Optimization Of Jet-Bit Hydraulics Using Impact Pressure
Authors: Smalling David A. , Key Thomas A. 
Published: December 2007

Bit hydraulic horsepower, impact force, and nozzle velocity are common criteria for optimizing jet-bit hydraulics. In this paper, a new improved criterion and technique based on maximum impact pressure is proposed. A method for quantitative evaluation of bit hydraulics is introduced also.

Impact pressure is the momentum change of the drilling fluid distributed over the impacted area at the hole bottom. This pressure, acting downward and to the sides, attempts to overcome chip hold-down pressure and remove cutting from beneath the bit. pressure and remove cutting from beneath the bit. Maximum impact pressure occurs when the pressure drop at the bits is from 45 to 80% of the pump pressure. The percent pressure drop at the bit for maximum impact pressure is dependent on the flow behavior of the drilling fluid and the nozzle area, number, and extension.

Past experience has shown that the use of extended or blanked-off nozzles improve penetration rates. Application of the concept of impact pressure quantitatively shows the improvement in bit hydraulics with extended or banked-off nozzles over bits with] conventional nozzle configurations. A comparison of impact pressure to differential pressure also can lead to relationship between bit hydraulics and drill rate.

Introduction

One of the basic features of drilling fluid is to remove the drilled cuttings from beneath the bit. The fluid pumped through the bit nozzles exist at high velocity and impacts the hole bottom. After impact, a crossflow develops that forces cuttings away from hole bottom and up the annulus. Penetration rate and overall drilling cost depend greatly on the cleaning action of the drilling fluid.

Kendall and Goins developed techniques for optimizing bit hydraulics based on three criteria: hydraulic horsepower, impact force, and bit velocity at the bit. Evaluation by hydraulic horsepower is based on the power expended as the fluid flows through the nozzles. The parameter optimized by the impact force method is the force produced by the momentum change after the fluid exists the nozzles and strikes the bottom. Jet velocity is based on the exit velocity from the nozzles. Few, if any, improvements have been made to these criteria since they were first presented.

Each of the above criteria is based on some parameter of the fluid calculated at the bit parameter of the fluid calculated at the bit nozzles. However, actual dislodging of the cuttings occurs on the hole bottom, a few inches beneath the bit nozzles. Therefore, the critical concern is not the fluid's action at the nozzles, but its action as it strikes the hole bottom.

In this paper we describe a new method for optimizing it hydraulics. It is based on maximizing the drilling fluid's impact pressure against the hole bottom. Impact pressure is the impact force distributed over the contact area, that area which the fluid initially strikes on the hole bottom. The impact force is that produced by the fluid's momentum change as it strikes the hole bottom.

Nozzle configuration appears to have an effect on penetration rate. Several authors have described improved drill rates with extended or blanked nozzle bits. However, presently used criteria have been unable to account for these improved drill rates. Outmans, in fact, has suggested a different optimum may exist for each nozzle size.

These increased penetration rates may be explained by the effect blanked or extended nozzles have on impact pressure. Impact pressure is a function of the bottomhole contact area of the fluid. With all other factors constant, blanking or extending nozzles reduces contact area. The reduced area increases the impact pressure, which, in turn, improves the bottom-hole cleaning.

As drilling fluid flows through the nozzles its streamline patters diffuse outward.