Mechanism of Wellbore Instability and Drilling Fluid Technology Countermeasures in Naturally Fractured Formations Under High In Situ Stress

The piedmont area of the southwestern Tarim Basin is characterized by piedmont compression, high in situ stress, frequent wellbore instability, and prolonged drilling cycles. Revealing the mechanisms of wellbore instability and proposing corresponding drilling-fluid countermeasures are crucial for safe and efficient drilling in this area. Through the analysis of in situ stress, actual drilling conditions, mineral composition, and physicochemical and mechanical properties of rocks soaked in drilling fluid, the wellbore instability mechanism in high-stress complex formations in the piedmont area of the southwestern Tarim Basin was evaluated. This study demonstrates that the mechanism of wellbore instability is governed by both geological and engineering factors. Geological factors include high in situ stress from tectonic compression, formation fragmentation near fault zones, and weak rock bonding strength. Additionally, engineering factors involve the hydration and dispersion of formation rocks induced by drilling-fluid interactions, which may promote the propagation of existing fractures and microcracks. Based on the identified wellbore-instability mechanism, a targeted “4 + 2 + 1” drilling-fluid strategy was proposed, integrating strong inhibition, effective fracture plugging and filtration control, and reliable cuttings transport and suspension. Laboratory evaluation showed that the optimized anti-sloughing polysulfonate drilling fluid achieved a hot-rolling recovery of 88% and an HPHT plugging-test filtration loss of 5.2 mL, demonstrating strong inhibition and effective plugging and filtration-control performance. Field application in 11 wells further demonstrated the practical applicability of the proposed strategy under the reported geological and operational conditions. In Well YT1, the average wellbore enlargement rate in the Permian interval was approximately 5%, indicating improved wellbore stability and drilling efficiency.