Summary

Aerated mud drilling has been recognized as having many advantages over conventional mud drilling, such as higher penetration rate, less formation damage, minimized lost circulation, and lower drilling cost. In some areas, the use of aerated mud as a circulating medium for drilling oil and gas wells is becoming an attractive practice. Maintaining an optimum combination of liquid and air flow rates is important in aerated drilling operations. However, most drilling operators are unclear on what constitutes the "optimum combination of the liquid and air flow rates." Guo et al. presented a mathematical approach to determining the flowing bottomhole pressure (BHP) for aerated mud drilling. This paper addresses the use of Guo et al.'s mathematical model to determine liquid and air volume requirements considering wellbore stability, pipe sticking, and formation damage as well as the cuttings-carrying capacity of the aerated mud. From a formation-damage-prevention point of view, the liquid fraction in the fluid stream should be as low as possible. However, a sufficient mud flow rate is always required to make the hole stable and to maintain the cuttings-carrying capacity of the aerated mud without injecting much air volume. This paper provides a simple approach to determining the liquid and air volume requirements for aerated mud drilling.

Introduction

Drilling cost is considered one of the major components of operating cost in the petroleum industry. Improving the penetration rate of drilling and reducing drilling problems, such as pressure-differential pipe sticking and lost circulation, have long been considered effective ways of decreasing drilling costs. The overbalance pressure, generally recognized as the most important among the many factors affecting penetration rate, is often defined as the pressure differential between the borehole pressure and formation fluid pressure. Formation pressures lower than the static pressure of a column of fresh water require the use of a lighter fluid, such as air, injected with liquid to obtain lower overbalance pressure to enhance penetration rate and to minimize lost circulation and pipe sticking as well as formation damage. Therefore, aerated mud drilling is becoming an attractive practice in some areas.

The commercial use of aerated mud drilling began only in recent years. Many of the problems associated with aerated mud drilling still have to be solved. One problem is the determination of air and liquid volume requirements. The objective of this paper is to provide a simple approach for determining the optimum combination of liquid and air injection rates for aerated mud drilling operations.

Aerated mud is defined as a fluid consisting of liquid (usually water), air, and drill cuttings. It can be shown that the theories on gas/solid two-phase flow developed for air and mist drilling and theories on liquid/gas two-phase flow developed for oil production through tubing are inaccurate when applied to aerated mud flow in an annular wellbore. Therefore, a mathematical model for describing three-phase (air, water, and cuttings) flow in a wellbore has been proposed by Guo et al. The modeling was validated by a comparison with field data obtained from aerated mud drilling. This paper uses the developed model to determine the liquid and air volume requirements considering cuttings-carrying capacity of the aerated mud, wellbore stability, pressure-differential pipe sticking, and formation damage.

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