Safe and Efficient Drilling Technologies for Shallow Karst Strata for Shale Gas Wells in Southeast Sichuan

In the northern part of the Dongsheng Anticline, located in the Southeast Sichuan Tectonic Belt of the Sichuan Basin, karst is developed in shallow strata, here karst caves have many levels, with thick fracture zones. In particular, drilling fluid is often lost during the drilling the strata above the 2nd member of Jialingjiang Formation, fracture zones of the borehole wall fall off seriously, which results in frequent downhole failures and abandonment of wells. In order to improve the drilling efficiency of shale gas wells, technical measures were taken, such as optimizing the casing program, improving the bottom hole assembly(BHA) by well sections, selecting drilling fluids by stages, and cementing the borehole wall with fracture zones by a combination method, which formed the safe and efficient drilling technologies for shallow karst strata for shale gas wells in southeast Sichuan, improving the drilling efficiency of shallow karst strata and shortened the drilling cycle. In addition, the technologies were applied in 10 wells in the northern block of the Dongsheng Anticline. As a result, the probability of downhole complications in well sections with shallow strata was reduced, and the well completion rate in shallow strata reached 100%. The rate of penetration (ROP) was increased by 25.0%, and the drilling cycle was decreased by 63.2%. Having safe and efficient drilling technologies for shallow karst strata for shale gas wells in Southeast Sichuan can provide technical support for efficient development of the block.