Investigation On Flow Heat Mass Transfer Mechanisms Of Supercritical Carbon Dioxide Fracturing

In the present study,molecular dynamics method was used to simulate the dissolution behavior and thickening effect of different thickeners in CO₂.The simulation results indicate that at 323.15K and30MPa,the 5wt%polyoxyethylene 6-nonyl phenol is miscible in CO₂ and can increase the viscosity of CO₂ by 23.8%.A transient 2D model considering the factors of CO₂ thermal properties,viscous heating source,Joule-Thomson effect,axial and radial heat transfer was proposed to investigate the wellbore temperature and pressure.The field data obtained from two CO₂ fracturing wells was used to verify the reliability of the model.The wellbore temperature and pressure were analyzed based on the investigation of the tubing and annulus injection well with the diameter of50.3 mm and depth of 3600 m.The simulation results show that the wellbore temperature increases with the increase of the well depth and the bottom hole temperature increases first and then decreases as injection time increases.The bottom hole pressure decreases first and then increases with the injection time.The axial pressure gradient decreases as injection rate increases and even becomes negative value.The critical depth increases as injection time increases.The CO₂ density and capacity are mainly affected by temperature,and both decrease when well depth increases.The simulation results indicate that the bottom hole temperature decreases and the bottom hole pressure increases as viscosity increases.The effect of viscosity decreases as injection time increases.Compared with tubing injection,tubing and annulus injection decreases critical depth and increases the duration of wellbore unsteady flow and heat transfer process.The fluid heat conduction in axial direction is small and can be neglected in the calculation process.The calculated bottom hole temperature is lower and pressure is higher without the consideration of axial solid heat conduction.In order to investigate the interaction between CO₂ and rock,the geological parameters of rock sample before and after CO₂ injection were measured,respectively.The experimental results show that in the rock the diameters of the main pores increase from 2?m-10?m to 50?m-60?m and the porosity increases from 0.12 to 0.14 due to the injection of CO₂.