A General Overview of Air Drilling and Deviation Control
Authors: Wilson G.E. 
Published: December 2007

Summary
What is the most economical way to drill a well? It may be by using air, drilling fluids (other than air or gas), or a combination of both as the circulating medium. This decision requires a thorough study of an area before drilling. Regardless of the circulating medium used, crooked holes can be a problem. Similar methods are used to prevent crooked holes in air and fluid drilling, but some of the techniques differ. Selection of the proper bottomhole assembly will determine the economical success of drilling a well with either air or fluid, but this choice is sometimes very difficult and demands special considerations.

Introduction
Considerable thought and preplanning are necessary before a well is drilled to determine the most economical drilling method. One option not always considered is drilling with air as the circulating medium. When conditions permit the use of air, the economic advantages are very significant.

What Makes Air Drilling Practical?
Hard formations that are relatively free from water are most desirable for drilling with air, because water intrusion into the wellbore is the greatest deterrent. Small quantities of water usually can be dried up or sealed off by various techniques, but a large water flow generally necessitates converting to another type of drilling fluid (other than air or gas).

Why Drill With Air?
It has been shown by past experience that air drilling usually increases the penetration rate by three or four times over that when drilling with mud, and only one-half to one-fourth the number of bits are required.Another big advantage is being able to detect the presence of hydrocarbons almost immediately and then continuously as the producing formations are penetrated.Wells cannot be drilled economically with fluid in some areas, so air or gas must be used. Use of natural gas as the circulating medium is not as prevalent as in the past because of a limited gas supply and high prices. A few conditions that necessitate drilling with air, and prohibit the use of fluids, include (1) sever lost-circulation areas, (2) sensitive producing formations that can be blocked or permanently damaged by drilling fluids, and (3) hard formation on or near the surface that require the use of an air hammer to drill.

Why Not Air Drill All Wells?
Not all wells can be drilled with air. A large majority of wells are drilled with some type of fluid, because drilling with air would not be practical. Some conditions that make drilling with fluid more favorable include (1) formations that contain large volumes of water, (2) high-pressure, high permeability formations that require heavy mud hydrostatic pressure to prevent blowouts, (3) unconsolidated formations that require wall cake and mud in the hole to prevent sloughing, and (4) airsensitive formations that will erode, causing a large overgauge hole.Some wells could be drilled with either air or fluid. In this case, economics will be the deciding factor. The question is: will the increased penetration rate and the savings on bits offset the additional cost per day of the compressors and other special equipment needed for air drilling?
JPT
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