Experimental and Numerical Study on Air Flow Behavior for a Novel Retractable Reverse Circulation Drill Bit of Casing-while-Drilling (CwD)

Bo Qi,José Luis Pastor,Zhichuan Zheng,Pinlu Cao,Mengke Wang,Wenbo He,He Yang,Baoyi Chen,Kun Bo

doi:10.1155/2021/3586572

Abstract

A reverse circulation Down-The-Hole (DTH) hammer drill bit in Casing-while-Drilling (CwD) processes is designed and applied to drilling under complicated formation. The drill bit is a special retractable drill bit with an exclusive reverse circulation gas channel. Using numerical simulations and experiments, the influence of the gas channel structure parameters of the drill bit, including the inner jet nozzles, flushing nozzles, suction channel, and other parameters, on its reverse circulation performance is analyzed, and the optimal gas channel structure parameters of the drill bit are determined to improve the reverse circulation effect. The results show that the flushing nozzles and inner jet nozzles have an important influence on entrainment performance. The entrainment rate η decreases as the flushing nozzle diameter increases and decreases as the inner jet nozzle diameter increases. An increase in the suction channel diameter can improve the reverse circulation effect of the drill bit. The spiral slot drill bit is more conducive to air being sucked into the central channel in the form of spiral flow, so it can improve the entrainment performance. The entrainment rate η can reach 23.4% with the optimum structured drill bit.

Highlights

Under complicated formations, wellbore instability is often accompanied by many kinds of borehole accidents, which cause economic losses and waste time
This paper introduces a new type of retractable DTH hammer drill bit of CwD that can form strong reverse circulation
Regarding the important influence of the structural parameters on the reverse circulation, this paper focuses on the research and analysis of inner jet nozzles, flushing nozzles, and suction channels using experiments and numerical simulations

Summary

Introduction

Under complicated formations, wellbore instability is often accompanied by many kinds of borehole accidents, which cause economic losses and waste time. The key to designing this new drilling system is to design a reliable drill bit that needs to have a good cutting bearing capacity and needs to be able to expand and contract freely at any time and form strong air reverse circulation so as to prevent annular air leakage between the casing and wellbore, as discussed elsewhere [19,20,21]. After the lower drill bit is completely opened, the central channel and the flushing nozzles of the upper and lower drill bits are connected, and the compressed air exhausted from the DTH hammer enters the retractable bit (see Figure 3). Due to the special gas channel structure of the drill bit, the additional air will be further sucked from the formation and the annular space between the wellbore and the drill bit. In order to study the influence of the gas channel structure parameters on bit reverse circulation performance, the influence of one parameter on the bit performance is considered, and the other parameters remain unchanged at their initial or optimal values

Numerical Solution

Experimental Measurements

Results and Discussion

Conclusions