Analysis on Wellbore Instability Mechanism of Jurassic Formation and Plugging Anti-collapse Drilling Fluid Technology

Well Qiantan-1 is a wildcat well located at the Niudong structure in the Qaidam Basin. To address the wellbore instability challenge in the Jurassic formation it encountered, a systematic study was conducted, including geological characteristic characterization, instability mechanism analysis, development of plugging and anti-sloughing drilling fluid, and its field application, by comprehensively adopting methods such as core observation, scanning electron microscopy (SEM), mineral analysis, mechanical testing, numerical simulation and field tests. The results show that the dominant mechanism of wellbore instability in this formation is the mechanical sliding of high-steep fractures combined with spalling of hard and brittle rocks, with insufficient thermal stability and poor plugging and inhibition performance of drilling fluid being the key engineering inducing factors. The developed temperature-resistant, plugging and inhibition synergistic drilling fluid system realizes the performance indicators of HTHP fluid loss ≤12 mL, filter cake thickness ≤2 mm and core rolling recovery rate ≥98% at a high temperature of 180 ℃ through optimizing temperature-resistant materials, compounding multi-level plugging agents and strengthening the inhibition system. Field application of this system has reduced the occurrence rate of wellbore collapse in the fifth spud by 80%, increased the rate of penetration (ROP) by 52%, and controlled the wellbore enlargement rate within 8%. The established instability criteria and prevention & control technology for hard and brittle formations with high-steep fractures provide a theoretical basis and engineering example for drilling in similar formations, and possess important on-site guiding value.