下扬子页岩气地质调查井的溶洞识别与处理

Karst cave identification and treatment for a Lower Yangtze shale gas geological survey well

下扬子地区页岩气地质调查井皖南地1井在奥陶系宝塔组灰岩地层1223.54~1226.34 m井段钻遇了溶洞,根据地质、地震、钻井、录井、岩心、测井曲线等资料中的溶洞识别特征和规律,通过分析钻具放空,泥浆失返,钻时为0或极低,岩心缺失,电阻率明显降低,密度明显减小,声波、中子数值明显增大等溶洞特征,判定钻遇了半充填、沿井眼轴向长2.8 m的溶洞。基于以上判断,并结合实际工况,通过对比分析桥接堵漏、水泥堵漏、顶漏钻进与套管封隔、波纹管堵漏、凝胶堵漏和移井位等常见溶洞处理方法的特点和使用范围,优选了清水强钻至完整地层后下套管封隔的处理措施,保证了皖南地1井顺利完钻并实现钻探目的。

Well WND1,a Lower Yangtze shale gas geological survey well,was drilled into a karst cave in Ordovician Baota Formation limestone strata.It can be diagnosed that a semi‑filled cave with axial length of 2.8m along the borehole has been encountered by analyzing the cave identification features and law,such as drilling string fall‑down,lost return of mud,drilling‑time as low as 0 or extremely low,core missing,significant decrease in resistivity,obvious decrease in density,increase in acoustic and neutron values.Based on the above diagnosis and in view of with the actual working conditions,comparison of the characteristics and application range of common treatment methods for karst cave,such as bridge plugging,cement plugging,drilling with lost circulation and sealing off with casing,bellow pipe plugging,gel plugging,well location relocation,was conducted with forced drilling with fresh water and sealing off with casing selected to drill to the intact formation and then casing set for sealing,which ensured the smooth completion of Well WND1 and the realization of drilling objectives.