Key core drilling technologies for complex strata on the eastern margin of the Xuefeng orogenic belt

Abstract:

The Taojiang area on the eastern side of the Xuefeng orogenic belt has been subjected to multi-stage tectonic superposition， resulting in fractured strata， interbedded soft and hard rocks， and well-developed fractures. Core drilling in this area faces prominent challenges， including borehole wall instability， severe drilling fluid loss， and difficult core recovery， necessitating the establishment of a systematic drilling technology system tailored to this region. Taking three typical boreholes （ZK0001， ZK1901， and ZK2402） in the study area as engineering cases， the lithological characteristics of the drilled formations and complex structural conditions were systematically analyzed. To address various downhole complexities， an integrated suite of techniques was applied， including serialized bit optimization， dynamic optimization of the three-stage borehole structure， improvement of coring tools for fractured formations， and adjustment of drilling parameters. Furthermore， a graded lost circulation control system based on leakage rate classification was established， featuring "loss prevention while drilling， bridging plugging， and composite gel plugging." Engineering practice demonstrates that： After optimizing impregnated diamond bits， the effective footage per bit in gravel-bearing sandstone intervals increased to 80~150 m； using side-jet bits combined with low weight-on-bit and low rotary speed drilling parameters， the core recovery rate in fractured manganese-bearing ore layers consistently exceeded 95% and locally reached 100%； the plugging system efficiently handled various levels of lost circulation with leakage rates ranging from 3 to 11 m3/h. In borehole ZK2402， weighted drilling fluid was employed for managed-pressure drilling in the confined water-influx zone， and the casing was permanently retained after completion， thereby achieving "one hole for multiple purposes" and transforming the exploration borehole into a rural water supply facility. This study establishes a key technology system for efficient and safe core drilling adaptable to the complex strata on the eastern side of the Xuefeng orogenic belt， providing a technical reference for mineral exploration drilling projects in similar complex intracontinental orogenic structural zones.