Study On Wellbore Thermal-Fluid Coupling Mechanism Of CO₂ Fracturing In Horizontal Wells Of Shale Gas Reservoirs

Shale gas is an efficient clean energy.Horizontal well+CO₂ fracturing can improve shale gas recovery,achieve geological storage,and improve the physical and chemical properties of shale reservoirs.There are significant differences in flow and heat transfer mechanisms between CO₂ strongly variable properties and steady fluids,which makes the coupling characteristics of flow and heat transfer in CO₂ fracturing wellbore unclear.Through theoretical analysis and numerical simulation,this paper reveals the distribution law of wellbore temperature and pressure during CO₂ fracturing of shale gas horizontal wells.The research results provide theoretical support for CO₂ fracturing technology and are of great significance for the sustainable,efficient and green development of shale gas reservoirs in China.Firstly,based on the study of the basic physical properties of CO₂ fracturing fluid,the variation characteristics of its thermal properties and rheology under temperature and pressure are analyzed.Secondly,the flow and heat transfer characteristics of CO₂ fracturing fluid in wellbore are analyzed,and the flow model and heat transfer model of CO₂ fracturing wellbore are established.The calculation methods of wellbore friction coefficient and wellbore heat transfer coefficient are compared and optimized.Finally,considering the synergistic effect of CO₂ variable physical properties,friction characteristics and heat transfer characteristics,a wellbore heat flow coupling model is established to realize the dual coupling numerical solution of wellbore axial and radial directions.The accuracy of the model is verified by field fracturing construction data,and the sensitivity analysis is carried out.The results show that the NIST Refprop software library is used to analyze the physical parameters of CO₂,and it is concluded that the physical parameters of CO₂ are most affected by temperature and pressure at the critical point,and the calculation problem that the Span-Wagner model and the Vesovic model are only applicable to specific temperature is solved.The wellbore friction coefficient is calculated by Colebrook equation,and the formula algorithm is optimized without iteration.By comparing the calculated values with the simulated values,the Driker formula and the Davis formula are used to analyze the convective heat transfer coefficient error of the inner and outer walls of the annular space.The CO₂ wellbore heat flow coupling model can well describe the interaction of CO₂ physical parameters,temperature and pressure during fracturing.The injection pressure has little effect on the bottom hole temperature and the length of the deviation section,which can be neglected in engineering.Under high displacement,the influence of injection temperature and geothermal gradient on bottom hole temperature is greater than that of bottom hole pressure.Wells with long horizontal section should take drag reduction measures to avoid excessive wellhead pump pressure.