Borehole Wall Instability Encountered in Drilling the Shawan Formation-Emeishan Basalt Formation in Southwest Sichuan: Mechanisms and Countermeasures

Severe borehole wall destabilization and pipe sticking caused by borehole wall collapse have long been encountered in drilling the Shawan formation-Ermeishan basalt formation in southwest Sichuan Province. A drilling fluid with high inhibitive capacity and plugging ability has been formulated based on the analyses of the mechanisms of borehole wall instability and field operations. In laboratory evaluation of the drilling fluid, the percent recovery of shale samples tested with the drilling fluid was 99.98%, the HTHP filter loss was 3.6 mL, the HTHP water filter loss was 4.8 mL, the HTHP filter loss tested on a sand bed was 4.4 mL. A plugging slug used with the drilling fluid and a technique of using the plugging slug was also designed. The formulation of the drilling fluid is:(0.5%-2.0%) bentonite + (0.5%-1.5%) PAC-LV + (5%-6%) SMP-3 + (2%-3%) JNJS-220 + (0.3%-1.5%) NaOH + (1%-3%) FT + (0.1%-0.3%) poly glycol + (0.4%-1%) CQ-SIA + (6%-8%) WND + (5%-10%) organic salt + 3% ultrafine calcium carbonate + barite. It was considered practical and feasible that mud loss control while drilling should be strengthened and the adhesive ability and viscoelasticity of mud cakes be improved. High quality mud cakes protect the borehole wall from being washed and keep drilling fluid out of borehole wall. This will mitigate secondary damage of borehole wall by downhole mechanical collision, drilling fluid washout and pressure surge.