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

Abstract:

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.