The aerated drilling fluid technique was introduced to a deep gas well drilling project as a solution to overcome the challenges posed by top fractured formations and highly water-sensitive shales. The top holes were originally drilled with a diesel oil-emulsion drilling fluid which was later changed to a high-bentonite content drilling fluid system. The aerated drilling fluid aimed at improving the overall drilling performance and addressing top hole geological complications.
The main objectives behind the introduction of aerated drilling were the mitigation of mud losses in Wasia and Shu’aiba formations and improvement of the penetration rate across the lowermost abrasive Biyadh interval. The aerated drilling fluid technique was successively implemented in four wells in different areas of the field. The system was a mixture of air and polymer-based fluid pumped inside a closed circulation system with the use of a rotating circulating head. The system provided the necessary viscosity to entrap the air downhole while allowing it to easily break out at surface in the separation equipment.
The technique proved to be an excellent solution for drilling through lost circulation zones across the Wasia and Shu’aiba fractured limestone formations. Equally important, this technique delivered much higher penetration rates while drilling across the Biyadh interval.
An effective oil-emulsion system used to be run to lighten the fluid column to prevent losses but was replaced by a high-bentonite drilling fluid for environmental reasons. The latter system was believed to seal the formation fractures due to its high gelation characteristics, thereby preventing losses. Despite being effective at preventing losses, this drilling fluid returned poor performance while drilling compacted clay intervals toward the end of the section. As a result, the casing point had to be set shallower thus complicating achieving the objectives of the subsequent hole section.
The results of implementation exceeded expectations both in terms of ROP improvement and control of downhole losses, reducing the section drilling times by more than two in comparison to the high-bentonite content drilling fluid and up to 35% over the oil-emulsion drilling fluid. This technique was also remarkable in reaching the designed casing point in all sections drilled.
This paper will describe in length the performance of the trial jobs, present the lessons learned from the implementation of this innovative drilling fluid technique and highlight the best practices utilized to make it a cost-effective solution.