Mechanism of wellbore instability and geomechanical control technology in strong Squeeze tectonic Zones—a case study of the Wuqia Tectonic Belt in Southwest Tarim

Abstract

Aiming at the challenges of frequent wellbore instability and the failure of conventional anti-sloughing technologies in the Wuqia tectonic belt of Southwest Tarim, which are caused by strong compression, high-steep structures, and formation fragmentation, this paper proposes a set of wellbore stability evaluation and geomechanical control technologies suitable for strong squeeze and fractured formations. The research focuses on analyzing innovative breakthroughs in core technologies such as multi-field coupling theory, deep rock mechanics characteristics research, and 3D geomechanical modeling. Based on the multi-field coupling theory, a basic framework for the interaction of stress, seepage, and chemical fields was established. Point load strength tests were conducted to analyze the strength characteristics of deep mudstone cavings. Through pre-drilling data such as logging data, a 3D geomechanical model of the study area was constructed to characterize its in-situ stress field with high precision. Simultaneously, based on instability indices, the mechanism of wellbore instability was analyzed. The mechanism analysis revealed that wellbore instability in this area is mainly controlled by high horizontal in-situ stress difference, strong stress concentration, and fracture development. Specifically, as the stress concentration factor and stress difference increase, the instability risk of traditional vertical wells is significantly higher than that of deviated wells with specific azimuths. Moreover, wellbore stability is highly sensitive to well trajectory (inclination and azimuth) and fracture occurrence. Furthermore, mudstone hydration leads to the decay of rock strength parameters over time, resulting in a significant narrowing of the safe drilling fluid density window as the drilling cycle extends. Based on this, a ‘Geology-Engineering’ integrated trajectory optimization decision-making method was proposed. Engineering practice shows that by sidetracking in Well Atu-105 and avoiding high-risk stress zones, the drilling problems in strong squeeze fractured formations were effectively solved. This research realizes the transformation from empirical drilling to data-driven scientific drilling, providing theoretical support for the safe exploration of complex structural oil and gas reservoirs.