TABLE OF CONTENTS Formation pressures less than the static pressure of a column of fresh water or oil require the use of a lighter fluid, such as air, to be injected with mud to obtain less overbalance and underbalance to enhance ROP and minimize lost circulation. Among the many factors affecting ROP, overbalance, which is often defined as the pressure differential between borehole pressure and formation fluid pressure, is generally recognized as the most important one. 1-7 Aerated drilling has been used in recent years to improve drilling rates.8 ,9 Many operators have also turned to aerated drilling to maximize the productive potential of low-permeability reservoirs. This is because aerated mud minimizes formation damage because of less liquid filtration and particle invasion into the pay zone during drilling. Therefore, aerated liquid drilling has become a major category of underbalanced drilling in the petroleum industry.
Aerated liquid drilling is referred to as aerated mud drilling in the petroleum industry. An aerated mud is a three-phase system consisting of gas (normally air), liquid (normally tapwater or oil), and solid (cuttings). The water constitutes the continuous phase, with air appearing as discontinuous bubbles. When air is the continuous phase and water appears as discontinuous droplets, the system is referred to as mist only if the cuttings are very small.
The hydraulics of an aerated mud differ substantially from those of conventional mud due to air content. They also differ from pneumatics in air and mist drilling due to the existence...
