加蓬G 区块破碎地层井壁失稳机制与钻井液技术对策

 加蓬G区块是中石化海外重点勘探开发区块，LPC和ANG复杂地层钻井施工中出现了严重的井壁失稳问题。通过破碎地层的泥页岩岩心表观特征分析、泥页岩钻屑清水浸泡试验、泥页岩黏土矿物分析和泥页岩滚动回收试验得出，以伊/蒙混层和伊利石为主的黏土矿物组分和微裂隙发育特征是加蓬G区块破碎地层井壁失稳的外在物质条件；钻井液滤液沿裂隙和微裂隙进入地层深部，使黏土矿物水化膨胀，使井壁失去平衡，是导致坍塌掉块的内在原因。制定了钻井液密度应力防塌、屏蔽封堵防塌、优质钻井液体系配方等综合防塌技术措施，在加蓬G区块进行了现场试验，G-9井和G-10井2口井LPC和ANG地层的平均井径扩大率分别降低到11.6%和12.1%，井身质量较邻井有显著的提高，取得了良好的井壁稳定效果。

Wellbore Instability Mechanism of Broken Formations and Drilling Fluid Countermeasures in G Block,Gabon

The block in Gabon is an important exploration and development block of Sinopec overseas. Wellbore instability existed in LPC and ANG formations during drilling operations. Through the analysis of shale core apparent characteristics in broken formations, shale cuttings water immersion test, shale and shale clay mineral analysis, and rolling recycling test, it is shown that the Illite / Smectite mixed layers, Illite clay mineral components and micro-fissures are the material conditions of the instability mechanism of broken formation of the G block Gabon. On the other hand, mud filtrates along fractures and fissures deep into the formation to cause hydration swelling of clay minerals and borehole loss of balance, which is the inherent mechanism leading to the collapse of blocks. Anti-sloughing drilling fluid density stress shielding comprehensive technical measures and drilling fluid system and quality recipes were developed. Field tests were conducted in G-9 and G-10 wells in Gabon. The LPC and ANG formation borehole calipers were reduced to 11.6% and 12.1% , respectively, as compared to the neighboring wells. The wellbore quality has been improved significantly, and good wellbore stability has been achieved.