UNDERBALANCED DRILLING WITH AIR OFFERS MANY PLUSES

Les Shale Baker
Hughes Inteq
Houston

The use of air drilling techniques offers several advantages over drilling with conventional mud systems.

In comparison with mud drilling, rates of penetration are significantly increased, resulting in less drilling time and fewer bits used. Some well bore problems, such as sloughing of sensitive shales, are virtually eliminated. Additionally, air drilling allows for the use of percussion drilling tools, which can improve drilling rates, and allows the detection of hydrocarbons almost immediately.

Underbalanced drilling usually takes place when the reservoir pressures have dropped substantially to levels below the hydrostatic pressure that is exerted by conventional water-based drilling fluids. In a low-pressure formation, fluids and solids invasion into the formation can be prevented by drilling at balanced or underbalanced conditions.

A pressure overbalance during conventional drilling can cause significant fluid filtrate invasion and lost circu- lation. Fluid invasion into the formation can lead to forma- tion damage, high mud costs, a need for expensive completions, and well productivity impairment.

Because underbalanced drilling creates a natural tendency for fluid and gas to flow from the formation to the borehole, successful underbalanced drilling depends upon the appropriate selection of circulating fluid. The use of a compressible fluid in the circulating system, referred to as air drilling, lowers the downhole pressure, allowing drilling into and beyond these sensitive formations. Other advantages include increased penetration rates (ROPs), improved drill bit performance, and contamination-free drill solids for ready detection of hydrocarbons.

Underbalanced drilling techniques involve using a compressible fluid (the most common gas compressed is air) in the circulating system to lower the downhole fluid pressure and inhibit development of problems associated with drilling low-pressure formations. Depending on specific drilling conditions, the compressed gas may be used alone or in conjunction with water and other additives.

Underbalanced drilling methods include gas continuous phase methods such as dry air (dusting) and mist as well as gas internal systems with stable foams and aerated fluids. These fluids provide a pressure gradient on the formation that is less than that exerted by a column of freshwater.

EQUIPMENT

The additional surface air/gas drilling equipment for providing and supplying the necessary volumetric air flow rate for the various types of air drilling techniques is readily available to rotary drilling contractors (Fig. 1(137385 bytes)). The compressors provide low-pressure air (100-300 psig) for drilling or for charging boosters. The compressors take ambient air at a specific rate, compress it to a specified pressure or to the limit of the unit, and deliver the compressed air through the standpipe and downhole. If a greater volumetric flow rate is needed, additional compressors can be added in parallel.

A booster is a positive displacement compressor that provides high-pressure air (600-1,500 Psig). It is designed to receive the volumetric air/gas flow from the compressors and then increase the pressure. If one booster cannot handle the pressure boost from several compressors, an additional booster can be added to the system. Figs. 1(137385 bytes),2(57253 bytes),3(60207 bytes) illustrate simplified plan layouts for the various surface equipment components.

• AIR HEADERS

  The line from the compressor to the standpipe should be large enough (usually 4 in. diameter) to minimize friction losses. The line should have a pressure relief valve to guard against high pressures for the compressors and other equipment and a check valve to prevent air or fluid backflow to the compressor. This line should have a pressure gauge, and the air line should have a connection to the braden-head for reverse circulation if necessary.

  The air header should also connect through a release, or blowdown line, to the blooey line. Thus, the compressors will not have to be shut down or taken off line during a con- nection. A three-way valve, or two standard valves, should be positioned so the rig crew can control air flow from the rig floor at all times.

  The manifolding should provide a valve to the standpipe so that, whenever necessary, the mud pumps may be used without pumping into the air line.

• BYPASS BLOWDOWN

  The main air header connects a bypass and choke to a muffler. When necessary to stop air injection, the bypass is opened and the main air header closed while the compressors are shut down.

  The bypass blowdown should be equipped with a muffler, often called a blowdown silencer or bypass muffler, to silence the discharged air.

• AIR SEPARATOR

  The air separator is a bowl-shaped vessel with a stack and is located at the end of the blooey line above the shakers, when mud or foam is used. The air separator allows most of the air to escape and helps conserve water.

  Care must be taken to ensure that the flow of fluid does not overflow the air separator. A full-opening valve in the blooey line can control fluid surges.

• BLOOEY LINE

  The blooey line carries exhaust air/gases and cuttings to the flare pit. The recommended length is 300 ft or more, and it should have a cross-sectional area equivalent to that of the annulus. The outlet end of the line should be perpendicular to the prevailing wind and should extend past the flare pit wall by 6 ft.

  The blooey line should be securely anchored and grounded along its entire length.

• BURN PIT

  Positioned at the end of the blooey line for an air (or gas) drilling operation, the burn pit should be located away from the standard mud drilling reserve pit. The burn

  pit will prevent any hydrocarbon liquids from burning or flowing into the reserve pit, thus preventing a reserve pit fire near the rig.

• CHEMICAL TANK

  The chemical pump injects liquid foamers, corrosion inhibitors, and water into the high-pressure air line. These pumps normally have small displacements and high-pressure ratings, with not more than 10 hp. The volume of the associated tank is usually not more than 10 bbl.

• DEGASSER

  The degasser, either centrifugal or vacuum-driven, is usually set up on the suction pit to remove air from the fluid.

• GAS SNIFFER

  The gas sniffer can be hooked into the blooey line to detect very small amounts of gas entering the return flow of air and cuttings from the annulus. The gas sniffer is located on the blooey line just after the return flow from the annulus.

• PILOT LIGHT

  A small pilot light or flame should be maintained at the end of the Money line to ignite any gas encountered while drilling. When drilling with natural gas, the flame should be extinguished until full flow is available in the blooey line.

• SAMPLE CATCHERS

  A small diameter pipe (-2 in.) is fixed to the bottom of the blooey line. The small pipe runs into the blooey line at an angle and is open to the return flow from the annulus passing through the blooey. A valve is placed on the small pipe outside the blooey line to facilitate sampling.

• SCRUBBER

  The scrubber removes excess water in the air stream to ensure that a minimum of moisture is circulated and to protect the booster in some cases.

• SOLIDS INJECTORS

  Solids injectors put hole-drying powders into the well bore to dry any weeping water zones or to reduce friction in a deep hole. The endless chain and the belt-type with pistons are most practical.

• SURGE TANK

  A surge tank is required only in aerated water or mud drilling. The surge tank prevents the air from blowing water or mud out of the system and onto the location and aids separation. Back pressure control chokes at the surge tank help control the downhole pressures and surging. Figs. 4 (66260 bytes)and 5 (68884 bytes) show a typical air drilling hookup and riser layout.

• BLEED OFF LINE

  This line bleeds off pressure within the standpipe, rotary hose, kelly, and the drill pipe to the depth of the top float valve. The line connects the standpipe manifold with the blooey line or mud flow line.

• KELLY

  A hexagonal kelly is recommended over the square type because of the effectiveness of the seal within the rotating kelly packer.

• ROTATING KELLY PACKER

  The rotating kelly packer, also called a rotating blowout preventer or rotating head, seals the annulus at the top of the bell nipple (banjo box). Air and cuttings are thus shunted through the blooey line or mud flow line.

• BITS

  The cutting structures of many oil field bits were never optimized for compressible fluid drilling (air drilling) because of their cone offset and the relatively soft grades of tungsten carbide inserts used. Also, the sealed journal bearings in oil field rock bits were primarily designed for fluid applications, and as such their reliability when used with air is decreased."

  The following are the basic design differences between air bits and conventional mud bits: cone angle, bit offset, bearings, gauge protection, and air passageways for cooling.

  Bit wear is not discernible during drilling. Thus, undergauge holes requiring reaming are common. Bits are usu- ally pulled on the basis of rotating time in the hole.

• BHA

  Drill pipe and bottom hole assemblies (BHAs) are the same as those required for mud drilling; however, effective string weight will be increased because buoyancy is negligible.

• FLOAT VALVE SUB

  Float valves are run at the top and bottom of the string. The bottom valve prevents backflow of cuttings into the string or steerable air motor, which could result in a plugged bit. The top valve retains high-pressure air within the drillstring during connections.

• INSTRUMENTATION

  Normal rig instrumentation is sufficient for most air drilling; however, the addition of a standard orifice-plate meter run is popular for measuring air or gas injection volumes. This meter helps establish known flow rates, which is essential for deviated or horizontal drilling.

  Installation of pressure gauges is recommended in the low and high-pressure lines, standpipe and, if aerated drilling, in the mud flow line (back pressure gauge). A thermometer in the air line near the standpipe and an automatic driller may be helpful in maintaining the normally low bit weights of air drilling.

WELL CONTROL

When high pressures are encountered in the producing formation during air drilling, the blowout of production fluids is evident at the blooey line as a large flare and burning oil in the pit. Prior to surface evidence, however, the standpipe injection pressure usually drops significantly (for open bit orifice drilling only) as the production zone is penetrated. This drop is due to the increased velocity of flow in the annulus as the producing formation injects fluid int the low-pressure annular space.

There is no such thing as kick in air drilling. There is no hydrostatic head or heavy drilling fluid in the annulus to contain injected materia from the formation, so this material rises to the surface with the air flow. Therefore in any higher-than-norma production zone, the air-drilled well is immediately in a blowout configuration.

If production zone pressure and rate of flow to the annulus from the formation are deemed too hazardous for normal air drilling operations, mud must be used as the drilling fluid.

DOWNHOLE AIR REQUIREMENTS

The circulation rate plays a critical role in the success or failure of any gas drilling operation. Volume requirements for a given application depend on a number of parameters present in the well bore. The following factors combine to determine lifting capacity in a gas drilling application: drilling depth, penetration rate, drill pipe size, hole size, type of compressible fluid used (air or gas), formation type, and cuttings size.

LIFTING CAPACITY

The lifting capacity of a compressible fluid must be great enough to provide adequate cuttings transport from the bit to the surface. Historically, the industry has accepted minimum fluid velocity values which obtain the minimum lift required, with the most widely used minimum annular velocity for ga being 3,000 fpm of standard air.

With a compressible mixture as the lifting medium, the actual design for volume requirements should be based on the area in the annulus where the lift is most difficult. The flow rate of air must exceed the sinking rate or slip velocity of the cuttings, or the cuttings will not be removed from the hole.

HOLE CLEANING

Hole cleaning in an air-drilled hole presents the same problems as in mud drilling. For dry air, the volume of air needed to clean a high-angle hole is much greater than that for a vertical well.

Hole cleaning problems are more pronounced when mist or foam is used because an even greater air volume is needed for drilling higher-angled sections.

Drill pipe rotation aids hole cleaning in an air-drilled hole because cuttings are continuously agitated and ground finer by the rotation, allowing the air to carry them out of the hole with relative ease. In these applications, the air stream makes a fluidized bed of the agglomerated cuttings and allows the drillstring to move freely! Experience has shown that the volume of air that will clean the hole while drilling with a rotary assembly will not necessarily clean the hole during downhole motor drilling with no drillstring rotation.

In high inclination or horizontal wells, cuttings fall to a maximum inclination. Thus, poor hole cleaning will be evidenced by excessive drag while the BHA is pulled through that section, and by the bridges encountered during a trip in the hole.

TORQUE AND DRAG

Two of the most significant problems in drilling with compressible fluids are torque and drag, caused by the friction between the drillstring and hole wall. The length of horizontal hole that can be drilled with air is shorter than that drilled with mud because eventually drag prevents the drillstring or casing from falling into the hole.

Drag is a function of the friction coefficient between the pipe and the hole wall. In a mud-filled hole, the friction coefficient is affected by the lubricity of the mud, which can be controlled with additives. With compressible fluids, however, there are essentially only three ways to affect the drag in the well: change the friction coefficient, change the directional profile of the well, or change the string weight.

Drag is a function of the friction coefficient and drill pipe normal force; if the friction coefficient is reduced by 500/c, the drag also will be reduced by 50%. Hole drag increases as the friction coefficient increases. Building angle at higher rates and to lower inclinations will yield a greater drag than building at low rates and to higher incli- nations.

Most torque and drag is caused by pipe tension in a dogleg. The greater the tension within a dogleg, the greater the dogleg. Reducing the tension in a dogleg reduces the torque and drag in the well bore.

There are no friction-reducing additives that can easily be added to a gas drilling medium. Foam or mist may increase lubricity, but the accompanying hole cleaning problems nullify any benefit. A typical friction coefficient for an air- drilled hole is 0.75, whereas friction coefficients of 0.2- 0.35 can be expected in mud-filled holes.

AIR DRILLING TECHNIQUES

There are four types of air drilling techniques, each with a specific purpose and application for drilling underbal- anced reservoirs: dry air drilling, mist drilling, stable and stiff foam drilling, and aerated fluid drilling. By choosing the right technique, the operator can successfully drill into and beyond low-pressure formations.

Reduced-pressure drilling systems provide fluid densities ranging from near 0 to 7 ppg. These low-density fluids are ideally suited for a variety of specialized drilling operations.

DRY AIR DRILLING

Typically, wells drilled with dry air systems (dusting) have fast penetration rates and greater footage per bit than any drilling fluid. They also have less-deviated holes, better cement jobs, better completions, and better production than those drilled with conventional mud systems.

The "air-dust" technique is used for drilling dry for- mations or where any water influx is slight enough to be absorbed by the air stream. Because the air does not contain any additives to stabilize the well bore or build a wall cake, dusting is not suitable for drilling unstable formations.

When water-saturated formations are encountered, dusting can become "slug drilling." As the fluid builds up in the annulus, it can wet any water-sensitive shales, which may lead to stuck pipe. Wet cuttings can stick together and to the pipe wall and will not be carried from the hole by the air flow. When the cuttings fill the annulus, a mud ring will form, stopping the flow of air, and the pipe will stick.

Also, as these large intermittent slugs of air and water move up the hole, they can cut out surface pits, and have a destabilizing effect on the walls of the borehole. As water influx increases, a number of warning signs are observable at the surface: Loss of returns, pressure buildup, and slugs of fluid at the blooey line.

If the hole cannot be dried, mist drilling should be used.

AIR-MIST DRILLING

Like dry air drilling, this system relies on the annular velocity of the air for cuttings transport out of the hole. Air-mist drilling is used when the amount of water influx is high enough to preclude air-dust drilling, but not so high as to cause hole cleaning problems. A pretreated drilling mud is injected with the air, and the combination returns to the surface as a mist. A small quantity of water containing a foaming agent (soap) is injected into the air stream at the surface, with the water mist being carried in a continuous air system.

The foaming agent reduces the interfacial tension of the water and cuttings in the hole and allows small water/cuttings droplets to be dispersed as a fine mist in the returning air stream. The cuttings and water are then removed from the hole without formation of mud rings and bit balling.

Proper amounts of water and soap must be added to achieve a nominally continuous flow of foam and cuttings and adequate separation of the cuttings. Obtaining the proper combination of water and soap is a trial-and-error process. The following guidelines are good starting points: 6-12 bbl/hr water and 1-2 qt to 3-4 gph soap (0.1-0.25% by volume in the water). These requirements are a function of the type and volume of influx water.

Many produced brines are effective defoamers, requiring use of additional soap. Produced oil requires a special type of soap. To determine the proper amount of water and soap to be injected, several rules of thumb are helpful:

• Air volumes for mist tend to be greater by 30-40% than for dry air drilling.

• Pressures generally run at 200-400 psi for mist, compared to 100-300 psi for dry air drilling.

• Insufficient air/soap leads to slugging, with attendant pressure increases.

STABLE FOAM DRILLING

This technique uses a stable air-in-mud emulsion. Stable foam is a mixture of fresh water, detergent, chemical additives, and compressed gases (nitrogen, carbon dioxide, natural gas, and air). Stable foam is mixed at the surface, preformed, and circulated a single time through the hole. This preforming eliminates problems with contamination. (Saltwater, oil, sulfides, and steam have all been successfully handled with this technique.) The foam readily separates into its gas and liquid components at the surface pit.

In mixing stable foam, compressed air or gas is fed through a foam generator. The water/detergent solution is prepared in a blender in a general range of 0.1-1.0 parts foaming agent to 100 parts of solution (1-2% foaming agent by volume).

Foam is formed by pumping the water/detergent solution through a venturi tube into the air/gas stream. The preferred range of gas-to-liquid ratio is 3-50 cu ft/gal, which can be adjusted according to downhole requirements. Injecting water into the air stream during stable foam drilling provides a mechanism for introducing other chemical additives to meet individual requirements of the well.

In stable foam drilling systems, annular velocities as low as 100 fpm are not uncommon. These lower annular velocities contribute to reduced hole erosion and large cuttings transportation to the surface. Operations using stable foam drilling systems are capable of effectively removing as much as 500 bbl/hr of downhole fluid influx.

Because stable foam drilling systems are air-internal systems containing high concentrations of foam and water, the potential for a downhole explosion or fire is virtually eliminated. This fact and the ability of the system to provide excellent water and cuttings transportation make stable foam systems one of the most versatile of all the reduced-pressure drilling systems.

STIFF FOAM DRILLING

Stiff foam drilling systems are an adaptation of stable foam drilling. Stiff foam systems incorporate bentonite and polymers into the stable foam to produce a foam with greater hole stabilizing properties needed for drilling large diameter hole sizes.

Stiff or stable foam, defined as a multiphase, metastable, compressible, non-Newtonian fluid, has a consistency similar to that of shaving cream. Like the annular velocity of the air in dry air and air-mist systems, the viscosity of this foam is the primary means for cutting transport. It offers superior fluid and cuttings-carrying capacity and requires significantly less horsepower.

There are several advantages to using a stiff or stable foam instead of the air-mist system:

• The foam generally requires less energy than air mist, and in certain formations where a lighter-than-mud system is needed for low fracture gradients, the erosion from the high annular velocities of the mist system could prohibit its use.

• Stiff foam is better able to remove produced water than mist. If the foam starts to separate, or if too many cuttings are present in the annulus because of sloughing or a too high ROP, the foam will show symptoms similar to slug drilling, with alternating returns of water and air. This condition requires a stiffer foam.

AERATED-FLUID DRILLING

Aerated mud is used when water influx is too great to be removed by mist or foam techniques. The sole purpose of the aeration is to lower the weight of the column of fluid on the formation and reduce the potential for lost circulation without changing the properties of the drilling fluid.

An aerated system is an air-internal fluid created by injecting air into a viscosified fluid or mud. Encapsulation of the air in the drilling fluid results in an expansion of the fluid and a reduced density per unit of volume. Cuttings transport in aerated fluid depends on the lifting and carrying properties of the fluid.

Combining the advantages associated with conventional drilling fluid and air drilling techniques, an aerated-fluid drilling system is well-suited for use in highly unstable formations where lost circulation is a concern. A balanced- pressure circulating system of aerated mud can effectively drill through low pressure, water-dominated reservoirs with full returns to the surface.

A mud that is suitable for aeration will have a number of basic properties: low gel strengths to facilitate breaking out of the air, low viscosity, and good corrosion-control characteristics.

After full circulation is established, properly con- trolled adjustments to the injected air and fluid volumes regulate the bottom hole fluid pressure. There is minimum loss of drilling fluid and cuttings to thief or production zones when the bottom hole pressure approaches or is below the formation pressure.

Each section of a well may require a change in the air- to-mud ratio, depending on a number of parameters, including hole geometry, mud properties, pump efficiency, hole problems, temperature, lost circulation rates, and water level in the well bore.

Aerated-fluid systems are the most corrosive of all reduced-pressure drilling methods. With proper selection of supply water, pH control, and use of advanced corrosion inhibitors, aerated fluid systems have been successfully used worldwide.

DOWNHOLE FIRES

Downhole fires are rare but spectacular. The aftermath of such incidents is impressive. Typically, the drill collars and bit are melted, making fishing operations impossible.

When hydrocarbons are encountered during air drilling, two conditions of the three necessary to create a downhole fire or explosion are present: fuel and oxygen. The third condition is ignition.

A downhole fire occurs when wet gas or gas and oil are present in the system at pressures and temperatures which cause them to ignite. The detonation or explosion is very similar to that which occurs in a diesel engine. Detonation occurs when the ignition temperature is reached. There are three phenomena which cause ignition during air drilling operations: a mud ring (seal between borehole and drillstring), downhole sparks, and a small hole in the drillstring.

The mud ring stops air circulation and allows gas to accumulate in a pressure chamber. When the gas-to-air ratio is in the 5-15% range, ignition will occur (Fig. 6 (60558)). The bit insert, collars, and drill pipe all cause sparks during drilling, and these will ignite if the proper fuel-to-air mixture is present.

When air flows through a pinpoint size hole (at 200-400 psig), friction across this hole can create sufficient heat to cause a hot spot which can lead to ignition if the correct fuel-to-air mixture is available.

Downhole fires can be prevented by prudent use of the mist pump. Introducing 30-40 bbl/hr of cold water into the system can cool the system sufficiently to prevent ignition temperature from being reached. As an alternative, inert diluents such as carbon dioxide and nitrogen can be effective.

DIRECTIONAL AIR DRILLING

Mud motors have been identified as having too many economic limitations when used with air. They are designed to be run using drilling mud as the power source, for lubrication, and for heat dissipation. The significant differences between drilling mud (incompressible fluids) and air (compressible fluids) have led to problems in the application of conventional mud motors when air is used.

The air volume required to clean the hole is three times greater than the recommended flow rate for the conventional mud motor."'When an air motor is used, even with high flow to maintain hole cleaning, the bit speed is kept low. Only a low differential pressure is required to provide sufficient torque for drilling.

Thus, the air drilling motor offers several advantages:

• No boosters are necessary.

• Efficiency is improved.

• The motor does not stall easily because of the more efficient relationship of torque/pressure.

• The motor is less likely to overspeed when it is pulled off bottom.

• The air motor is suitable for drilling with compressible fluids and standard drilling muds.

The air motor is a steerable motor drilling system that combines directional and straight hole drilling capabilities. In addition, it can achieve a variety of build rates.

Generally, in one run it can establish the desired direction and inclination for the surface interval of a direc- tional well. With total directional control, dogleg severity is more precisely managed, and problems are often reduced in the critical hole section.

SURVEYING

Very little research information is available regarding surveying high-angle or horizontal air-drilled wells. In compressible fluid drilling, conventional survey instru- mentation encounters severe conditions. Because there is no drilling fluid to help dampen the effect of vibration and resonance, instruments are sometimes quickly put out of action.

Electromagnetic measurement-while-drilling systems are now used in air-drilling applications. Inadequate signal transmission because of formation resistivity and depths greater than 4,0005,000 ft remain the main problems, however.

The cartridge data transmission system and wet connects use a special, rugged steering tool. A hard wire from the steering tool through the drill pipe and kelly to the surface relays the survey information to computer equipment on the surface.

This system provides the advantages of steering tool operation while overcoming the limitations of other con- ventional survey techniques."

COMPLETIONS

Historically, air drilling is conducted in competent, consolidated formations where production is usually from an open-hole completion. There are other completion methods for the various air-drilling techniques:

• For dry air (dusting), one completion method is to run production casing in a dry hole. One example is a 4,000-ft gas well drilled in the Dakota or Morrison formation in New Mexico.

  The procedure is to air drill to depth, run a log for depth, run casing with a float collar and guide shoe, land casing on bottom and fill with water, displace the water with cement using the single-stage method, blow water from the casing and dry the pipe, and then drill the shoe and producing formation.

  After air drilling, the producing formation is unconta- minated and ready to be put on Production or to receive any other completion method.

• Another completion method is used when the producing zone is to be drilled through and tested with air, and the production casing is to be set above the zone.

  The procedure is the same as that in the first method, except the following are added: set an open hole bridge plug above the producing zone (to prevent gas from reaching the rig floor and to isolate the zone from fluid contamination while cementing); blow out water after cementing; drill out float, shoe, and bridge plug; and then clean out the well with air to bottom."

• With a stable foam, a foam-gravel-pack hole completion in an underpressured reservoir is possible. Figs. 7 (61726 bytes)and 8 (39921 bytes) show the 15,000-ft gas well and equipment layout for in operation in California.

  The objective was to minimize formation damage during the completion phase. The completion program consisted of a 300- ft foam gravel pack with a 3/2-in., wire-wrapped screen in the open hole section."

UNDERBALANCED DRILLING

Generally, underbalanced drilling with air should be considered if any of the following conditions occur:

• Drilling in areas of hard rock because the formations are generally impermeable and therefore minimize water inflow

• Lost circulation prevalent (with air drilling, this problem is virtually eliminated.)

• Formations sensitive to excessive damage by drilling fluids

• Adequate strength in formations being drilled through to withstand mechanical stresses without collapse

• Limited ground water flow

• Unlikelihood of high pressure formations or hydrogen sulfide gas

• Cavernous areas

• Contamination of production zones.

• Areas where the drilling rate is highly sensitive to bore hole pressure (Because of the reduction in hydrostatic head when using air, substantial rates of penetration are realized.)13

In all of these cases, economics will be the deciding factor. The question in each case will be whether the increased penetration rate and savings on bits, completion, and production costs will offset the one-time additional costs of compressors and other equipment needed for air drilling.

ACKNOWLEDGMENT

The author would like to thank Frank Radez with Baker Hughes Inteq for his editorial contribution.

AUTHOR