Study On The Temperature Field Of Horizontal Wellbores During CO₂ Fracturing In Shale Gas Reservoirs

The CO₂fracturing technology,which can achieve well stimulation,oil/gas recovery enhancement and CO₂storage simultaneously,has broad application prospects.During the CO₂fracturing,the variation of the wellbore temperature and pressure will change the CO₂sand carrying capacity and the fracturing effectiveness and finally affect the performance of the management.However,CO₂has high compressibility and high friction,thus the coupling of physical parameters and pressure is required,which makes the physical process very complicated.It is still lacking a profound study on the temperature field of horizontal wellbores during CO₂fracturing in shale gas reservoirs.In this dissertation,the properties of CO₂and shale are investigated by calculation models and laboratory experiments respectively,and the phase characteristics of CO₂during fracturing process are analyzed in combination with the three-phase diagram.Based on these characteristics,the temperature field of the horizontal wellbore during CO₂fracturing in shale gas reservoirs is studied.Firstly,the Laplace method is used to obtain the novel analytic solution of the wellbore temperature equation,which can overcome the problem that the Hasan's solution is not suitable for the fracturing process.The accuracy and efficiency of the novel analytic model are illustrated by comparing with the Hasan's model and the numerical model.However,the application of the analytic solution on the prediction of wellbore temperature during CO₂fracturing has some limitations.In order to overcome the limitations of the analytical model,the wellbore energy equation considering the pressure work and viscous dissipation of the CO₂fracturing process is derived and coupled with the continuity equation,momentum equation and the properties model,and the solution region is simplified according to the horizontal wellbore flow characteristics to establish a 1D+1D transient fully-coupled model,in which the CO₂flow is considered as the one-dimensional non-isothermal flow and the wellbore and formation is considered as the one-dimensional transient heat transfer.The model is dispersed by the fully implicit finite difference method and the MATLAB is used here to code and calculate the equations.The 1D+1D transient fully-coupled model is verified by the measured data of vertical wells during CO₂flooding and CO₂fracturing and the simulation data of a commercial software for non-isothermal flow in a horizontal tube.In addition,the test of convergence and the grid independence validation is also performed.Secondly,the temperature change mechanisms including the wall heat transfer,pressure work and viscous dissipation in CO₂fracturing process is studied.The contribution ratio of the three temperature changing mechanisms to temperature field under various injection conditions are compared to illustrate the difference between the CO₂fracturing condition and the slow injection rate condition?CO₂flooding,CO₂storage and CO₂heat recovery?.Finally,the 1D+1D transient fully-coupled model is used to analyze the sensitivity of the treating parameters,formation parameters and wellbore parameters of the horizontal wellbore during the CO₂fracturing in shale gas reservoirs and quantitatively characterized the influence of key parameters on the horizontal wellbore temperature field.Based on theoretical analysis and numerical simulation,the dissertation reveals the distribution law of temperature and pressure of the horizontal wellbore during CO₂fracturing in shale gas reservoirs.The research results will provide theoretical basis and technical support for accurately wellbore temperature field and pressure prediction and fracturing designs of CO₂fracturing in horizontal wells of shale gas.