A comprehensive study on evaluating drainage capability of air reverse circulation down-the-hole hammer drill bits via numerical simulation and experimentation

The air down-the-hole (DTH) hammer reverse circulation drilling technique offers high drilling efficiency and preserves geothermal reservoirs, making it extensively used in geothermal well construction. However, because air is used as the circulation medium, this technique is highly sensitive to water production during formation. An excessive inflow rate of water can deteriorate the air circulation performance in a borehole, substantially limiting the applicability and progression of this technology. To assess the sensitivity of air DTH hammer reverse circulation drilling to the formation water inflow rate and to elucidate the mechanisms by which groundwater influences reverse circulation, we utilised computational fluid dynamics (CFD) multiphase modelling to conduct a series of simulations complemented by laboratory experiments. The results demonstrate that reverse circulation efficiency decreases significantly with increasing formation water inflow rate, with 0.2 kg/s identified as the critical threshold for effective drainage using 150-mm-diameter bits. However, elevating the air supply rate effectively improves drainage performance—increasing the air supply from 0.06 kg/s to 0.24 kg/s enhanced water outflow efficiency by about 82 % in this study. Furthermore, our research led to the optimization of conventional ejector-type reverse circulation drill bits, providing essential design principles for drill-bit structures. This study may provide valuable insights into the application of air down-the-hole hammer reverse circulation drilling technology in geothermal drilling and promote the research and development of high-performance drilling equipment.