Aerated Drilling is a technique whereby gas is injected into a column of unweighted drilling fluid (water-, oil-, or synthetic based) to reduce its specific gravity (s.g.), to as low as 0.6. If there is a chance that hydrocarbons (oil, gas) may enter the wellbore then the gas used is usually membrane nitrogen or sometimes even natural gas from a nearby pipeline or wells. If no hydrocarbons are to be expected, such as on geothermal wells, then air can be used. The gas can be injected in a number of ways, namely:
Standpipe Injection is the most common and simplest way of aerating the drilling fluid, by injecting compressed gas into the rig's standpipe at desired gas/fluid ratios at or above surface injection pressures. The air will combine with the drilling fluid and yield a (gas/cut) drilling medium with the air phase compressing through the drill string and gradually expanding as it travels up the annulus, improving velocities and lightening the hydrostatic pressure.
Parasite String Injection is done by running small-diameter tubing on the outside of the intermediate casing to a pre-determined depth, where it is ported into the wellbore. The gas is then injected through the tubing and into the wellbore. The depth of the injection port is determined by the maximum pressure reduction anticipated to avoid lost circulation. The total pressure reduction being used is a function of tubing depth, air mud ratio and mud weight.
Concentric String Injection is similar to the above procedure of a gas lift of the fluid column by injecting gas into the annulus at a given depth. However in this case a liner is run in the well to provide a secondary annulus between the drillpipe and the existing casing, thus allowing injection via this inner cavity.