AIR DRILLING HAS SOME PLUSES FOR HORIZONTAL WELLS

Richard S. Carden
Grace, Shursen, Moore, & Associates Inc.
Amarillo, Tex.

Drilling horizontal wells with air as the circulating medium is not a common practice; however, air has some distinct advantages over drilling mud. They are:

Significant increase in rate of penetration which leads to shorter drilling time
Elimination of lost circulation problems, especially in areas of very low bottom hole pressures
Continual drill stem test of potential producing formations
Minimal damage to the formation.
Unfortunately, there are some disadvantages to drilling with air:
Downhole motor life is shorter and less predictable.
No measurement-while-drilling (MWD) system is currently available that will work consistently in air drilling environments.
Hole cleaning is a problem at inclinations above 50.
The horizontal section length is reduced because of the increased friction (drag) between the drillstring and borehole.
The types of lithologies and targets are limited.

Several horizontal wells have been successfully drilled with air or foam since 1986. At a minimum, operators drill the horizontal section with air or foam to eliminate lost circulation problems in low pressure or partially depleted reservoirs and to reduce formation damage due to drilling fluid invasion.

However, problems have been encountered in drilling horizontal wells with air. Not all of the problems are unique to air drilling, but some may be exaggerated by the conditions in an air-drilled hole.

DOWNHOLE MOTORS

Positive displacement motors (PDM) are used to build inclination and frequently to drill the horizontal section of medium and long-radius horizontal wells.

These motors, used in directional drilling since the 1960s, are designed to be run using drilling mud as the power source. The significant differences between drilling mud and air have led to problems in the application of these motors to air drilling.

Because air is compressible, the flow rate changes with pressure. Also, because of its much lower lifting capacity, air requires annular velocities much greater than mud. However, the higher air volumes exceed the recommended flow rates for the motors, often causing premature failure.

Typically, the air volume required to clean the hole is three times greater than the recommended flow rate for the motor. To prevent motor problems caused by an excessive air rate, the flow through the motor must be reduced. Some of the air can be diverted by placing a jet sub above the motor, which will allow some air to escape from the drillstring without passing through the motor (Fig. 1).

In motors with hollow rotors, the bypass valve can be replaced by an orifice, allowing some of the excess air to pass inside the motor (Fig. 2). In either case, the orifice size must be predetermined to divert the necessary amount of air to the annulus instead of through the motor.

Jets should also be placed in the bit to provide adequate bottom hole cleaning and extra cooling for the motor. The jets can be designed for bottom hole cleaning based on the method presented by Lyons, but a pressure drop of 200-300 psi (1,379-2,068 kPa) is usually sufficient.1

The expansion of air through the bit nozzles provides the cooling for the bit and motor, which helps the motor run longer.

However, friction between the rotor and stator causes a buildup of heat inside the motor.

In most cases, mist or foam is injected into the air stream to provide lubrication for the rotor to lessen the friction. When water (included in the mist or foam) is introduced in the well bore, enough must be used to completely wet the borehole and the generated cuttings. Otherwise, a mud ring will form, and the drillstring may become stuck. As a general rule, a minimum rate of 10 bbl/hr (1.59 cu m/hr) is used even though the motor does not require that much for ample lubrication. This slight excess amount of water may help prevent the drill pipe from sticking.

Water added into a well bore sometimes causes shale stability problems; thus, a dry hole may be desirable. As an alternative to water, a small quantity of oil added to the airstream can also provide effective motor lubrication.

Injection rates of 5 gph (18.9 l./hr) will provide ample lubrication. Too much oil will cause the drill cuttings to become slightly wet, which can stick the drillstring. Therefore, oil injection rates should be limited.

Recently, a positive displacement motor designed specifically for air drilling has been developed to operate without requiring lubrication. Experience shows that the motor is reliable, and it will become more effective through design improvements and experience.

MWD EQUIPMENT

Typical MWD equipment pulses the mud system to send information from the bottom hole assembly to the surface. Because air is compressible, it cannot be pulsed effectively. Therefore, conventional mud-pulse MWD technology does not work in an air-drilled hole. A different MWD system, electromagnetic measurement-while-drilling (EMWD), has been developed to operate in air-drilled holes by using radio waves to send information to the surface.

EMWD systems have been used in air-drilled holes with mixed results. Signals do get back to the surface with correct information, but tool reliability has become a problem. Frequent EMWD failures have occurred in air-drilled holes because drilling conditions are rougher than in mud-filled holes which have fluid to dampen vibrations.

EMWD systems are not yet durable enough to work consistently in the rough, air-drilled hole environment, but with experience and continued improvements, EMWD shows much promise.

Currently, downhole motors are oriented with a steering tool that sends information to the surface using a single-conductor wire line. Some steering tools have the same problem as EMWD: Vibrations in the well cause frequent failures. One method to reduce vibrations at the steering tool is to place jets in the bit, but this does not entirely eliminate the problem. Because steering tools tend to have a higher failure rate in air-drilled holes, selection of an appropriate steering tool is essential.

There are two methods used to survey the horizontal section, but both involve tripping the drillstring. Once the well angle exceeds about 70, the steering or survey tool can no longer fall down the hole and cannot be pumped down with air. Unlike mud, the air passing by the heavy tool does not generate enough drag to carry it down the hole.

One method of surveying uses an electric line and a side entry sub with a latch-in assembly (Fig. 3). To survey the horizontal section, the drillstring is pulled from the hole until the bit is at an inclination of 70. A side entry sub is installed in the drillstring, and the survey tool (singleshot or steering tool) is run to the bit on an electric line. The drillstring is then tripped back to bottom with the remainder of the wire on the outside of the drillstring. After reaching total depth, a survey is taken and the drillstring tripped back out of the hole to the side entry sub. The survey tool and side entry sub are removed, and the drillstring is run back to bottom to continue drilling. Surveying the horizontal section in this manner is both time consuming and expensive.

Another method of surveying reduces the time and cost by eliminating the side entry sub and electric line. However, the drill pipe still must be tripped to a minimum inclination of 70.

A singleshot (steering tool cannot be used) is run on a slick line with a releasing overshot. Upon entering the monel drill collars, a monel sensor activates the releasing overshot, disconnecting the singleshot from the slick line. The slick line is removed from the hole, and the drillstring tripped back to bottom to take the survey.

After the survey the pipe is tripped back to 70 where the singleshot can be retrieved by using a standard overshot on the slick line. The benefits of this method include a reduction in rig time because tripping is much quicker without an electric line on the outside of the pipe and lower wire line expenses because the slick line and releasing overshot are significantly less expensive than the electric line.

Without an effective MWD system for air-drilled holes, it is much more difficult to use steerable systems in the horizontal section. Because steerable systems have to be oriented by steering tools which require tripping the pipe in wells over 70, these systems frequently lose their cost benefits which typically result from a reduction in the amount of tripping.

Furthermore, PDM life is shorter and less predictable in air environments. Thus, rotary assemblies are ordinarily used to drill the horizontal sections, using motor corrections as necessary.

HOLE CLEANING

As in mud drilling, hole cleaning in an air-drilled hole is a problem. At inclinations above 50, cuttings no longer fall back to bottom and instead lie on the low side of the hole. For dry air, the volume of air needed to clean a high-angle hole is much greater than that for a vertical well. As a general rule, the volume should be twice the volumes recommended by Angel.2

Hole cleaning problems are more pronounced while using mist or foam because an even greater volume is required during drilling of the high-angle section. Unfortunately, the optimum volume is unknown. For this reason, it is desirable to run downhole motors using oil as a lubricant rather than mist or foam.

Drill pipe rotation aids hole cleaning in an air-drilled hole because the cuttings are continuously agitated and ground finer by the rotation, allowing the air to carry them out of the hole with relative ease. Experience has shown that a given volume of air will clean the hole while drilling with a rotary assembly but will not clean the hole while drilling with a downhole motor with no drillstring rotation.

HOLE FRICTION

The length of horizontal hole that can be drilled with air will be less than that drilled with mud because eventually drag will prevent the drillstring or casing from falling in 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.

There are no friction-reducing additives that can easily be added to air. Foam or mist can increase lubricity, but the accompanying hole-cleaning problems nullify the effect.

A typical friction coefficient for an air-drilled hole is 0.45, whereas friction coefficients in mud-filled holes range from 0.2 to 0.35. Hole drag increases as the friction coefficient increases. Fig. 4 is a plot of hook load vs. horizontal hole length for 5-in., 20-lb/ft (14.0-cm, 2.77-kg/m) casing at 2,600 ft (792 m) true vertical depth with various friction coefficients. When the hook load falls below zero, the pipe will no longer fall into the hole by itself, thereby limiting the amount of horizontal hole section that can be drilled.

TARGET CONSTRAINTS

The types of lithologies that can be drilled with air are limited, with older, consolidated rocks the most applicable to air drilling. Some of these hard rock formations do not need the pressure forces of fluid to support the borehole wall. Less consolidated formations are not well-suited to air drilling because they may have a tendency to slough.

The amount of water that can be accommodated in an air-drilled hole is limited because too little can lead to sticking and too much can negate the benefits of air drilling.

If the formations above the target reservoir produce an abundance of water, that portion of the well must be drilled with fluid. The horizontal section can still be drilled with air provided that casing is set through the water-producing strata. (A cost analysis must be calculated to determine if an extra casing string is economical.)

Air drilling cannot continue when excessive amounts of oil or gas are released from the producing formation. The gas presents an obvious fire hazard while tripping. Normally, up to 5,000 Mcfd (141,584 cu m/day) can be kept off the rig floor through a blooie line with proper jetting configurations. Large quantities of oil are a problem because the oil must be collected safely in a pit or tank, picked up from the location, and cleaned for sale.

With the lack of a cost-effective steerable system, thin reservoirs cannot be drilled efficiently with air. A target thickness of 50 ft (15.2 m) is a minimum when using rotary assemblies because the build and drop tendencies of rotary-hold assemblies are difficult to maintain below 0.25/100 ft (0.25/30.48 m). In thin targets, too many motor-correction runs would be required to make air drilling beneficial.

Air drilling horizontal wells can be a beneficial alternative for certain applications. The operator must be aware of the limitations and advantages of air drilling to optimize its drilling operations. Many standard practices used in air-drilling vertical holes have to be modified continuously as the hole angle increases to horizontal. As with any horizontal drilling operation, careful planning is one of the keys to a successful well.

REFERENCES

Lyons, W.C., Air and Gas Drilling Manual, Chapter 4, Gulf Publishing Co., 1984, pp. 46-52,
Angel, R.R., Volume Requirements for Air and Gas Drilling, Gulf Publishing Co., 4th printing, July 1985.