Study on Structure Parameters of Reverse Circulation Drill Bit Secondary Injector Device Based on Injector Coefficient

Abstract The hollow-through DTH hammer reverse circulation continuous coring drilling technology has been successfully applied to well drilling. During its application, reverse circulation drilling sometimes does not work in broken formation or under leakage formation condition. Difficulty in sampling is another problem. These problems are primarily caused by the injected gas. It comes into the fissures and not drives the reverse circulation, thereby failing to generate an adequate pressure at the hole bottom to deliver the cuttings into the central channel. The reverse circulation bit is the pivotal factor in the formation of reverse circulation. To improve the reverse circulation effect, this paper proposes that a secondary injector device be set at the central hole of the original reverse circulation bit. The injector device used for the hollow-through DTH reverse circulation bit is a gas injector designed with the gas injecting principle. Based on the structure principle of the injector and the actual structure of the DTH reverse circulation bit, it is used the bottom nozzle of the bit as injector nozzle and add a pressure diffusion slot, so a multi-nozzle injector structure at the bottom of the bit is formed. The secondary injector device can effectively improve the reverse circulation effect of the hollow-through down the hole (DTH) hammer drilling technology. The influence of the four structural parameters, namely, secondary injector hole diameter d, center hole diameter D, injector hole angle θ, and distance h from the injector hole to the reverse circulation drill bit bottom, on the injector coefficient n was studied by computational fluid dynamics (CFD) simulation to determine the structure size. The CFD results were verified by experimental method. The results showed that the reverse circulation effect changed with the changes in the secondary injector device structure parameters d, D, θ, and h. A combination of structure parameters allowed the drill bit to achieve the best reverse circulation effect. Considering the effects of the four parameters, d=11 mm, D=44 mm, θ=30°, and h=180 mm were the more rational parameters. Moreover, laboratory experiments were conducted to verify the CFD simulation results. The simulation results that showed the variation of n with different structure parameters were compared with the experimental test results. The comparison showed an error of less than 15% between both results, further verifying the rationality of the structure parameters. The hollow-through DTH hammer-based reverse circulation continuous coring drilling technology is featured by reverse circulation of fluid medium through the entire hole and continuous upward conveyance of the cores during drilling. This paper studies the structural parameters of the secondary injector device of the reverse circulation drilling bit in order to improve the rate of core recovery.