Study On The Off-bottom Well Killing Method

Blowout is one of the serious accidents in the drilling process.Improper handling of blowout accidents can easily cause serious consequences.Timely and effective well control measures are an important guarantee to avoid the harm and heavy losses of blowout accidents.However,there are few researches on the off-bottom killing methods and related theoretical models,and there is a lack of analysis methods for the design of engineering parameters of the snubbing and killing.There are few calculation models and algorithms for transient off-bottom wellbore pressure of Y-type gas-liquid two-phase flow considering gas slippage,migration and expansion;at the same time,there is no off-bottom killing module in domestic well control design software;off-bottom well killing operation is mainly based on empirical analysis,which leads to low success rate of well killing.Therefore,it is of great significance to carry out the research on the off-bottom killing method.This paper starts with the stress state of drill string during off-bottom snubbing,establishes the comprehensive calculation model of the jacking force of drill string and the analysis model of the stability of drill string,reveals the variation law of the forced downward force and the safe maximum stroke under different working conditions,and obtains the analysis method of off-bottom killing well.Fluent software is used to simulate the gas-liquid two-phase flow during off-bottom dynamic killing.The development and transformation mechanism of gas-liquid flow pattern and gas migration characteristics are revealed.The transient wellbore pressure calculation model of off-bottom dynamic killing is established by coupling gas-liquid twophase flow and formation seepage,and the design method of hydraulic parameters of off-bottom dynamic killing is obtained.Based on the above results,the software module of off-bottom dynamic killing is developed by using programming language.The software module can provide guidance for the design and analysis of off-bottom dynamic killing parameters.The results show that the jacking force generated by the pressure in the wellbore increases with the increase of the wellhead casing pressure,and the required forced downward force increases.With the increase of the depth of the drill string running,and the required forced downward force decreases,the maximum safe stroke of the snubbing tool increases continuously,and finally becomes the rated maximum safe stroke of the tool.When the wellhead casing pressure is large and there is a risk of drill string jacking,reasonable measures should be taken to reduce the wellhead casing pressure or increase the forced downward force,and at the same time,the current equipment operation stroke should be reduced as much as possible.During the well killing period,under the impact and crushing of high liquid displacement,the gas in the well section near the bit can not gather and migrate upward,and the gas and liquid are mixed violently,forming local agitated flow or dispersed bubble flow.During off-bottom dynamic killing,the short-circuit circulation occurs in the section above the bit,and the gas is accelerated to migrate in the section above the bit;the increase of bit position depth is conducive to increase the circulating friction,reduce the gas invasion rate,and improve the circulating exhaust efficiency.At the same time,the effect of different kill parameters on bottomhole pressure regulation is more obvious,and the increase of bottomhole pressure is greater under the same kill rate.Killing displacement change has the most direct,rapid and effective regulating effect on bottom hole pressure.However,the changes of viscosity and density of killing fluid have hysteresis effect on bottom hole pressure.