Study On The CO₂ Flooding Method Of Reducing Miscible Pressure In Low Permeability Reservoir

Oil is still a very important energy source for every country in the world.The consumption trend of oil as a non-renewable resource is growing for every country.Especially,our country is in the stage of high-speed development.Our foreign dependence ratio of oil is increasing year by year,that has reached nearly 70%in 2017.Under the increasingly complex international situation,how to achieve a stable production of 200 million tons of crude oil per year is a major strategic issue concerning modernization and energy security of our country.With the rapid development of China's economy,the conventional oil and gas resources have been unable to meet the demand for oil and gas resources.As an important strategic supplement to conventional energy resources in the future,low permeability oil and gas resources has become the focus of research and development.China has large reserves of low permeability oil and gas resources.In the proven reserves of crude oil,the proportion of low permeability oil reserves is high and the development potential is huge.In recent years,the low permeability reservoirs discovered in China accounts for more than half of the newly discovered oil and gas reservoirs.Therefore,the exploration and research of low permeability reservoirs is of great significance to the development of oil industry and national economy.Water flooding oil production technology in domestic has formed a set of very mature technology.But the low permeability reservoir is difficult to start up effectively due to small pore throat,high capillary pressure and low permeability.So it is more suitable for most low permeability oilfields to be developed by gas injection.In the choice of gas source,carbon dioxide becomes the object of choice of many oil fields due to its own advantages.CO₂ for oil displacement compared to other gas drive technology has many advantages,such as wide applicable scope,good miscibility with crude oil,low cost,recyclable,and significantly improve the recovery efficiency and so on.Therefore,the technology of producing crude oil by injecting carbon dioxide has been widely used in the world.At the same time,carbon dioxide flooding can also solve the problem of carbon dioxide sequestration to a certain extent,reduce the emission of greenhouse gases into the atmosphere and ease the pressure of environmental pollution.Under the background of energy shortage and energy saving and emission reduction,carbon dioxide flooding has a broad application prospect.At present,the main technologies to improve oil and gas recovery by using carbon dioxide are miscible flooding and non-miscible flooding.The results of field test and laboratory experiment show that the improved recovery of miscible flooding is better than that of non-miscible flooding.However,one of the key factors for miscible flooding is the minimum miscible pressure.The minimum miscibility pressure is affected by many factors,such as crude oil properties,injected gas composition,formation temperature and formation pressure and so on.The minimum miscibility pressure of the target area in this study is relatively high.Due to the limitations of the reservoir conditions and development conditions,the formation pressure cannot reach the minimum miscibility pressure,which results in the inability to realize miscibility flooding within a large range of the area and reduces the development effect.Therefore,it is urgent to find effective techniques and methods to reduce the minimum miscibility pressure according to the actual situation of the target area,so that the miscibility flooding can be fully realized in the near and far well areas of the reservoir.The purpose of this study is that on the basis of investigation of relevant research results at home and abroad,through the combination of indoor physical simulation and numerical simulation method,we can determine the method for reducing the minimum miscible pressure of the target area.That provides strong technical support and theoretical basis for realizing integrated miscible flooding and further improving oil recovery of the target area.The following main understandings have been got by the research:(1)Through the study of geological features,development evaluation and physical characteristics of crude oil in the target area,the technical direction of expanding miscible flooding range by reducing the minimum miscible pressure is determined,which provides the basis of geological features,development status and physical characteristics of crude oil for further improving the recovery.(2)The minimum miscible pressure(MMP)of crude oil and carbon dioxide system in the target area is 29.73MPa by long tube displacement experiment and interfacial tension method,which are widely accepted by researchers at home and abroad.The minimum miscible pressure measured by the method of long tube displacement experiment is 29.6MPa and the minimum miscible pressure measured by the method of interfacial tension was29.85MPa.The minimum miscibility pressures measured by the two methods are very close,and the average of 29.73MPa is determined as the minimum miscibility pressure in the target area.Meanwhile,in order to optimize the empirical formula for rapid prediction of minimum miscible pressure in similar areas of target areas,7 empirical formula methods applicable to the target area are selected to predict the minimum miscible pressure.The units and related parameters of each empirical formula are unified.From the prediction results of the empirical formula method,the results obtained by the empirical formula method are significantly different.The average value predicted is 26.57MPa.Comparison with the minimum miscible pressure of 29.73MPa identified in the target area,the empirical formula with the smallest relative error is The Petroleum Recovery Institute?(PRI_?).The minimum miscibility pressure is predicted to be 28.73MPa.The relative error is 3.36%.The PRI_?can be used as an empirical formula for the rapid prediction of minimum miscible pressure in similar areas of target areas.(3)According to the mechanism of carbon dioxide flooding,the actual condition of target area and the minimum miscible pressure,the way to improve the development effect is to reduce the minimum miscible pressure.Based on the analysis of the conditions for carbon dioxide and crude oil to be miscible and the factors affecting the minimum miscible pressure,and combined with the crude oil composition effect of carbon dioxide injection,the method of using the ester oil-dissolved surfactant citric acid isobutyl ester and citric acid isopentyl ester as primary reagents to reduce the minimum miscible pressure is determined.The chemical reaction equations,formulations and preparation processes of two primary reagents are determined.The two reagents can not only dissolve in crude oil to reduce the viscosity of crude oil,but also dissolve in supercritical carbon dioxide to reduce the interfacial tension between crude oil and carbon dioxide.(4)The results of the minimum miscible pressure test of citric acid isobutyl ester and citric acid isopentyl ester with different slug sizes indicate that:The minimum miscibility pressure measured gradually decrease with the increase of citric acid isobutyl ester and citric acid isopentyl ester injection slugs size,but the decrease is getting smaller and smaller.The optimal injection slug size is 0.003pv.Under the condition of the slug size,the minimum miscibility pressure measured after adding citric acid isobutyl ester slug is 23.5MPa,and the minimum miscibility pressure is decreased by 6.23MPa.The minimum miscibility pressure measured after adding citric acid isopentyl ester slug was 24.1MPa,and the minimum miscibility pressure is decreased by 5.63MPa.It can be seen that the reduction of minimum miscibility pressure by adding citric acid isobutyl ester slug is larger and the effect is better.(5)Four simulation schemes of artificial core displacement are developed,and the recovery of miscible flooding stage in conventional schemes without optimized reagent is5.4%.Among them,the scheme with the highest recovery of miscible flooding is scheme 4:isobutyl citrate citric acid isobutyl ester slug of 0.003pv is injected after water content reaches98%in water flooding,and then CO₂ of 1.2pv is injected.In this scheme,the recovery of miscible flooding stage is 17%,which is 11.6 percentage points higher than that of conventional CO₂ miscible flooding scheme without optimized reagent.It can be seen that injection of citric acid isobutyl ester under the same experimental conditions can reduce the minimum miscible pressure,expand the affecting range of miscible flooding,and thus improve the oil displacement effect of carbon dioxide miscible flooding.(6)With comprehensive application of the geological characteristics research results of the target area,the fine geological model of the target area is established by Petrel modeling software.Combined with the dynamic and static characteristics of the target area,the numerical simulation model of the target area is established by CMG numerical simulation software.The fitting of geological reserves and history are completed.Based on the results of historical fitting,ten CO₂ injection schemes are designed.The recovery of the basic prediction scheme without injection of preferred reagent is 7.11%.The optimal scheme is the fourth after comparison and analysis:on the basis of the current production system of each well,the0.003pv slug of citric acid isobutyl ester is first injected,then continue production with the current production system of each well,and predict the development index.It is predicted that the cumulative oil production of the target area will be 55.21×10⁴t after 10 years,and the recovery will be 11.47%,which is 4.36 percentage points higher than that of the basic prediction scheme.