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Air Drilling –

Air drilling—also generally known as pneumatic percussion drilling[1] —is an underbalanced drilling (UBD) approach in which gases, often compressed air or nitrogen, are used to cool the drill bit and elevate the cuttings of a wellbore instead of conventionally used liquids. Recognized for being extra environment friendly and cheap than conventional drilling, air drilling nonetheless has drawbacks and skeptics, regardless of its 60-year history of use in the industry.

1 History
2 Varieties 2.1 Mud Drilling
2.2 Mist Drilling
2.3 Foam Drilling
2.4 Aerated Drilling
2.5 Nitrogen Membrane

History
The primary recorded use of air drilling was within the early 1860s. A piston-sort compressed air mechanical drill bit bored an 8.5-mile-lengthy Mont Cenis Tunnel in the Alps. Air drilling turned a preferred various to rotary drilling in the late 1940s and early 1950s.[2] Because of restricted air compression tools pennsylvania oil refineries to correctly clear the annulus because the effectively was drilled, air-drilled holes have been normally limited to shallow wells (<6000 ft.). But by the late 1970s, air-drilled holes became deeper when larger volume air compressors and high-pressure boosters were developed. The use of high-pressure air compression equipment rose after the downturn of the oil and gas industry in the 1980s because of the development of a high-energy air hammer and diamond-enhanced hammer bits. The hammers and bits greatly increased the rate of penetration and footage in such air drilling areas as the Appalachian and Arkoma Basins, thus reducing drilling costs in these areas. These new developments also opened the door for deeper air drilling applications by decreasing both the number of bit trips and the need to downsize the hole's diameter from gauge wear.[3]

Types
The type of air drilling required depends on drill site conditions, including presence of wellbore fluid influxes or oil-based mud. Air drilling methods include dust drilling, mist drilling, foam drilling, aerated drilling, and nitrogen membrane.

Fig. 1 – Air Drilling Techniques[4]
Dust Drilling

Dust drilling is another term for air drilling; compressed air is the sole circulating medium. Because no fluid is injected, the annular returns are “dust.” Dust drilling provides an ideal environment for use with air hammers, is the least expensive type of air drilling, requires no fluid system for cleanup, provides maximum penetration rates, and extends drill bit life. However, dust drilling cannot effectively handle wellbore fluid influxes, those influxes will wet cuttings and result in mud rings in the annulus, and there is a risk of a down-hole fire if mud rings are not eliminated. Switching to mist or foam drilling would allow continued air drilling in the presence of water.[4]

Mist Drilling
Mist drilling is air drilling with liquids, generally water, soap, and chemical inhibitors. The water and soap mixture is added to the air stream at the drilling surface at a controlled rate to improve annular hole cleaning. Many different mediums can be used for mist drilling (water, surfactants, etc.) The annular pressure increases in mist drilling, so the rate of penetration will usually be lower than in dust drilling. In mist drilling, the rate of penetration is higher than in conventional mud drilling, drilling can proceed while producing fluids, hold cleaning capacity improves, risk of downhole fires decreases, and no nitrogen is needed. But the penetration rate is still slower than in dust drilling and water influx makes misting uneconomical. If large liquid influxes are encountered, foam or aerated mud drilling are more viable options.[4]

Foam Drilling
In foam drilling, water, surfactants, and air are combined to create a stiff foam. The foam is then circulated as a drilling fluid. The cuttings carrying capacity is 6-7 times greater than dust drilling, and the required annular velocity for optimum hole cleaning is significantly lower. The lower air volume equals less air equipment is required than in dust or mist drilling. Holding back pressure on the annulus can reduce water influx and/or maintain hole wall stability. But foam drilling has its drawbacks: surface requirements, or pits, for foam can become a problem; large pits must be built to contain foam and allow time for the foam to settle; the cost of chemicals to break down foam can be high; a large influx of fluids can break down the foam, reducing hole cleaning.[4]

Aerated Drilling
Air or Nitrogen is added to the liquid phase of the drilling fluid, lowering the effective mud weight, in aerated drilling. The air or nitrogen is injected directly into the standpipe, using parasite string, or using concentric casing strings. Corrosion inhibitors are highly recommended in this method. Nitrogen must be used with oil based mud or when working with a closed loop system (closed separator), and it is highly recommended when oil or condensate influx is expected. Aerated drilling can be used with most types of drilling fluids, allows for the adjustment of bottomhole pressures by changing the gas injection rates, and increases penetration rates by lowering the annular pressure on the formation.[4]

Nitrogen Membrane
Like mist drilling, Nitrogen membrane drilling minimizes chance of downhole fire. Membrane units usually reduce operating costs when compared to cryogenic (liquid) nitrogen drilling and transportation problems related to liquid nitrogen are eliminated.[4] The US patent for nitrogen membrane drilling is held by Weatherford, which owns the largest fleet of on-site generated membrane Nitrogen Production Units in the world.[5]

Functions
The most common air drilling applications include: hard rock drilling where rate of penetration is less than 15 ft./hr. using mud, areas that have deviation problems with conventional BHA’s and use light WOB, lost circulation issues, and pay zones that are sensitive to formation damage.[4]

Advantages
As a result of air is the perfect low density drilling medium, air drilling provides many advantages. To achieve the best results and best financial system, several factors have to be thought of for air drilling. The best circumstances for air drilling contain laborious, dry formations that produce comparatively few formation liquids. As soon as the formation is completely dry, or the inflow of liquids is small enough to be absorbed within the air stream, the drill cuttings return to the floor as mud. The method permits for the fast and sustained evaluation of hydrocarbons.[6]

Other advantages of air drilling are low cost, increased price of penetration, extended bit life, superior control in cavernous and misplaced circulation areas, and minimal injury to liquid-delicate pay zones.[6]

The drill string all the time remains on the bottom when gasoline is encountered, which is a tremendous advantage in properly control. If no gasoline is in the outlet when a visit is made, no fuel will likely be in the hole when the new bit is returned to the underside. Generally holes filled with mud will allow fuel to enter the well bore because of reduced hydrostatic stress, creating nicely management points. With air drilling, gasoline that has already been penetrated will enter the properly bore on journeys, however the quantity of gasoline is a recognized quantity that may easily be jetted away from the rig and operating personnel.[6]

Elevated fee of penetration occurs because the low density of air or gasoline used minimizes hydrostatic pressure and aids with fracturing.[1] The rate of penetration in air drilling has been recorded at up to 200 ft/hr in comparison with 30 ft/hr in typical drilling.[6]

Disadvantages
Massive water-bearing formations are the largest enemies of air drilling, and the speed of formation water inflow that can be dealt with will not be outlined. Nevertheless, when water is encountered, mist, foam, aerated, or slug drilling can be used. Other disadvantages to air drilling include: attainable downhole fires and explosions, sloughing of formations (when dry or wet), and delicate pennsylvania oil refineries formations. Such disadvantages scale back air drilling’s effectivity, but trendy air tools can handle the challenges. Another detriment of air drilling is bits going out of gauge, which is prevalent when arduous, abrasive quartzite sands are drilled.[6]

↑ 1.0 1.1 White, Jeff. 2014. ‘ ‘Air Drilling Improves Efficiencies, Cost’ ‘. The American Oil & Gas Reporter 57 5: 47-49.
↑ Malloy, Kenneth P. 2007. Taking one other take a look at the risk profile for air drilling in presence of hydrocarbons. Drilling Contractor March/April: 66-seventy three. http://drillingcontractor.org/dcpi/dc-marapr07/DC_Mar07_malloy.pdf.
↑ Patin, Michael, Orr, Alan, and Meyers, John. 2007. Optimizing Operational Parameters can “Save You Money” in addition to Improve Bit Life and ROP. Offered at the AADE Nationwide Technical Conference & Exhibition, Houston, 10-12 April. http://www.b2i.cc/Doc/1233/95324.pdf.
↑ four.0 four.1 4.2 four.Three 4.4 four.5 4.6
↑ Weatherford. Membrane Nitrogen. Net web page. http://www.weatherford.com/Merchandise/Manufacturing/PipelineSpecialtyServices/MembraneNitrogen/
↑ 6.0 pennsylvania oil refineries 6.1 6.2 6.3 6.Four Cooper, L. W. Hook, R. A. & Payne, B. R. 1977. Air Drilling Strategies. Presented at the SPE Deep Drilling and Production Symposium, Amarillo, Texas, USA, 17-19 April. SPE-6435-MS. http://dx.doi.org/doi:10.2118/6435-MS
Noteworthy papers in OnePetro

Adewumi, M. A. & Tian, S. 1989. Hydrodynamic Modeling of Wellbore Hydraulics in Air Drilling. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/19333-MS

Adewumi, M. A. & Tian, S. 1990. Evaluation of Air Drilling Hydraulics. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/21277-MS

Cooper, L. W. Hook, R. A. & Payne, B. R. 1977. Air Drilling Techniques. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/6435-MS

Hartley, R. C. Weisbeck, D. H. Robert, S. & Smith, M. A. 2011. The Profitable Evolution Of An LWD Rotary Steerable System For Air Drilling. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/140260-MS

Malloy, K. P. Medley, G. H. & Stone, R. 2007. Air Drilling in the Presence of Hydrocarbons: A Time for Pause. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/108357-MS

Nas, S. W. Gala, D. M. & Cox, P. 2010. Deep Air Drilling Software to reinforce Rate of Penetration in Extraordinarily Arduous, Abrasive and High Temperature Surroundings. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/132048-MS

Pletcher, J. P. Scarr, A. Smith, J. Swadi, S. N. & Rogers, C. 2010. Application of Air Percussion Drilling Improves Drilling Effectivity in Horizontal Sandstone Wells. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/135308-MS

Ramalho, J. 2007. Changing the appear and feel of Underbalanced Drilling. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/108358-MS

Vieira, P. Lagrandeur, C. & Sheets, K. 2011. Hammer Drilling Know-how – The Proved Answer to Drill Hard Rock Formations in the Middle East. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/140312-MS

Wilhide, S. Smith, J. Doebereiner, D. Raymond, B. Weisbeck, D. H. & Ziemke, B. 2010. First Rotary Steerable System Drilling with Dry Air is Used to Further Improve Low Value Growth of an Unconventional Gas Reservoir. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/135471-MS

Zhao, Z. Gao, D. & Zheng, D. 2010. Mechanism of Effectively Deviation in Air Drilling and Its Management. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/130201-MS

Zhu, H. Lin, Y. Meng, Y. Zhao, S. Liu, D. & Luo, F. 2010. Influence of Relevant Parameters on Hole Cleaning and Pipe String Erosion in Air Drilling. Society of Petroleum Engineers.