超临界CO_2钻井井筒压力温度耦合计算

Wellbore temperature and pressure coupling calculation of drilling with supercritical carbon dioxide

超临界CO2钻井过程中,井筒温度和压力对CO2的密度、黏度、导热系数、热容等物性参数影响较大,这些参数的变化对温度和压力产生反作用。利用Span-Wagner基于亥姆霍兹自由能的气体状态参数计算方法,建立了超临界CO2钻井井筒压力温度耦合计算模型,并以超临界CO2连续油管钻井为例进行了实例分析。计算结果表明,钻杆压力和环空压力都随着井深的增加而增大,钻杆内的CO2相态变化点比环空深50 m;钻杆中温度略低于环空中的温度;随着井深的增加,CO2密度缓慢降低,变化率逐渐减小;距井底20 m深处,环空中CO2密度突然增大,钻杆中CO2密度也有所增大。本文模型与文献中采用P-R状态方程的计算结果相差较大,主要原因是文献中的模型没有考虑井筒中CO2物性参数的变化及其对温度和压力的影响,本文模型计算精度高。

Wellbore temperature and pressure have great influence on CO2 density,viscosity,thermal conductivity,heat capacity and other physical properties,the change of these parameters will react against the wellbore temperature and pressure.The Helmholtz free energy state equation of gas put forward by Span and Wagner was adopted to build the pressure and temperature coupling calculation model,and coiled tubing drilling was taken as an example for analysis.The results indicated that the pressure of drill pipe and annular increases with the well depth increasing,and the state change point in the drill pipe is 50 meters deeper than that in the annular.The temperature in the drill pipe is a little lower than that in the annular.The density of CO2 decreases slowly with the well depth increasing,and the changing rate decreases gradually.At the depth of 20 meters to the bottom hole,the density of CO2 in the annular increases abruptly,and the density of CO2 in the drill pipe increases too.The calculation error between the proposed model and the P-R model is large,the main reason is that the referenced model doesn＇t consider the physical parameters change and its influence on the temperature and pressure,and the proposed model has a high calculating precision.