AIR/GAS DRILLING

The extreme under-balanced that results when drilling with air or gas allows large disc-shaped cuttings to break from the formation with the impact of the bit tooth. These cuttings are degraded to dust as the turbulent airlifts them to the surface. Solids control while air drilling (including natural gas and nitrogen) consists of controlling atmospheric pollution, collecting samples, and disposing of cuttings and liquids. Normally there is no recovery or reuse of the air or gas except for a few occasions in natural-gas drilling when the gas is recycled to the gas plant. An elaborate separator/cleaning/recompressing system is required to recycle the gas. It is usually more economical to flare the gas than to recover and clean it (see Figure 1.)

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UNDERBALANCED DRILLING FUNDAMENTALS

Underbalanced drilling (UBD) is defined as ‘‘deliberately drilling into a formation in which the formation pressure, or pore pressure, is greater than the pressure exerted by the annular fluid or gas column’’ (IBD HSE Forum, IADC 2002). In this respect, ‘‘balanced’’ pressure drilling is a subcategory of underbalanced drilling because the annular pressure may fall below the formation pressure during pipe movement. Underbalanced drilling is used to avoid or limit lost circulation and as a method to protect reservoirs, prevent differential sticking, and increase the drilling rate.

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RECOMMENDED SUCTION PIPE CONFIGURATIONS

In addition to selecting the proper suction pipe diameter and having adequate NPSHA, the submergence level and suction pipe configuration must be considered. Submergence level is the depth of the suction pipe inlet below the liquid surface. If an inadequate submergence level exists, an air vortex will form that extends from the liquid surface to the inlet of the suction pipe. This will introduce air into the system, resulting in either turbulent flow patterns or vapor locking of the pump. Amount of submergence required varies with velocity of the fluid. Fluid velocity is controlled by flow rate and pipe diameter. Refer to Figure 1. to determine submergence required based on fluid velocity (fluid velocity can be found in Friction Loss (Centrifugal Pumps Velocity Measured), in the column ‘‘V (ft/sec)’’).

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