Experiment And Neural Network Prediction On The Mimimum Miscibility Pressure Of CO₂-Alkanes System

As a major greenhouse gas,CO₂ has an increasingly serious impact on global warming.Injecting CO₂ into oil reservoir for miscible displacement can improve oil recovery on the one hand,and on the other hand,it can store a large amount of CO₂ and alleviate the greenhouse effect,which is an effective emission reduction technology.In oil displacement engineering,the minimum miscibility pressure and diffusion coefficient of CO₂ and crude oil are two important parameters,which greatly affect the displacement effect and storage efficiency.In this paper,the minimum miscible pressure and diffusion coefficient under the condition of formation temperature and pressure are measured experimentally,and the minimum miscible pressure is predicted by using neural network model.Firstly,the experimental system of interfacial tension measurement based on drop shape analysis method is established.The interfacial tension of CO₂-pure alkanes and CO₂-mixed alkanes under multiple temperature and pressure conditions is measured.The minimum miscible pressure is obtained by linear fitting of the interfacial tension data.It is found that the interfacial tension has a linear negative correlation with pressure,and the interfacial tension increases with the increase of carbon number of alkanes.The minimum miscibility pressure has a linear positive correlation with temperature and carbon number of alkanes.Secondly,98 minimum miscible pressure datasets of CO₂-crude oil are obtained from the literature data,and three models of wavelet neural network,feedforward neural network and radial basis function neural network are established based on neural network algorithm.The minimum miscibility pressures of different oil samples and gas environments were predicted with the content of intermediate gas,volatile gas,n-pentane and n-hexane,average molecular weight of high carbon alkanes and gas mole fraction as input parameters,Then,the influence of input parameters on the minimum miscibility pressure in CO₂-crude oil system is further analyzed by using the model.Finally,the diffusion coefficients of CO₂-alkanes were measured by using the above experimental system and based on the dynamic droplet volume analysis method.It is found that the diffusion coefficient of CO₂-mixed alkanes increases with the increase of pressure at the same temperature;the diffusion coefficient of CO₂-mixed alkanes decreases with the increase of temperature under the same pressure.The diffusion coefficient between gas and liquid is mainly affected by the joint action of temperature and pressure,which affects the viscosity of droplets and changes the ability of gas-liquid diffusion.The closer the pressure environment is to the minimum miscibility pressure at this temperature,the greater the increase of gas-liquid two-phase diffusion ability.In this thesis,the interfacial tension,minimum miscible pressure and diffusion coefficient of CO₂-oil are comprehensively analyzed by experiments and models,which can provide data and theoretical support for CO₂ miscible displacement technology.