Research On EOR Of CB Heavy Oil Reservoir With Nonhydrocarbon Gas Assisted By Viscosity Reducer

CB reservoir is located in the southeast of Chengdao oilfield,which is an offshore heavy oil reservoir.CB reservoir is characterized with large geological reserves,high oil viscosity,worse exploitation and difficult in extracting.Heavy oil is usually recovered by thermal recovery.However,due to the high viscosity of crude oil and serious reservoir heterogeneity,the formation energy shortage,limited steam heating sweep and low oil production speed appear after multiple cycles of heavy oil thermal recovery.So cold recovery of heavy oil has been applied in many oilfields.Cold recovery of heavy oil cold production methods are mainly CO₂ huff-n-puff and N₂ huff-n-puff.However,the single huff-n-puff will cause serious gas channeling and low recovery factor after multiple cycles of production.During the process of huff and puff production,viscosity reducer or foaming agent was added to reduce the viscosity of crude oil,Reduce crude oil viscosity,plugged the large channel with high permeability,and improve the production degree of reservoir.In this text,according to the characteristics of CB reservoir,the viscosity reducer was selected and evaluated by evaluating the ability of viscosity-reducing,emulsion stability,interfacial performance and foaming.The experimental study of non-hydrocarbon gas assisted by viscosity reducer was carried out in laboratory.The mechanism of enhancing oil recovery with non-hydrocarbon gas assisted by viscosity was studied in depth,and the efficient exploitation mode was optimized.On this basis,the development mode optimization of non-hydrocarbon gas and viscosity reducer and injection and production parameters optimization of non-hydrocarbon gas and viscosity reducer were carried out by numerical simulation.The results show that the viscosity reduction rate of the optimized ZQQ viscosity reducer is 92.3%at 80℃.The viscosity reducer and heavy oil can form a stable emulsion,which can effectively reduce the oil-water interfacial tension and gas-liquid interfacial tension.The foaming performance of ZQQ viscosity reducer was good and can form good plugging effect.The non-hydrocarbon gas huff and puff experiment found that the combination of N₂/CO₂ and viscosity reducer can inhibit the gas channel,reduce the viscosity of crude oil,increase the flow kinetic energy of crude oil,and play a 1+1>2 effect.Under the same conditions,the recovery rate was better than that of single non-hydrocarbon gas huff-n-puff or single viscosity reducer huff-n-puff,and the stability was higher,and the amount of remaining oil distribution was significantly reduced.The total recovery rate of N₂ huff-n-puff assisted by viscosity reducer and CO₂ huff-n-puff assisted by viscosity reducer were 34.98%and 35.52%respectively.The effect of dissolution of carbon dioxide on improving oil recovery was not obvious when the viscosity reducer greatly reduces the viscosity.CB reservoir belongs to the offshore oil field,which has strict requirements on gas source,corrosive of CO₂ and high requirements on equipment.N₂ huff-n-puff assisted by viscosity reducer was more economically applicable to CB reservoir.The results of CMG numerical model show that the development effect of horizontal well was better than vertical well.The production of producing well increases by 66%,and the economic benefit increases by about 86%.When the injection-production well spacing was 400m,the cumulative oil production reaches the maximum.The optimal water injection rate of CB reservoir was 80m³/d,the time of water injection was 0～2 years,and the concentration of surfactant was 0.2%.The injection rate of N₂ was 20,000 m³/day,the concentration of viscosity reducer was 0.3%,the injection time was 20 days,and the steaming time was 20 days.Initial oil production per well was 110 cubic meters per day.