砾石层空气钻井井壁失稳机理研究及应用

RESEARCH ON MECHANISM OF WELLBORE INSTABILITY IN GRAVEL FORMATIONS DURING AIR DRILLING AND APPLICATION

塔里木盆地库车山前分布巨厚砾石层(1 000~5 000 m),采用常规方式钻井面临单只钻头进尺少,机械钻速低,建井周期长等问题。塔里木油田自引进空气钻井开展现场试验,取得了较好的提速效果,但也存在井壁失稳,井底沉砂较多,甚至发生卡钻的恶性事故。文章首先通过对传统井壁稳定模型分析,将孔隙压力的影响去掉,得到适合于空气钻条件下的井壁稳定模型。同时运用新的井壁稳定模型进行井壁失稳全过程分析,提出了沉砂主要是由于井壁形成坍塌椭圆时的坍塌物质造成的,坍塌椭圆的形成有利于保持井壁稳定,解答了现场沉砂量多且沉砂速度不同的疑问。为解决井壁失稳卡钻的问题,并基于以上研究成果结果得出空气钻适合于砾石分布均匀的小砾岩地层,对于大和中粒径砾石层,需考虑胶结强度对井壁稳定的影响的重要结论,为下步空气钻井井位优选提出了新的优选准则。

Extremely thick gravel formations( 1000 ~5000 m) are distributed in KucheP iedmont in Tarim Basin. The conventional drilling in this areafaces the problems of not much footage per drill bit,low ROP and long well construction periods. After Tarim Oilfield carried out on-site tests of air drilling,ROP improvement is obtained. However,there still are borehole instability,sand settling at the holebottom,and serioussticking accidents. First in this paper the analysis of the traditional wellbore stability model is madeand the influence of pore pressure is removed,and a wellbore stability model suitable for air drilling conditions is obtained. By this new wellbore stability model,the whole process of wellbore instability is studied,the bottom sands is found to be mainly caused by the collapse of the wellbore when the collapse ellipse is formed. The formation of the collapse ellipse is conducive to maintaining thewellbore stability,the large amount and differentvelocitiesof sand settling occurred is then understood. To solve the problem of borehole instability caused sticking,on basis of the above research results,new air drilling candidate selection criteria are proposed. The study shows air drilling is suitable for uniformly distributed small size gravelscontained formations. For the large and medium-sized gravel layers,the influence of cementation strength on the wellbore stability should be considered.