Aerated mud is now a proven technique for drilling lost circulation zones in ADCO's fields. Severe lost circulation problems encountered while drilling surface aquifer formations were traditionally cured by using oil to lighten water based mud. Aerated drilling is one method that eliminates any environmental impact of discharging oil to the desert. As a pilot, four wells were drilled in Abu Dhabi Onshore field utilizing aerated mud. All proved to be operationally successful achieving the goal of drilling without using crude oil to lighten the mud. The extra cost of the air compressors was offset by eliminating crude oil and waste treatment cost. Air drilling time was reduced 16% by the 4th well, but further improvement in bit hydraulics is required to increase penetration rates. The original air package design was to deliver 1500 SCFM of air at 1350 psi. A booster to increase the air discharge pressure to 1900 psi was employed for the last three wells. This increased the hydraulic energy delivered to the bit. Using greater air compression capacity with optimized bit nozzles played the key role in improving bottom hole cleaning and increasing the rate of penetration.
Historically the surface hole section was drilled with water based mud lightened to 56 pcf with crude oil additions to prevent losses. Environmental concerns and economic considerations for crude oil consumption and waste pit clean-up promoted the search for a means of improving drilling conditions for the 17.1/2" surface hole by:-
Replacing the oil/water mud mixture with aerated mud and thus controlling losses throughout the 17.1/2" hole section.
Improving the drillability of hard limestone formations by reducing hydrostatic overbalance pressure.
Engineering Studies indicated one solution to the loss circulation problem lay in the use of aerated mud drilling. It was obvious from the beginning that straight air drilling or foam drilling was not practical because of high potential water flows from the aquifer zones that had to be penetrated. Blind drilling was not economically feasible in the desert operation. Fluid pumping rates of 850 to 950 GPM are normally used to drill and clean the 17.1/2" hole. Blind drilling requires about 25,000 to 30,000 BWPD for about six days assuming complete losses. Three shallow water supply wells are normally drilled for each location. These wells produce a total 5000 BWPD on average. The balance of 20,000 to 25,000 BWPD required for blind drilling must be hauled by trucks or produced by drilling additional water supply wells, both of which are expensive. Furthermore, known problems associated with blind drilling such as frequent drill string twist-offs limit the acceptance of blind drilling. Aerated mud was seen as an operational, economical and environmentally friendly solution to combat massive mud losses into the three separate water source aquifers at 1400, 2800 and 4500 feet. Using aerated mud made drilling through these low pressure, water-bearing reservoirs with full returns to the surface possible. Air injection through the stand pipe is the simplest way of lowering the density of the drilling fluid.