It is an object of the present invention to provide an SLIDING STABILIZER ASSEMBLY improved method and apparatus for drilling generally
n A ... r\v Tuc Txta/cxttt/^vt horizontal gas relief holes in coal beds.
BACKGROUND OF THE INVENTION T. . f °. u- * * -j u J
It is a further object to provide such a method and
1. Field of the Invention 5 apparatus that will enable an operator to follow the This invention relates to a method and apparatus for contour of a coal bed when drilling a gas relief hole
drilling gas relief holes through coal beds, and more without the necessity of removing the drill string to
particularly to a method and apparatus for maintaining effect equipment changes for changing the direction of
a rotating drill bit along a trajectory parallel to the drilling.
bedding planes of subterranean coal beds. 10 The accomplishment of the foregoing as well as other
The use of rotary drilling to form long horizontal gas objects and advantages is obtained by the present inven
relief holes in coal beds is known in the art as a means tion, and will be apparent from consideration of the
for degasifying a coal bed in advance of mining. These following detailed description thereof.
gas relief holes are either vented or connected to a , „„„„ „ „„ m„ „„ . ,„„
vacuum source to remove methane from the coal bed. ^ BRIEF DESCRIPTION OF THE DRAWINGS
The greatest problem encountered in drilling these gas FIG. 1 is a view of a length of drill rod having a slot
relief holes is that of maintaining the bit trajectory par- formed according to the invention,
allel to the coal bed such that the resulting holes are FIG. 2 is an orthogonal projection of a stabilizer in
actually through the coal bed rather than through an accordance with the invention.
overlying or underlying formation. 20 FIG. 3 is a view of a length of drill rod with a stabi
2. Description of the Prior Art lizer positioned adjacent a drill bit.
The state of the art to which the present invention FIG. 4 is a view of a length of drill rod with a stabipertains is set forth in detail in a Bureau of Mines Report lizer positioned remote from a drill bit. of Investigations published in 1975 numbered 8097 and FIG. 5 is a view illustrating the apparatus of the inentitled "Rotary Drilling Holes in Coalbeds for Degasi- vention in use.
fication", by Cervick et al, available in the U.S. Depart- „„„n
ment of the Interior Library. That report describes the S^tsot^^
use of rotary drill bits attached to drill rods and main- EMBODIMENT
tained in a desired trajectory by a combination of bit 3Q The following detailed description, in conjunction
thrust, rotational speed and drill rod centralizer spacing. with the drawings, describes the most preferred version
That report further notes that locating a centralizer or of the apparatus and method of the invention,
stabilizer near the drill bit will cause a slight upward It is often desirable in degasifying a coal bed to drill
trajectory to the bit with proper drill thrust and bit a series of gas relief holes for distances of 300 meters
rotational speed, and further notes that a downward 35 and more into the coal bed. Maintaining the bit on a
trajectory can be obtained by locating a centralizer horizontal trajectory parallel to the bedding planes of
several meters behind the bit. However, relocating a the coal bed is difficult. The natural tendency of the bit
centralizer to facilitate a change in bit trajectory due to during horizontal drilling is to arc downward due to the
change in bed dip or to the bit straying out of the coal forces of gravity. Other factors such as inclusions in the
bed for any reason has previously involved removal of 40 coal bed may also cause the bit to tend to deviate from
the entire drill string. Such a procedure is time consum- the plane of the coal bed. It is generally desirable to
ing and unproductive. The use of two centralizers as maintain the bit trajectory within one degree of the
suggested in the report also presents problems. plane of the coal bed being drilled through.
U.S. Pat. No. 3,131,778 describes a device for chang- Prior to this invention, there were several approaches
ing the direction of a hole being drilled, the device 45 taken to maintain the bit trajectory along the desired
including a housing having a shaped slot with lateral path. The primary factors affecting the direction of
offsets. A drill bit is connected to the housing by a pin drilling are bit thrust and bit rotational speed. As a
movable in the slot. However, the apparatus described general rule, decreased thrust and increased rotational
in that patent is designed for drilling generally vertical speed tend to cause a downward effect on bit trajectory,
boreholes, and is not suitable for drilling generally hori- 50 while increased thrust and reduced rotational speed
zontal gas relief holes in coal beds. tend to cause an upward trajectory. It is also known
There has been a continuing need, prior to the present that positioning a stabilizer or centralizer on the drill
invention, for an improved method and apparatus for rod near the drill bit increases the tendency of the bit to
controlling the trajectory of horizontal gas relief holes move upwardly, while positioning a stabilizer several
in coal beds. 55 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 According to the present invention, the horizontal trajectory of the hole. An experienced operator must trajectory of a rotating drill bit in a coal bed is con- consider the above as well as other factors in controltrolled by use of a stabilizer or centralizer selectively 60 ling the trajectory of the hole. Using prior art procepositioned on a drill rod behind a drill bit. The stabilizer dures, it was sometimes necessary, upon encountering a may be moved relative to the drill bit without removing downward dip in a coal bed, to remove the drill string the stabilizer from the borehole, thereby saving time from the hole and remove a stabilizer from adjacent the and expense when the trajectory of the advancing bore- bit. This procedure is time-consuming and costly, and it hole must be changed to accommodate a change in bed 65 would be desirable to be able to reposition the stabilizer dip of a coal bed being drilled or to prevent the bit from on the drill rod without the necessity of removing the straying out of the coal bed due to bit deviation from drill string from the hole, particularly after the hole has any cause. been drilled a substantial distance into the formation.