纯液态CO_2压裂非稳态过程数值模拟

NUMERICAL SIMULATION ON THE INITIAL UNSTABLE STAGES OF LIQUID CO_2 FRACTURING

为了解纯液态CO2压裂初始井底压力和温度随时间的演化规律,对压裂液初期非稳态过程进行了数值模拟。从模拟的结果看:井底液体CO2在压裂的初期会经历较大的温度和压力变化,液体CO2会因受热而发生相态的变化和体积的膨胀,最大膨胀幅度达17.2,而其重位压头的变化则是引起井底液体CO2压力变化的主要因素。一般在压裂2～5min后井底温压即可稳定,稳定后的温度和压力以及稳定所需要的时间主要与压裂液排量、井深有关。如果气井太深,低温液体CO2会在井筒附近地层造成巨大的温度梯度,这有可能会引起井筒周围地层热应力的剧烈上升,从而有利于井筒射孔附近地层的开裂。

The initial unstable stage of pure liquid CO2 fracturing process was studied numerically to clearly understand the evolvement of bottom hole temperature and pressure.The numerical simulation results show that the bottom hole temperature and pressure will vary considerably at the beginning of fracturing:CO2 will be heated,which gives rise to the change of its phase from liquid to supercritical,and of its volume maximum expansion up to17.2%;besides,the variation of CO2 hydrostatic pressure is the main factor influencing the change of the bottom hole pressure.After about 2-5 minutes fracturing the bottom hole temperature and pressure will become stable,which lie on the flux of CO2 and the well depth.The low-temperature liquid CO2 may arouse a great temperature gradient and thermal stress at a relatively deep formation around the oil and gas wellbore,which is beneficial to fracturing.