Structural design and flow field characteristics analysis of a new spin-on multi-hole jet drill bit for radial horizontal wells

Abstract

In order to improve the rock breaking and drilling capacity of the jet bit in radial drilling, a new spin-type multi-hole jet bit was designed. The RNG K-ε turbulence model is used to analyze the flow characteristics of the internal and external flow fields of the designed jet bit, and the influence of the rotational speed and nozzle distribution on the flow field characteristics of the jet bit is analyzed to provide a basis for the design and structural parameter optimisation of the jet bit in radial horizontal wells. The results show that the internal flow field of the jet bit can be divided into four zones: the forward jet zone, the bottomhole overflow zone, the reverse overflow zone and the low-speed reverse flow zone. The reverse jet consists of six straight jets uniformly distributed along the circumferential direction and arranged at equal angles around the axis of the rotating body in the flow field. The reverse jet also impinges on the well wall to form an overflow zone, which provides self-propulsion and acts as a reaming agent. As the injection distance increases, the axial velocity rapidly decreases to zero and the decay velocity of the three groups of jets gradually approaches at the impact wall. Rotational speed has an effect on the decay of the flow velocity at the centre of the forward jet axis. When the angle of inclination is very small, the rotational speed has almost no effect on the decay of the axial velocity of the jet. The greater the angle of inclination, the greater the linear angular velocity at the jet exit, the more the jet mixes with the surrounding crossflow, the faster the dissipation of jet energy and the faster the jet decays along the axis. Controlling the magnitude of the rotational speed has to be taken into account in the design. Initially, the optimum values of the different structural parameters were determined as the central nozzle coinciding with the drill axis, the central forward nozzle deflection angle of 20°, the external nozzle deflection angle of 20° and the external nozzle tension angle of 15°.