RESEARCH ON THE WELLBORE INSTABILITY MECHANISM OF AIR DRILLING TECHNOLOGY IN CONGLOMERATE FORMATION

摘要

There are huge gravel beds (1000-5000m) in front of Kuqa Mountain in Tarim Basin. Using conventional drilling methods, the drill bit footage is short, the rate of penetration is slow, and the whole construction period is long. The performance has been significantly improved by introducing air drilling. However, there are still some serious problems during air drilling, such as wellbore instability, more bottom-hole thick sand sinking in the bottom hole, and sticking. Air drilling does not need to consider the influence of pt)rc pressure. Based on traditional wellbore stability analysis model,a wellbore stability model suitable for air drilling is established. After the analysis of the whole process of instability by using the new wellbore stability model, it is pointed out that the sand sinking is mainly caused by the collapse material when the wellbore collapses into an ellipse. The formation of the collapse ellipse contributes to maintain the wellbore stability and resolving the doubts of the large amount of sand and the different speed of sand sinking in the field. Based on the above research results, it is concluded that air drilling is suitable for small conglomerate strata with uniform gravel distribution. For large and medium size gravel strata, the influence of cementation strength on wellbore stability should be considered. Our research proposed a new optimi7i)tion criterion is proposed for tlic next optimization of air drilling well location.