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This paper details the lessons learned in late 2007 and early 2008 whilst air drilling vertical gas wells in the Travis Peak formation of the western East Texas deep Bossier play in Amoroso Field and applying the lessons learned in the US to a deep appraisal well drilled in Northern China.
The deep Bossier represents one of the most active onshore plays in the United States. Deep Bossier wells are 14,000 ft (4,267m) to 16,000 ft (4,876m) deep and intersect shale and sandstone formations ranging between 2,000 ft (610m) and 3,000 ft (914m) thick. The Travis Peak is also known as ‘tragic peak' during conventional drilling operations because the sandstone is hard and abrasive in nature, leading to extreme low rate of penetration (ROP). Air Drilling technology was utilized to increase the ROP through the formation at depths of 10,000 ft (3,047m) and greater. The deep exploration wells in the Daqing area of Northern China are drilled to depths of 20,000ft (6,095m) and greater in an attempt to explore the deep volcanic reservoir potential in the area.
In the US the 8-1/2-in hole size was drilled using air hammers and roller cone bits using straight air or nitrogen. Temperatures in the Travis Peal formation were reported to be above 300°F (148°C). Temperatures in Northern China were reported to be as high as 400°F (204°C) in the 12-1/4-in hole size.
The fit-for-purpose equipment needed for the air drilling sections of these wells comprised of air hammers capable of drilling in high temperatures, hammer bits with full diamond inserts, high-temperature float valves, air compressors, nitrogen production units, mist injection systems, and rotating control diverters.
The lessons learned in the US were applied to the well in China where similar results were encountered. The drive to enhance air drilling to extreme depths and high temperatures previously believed to be off limits proved to be successful in both parts of the world.
Because underbalanced drilling creates a natural tendency for fluid to flow from the formation into the borehole, successful underbalanced drilling depends on appropriate selection of circulating fluid. Under these conditions, use of conventional mud systems often results in lost circulation, formation damage, high mud costs and a need for expensive completions. Use of compressible fluids, on the other hand, can inhibit or eliminate many of the problems associated with drilling in environments in which formation damage is likely.
Use of a compressible fluid in the circulating system, referred to as air drilling, lowers the downhole fluid pressure, allowing drilling into formations where loss of circulation and damage to productive formations are problems of major concern.
Other advantages to air drilling include increased penetration rates, improved drill bit performance, and contamination-free drill solids for ready detection of hydrocarbons.
Reduced pressure air drilling techniques include not only gas continuous phase methods and use of dry gas and gas mist, but also gas internal systems with stable foams and aerated fluids.
This discussion of problems related to underbalanced drilling addresses types of drilling equipment used, and provides an overview of experience gained from both successful and unsuccessful wells.
The concept of using compressible fluids (i. e., gases) as a drilling media to remove cuttings from a drilled hole was first recorded in a United States patent issued in 1866. Since then science and technology have transformed this idea into a sophisticated industry of specialized drilling techniques.
Reduced pressure drilling techniques involve using compressed gas, (most commonly, atmospheric air), as the circulating fluid. Depending on specific drilling conditions, this gas may be used alone, or in conjunction with water and other additives.
The use of air or gas as a circulating medium was introduced in the early 1950's. Even though initial attempts were crude, significant increases in penetration rate and bit life were obtained. Since these initial attempts, development of air and gas drilling techniques have expanded and are widely accepted today as a method to reduce drilling times and cut cost of many wells. Along with the time and resultant dollar savings, other advantages such as immediate and continuous hydrocarbon detection, minimum damage to liquid sensitive pay zones, better control of lost circulation, and cleaner cores are obtained.
Today's air drilling technology is attributed to many drilling people whose initiative and accumulative experience have refined the method and determined situations where the technique is most applicable. The lack of understanding, rather than experience, is often the reason for not accepting air drilling. Drilling with air does involve special consideration in the use of equipment and drilling techniques that are not commonly encountered with other drilling media. For example, air, unlike fluids, compresses readily and requires a somewhat more sophisticated engineering approach to achieve the desired results.
This paper discusses the mechanics of air drilling, modifications such as mist or foam drilling, unique equipment requirements, and downhole problems that have been encountered. Special attention is given to presenting techniques developed to prevent or control downhole problems.
Mechanics of Air Drilling
Air is the ultimate low density drilling media. In order to achieve optimum results and greatest economy from air drilling, there are several factors which should be considered. Hard formations which are dry or produce relatively few formation liquids provide the best results while air drilling. When the formation is completely dry, or the influx of liquids is slight enough to be absorbed in the air stream, the drill cuttings return to the surface in the form of dust. Also, this allows for immediate and continuous evaluation of hydrocarbons.