HORIZONTAL DRILLING WITH AIR

[nima.mohamadin]

INTRODUCTION
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
1.
Penetration rates are significantly increased leading to shorter drilling time and lower costs
2.
Elimination of lost circulation problems especially in areas of very low bottomhole pressures
3.
Continual drill stem test of potential producing formations
4.
Minimal formation damage


Unfortunately, there are some disadvantages to drilling with air
1.
Downhole motor life is shorter and less predictable than with fluid
2.
No MWD system is currently available that will work consistently in an air environment
3.
Hole cleaning is a problem at inclinations above 50° with air and foam systems
4.
The horizontal section length is reduced because of the increased friction (drag) between the drill string and borehole
5.
The types of lithologies and targets that can be drilled with air are limited

Horizontal drilling with air and foam systems can, and has been successful. With careful planning, all of these problems can be minimized or eliminated
Several horizontal wells have been successfully drilled with air or foam since 1986. Operators are drilling, at a minimum, the horizontal section with air or foam to eliminate lost circulation problems in low pressure (partially depleted reservoirs) and to reduce formation damage due to drilling fluid invasion
Problems have been encountered in drilling the horizontal wells. Not all these problems are unique to air drilling but are sometimes exaggerated by the conditions in an air hole. By changing the conventional method of operations in an air hole and anticipating potential problems, air drilling can be cost competitive and even less expensive. The potential problems will be enumerated along with the experience to date in solving the problems

​

 

[nima.mohamadin]

DOWNHOLE MOTORS

​

Medium and long radius horizontal wells are drilled using downhole motors to build inclination and frequently to drill the horizontal section of the well. These positive displacement motors (PDM) have been used in directional drilling since the 1960's. The motors are designed to be run using drilling mud as the power source. Unfortunately, there are some significant differences between drilling mud and air. Air is a compressible fluid and the flow rate changes with pressure. Air also has a much lower lifting capacity than mud. The annular velocities must be much greater for air. Unfortunately, the higher air volumes exceed the recommended flow rates for the motors and they fail prematurely

Typically, the air volume required to clean the hole is three times greater than the recommended flow rate for the motor. There is a way to reduce the flow through the motor. Some of the air can be diverted prior to passing through the motor section. This can be accomplished by placing a jet sub above the motor which will allow some of the air to escape from the drill string without passing through the motor. In the motors with a hollow rotor, the bypass valve can be replaced by an orifice. In each case, the orifice would be of a predetermined size to divert the necessary air to the annulus without the air passing through the motor section

​

Jets should also be placed in the bit to provide adequate bottomhole cleaning and extra cooling for the motor. The jets can be designed for bottomhole cleaning based upon the method presented by Lyons;1 though, a pressure drop of 200 to 300 psi (1,379 to 2,068 kPa) is usually sufficient. The expansion of the air through the bit nozzles provides the cooling. Friction between the rotor and stator within the motor causes a buildup of heat. Cooling at the bottom of the hole helps the motor run longer

The motor also requires lubrication. In most cases, mist or foam is injected into the air stream to provide the lubrication. When water (included in the mist or foam) is introduced in the wellbore, sufficient quantity must be used to completely wet the borehole and the generated cuttings. Otherwise, a mud ring will form and the drill string will become stuck. As a rule, a minimum rate of 10 BPH (1.59 m3 per hour) is used even though the motor does not require 10 BPH (1.59 m3 per hour) for lubrication

​

Injection of water into a wellbore sometimes causes shale stability problems and a dry hole may be desirable. Injecting a small quantity of oil into the air stream can also provide effective motor lubrication. Injection rates of 5 GPH (18.9 liters per hour) will provide ample lubrication. Too much oil will cause the drill cuttings to become slightly wet which can stick the drill string. Therefore, oil injection rates should be limited

Recently, a positive displacement motor designed specifically for air has been developed. Experience shows that the motor is reliable. The advantage of the air motor is that no lubrication is required

​

 

[nima.mohamadin]

MWDEQUIPMENT[FONT=&quot][/FONT]​

​

A typical MWD pulses the mud system in order to send information to the surface. Air is a compressible fluid and cannot be easily pulsed. Therefore, mud pulse MWD technology does[FONT=&quot][/FONT]​

​

[FONT=&quot]not work in an air hole. An electromagnetic measurement while drilling (EMWD) system will work in an air hole. The EMWD system sends information to the surface using radio waves instead of pulsing the standpipe. [/FONT] [FONT=&quot]EMWD systems have been used in air holes with mixed results. Signals do get back to the surface and the information is correct. Reliability is the problem with the EMWD. Frequent failures have been experienced on air holes. Drilling conditions in an air hole are rougher than that experienced in a mud filled hole. There is no fluid in the hole to dampen vibration. The EMWD is not yet durable enough to work consistently in an air hole; but with experience and improvements, the EMWD should work well. [/FONT]

[FONT=&quot]Currently, the motors are oriented with a steering tool that sends information to the surface using a single conductor wireline. Some steering tools have the same problem as the EMWD. Vibrations in the well cause frequent failures. In fact, all steering tools will experience more failures in an air hole. Selection of the right steering tool is essential. Placing jets in the bit also reduces the vibration experienced by the steering tool.[/FONT][FONT=&quot][/FONT]​

​

[FONT=&quot]A side entry sub is required along with a latch in assembly for the steering tool. Above an inclination of 70°, the steering tool will no longer fall in the hole and cannot be pumped down with air. Unlike mud, the air passing by the steering tool does not generate enough drag to carry a heavy tool down the hole. The steering tool must be installed through a side entry sub above 70° and tripped to bottom with the drill string. [/FONT] [FONT=&quot]The horizontal section of the well is surveyed with a single-shot or the steering tool as a single-shot instrument. Neither instrument can be pumped down the pipe for reasons given previously. For this reason, a reliable MWD system would be extremely advantageous. [/FONT]

[FONT=&quot]There are two methods that have been used to survey the horizontal section. Both involve tripping the drill string. The first method uses an electric line with the side entry sub. The drill string is pulled from the hole until the bit is at an inclination of 70°. A side entry sub is installed and the survey tool (single-shot or steering tool) is run to the bit on an electric line. Then, the drill string is tripped back to bottom with the remainder of the wire on the outside of the drill string. After reaching TD, a survey is taken and the drill string is tripped back out of the hole to the side entry sub. The survey tool is removed and the drill string is run back to bottom to continue drilling. As should be evident by now, surveying the horizontal section is time consuming and expensive.
Without an MWD system for air holes, it is much more difficult to use a steerable system in the horizontal section. The steerable system has to be oriented by a steering tool, and the drill string must be tripped to 70° in order to install the steering tool. The cost savings associatedwith steerable systems are derived from reducing the amount of tripping necessary to drill the well. In an air hole, tripping is required anyway, so steerable systems are frequently not cost effective especially since motor life is shorter and less predictable. Ordinarily, rotary assemblies are used to drill the horizontal sections using motor corrections as necessary.
[/FONT][FONT=&quot][/FONT]
​

​

 

[nima.mohamadin]

HOLE CLEANING

​

Like mud drilling, hole cleaning in an air hole is a problem. At inclinations above 50°, cuttings will no longer fall back to bottom and will lay on the low side of the hole. The problem is more pronounced when using mist or foam. Even when dry air is used, the volume needed to clean the hole is more than a vertical well. As a rule, the volume should be twice the volumes recommended by Angel. Even more volume is required when drilling with mist or foam. The exact volume is unknown but it is generally assumed to be at least 2.5 times the volumes recommended by Angel. For this reason, it is desirable to run downhole motors using oil as a lubricant

Drill pipe rotation aids hole cleaning in an air hole. Experience has shown that the same 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 (no drill string rotation). The cuttings are agitated and ground finer by the rotation of the drill pipe allowing the air to carry them out of the hole.

​

 

[nima.mohamadin]

HORIZONTAL SECTION LENGTH

​

The length of horizontal hole that can be drilled with air will be less than that with mud. At some point, drag will prevent the drill string or casing from falling in the hole. The drag is a function of the friction coefficient between the pipe and the wall of the hole. 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 attendant hole cleaning problems nullify the effect. The cuttings will create additional drag

A typical friction coefficient for an air hole is between 0.40 and 0.50. Mud filled holes range from 0.15 to 0.40 depending upon the type of mud
​