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Abstract

An important function of drilling mud is the control of formation fluid pressure to prevent blowouts. The density of the mud must be raised at times to stabilize incompetent formations. Any substance that is denser than water and that does not adversely affect other properties of the mud can be added to raise the density to some extent. Obviously, the specific gravity of the weighting agent is of primary importance, especially in very heavy muds. The fractional volume occupied by the added solid is a major limiting factor in its use. In this work a simple-to-use correlation has been developed to predict the effect of the specific gravity of the weighting material and the density of mud on the solids concentration of weighted muds. Estimations are found to be in excellent agreement with the reliable data in the literature with average absolute deviation being less than 0.3%.

Introduction

Drilling fluids, also referred to as drilling mud, are added to the wellbore to facilitate the drilling process by suspending cuttings, controlling pressure, stabilizing exposed rock, providing buoyancy, and cooling and lubricating  (Ohara and Wojtanowicz 1995; Guo et al, 1996 ). Drilling fluids also help to control pressure in a well by offsetting the pressure of the hydrocarbons and the rock formations (Gallino et al 1999; Reinboth et al 2005). Weighing agents are added to the drilling fluids to increase its density and, therefore, its pressure on the walls of the well. Cost is important, but there are other practical restrictions on the weighting material to be selected (Al-Safran et al 2008; Emofurieta and Odeh 2010). The solubility of salts limits their range of usefulness, and there are other problems associated with the use of such systems (Hamed and Belhadri 2010). Various finely-ground solid materials have been used to successfully raise drilling mud density. Several factors in addition to chemical inertness and specific gravity affect the use of a substance as a weighting material. First, the substance should be available in large quantities (Abdou and Ahmaed 2010; Al-Bazali et al 2008). It should be easily ground to the preferred particle-size distribution, and relatively nonabrasive. It should also be moderate in cost, and not injurious or objectionable to the drilling crew or the surroundings (Gray and Darley, 1980).

In view of the above mentioned issues and the importance of drilling mud in petroleum engineering, it is necessary to develop an accurate and simple correlation which is easier than existing approaches, less complicated and with fewer computations for predicting the solids concentration of weighted mud (volume or weight percent) as a function of the specific gravity of the weighting material and the density of mud.