超临界二氧化碳压裂井筒温压及相态控制研究

Study on Wellbore Temperature&Pressure and Phase Control in Supercritical Carbon Dioxide Fracturing

超临界二氧化碳压裂液对温度、压力较为敏感,准确地预测注入过程中的井筒温压及相态直接影响着最终的压裂效果。因此,建立了考虑轴向导热、焦汤效应、膨胀(压缩)做功、摩擦生热热量分配的超临界二氧化碳压裂井筒瞬态温压模型,模拟分析了注入温度、施工排量、降阻效果、油管尺寸对井筒温压及相态的影响。研究结果表明,井筒温度降低导致的二氧化碳密度增加、流速降低,使得井口压力随井底温度同步降低。注入温度越高、施工排量越小、降阻率越高、油管尺寸越大,井底温度越高、井口压力越低。其中,井口温度增加10℃,井底温度增加约为7℃;降阻率提高20%,井口压力降低约7MPa。提高注入温度及流动通道的横截面积、降排量的同时使用稠化剂(降阻剂)可促使二氧化碳在井底达到超临界态。研究成果对超临界二氧化碳压裂的优化设计及现场应用具有较强的指导意义。

Supercritical carbon dioxide fracturing fluid is sensitive to temperature and pressure,and accurate prediction of wellbore temperature,pressure and phase state during fracturing directly affects the final fracturing effect.As a result,a transient wellbore temperature and pressure model of supercritical carbon dioxide fracturing considering axial heat conduction,Joule-Thomson effect,expansion/compression work,and frictional heat was established.Based on the model,the effects of injection temperature,displacement,drag reduction effect,and tubing size on the wellbore temperature,pressure and phase state were analyzed.The results show that the decrease of wellbore temperature leads to an increase in carbon dioxide density and a decrease in flow velocity,which causes the wellhead pressure to decrease simultaneously with the wellbore temperature.The higher the injection temperature,the smaller the displacement,the higher the resistance reduction rate,the larger the tubing size,the higher the bottom hole temperature,and the lower the wellhead pressure.Among them,the wellhead temperature increases by 10C,and the bottom hole temperature increases by about 7C;the resistance reduction rate increases by 20%,and the wellhead pressure decreases by about 7 MPa.Increasing the injection temperature,the cross-sectional area of the flow channel,and reducing the displacement while using the thickener/resistance reducer can promote the carbon dioxide to reach the supercritical state at the bottom of the well.This article has strong guiding significance for the optimization design and field application of supercritical carbon dioxide fracturing.