The Study On Phase Characteristics Of Crude Oil During Gas Injection In Tight Reservoir

The HH oil field is a typical tight sandstone reservoir.Because in such reservoirs,porous media has characteristics such as large surface area,small pore size,large capillary force,they cause that only a low degree of recovery can be reached,and the conventional development methods has poor efficiency or cannot be effectively applied.Currently,facing problems such as high water injection pressure,low water absorption index,and low-efficient water injection in the process of waterflooding,gas injection development research is required to further improve oil recovery.The properties of crude oil in oil reservoirs will change after gas injection,which will then affect the gas injection effect.Therefore,before gas injection,it is necessary to carry out laboratory experiments on the property changes of crude oil before and after gas injection,to provide reliable theoretical basis for gas injection development and oil production.The current researches on the phase characteristics of crude oil are all based on the premise that the change of the fluid phase in the reservoir is not affected by the porous media.Since the phase behavior of formation fluids occurs in oil and gas reservoir stratums,the effect of porous media on the phase state exists objectively.This effect is mainly manifested in the interface adsorption between the formation fluid and the porous media,capillary pressure,etc.Therefore,the phase characteristics of the crude oil in the porous media can more accurately reflect the phase transition law of the crude oil in the reservoir.On the basis of compound crude oil samples of HH oilfield,this paper performed gas injection expansion experiments of three kinds of gases:N2,CH4,and CO2;The ADSA technology was used to determine the change of equilibrium interfacial tension with pressure in three different systems:the CH4/crude oil,N2/crude oil,and C02/crude oil under reservoir conditions.Then,the test method for phase state of crude oil in porous media was designed,and the influences of different crude oils,different permeabilities,and different pressures on the phase state of crude oil were studied;in addition,through literature researches,the gravitational term of the existing state equations and the interaction correlations were modified,and has established the prediction model for the phase behavior of the crude oil in the PVT cylinder and in the porous medium respectively.The main achievements are listed as follows:(1)Gas injection expansion experiments showed:injection of N2,CH4,and CO2 can improve the physical properties of crude oil,and make the volume of the crude oil expand as well as reduce the viscosity,thus increase its fluidity;comparing the three types of injected gas,CO2 has the best ability of reducing viscosity and dissolving crude oil,followed by CH4,and N2 is the worst.2)Tests of interfacial tension between injected gas and crude oil showed that under the reservoir temperature,the interfacial tensions of N2/crude oil,CH4/crude oil and CO2/crude oil system all decrease with the increase of pressure;the dropping degree of CO2/crude oil system is largest,mainly because of the strong solubility of CO2 in crude oil.(3)The phase state experiments of crude oil in porous media showed that the bubble pressure of formation crude oil in porous media was significantly lower than that in PVT cylinder;the lower the permeability of core is,the more greatly the bubble point pressure in porous media decreases;the more the content of saturated CO2 in crude oil is,the less the bubble point pressure in porous media decreases.4)By modifying the equation of state between injected gas and crude oil,the prediction model of phase behavior of crude oil after gas injection in PVT cylinder is established as follows:PT state equation(including the newly established correlation between components)+ Pedesen "+" component partition method + Kester-Lee critical properties of pseudo-component calculation correlation;the prediction model established for the calculation of the phase state of crude oil after gas injection in porous media is:SRK state equation + Pedesen "+" component partition + Kester-Lee pseudo-component critical property calculation correlation + The newly established a-function and inter-components reaction correlation;the new model was well fitted to the experimental data.