Fuzzy-ball drilling fluid technology in CBM of multilateral horizontal well Qinping 12-11-3H with six branch holes

The No. 3 coalbed in Qinshui Basin is high rank coal seam, the drilling of coalbed methane encountered rockfall, collapsing, lost circulation, sticking and the rate of successful drilling was very low. The third spudding of Qinping 12-11-3H Well was a multi-lateral horizontal well consisting of two main branches and six branches; the fuzzy-ball drilling fluid was applied in 13 days and drilled 4 189.49 m. According to the different downhole situations and solid-control equipment, four kinds of fuzzy-ball drilling fluid additives: core-forming agent, film-forming agent, layer-forming agent and fuzzy-forming agent were added to adjust the drilling fluid properties. The drilling fluid density was 0.96-1.08 g/cm3, plastic viscosity was 7-17 mPa·s, dynamic shear force was 4.0-10.22 Pa, and coalbed methane encountering rate was 95%. The drilling fluid met the geological requirements and the average ROP was 12.65 m/h, which had improved by 11.55% when compared with adjacent wells. This drilling tested the capacities of the fuzzy-ball drilling fluid, including the accommodating capacity of high percentage of solid phase , plugging capacity in leaking formations, processing capability for well collapsing and pipe sticking, and the maintaining performance capacity during gas invasion. It is suggested that auxiliary solid-control equipment is needed to further improve the performance of the drilling fluid.