Sliding stabilizer assembly

Apparatus and method for drilling generally horizontal holes through subterranean coal beds for release of methane gas from the coal beds are described. A sliding stabilizer on a drill rod is selectively positioned to provide elevational control to a rotating drill bit. The stabilizer is keyed to a slot in the drill rod, and lateral offsets in the slot are used to retain the stabilizer in the desired position on the drill rod.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method and apparatus for drilling gas relief 
holes through coal beds, and more particularly to a method and apparatus 
for maintaining a rotating drill bit along a trajectory parallel to the 
bedding planes of subterranean coal beds. 
The use of rotary drilling to form long horizontal gas relief holes in coal 
beds is known in the art as a means for degasifying a coal bed in advance 
of mining. These gas relief holes are either vented or connected to a 
vacuum source to remove methane from the coal bed. The greatest problem 
encountered in drilling these gas relief holes is that of maintaining the 
bit trajectory parallel to the coal bed such that the resulting holes are 
actually through the coal bed rather than through an overlying or 
underlying formation. 
2. Description of the Prior Art 
The state of the art to which the present invention pertains is set forth 
in detail in a Bureau of Mines Report of Investigations published in 1975 
numbered 8097 and entitled "Rotary Drilling Holes in Coalbeds for 
Degasification", by Cervick et al, available in the U.S. Department of the 
Interior Library. That report describes the use of rotary drill bits 
attached to drill rods and maintained in a desired trajectory by a 
combination of bit thrust, rotational speed and drill rod centralizer 
spacing. That report further notes that locating a centralizer or 
stabilizer near the drill bit will cause a slight upward trajectory to the 
bit with proper drill thrust and bit rotational speed, and further notes 
that a downward trajectory can be obtained by locating a centralizer 
several meters behind the bit. However, relocating a centralizer to 
facilitate a change in bit trajectory due to change in bed dip or to the 
bit straying out of the coal bed for any reason has previously involved 
removal of the entire drill string. Such a procedure is time consuming and 
unproductive. The use of two centralizers as suggested in the report also 
presents problems. 
U.S. Pat. No. 3,131,778 describes a device for changing the direction of a 
hole being drilled, the device including a housing having a shaped slot 
with lateral offsets. A drill bit is connected to the housing by a pin 
movable in the slot. However, the apparatus described in that patent is 
designed for drilling generally vertical boreholes, and is not suitable 
for drilling generally horizontal gas relief holes in coal beds. 
There has been a continuing need, prior to the present invention, for an 
improved method and apparatus for controlling the trajectory of horizontal 
gas relief holes in coal beds. 
SUMMARY OF THE INVENTION 
According to the present invention, the horizontal trajectory of a rotating 
drill bit in a coal bed is controlled by use of a stabilizer or 
centralizer selectively positioned on a drill rod behind a drill bit. The 
stabilizer may be moved relative to the drill bit without removing the 
stabilizer from the borehole, thereby saving time and expense when the 
trajectory of the advancing borehole must be changed to accommodate a 
change in bed dip of a coal bed being drilled or to prevent the bit from 
straying out of the coal bed due to bit deviation from any cause. 
It is an object of the present invention to provide an improved method and 
apparatus for drilling generally horizontal gas relief holes in coal beds. 
It is a further object to provide such a method and apparatus that will 
enable an operator to follow the contour of a coal bed when drilling a gas 
relief hole without the necessity of removing the drill string to effect 
equipment changes for changing the direction of drilling. 
The accomplishment of the foregoing as well as other objects and advantages 
is obtained by the present invention, and will be apparent from 
consideration of the following detailed description thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following detailed description, in conjunction with the drawings, 
describes the most preferred version of the apparatus and method of the 
invention. 
It is often desirable in degasifying a coal bed to drill a series of gas 
relief holes for distances of 300 meters and more into the coal bed. 
Maintaining the bit on a horizontal trajectory parallel to the bedding 
planes of the coal bed is difficult. The natural tendency of the bit 
during horizontal drilling is to arc downward due to the forces of 
gravity. Other factors such as inclusions in the coal bed may also cause 
the bit to tend to deviate from the plane of the coal bed. It is generally 
desirable to maintain the bit trajectory within one degree of the plane of 
the coal bed being drilled through. 
Prior to this invention, there were several approaches taken to maintain 
the bit trajectory along the desired path. The primary factors affecting 
the direction of drilling are bit thrust and bit rotational speed. As a 
general rule, decreased thrust and increased rotational speed tend to 
cause a downward effect on bit trajectory, while increased thrust and 
reduced rotational speed tend to cause an upward trajectory. It is also 
known that positioning a stabilizer or centralizer on the drill rod near 
the drill bit increases the tendency of the bit to move upwardly, while 
positioning a stabilizer several meters behind the drill bit tends to 
cause a downward trajectory of the bit. Other factors such as formation 
hardness, bit type and drill rod weight can affect the trajectory of the 
hole. An experienced operator must consider the above as well as other 
factors in controlling the trajectory of the hole. Using prior art 
procedures, it was sometimes necessary, upon encountering a downward dip 
in a coal bed, to remove the drill string from the hole and remove a 
stabilizer from adjacent the bit. This procedure is time-consuming and 
costly, and it would be desirable to be able to reposition the stabilizer 
on the drill rod without the necessity of removing the drill string from 
the hole, particularly after the hole has been drilled a substantial 
distance into the formation. 
The apparatus according to the preferred embodiment of this invention will 
now be described in detail. Referring to FIG. 1, a length of drill rod 10 
is shown having a slot 11 formed along the length thereof. Slot 11 inludes 
a lateral offset 12 at the forward end of slot 11 and a lateral offset 13 
at the rearward end of slot 11. The slot 11 may be from three to ten 
meters long, but preferably is about five to seven meters long. Drill rod 
10 includes a drill bit 14 attached at the forward end thereof. 
Referring to FIG. 2, a stabilizer 15 including a cylindrical section 16 and 
helical vanes 17 attached to the outer surface thereof is shown. A key 18 
protrudes from the inner wall of cylindrical section 16 to guide the 
stabilizer 15 along the length of drill rod 10 due to the action of key 18 
and slot 11. Key 18 is also adapted to fit in the lateral offsets 12 and 
13 upon being positioned adjacent thereto and upon clockwise rotation of 
drill rod 10. Key 18 can be released from lateral offsets 12 and 13 by 
counter-clockwise rotation of drill rod 10 allowing longitudinal movement 
of stabilizer 15 along the length of slot 11. 
FIGS. 3 and 4 respectively illustrate the forward and rearward position of 
stabilizer 15 on drill rod 10. In the embodiment illustrated, the drill 
bit 14 is intended for clockwise rotation, such that the rotation of drill 
rod 10 and the forward thrust will both tend to secure key 18 in lateral 
offset 12 when the stabilizer 15 is in the position illustrated in FIG. 3. 
When it is desired to re-position stabilizer 15, drill rod 10 is pulled 
back slightly, rotated counter-clockwise a fraction of a turn, and then 
moved forward such that key 18 moves rearwardly relative to slot 11. 
Continued drilling will cause central stabilizer 15 to work back along the 
length of drill rod 10 until it reaches lateral offset 13, where it will 
then be secured in the rearward position. If it is desired to re-position 
stabilizer 15 from the forward to the rear position prior to any 
additional drilling, it is only necessary to withdraw the drill rod 10 a 
distance equal to or slightly greater than the length of slot 11, followed 
by slight counter-clockwise rotation of the drill string and forward 
movement of the drill rod 10. 
It will be apparent to those skilled in the art that additional flexibility 
can be designed into the apparatus by providing additional lateral offsets 
along the length of slot 11 such that the stabilizer could be positioned 
at any desired distance from the drill bit by appropriate manipulation of 
the drill rod. Also, the apparatus could be used with only the forward 
offset 12, as the centralizer would tend to stay in the rearward position 
during drilling even without the rear lateral offset 13. However, the rear 
lateral offset 13 is helpful in maintaining the stabilizer in the rearward 
position under certain operating conditions. 
The method in accordance with the invention will now be described, having 
reference to FIG. 5. In FIG. 5, a borehole 20 is shown extending into a 
coal bed 19 in a generally horizontal direction. The stabilizer 15 is 
positioned adjacent bit 14. The coal bed 19 forward of the drill bit 14 is 
shown dipping downwardly from the horizontal, such that the trajectory of 
the bit 14 must also dip downwardly to maintain the borehole 20 within the 
bed 19. To accomplish this, the operator would withdraw the drill rod 10 a 
distance at least equivalent to the length of slot 11, rotate drill rod 10 
slightly counter-clockwise to release key 18 from forward offset 12 in 
slot 11, and then move drill rod 10 forwardly causing drill rod 10 to 
advance past stabilizer 15 until the key 18 contacts the rear of the slot 
11. Upon resuming drilling, key 18 is secured in rear offset 13, and with 
the stabilizer 15 located several meters behind bit 14, the trajectory of 
the borehole 20 will tend to drop, enabling the borehole to be maintained 
within the coal bed 19 until the bed again resumes a horizontal or upward 
direction, at which time the stabilizer 15 can again be positioned 
adjacent the drill bit without the necessity of removing the entire 
drilling apparatus from the borehole. 
The foregoing detailed description of the preferred embodiment of the 
invention is intended to be exemplary rather than limiting, and it will be 
appreciated that numerous variations and modifications could be made 
without departing from the true scope of the invention, which is to be 
defined by the appended claims.