Aerated or Nitrified Fluids
Aerated Drilling is a technique whereby air or Nitrogen is injected into an unweighted drilling fluid whether it be water based fluids, oil based, or synthetic based fluids to reduce its hydrostatic head. The injection air or Nitrogen can reduce the total hydrostatic head to as low as 0.6 specific gravity. Nitrogen is used if there is any chance that hydrocarbons may be present to minimize the chance of a down hole fire or explosion. Natural gas can be used if feasible as in the case of gas being available from a nearby pipeline or wells. If no hydrocarbons are to be expected, such as on geothermal wells, then air can be used. The following are the different ways air, nitrogen or natural gas are used to reduce the hydrostatic head of the drilling fluid.
- Standpipe Injection is the most common and simplest way of aerating the drilling fluid. Compressed gas (Air, Nitrogen, or Natural Gas) is injected into the rig's standpipe at desired gas/fluid ratios at or above surface injection pressures. Advantages of this technique is that the reduction in Hydrostatic head is gained over entire vertical depth. The Wellbore does not have to be specifically designed for underbalanced condition. Less gas is required to achieve a given pressure compared to parasite injection, and Penetration rates may be improved. Disadvantages of this technique include: (1) an overbalanced condition may occur if the well is shut down and (2) exotic MWD systems are required.
- Parasite String Injection is done by running small-diameter tubing on the outside of the intermediate casing to a pre-determined depth, where the gas is then injected through the tubing and into the wellbore. Advantages of this technique, There are no operational differences, Constant bottom hole pressure is achieved, and Standard MWD equipment can be used. Disadvantages of this technique include: (1) additional costs are incurred, (2) additional time is required, and (3) larger diameter surface casing is required.
- Concentric String Injection is similar to the above procedure of a parasite string injection 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. The advantages and disadvantages are the same as the Use of a Parasite string.