Experimental study and application of anti-collapse drilling fluid for directional long boreholes passing through soft coal (rock) strata

Aiming at the problems of borehole wall instability and hole collapse caused by unfavorable geological conditions during directional drilling in soft coal (rock) strata in underground coal mines, an optimization study of drilling fluid formulation was carried out to ensure borehole engineering quality and mine safety production. Taking No. 2 coal seam in the fifth mining district of Yaojie No. 3 Mine in Gansu as the research object, experimental research on drilling fluid formulation parameters was conducted, and suitable formulation parameters for this coal mine were selected, so as to improve the inhibitive property of the drilling fluid and achieve hole protection for directional long boreholes in soft coal (rock) strata. The experiments show that the drilling fluid with a formulation of 3% KCl, 0.3% coal-rock drill cuttings, 2% inhibitor, and 94.7% water has the best test effect. During trial drilling in the roof mudstone stratum of No. 2 coal seam in Yaojie No. 3 Mine, no borehole collapse occurred. The selected drilling fluid formulation parameters exhibited good hole protection performance in the trial in the roof mudstone stratum of No. 2 coal seam in Yaojie No. 3 Mine, Gansu, and have practical application value.