Experimental study on solid-free anti-sloughing drilling fluid suitable for deep core drilling projects

To deal with the problems of wellbore collapse, mud making and mud lining in drilling strings, a solid-free anti-collapse drilling fluid with polyvinyl alcohol as inhibitor was developed. The inhibition effect of commonly used inhibitors was studied by shale hot rolling recovery test, critical addition of bentonite test, and mud boulder soaking test. The types and dosage of viscosifiers and water loss reducers were determined by compatibility test, and the influence of different pollutants on the comprehensive properties of the formulated drilling fluid were compared. Experimental results showed that compared with the conventional inorganic salt such as NaCl and KCl, polyvinyl alcohol had a stronger inhibition effect which was positively correlated with polymerization degree. XC, SPNH and FT-1 could be used as viscosifier and water loss reducers to prepare the drilling fluid, respectively. The performance of formula drilling fluid under the pollution of 35%NaCl, 4% CaCl₂ and 12% calcium bentonite can be stable in the temperature of 110 ℃, indicating that the prepared drilling fluid can be used in the strong mud making strata, fractured strata, salt bed and other complex strata in the depth of 3 000 m.