Application of New Hole Cleaning Optimization Method within All Ranges of Hole Inclinations
Authors: Mohammadsalehi Mehdi , Malekzadeh Nozar 
Published: March 2015

Abstract
During drilling operation, as inclination of well deviates from zero degree in vertical parts to 90 degree in horizontal and extended reach wells, hole cleaning and cutting removal condition imposes more influence on drilling performance and encountering associated problems with poor hole cleaning. Some models have been developed to predict the minimum flow rate required to clean the hole, such as Larsen's model, which works within inclinations from 55 to 90 degree. In this paper Larsen's model is combined with Moore's correlation, which is used to find the slip velocity of the cuttings in vertical wells (0 degree inclination) to find the best mud flow rate to have both good hole cleaning condition and achieve maximum drilling hydraulics. Calculations performed for a well prior to drilling and the results has been applied during drilling operation as modifications in flow rate and mud rheological properties. To demonstrate effectivity of results, parameters such as drilling torque, amount of returned cuttings on shaker regarding to rate of penetration, rate of penetration and occurrence of problems associated with poor hole cleaning has been observed and recorded both before and after applying optimization results. Comparison results show minimizing hole problems associated of settling the cuttings. This approach is valuable both for pre-drilling and on-site drilling operations, which will result more efficient drilling practices thus reduction of total cost of a drilled well.

Introduction
Good hole cleaning during drilling operations plays a major role in the operation performance. As the number of deviated and horizontal wells grow rapidly, this phenomenon needs more attention. Investigations and experiments about cutting removal have been performed mainly for vertical wells (zero degree inclination). However, with increasing the well inclination, due to cuttings settlement, a "bed" of solids is formed along the bottom and low side of the hole. Novel methods of prevention and controlling the formation of this cuttings bed have been proposed over the years such as the addition of special drilling additives (viscosities) that enhance the cutting transport ability of the drilling fluid. Unfortunately, these methods are not efficient in preventing the formation of cuttings bed and even cause other problems such as excessive equivalent circulation density.
Poor hole cleaning can result in pipe sticking, reduction in rate of penetration, excessive torque and drag, and generally increases the total cost of drilling projects and extends the expected project time. Many factors influence efficiency of cutting removal. Amongst them, two main parameters that can be controlled directly in drilling wellsite are mud flow rate (GPM) and drilling mud rhelogical properties. According to these facts, methods been introduced to find the minimum flow rate required to properly remove the cuttings from the well for a specific drilling condition and drilling fluid rheological properties. Larsen's model calculates the minimum flow rate for cuttings removal within well inclinations of 55 to 90 degrees. Moore's model is used to find the slip velocity of cuttings in vertical wells. In this study, based on combination of these two models, a new approach has been developed that predicts and calculates the minimum flow rate for optimum cuttings removal from 0 to 90 degrees of inclination. This approach is used in a directional well to correct the well cleaning program during drilling operation. To find out how efficient this method can do, some parameters such as amount of returned cutting, torque and drilling rate has been observed and presented in graphs both after applying and before changing the parameters. As the results show, by applying optimum flow rate cutting removal performance increases. Additionally because drilling hydraulics is considered in calculations, max bit horsepower or jet impact regarding to formation is achieved as well [1, 4].