Abstract: In order to solve the problem of exfoliation and collapse of pyroclastic rocks during drilling, the authors studied the effect of drilling fluid performance on the disintegration of pyroclastic rocks. The samples of pyroclastic rocks in the Akechukesai mining area of Qinghai were taken to analyze the mineral composition of rocks, and the sample drying and soaking cycle experiments were carried out. The relationship between the main parameters of the original 1^# and 2^# drilling fluids and the slake durability index was analyzed by multiple linear regression analysis. It shows that the correlation between the dynamic performance ratio, static shear force, water loss, funnel viscosity of the drilling fluid and the disintegration of pyroclastic rocks can reach 83.37%. The effect of dynamic plastic ratio on the collapsing of pyroclastic rocks is particularly significant. By adjusting the mass fraction of hydrolyzed polyacrylamide and lignite resin to improve the dynamic fluid ratio and stability of the drilling fluid, the drilling fluid formulation is optimized as:water +4% bentonite +3% sodium carbonate +0.1% sodium hydroxide +0.15% medium viscosity sodium carboxymethyl cellulose +0.5% low viscosity sodium carboxymethyl cellulose +2% lignite resin + 0.08% hydrolyzed polyacrylamide. The field application results show that when drilling to the depth of 429.00 m, the thickness of residual cuttings at the bottom of the hole with the optimized drilling fluid is 89.7% lower than that with the 1^# drilling fluid, and 80.3% lower than that with the 2^# drilling fluid.

Key words: Akechukesai, pyroclastic rock, slake durability index, drilling fluid properties