水平井油基钻井液气侵溶解气膨胀运移规律研究

天然气进入井筒并溶解于油基钻井液会对钻井安全产生潜在威胁，溶解气随钻井液运移到井口附近突然发生析出并膨胀，使气侵早期监测及预警的难度显著增加。为了降低井喷发生概率和钻井作业风险，针对水平井油基钻井液溶解气运移规律进行了研究。采用气液两相流模型，模拟了不同初始气侵速率情况下甲烷溶解气和游离气随钻井液在水平段和垂直段的运移过程，得到环空中钻井液的流速变化、甲烷析出过程、井筒环空气液两相流流型变化、泥浆池增量随时间变化和截面含气率沿程分布等规律。模拟结果表明，油基钻井液发生气侵时的初始进气率存在临界值；在小气侵速率情况下，侵入环空的甲烷将以溶解气的形式运移到井口，在井口分离释放，对钻井安全构成的威胁较小；而当气侵量超过临界值时，在环空上部发生气体分离，分离点迅速下移，如不及时控制，在极短时间内便可演化为井喷，危险性极大。

Study on swelling and migration laws of dissolved gas during gas kick of oil-based drilling fluid in horizontal well

The phenomena that natural gas enter the wellbore and dissolve in the oil-based drilling fluid will lead to potential threaten to the drilling safety, and the dissolved gas migrating to the wellhead with the drilling fluid may occur evolution and swelling suddenly, which make the early monitoring and warning of gas kick more difficult. In order to reduce the probability of blowout and the risk of drilling operation, the migration laws of dissolved gas for oil-based drilling fluid in horizontal well were studied. The migration processes of methane dissolved gas and free gas with drilling fluid in the horizontal section and vertical section under different initial gas kick rate were simu-lated by using the gas-liquid two-phase flow model, and the following laws were obtained, such as the change of flow velocity for drilling fluid in annulus, the methane evolution process, the change of flow regime for gas-liquid two-phase flow in wellbore annulus, the change of pit gain against time, and the distribution of sectional void frac-tion along the wellbore, etc. The results showed that the initial gas inflow rate exists a critical value when the oil-based drilling fluid occur gas kick. When the gas kick rate is small, the methane cutting into annulus will migrate to the wellhead as the form of dissolved gas, and separate and release at the wellhead, which causes a smaller threaten to the drilling safety. When the gas inflow rate exceeds the critical value, the gas separation will appear at the upper part of annulus, then the separation point moves down quickly. If not controlled in time, it will evolve into blowout in a very short period of time, with a great danger.