed to triangle wave under high frequency. Thepulsed DC is not suitable to be used in electrostatic coalescer containing electrode coated withinsulation layer. The power consumption is related to wave form, frequency and voltage. The power consumption of sine AC is less than that of pulsed DC, pulsed AC and square waveform when f is50Hz，while which of pulsed DC is the least whenf is1000Hz.The high-voltage electrode coated with insulation layer is designed and the electrostaticcoalescer with multi electrodes is manufactured. The results indicate that the coalescenceeffect is obvious under high electric field strength and the insulated electrode can effectivelyprevent electric breakdown. The optical electric field strength decreases with the increase ofwater content. High electric field intensity and long residence time are conductive to waterdroplets coalescence, but if the electric field intensity is higher than critical one, the waterdroplets will rupture. Coalescer with multi electrodes can enhance the flow rate and moreeconomical. When electrode spacing is small, the power consumption is low but the electricfield is badly distorted. If the electric field can effectively promote water droplet coalescence,the electrode spacing can be decreased to avoid adverse effect.The compact electrostatic coalescer skid mounted system is manufactured. From fieldtest, it can be found the water content of crude oil after processed is satisfied with request.The optical electric field is equal to that in lab. The electrostatic coalescer with4channels isdesigned and used in oil field. The results indicate that the coalescer work well, safe andreliable. The capacitance is high and water content of treated crude oil meet the request of oilfield. The power consumption of coalescer is lower than traditional one and has little effect onthe production. This equipment can solve problems in oil field and has important practicalsignificance.From micro scale experiment, it can be found the coalescence manner is mainly dipolecoalescence, oscillation coalescence and chained mode coalescence under sine AC, squarewave and pulsed AC electric field. Under different frequencies, the droplet chain can be found.It is easier to form droplet chain under conditions of larger droplets and higher water content.When electric field is applied, dipole coalescence occurs to form larger droplets and thenchain coalescence happen. There are3kinds of chain coalescence. Under pulsed DC electricfield, dipole coalescence and electrophoresis coalescence mainly happen. At the same time,chain coalescence is not found. The length of droplet chain and droplet diameter in squarewave and sine AC electric field is higher than those in pulsed AC electric field.