Construction And Experimental Study Of Heavy Oil In-situ Emulsifying System For Enhanced Oil Recovery

With long-term and continuous development of conventional reservoirs,heavy oil reservoirs play an increasingly important role in solving the problem of energy shortage.Heavy oil is mainly recovered by thermal and non-thermal methods,among which water injection is the most economical development method.However,water flooding for heavy oil usually faces serious water channeling,thus leading sweep efficiency and oil recovery.Therefore,it is very urgent to adopt certain technical means to further improve the recovery of heavy oil reservoir after water flooding.This work mainly researches the feasibility of in-situ emulsification flooding to further enhance the recovery of heavy oil reservoirs after water flooding.All research in this work is based on X reservoir in Xinjiang Oilfield and it is found that X reservoir is a common heavy oil reservoir with crude oil viscosity of 1440 mPa s and density of 0.911 g/mL at reservoir temperature.In addition,heavy oil of X reservoir has a high content of asphaltene,resin and petroleum acid,which are all natural surfactants of heavy oil.Laboratory experiments showed that it was very easy to form viscous W/O emulsion by purely mixing heavy oil and formation water of X reservoir.When water content was 70%,the viscosity of W/O emulsions was 5123mPa·s,3.66 times higher than that of heavy oil.The extremely high emulsion viscosity was unfavorable to the flow of crude oil in porous media.In order to further improve development efficiency of heavy oil after water flooding,the in-situ emulsifying system VMPS has been obtained through emulsifier combination.VMPS could decrease oil-water interfacial tension to 10-2mN/m and produce O/W emulsions with viscosity lower than heavy oil under certain water content conditions,instead of viscous W/O emulsions with higher viscosity than heavy oil.Furthermore,as VMPS concentration increased,rock wettability changed from oil-wet to water-wet,and adsorption quantity of the emulsifying system VMPS on formation sand and quartz increased first and then remained constant.The evaluation experiments of emulsion properties showed that with increase of shear rate,shear time and VMPS concentration,emulsification efficiency became higher and more stable emulsions were produced.As salinity increased,emulsion stability decreased.The emulsion stability increased under both acidic conditions(pH<7)and alkaline conditions(pH>7).Emulsions produced under alkaline conditions were extremely stable,which may be related to petroleum soap reacted by NaOH and petroleum acid under alkaline conditions.Oil displacement experiments showed that injection parameters and reservoir properties had a great influence on development efficiency of in-situ emulsification flooding.Considering both efficiency and economic input of in-situ emulsification flooding,0.2mL/min was chosen as the optimal injection rate,0.4%was chosen as optimal injection concentration and 0.4PV was chosen as optimal slug volume,respectively.As permeability increased,recovery factor of in-situ emulsification flooding and subsequent water flooding increased first and then decreased.The maximum value was 16.31%when permeability was 60.89mD.In addition,by comparison,it was found that in-situ emulsification flooding had better oil recovery effects(recovery increased by 16.23%)than surfactant flooding(recovery increased by 8.65%)for heavy oil in X reservoir.Under heterogeneous conditions,the displacement effect of in-situ emulsification flooding becomed worse when permeability gradient increased.In-situ emulsification flooding could effectively start low permeability layer when permeability gradient was less than 5.2.When emulsifying system was injected at different watercut of core outlet,it was found that lower watercut of core outlet would make liquid production fraction bigger in low permeability reservoir and thus oil recovery increment was higher.Triple parallel flooding experiments showed that in-situ emulsification flooding technology could start medium and low permeability layers when permeability variation coefficient was less than 0.65.