Intermittent Nozzle Fluid Flow and Its Application to Drilling
Authors: Ghalambor A. , Hayatdavoudi A. , Akgun F. , Okoye C.U. 
Published: December 2007

Summary.
The effect of intermittent bit nozzle fluid flow on chip removal is examined in drilling operations. Theoretical equations for a nozzle that would provide intermittent flow have been developed. The proposed modified design equations were used to calculate the impact-pressure gradients under the bit. The calculated values of the impact pressure for both regular and modified nozzles show that the modified nozzle provides a decrease in the impacted area under bit-jet pressure while the impact-pressure gradient is increased, resulting in improvement in chip removal from under the bit.

Introduction
This study examines the use of intermittent nozzle fluid flow to increase penetration rate (ROP) in drilling operations. Impact-pressure evaluations for a regular nozzle and for a nozzle with a rotating disk showed that the latter provides a better pressure profile for the purpose of chip removal. pressure profile for the purpose of chip removal. Impact-pressure profiles were obtained with McLean's calculation technique. Certain modifications, however, were required for a nozzle with a rotating disk. The modified nozzle does not have a constant diameter through which fluid flows. Therefore, variable diameters are inserted in the equations. The required fluid and nozzle data were obtained from published data. Results obtained in this work were compared with published data. Results obtained in this work were compared with published experimental works for a regular nozzle, and the match published experimental works for a regular nozzle, and the match was good. A heavier mud density was also used in the same equations to investigate the effects of different mud densities on the impact-pressure gradients. It was found that the higher the mud density, the greater the impact-pressure gradient. Higher ROP's are an effective means of reducing drilling cost. Minimum-cost ROP demands the best possible use of hydraulics and energy at the bit. As the drilling fluid flows through the bit nozzles across the hole bottom and upward to the surface, friction and turbulence cause a large amount of energy loss. Therefore, in any general attempt to reduce drilling costs, fluid flow at the hole bottom must be examined. There is no universal agreement about what the most important factors are at and below the bit. Attempts have been made to increase the ROP by optimizing bit weight, rotary speed, and drilling-fluid flow rate (hydraulics). Kendal and Goins studied bit-jet hydraulics. They outlined hydraulics programs for maximum jet velocity, maximum jet impact, and maximum jet horsepower. Van Lingen found that drilling rates increased if the bit nozzles were extended toward the bottom of the hole. Feenstra and van Leeuwen showed that for impermeable rock, increasing the jet velocity influences drilling rate more than increasing the flow rate. The last two studies were performed in a laboratory and were made with a full-size bit. Eckel used a microbit and low-permeability rock, and showed that the drilling rate is a function of the Reynolds number of the fluid involved in the flow rate, nozzle diameter, and fluid density and viscosity. All the studies mentioned suggest that jet velocity plays an important role in establishing the drilling rate. The "jet action" keeps the bit teeth and hole bottom clean and overcomes the "chip holddown effect." In a more basic study, McLean measured the impact pressure and crossflow on a simulated hole bottom under a jet bit, concluding that crossflow velocity was the more important factor. He determined the relationship between crossflow and vertical velocity distribution, kinetic energy flux, and shear stress, all of which have a significant effect on the cleaning of the hole bottom and the bit. Maurer and Myers showed that bottomhole pressure has an effect on drilling rate. They found that as the difference between mud pressure and formation pressure increases, the bit tooth forms a smaller crater. The net effect would appear to be a decrease in drilling rate as the overbalance pressure increases. During the jet-bit drilling operation, jet impingement produces two mechanisms to clean the bottom of a borehole, as shown produces two mechanisms to clean the bottom of a borehole, as shown in Fig. 1. One is an impact-pressure wave in the immediate area of jet impingement. The other is crossflow. The crossflow is the flow parallel to the hole bottom that results from the impingement of parallel to the hole bottom that results from the impingement of the jet on the surface of the hole bottom. This mechanism is similar to the cleaning of dirt or debris from a patio by a jet nozzle attached to a garden hose. The crossflow velocity actually transports the chips in many forms - e.g., tumbling, saltation, jumping, grinding. The magnitude of the crossflow, assuming the fluid is incompressible and the flow is in steady state, is proportional to the square root of the product of jet velocity proportional to the square root of the product of jet velocity (in vertical direction) and the total flow rate divided by the hole diameter: vc = K qv/dh. Considering the evidence presented so far, it is apparent that the fluid velocity through and beyond the nozzles and the pressure existing at and near the hole bottom both play important pressure existing at and near the hole bottom both play important roles on rock drillability. This study consists of simultaneous consideration of rock failure and nozzle fluid flow. Therefore, the improved nozzle takes into account the effect of nozzle fluid flow on rock failure.

Development of the Impact-Pressure Formulation
The properties of impact-pressure waves were investigated in the same model as McLean's crossflow. The pressure gradients in an impact-pressure wave are a good measure of the concentration and intensity of the pressure wave. These gradients appear to be a suitable measure of the cleaning capacity of the wave. McLean analytically and experimentally investigated the nature of the impact waves generated on an unrestricted flat surface by an impinging jet. As a jet leaves the nozzle, the edges of the jet will begin to mix with and entrain surrounding fluid. This causes a transfer of momentum from the jet to the surrounding fluid and accelerates the transfer while velocities in the jet are reduced. General features of this flow are shown in Fig. 2. The region of flow in which the velocity in the jet is unaffected by mixing and is still equal to the exit velocity at the nozzle is called the potential core. The velocity distribution in that portion of the jet that includes the potential core has been described, but means of predicting the length of the core have not been given. Reported lengths usually vary from about 4 to 18 times the diameter of the nozzle.
JPT
P. 1021