Underbalanced drilling offers significant advantages in terms of increased rate of penetration (ROP), less formation damage, reduced lost circulation material, decreased cost of cuttings disposal, and increased production. Underbalanced drilling injects gas into a mud column to lower the overall equivalent mud weight to create a drilling environment where the pressure in the wellbore is kept lower than the fluid pressure in the formation being drilled. Air is the ultimate underbalanced fluid, but diminishes the efficiencies of mud motors, and prevents the use of mud pulse telemetry MWD tools due to the lack of an incompressible fluid. With air drilling, the only fluid injected into the well is a small amount of oil needed to prevent corrosion. Downhole mechanical forces are usually more violent due to the lack of a fluid column for dampening as well as the higher air volumes going through the bottom hole assembly (BHA) for cuttings flow. Common drilling technologies to address air drilling include Electromagnetic Telemetry (EM), mud motors, and downhole air hammers, but reliability issues are particularly prevalent, especially for the EM MWD tools and downhole mud motors.

Air drilling has become popular especially in the Marcellus and Utica shale reservoirs in the Northeast United States because of higher ROP and less formation damage. As an example, of the 111 rigs drilling in the Marcellus Shale, 27 rigs are drilling underbalanced and 23 are being drilled with air. A unique drilling system incorporating the use of downhole mud motors, EM MWD, and air hammers has been specifically designed and ruggedized to address downhole shock and vibration encountered in air drilling. Use of this system has resulted in significant reduction of non-productive time (NPT) while drilling with air.

This paper will describe how air drilling is being successfully utilized in the unconventional reservoir of the Marcellus shale in the Northeast United States. Drilling fluids and their affect on various pressure regimes will be discussed. The new drilling system will be described and drilling parameters highlighting the differences between mud and air drilling will be provided. Modifications to the BHA to increase reliability will be discussed, and success metrics presented.

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