Selecting the Drillbit and Drillstring

Before selecting the bits for your drilling program, the data that you gathered as discussed in Section 1 should provide information about formation types, drilling techniques, and commonly used hydraulics.

The bit records of offset wells should be included in that data. If not, this type of information can often be obtained from bit suppliers. A review of the offset bit records will help to estimate the number and types of bits to use.

You will determine the size of the drillbits based on the sizes of the holes for the surface casing and production casing, which you selected in step 4, Selecting Hole Size, earlier in this chapter.

The bits most commonly used in drilling coalbed methane wells are tri-cone rotary bits and percussion bits.

Tri-Cone Rotary Bits

The sealed bearing,tri-cone rotary bit is the most common and the most versatile bit used in the oil and gas industry. These bits are available for drilling a variety of different formations.

A specially designed tri-cone rotary bit is available for air drilling. This bit contains ports which allow air to flow through the bearing assembly for cooling. Most tri-cone air bits are open port bits and are thus more susceptible to corrosion than sealed bearing tri-cone rotary bits.

If you drill with air only, a tri-cone air bit may provide the longest bit life. However, if you plan to alternate drilling air mist and water, a sealed bearing bit will likely last longer. You should consider using sealed bearing bits to provide the flexibility of drilling with either air-mist or water.

Percussion bits

Percussion bits are used in combination with air hammers. This type of bit is used exclusively for drilling hard formations with air or air-foam mist. As discussed earlier in step 6, Selecting a Drilling Technique, percussion drilling is necessary when drilling hard formations at very shallow depths. Percussion bits with air hammers cannot be used in soft or sloughing formations. A typical percussion bit and air hammer is shown in Figure 2-5. If you encounter a soft formation, such as the Cretaceous in the Southwestern part of the Black Warrior Basin, you should use tri-cone rotary bits with fluid.

At the Rock Creek project, the surface holes were drilled with rotary bits because the first several feet of the hole are in a soft formation. Because the State Oil and Gas Board of Alabama required less surface casing then than it does now, drilling was continued with the rotary bit down to the setting depth for the surface casing. After setting surface casing, the production holes at Rock Creek were drilled with percussion bits and air hammers.

Drill Collars

To select the number of drill collars for the drillstring, you must consider the weight-on-bit that the operator or drilling contractor has determined necessary to drill the hole. You can determine the opti-mum weight-on-bit by conducting drilloff tests or by estimating it from offset bit records. For more information on determining weight-on-bit, you may consult with drilling contractors in your areas of interest as well as drillbit suppliers.

When air drilling, the drillstring and bottomhole assembly (BHA) are subjected to high vibration loads. This vibration is often extreme on the bottomhole assembly and the connection between the BHA and the drillpipe, especially when drilling hard formations. To protect the drillstring and the drillstring/BHA connection, you should design the drillstring so that the neutral point between axial, tensile, and com-pressive stresses during normal drilling is located in the drill collars.

You can calculate the length of drill collars needed to achieve this condition by using this equation:

Length of drill collars = BW ,ft (BF) (CW) where:

BW = Desired bit weight, lb

BF = Buoyancy factor, dimensionless

(The BF for air is 1.0 because the collar weights are measured in air.)

CW = Collar Weight (in-air), lb/ft

Industry experts recommend adding ten percent to this calculation to account for unforeseen forces such as bounce, hole friction, hole deviation, etc.

Operators in the Black Warrior Basin typically run enough 6-inch collars to provide a weight-on-bit of approximately 5000

pounds/inch for tri-cone bits and 500 pounds/inch for air-hammer bits.

Stabilizers are sometimes run in the drillstring to control hole deviation. The operator must usually decide what arrangement of stabilizers, if any, to run. When determining the type and number of stabilizers to run, you should consider the desired weight-on-bit, penetration rate, and type of formations to be drilled. To learn what arrangements of stabilizers work best in your area of interest, you should consult with drilling contractors in the area.

In most parts of the the Black Warrior Basin, drilling contractors do not use stabilizers because controlling hole deviation is not a

problem. Most of the wells in the basin are drilled with air or air mist. Because air drilling requires less weight-on-bit than fluid drilling, there is less tendency for the bit to “walk,” or deviate.

However, in a few parts of the Black Warrior Basin stabilizers are needed to prevent deviation. These are areas where the formations are stressed by extensive faulting and folding. When stabilizers are used, the typical bottomhole assembly includes:

❖ Drillbit

❖ Percussion hammer

When selecting drillpipe, you should base your selection on the worst case drilling scenario. If you are drilling wells in a devel-oped area, consult with drilling contractors in the area. They likely have gained enough experience to recommend drillpipe designs that work effectively in that area. In the Black Warrior Basin, most drilling contractors use 4-1/2 inch drillpipe. For more information on designing drillstrings, refer to Additional Resources at the end of this chapter.

Drillpipe

Stabilizers

❖ Short drill collar

❖ First stabilizer

❖ Full drill collar

❖ Second stabilizer

Check Valves

You should install check valves at specific intervals in the drillstring to:

❖ Prevent backflow of cuttings into the drillstring during connections or other shut-downs that would otherwise plug the bit.

❖ Reduce the volume of air that must be bled off when mak-ing a connection.

To learn what combination of check valves works best in your area of interest, consult with drilling contractors experienced in the area.

For the Rock Creek Project, check valves were usually placed at intervals of 400 feet in the drillstring.

Because coals have a low mechanical strength, you must design the cementing program to prevent the weight of the cement from fracturing the coal formations. You can avoid fracturing coal formations during cementing by selecting proper cement and additives and proper cementing techniques.

To select a cement that is strong enough to provide a sufficient bond, but that will not fracture the coal because of its weight, follow these procedures:

Selecting Cement and Additives