起下钻过程中井喷压井液密度设计新方法

A New Design Method of Killing Fluid Density against Blowout during Tripping

钻井起下钻过程中,操作不当容易导致溢流或井喷,而此时钻头不在井底,U形管原理不能描述此时的井眼状况。针对该种情况,基于气液两相流理论,通过分析钻头不在井底时溢流、关井和压井期间的井筒流体特性,建立了钻头不在井底的Y形管模型。利用Y形管的3个分支结构代表钻头不在井底工况下钻柱、环空和钻头以下井眼3部分,钻柱和环空部分的井筒压力特性可以用典型U形管原理分析,当分析关井和压井期间井底压力等压井参数时,应考虑钻头以下井段的流体特性,即Y形管结构下部分支的流体特性,据此,应用Y形管描述钻头不在井底工况下整个井筒的压力特性。基于该理论,给出了根据关井压力恢复曲线读取关井压力的时机,推导出了钻头以下井段的流体密度计算公式,并给出了压井液密度设计方法。实例计算表明,设计压井参数时,钻头以下井段的含气性对地层压力获取和压井液密度求取具有较大影响,因此,在设计压井参数时应首先分析其含气性。

During tripping, improper operation might lead to kick or even blowout when wellhole condi- tions could not be described by Utube theory if the bit was off-bottom. Based on the theory of two phase flow, a Y-tube model is built through analysis of wellbore fluid properties in overflow, shut-in and well kill- ing when the bit is off-bottom. Three branches of Y-tube represent the drill string, annulus and wellbore below the bit, respectively. For the drill string and annulus, the wellbore pressure may be analyzed by the typical Y-tube theory. Fluid properties in interval below the bit（namely lower branch of Y-tube）should be considered, when the changes of killing parameters（e, g. bottomhole pressure）during shut-in and killing are investigated,Then, the characteristics of entire wellbore pressure with bit off bottom are described with Y- tube theory. Based on this theory, the moment at which the shut-in pressure is determined from shut-in pressure buildup curve is proposed,the density calculation formula of fluid in interval below the bit is de- rived, and the design method of killing fluid density is given. Actual calculation indicates that gas potential in the interval below bit is significant for acquiring the formation pressure and killing fluid density. So, the gas potential should be analyzed first in design of killing parameters.