Bit Hydraulics Analysis for Efficient Hole Cleaning
Authors: Lim Kien Ming , Chukwu G.A. 
Published: December 2007

Abstract
Hydraulic horsepower and jet impact force are two criteria used to optimize drilling hydraulics. Ah optimized drilling hydraulics program can lead to good bottom hole cleaning, resulting to increase in penetration rate. Experiments on bottom hole cleaning with a three nozzle bit and a two nozzle bit were performed using an in house flow loop drilling simulator for vertical or near vertical wellbores to investigate the pressure and flow parameters that contribute to efficient borehole cleaning. A comparative analysis of hydraulic horsepower (HHP) and jet impact force (JIF) was made to study the impact of each force on hole cleaning.
Experimental results show that increase in drillstring rotational speed reduces apparent slip velocity but increases drag coefficient on the transported cuttings. Optimized values of the ratio of the pressure loss across the bit nozzles to the circulating pressure loss for both the hydraulic horsepower and jet impact force are presented in this work as new and important criteria for analyzing drilling hydraulics.

Introduction
The ability of the drilling fluid to effectively transport cuttings from the bottom of the hole depends upon the hydraulics of the system and the properties of the drilling fluid. As bottomhole cleaning is the major factor causing the decrease in drilling rate with depth, the proper use of hydraulic practices can eliminate such a drilling problem. Figure 1 gives a clear view of the concept of the hydraulic principle. The essence of a good hydraulic system is to provide adequate impingement to hydraulic power at the formation. As soon as the bit teeth crater and fracture the rock, the bit nozzle will provide jet force to transport the debris of cuttings to the surface. Insufficient hydraulic power will lead to regrinding the cuttings in the bottomhole instead of transporting them to the surface. This hole cleaning deficiency can cause accumulation of cuttings in the bottomhole and consequently impede the rate of penetration.
Drilling hydraulics is associated with the related effect of drilling fluid viscosity and flow rate, drill pipe rotational speed, and system pressure losses. Bit hydraulics is related to the effects of nozzle sizes, number of nozzles, the jet velocity of drilling fluid passing through the bit nozzle, and the pressure loss across the bit. Optimized hydraulics reduces the overall drilling cost. Two criteria that are commonly used to analyze bit hydraulics for hole cleaning are the Hydraulics Horsepower (HHP) and Jet Impact Force (JIF). However, today there is still no agreement on which one of these criteria provides a better analysis for hole cleaning.
The present study was performed in two stages. The first stage was to experimentally investigate the rate at which cuttings are transported in a vertical annulus at different simulated borehole and drilling fluid conditions. The experiment was performed in a simulated subsurface borehole environment which comprised of a flow loop with rotating pipe and varying nozzle sizes. The drilling fluid used was a transparent clay-free KELZAN XC biopolymer, designed to permit visual observation of cuttings movement.
The second stage of the study was to analyze the bit hydraulics using the HHP and JIF. HHP is the rate of work done by a fluid at a given pressure.
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