A simulation and experimental study of bottomhole flow field in a large-diameter rescue borehole

Objective In mine rescue, reaming using a large-diameter down-the-hole (DTH) hammer faces is challenged by complex bottomhole flow fields and difficult slag discharge. This study aims to address these challenges.

Methods To reveal the influence patterns of drilling parameters on the bottomhole flow field and slag discharge efficiency, this study determined the optimal drilling parameter combination. Then, using the computational fluid dynamics (CFD)-discrete phase model (DPM) method, this study established a numerical model for the slag discharge and bottomhole flow field during the reverse circulation drilling using a DTH hammer. Relying on this model, this study analyzed the variation patterns of the flow velocity distribution, cutting transport, and slag removal rate of the bottomhole flow field under a rate of penetration (ROP) of 3 m/h. Furthermore, this study developed a simulation device of bottomhole multi-phase flow for reverse circulation drilling of a large-diameter rescue borehole. This device allows for the adjustment of various drilling parameters and the maintenance of records of the transport trajectory of cuttings at the bottomhole. Using this device and simulated cuttings, this study conducted experiments on bottomhole two-phase flow during reverse circulation drilling and, accordingly, investigated the influence patterns of the drilling fluid flow rate and drill bit’s rotational speed on the flow field. Additionally, orthogonal experiments involving three values of the drilling fluid flow rate and drill bit’s rotational speed were designed based on the experimentally validated numerical model.

Results The results indicate that with an increase in the drilling fluid flow rate, the slag removal rate increased, while the increasing magnitude diminished. With an increase in the drill bit’s rotational speed, the slag removal rate increased initially and then decreased, peaking at 15 r/min. The orthogonal experiments indicate that the highest slag discharge performance was achieved under a relatively high drilling fluid flow rate and a moderate rotational speed of the drill bit. Under ROPs ranging from 3 m/h to 6.2 m/h, the drilling parameter combination that contributed to the highest slag discharge efficiency comprised a drilling fluid flow rate of 216 m³/h and a rotational speed of the drill bit of 20 r/min.

Conclusions The results of this study provide a theoretical reference for selecting rational drilling parameters for large-diameter rescue wells, assisting in enhancing the borehole-forming efficiency of drilling rescue engineering.