Abstract: Aiming at the engineering problem of wellbore instability in Permian coal-rock strata in Huainan mining area,
the coupling instability mechanism of ′coal-rock-drilling fluid-consolidation agent′ multiphase medium is studied,and the wellbore
stability control theory suitable for Permian broken soft coal roof is established. Through XRD,SEM and macro-micro mechanical
analysis,the instability essence of ′clay mineral enrichment (illite accounted for 28. 7%),micro-fracture network development
(connectivity rate of 65%),and significant stress sensitivity (confining pressure gradient of 25 MPa/ km)′ was revealed.
The mechanical model of ′borehole-surrounding rock-consolidation agent′ is constructed under the framework of elastic
mechanics. It is confirmed that the consolidation measures can reduce the concentrated stress of the borehole wall by 6. 0 MPa
(a decrease of 22%) and the radius of the plastic zone by 41%. Based on this,an engineering process of ′low-pressure progressive
injection + fine water long-flow glue addition′ was proposed. After field application in Xinxie-1L well,the hole enlargement
rate was reduced from 28% to 9%,and the problem of collapse and sticking was successfully solved. The research results
not only provide a complete theoretical support of ′mechanism-model-process′ for wellbore stability in Permian coal-bearing
strata,but also provide a replicable technical paradigm for drilling engineering in similar complex coal-bearing strata.