Mist drilling is used when small amounts of fluid invade the well bore from a wet formation zone. A small amount of foaming/ inhibitor agent is mixed with water and added to the compressed air. Mist Drilling maintains high penetration rates while handling the fluid influx. The quantity of fluid entry that can be handled effectively is dependent on the depth and fluid content.
A major condition which governs the success of a Mist Drilling system is the type of formations exposed. Dry hard, wet hard, fractured, deviated, and low pressure formations are good Mist Drilling prospects. Water sensitive shales or sloughing formations are not good Mist Drilling formations.