???????????????????????

欠平衡钻井是开发低压低渗透储层和枯竭油气藏非常有效的钻井技术。它不但可将地层损害减小到最低程度，而且在钻井过程中有机会对储层进行评价和描述，在钻遇复杂地层中可防止漏失和地层压差卡钻，并能大幅度提高钻速。欠平衡钻井循环系统是一个多相流体流动的非稳定压力系统，它受注入流体气液比，储层流体二次充入，地面回压等因素控制。为此，分析了这个多相流循环系统设计中应考虑的一些主要问题。并通过井底压力模拟计算表明：循环系统在静压控制区工作时，气相注入速率的微小变化，将引起井底压力突变，该区为循环系统不稳定区，欠平衡钻井操作应避免在该区工作而引起井眼失稳和过平衡压力。循环系统在摩阻控制区工作时，对气相注入速率和地层流体进入系统，井底压力变化平缓，因此该区为循环系统稳定区，欠平衡钻井作业在该区能保证设计负压差值。但随着气相注入速率的增加，井底压力增加，此时应控制氮气注入速率，节约钻井成本。

ANALYSIS OF THE MULTIPHASE FLOW CIRCULATING SYSTEM OF UNDERBALANCE PRESSURE DRILLING

Underbalance pressure drilling is a very effective drilling technique for developing low pressure and low permeability reservoirs and depleted oil and gas pools,which not only can make the formation damage reduced to minimum,but also provides a chance to evaluate and describe reservoir in the course of drilling,and can make the leakage and differential sticking avoided when complicated formations are drilled as well as can make the penetration rate greatly raised.The circulating system of underbalance pressure drilling is a multiphase fluid flowing unsteady pressure system,which is controlled by such factors as the air liquid ratio of pumped fluid,second invaded reservoir fluid,surface back pressure and state equation etc.For this reason,some main problems considered in the design of this multiphase flow circulating system are analyzed.The analog computation of bottom hole pressure shows that when the circulating system is running in static pressure controlled zone,a little change in gas phase injection rate will cause a sudden change of bottom hole pressure,thiszone being an unsteady zone of the circulating system.Underbalance pressure drilling operation in such zone should be avoided to cause hole unstability and overbalanced pressure.When the circulating system works at frictional drag controlled zone,nitrogen and formation fluid enter in the system,which makes the change of bottom hole pressure gentle,therefore,this zone is a stable one of circulating system and the underbalance pressure drilling operation in this zone can obtain a designed negative pressure differential value.But the bottom hole pressure will raise with the enhancement of gas phase injection rate.At this moment,the nitrogen injection rate should be controlled to cut down on drilling cost.