Apparatus for directional drilling

Abstract

 A method and apparatus for drilling a substantially horizontal borehole through a subterranean mineral bed with a drilling assembly including a drill shaft 10 connected to a down-hole motor 12 and drill bit 13. A deflection device 26 is positioned between the down-hole motor and the drill bit. The deflection device applies a predetermined perpendicular force to the drill bit causing the borehole to deviate from the drill shaft axis in the direction of the force. The method includes periodically surveying the borehole to determine its position relative to the top or bottom of the mineral bed, and rotating the drill shaft through a half turn sequentially to provide an alternating upward and downward trajectory to the borehole within the upper and lower boundaries of the mineral bed.

Description

[0001] This invention relates to a method and apparatus for drilling generally horizontal boreholes in a subterranean earth formation, and more particularly to a method and apparatus for adjusting the trajectory of such a borehole to maintain same within the upper and lower boundaries of a mineral bed such as a seam of coal.

[0002] The drilling of long generally horizontal gas drainage holes in coal beds is known in the art as a method for degasifying a coal bed in advance of mining. The gas drainage holes drilled in this manner are either vented or connected to a vacuum source to remove methane from a coal bed. The greatest problem encountered in drilling these gas relief holes is that of maintaining the bit trajectory within the coal seam such that the resulting holes are actually through the coal seam rather than through an overlying or underlying formation. Control of borehole trajectory is also important in exploratory drilling for coal or other minerals.

[0003] A directional drilling system including a drill shaft, a down-hole fluid motor connected to a rotatable bit, and a deflection means is described in U.S. Patent No. 3,260,318. The deflection means described therein is a shaft housing having a slight angle formed therein to divert the drill bit from the axis of the drill shaft. Rotation of the drill shaft and housing through part of a turn changes the direction of the borehole relative to the drill shaft. However this apparatus has been found to give poor results in controlling the trajectory of generally horizontal boreholes.

[0004] Viewed from one aspect the present invention provides drilling apparatus comprising a down-hole motof a rotatable shaft for connecting said motor to a drill bit, a non-rotating housing extending from said motor over said shaft, and a deflection member carried by said housing, said deflection member extending radially beyond said housing at one side thereof sufficiently to apply a deflection force to the wall of a borehole during drilling.

[0005] In operation the deflection member applies a predetermined radial force to a drill bit, causing the trajectory of the borehole being drilled to be deflected in the direction of the applied force.

[0006] Viewed from another aspect the invention provides a method of drilling a substantially horizontal borehole through a subterranean mineral bed with a drilling apparatus having a down-hole motor which provides rotation to a drill bit, wherein:

(a) a deflection member having an outwardly biased wall contacting surface is provided on said drilling apparatus between said motor and drill bit and oriented relative to said borehole;

(b) said borehole is extended for a distance into said mineral bed while maintaining the orientation of said deflection member;

(c) information regarding the trajectory of said borehole is obtained; and

(d) said deflection member is rotated through part of a turn whereby upon resumption of drilling the trajectory of said borehole is modified.

[0007] An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which:-

Figure 1 is an illustration of a drilling apparatus showing the relation of a deflection member of the invention to the other parts of the apparatus;

Figure 2 is an exploded view showing the structural details of a preferred embodiment of the deflection member;

Figure 3 is an axial cross-sectional view of the apparatus in the region of the deflection member; and

Figure 4 is a transverse cross-section taken along the line 4-4 of Figure 3.

[0008] The relationship of a deflection member of this invention to the other parts of a drilling apparatus is shown generally in Figure 1.

[0009] Figure 1 shows a drill shaft 10 connected to an orienting sub 11 and a down-hole motor 12. Motor 12 is preferably a Moyno type fluid motor having a rotatable shaft (not shown) extending through a shaft housing 14 and connected to drill bit 13. Shaft housing 14 extends between motor 12 and bit 13. A deflection member 15 is mounted over a section of shaft housing 14, and extends radially beyond shaft housing 14 at one side. The portion of the deflection member extending beyond the shaft housing includes a wall contacting surface 26 which contacts the wall of borehole 16 being drilled through a mineral bed.

[0010] The drilling apparatus including the drill shaft orienting sub, motor, shaft housing and drill bit is preferably of the type generally described in U.S. Patent No. 3,260,318. This type of drilling apparatus, without the deflection member of this invention, has been widely used in the drilling industry.

[0011] The details of the preferred version of the deflection member of this invention are shown in Figures 2, 3 and 4.

[0012] Shaft housing 14 is machined to provide a reduced section including a top surface 17, sides 18 and 19, and bottom surface 20. Shaft housing 14 includes an internal bore 29 through which a rotatable shaft (not shown) connecting the fluid motor and the rotating drill bit extends. The main body element 21 of deflection member 15 is formed to fit closely over sides 18 and 19 of the reduced section of shaft housing 14 and to be slidable with respect thereto. Recesses 22 and 23 in main body element 21 are provided for containing springs 24 and 25 which urge borehole wall-contacting surface 26 of deflection member 15 radially outward from shaft housing 14. Bottom body element 27 attaches to main body element 21 with fasteners 28 to encompass the reduced section of shaft housing 14 and to compress springs 24 and 25. The deflection force provided by the springs depends to some extent on the particular drilling appparatus and the rate of direction change desired. When the borehole has a diameter of from 75 to 150 mm (preferably 75 to 100 mm) a deflection force of at least 225 newtons is preferred. In order to provide the preferred rate of angle build of about ₀.5⁰/3 m in a coal seam, a deflection force of 450 to 900 newtons is generally adequate.

[0013] Figures 3 and 4 show the assembled deflection member. When it is assembled and unrestrained, the springs 24 and 25 act to position the deflection member so that bottom body element 27 abuts the bottom surface 20 of housing 14, and wall contacting surface 26 extends outwardly beyond the shaft housing 14 sufficiently to apply the desired deflection force to the wall of a borehole during drilling..

[0014] The apparatus described above is particularly suited for drilling generally horizontal degasification holes in underground coal seams to reduce the methane concentration in a mine working area in advance of mining the seam. To be effective, the degasification holes must be kept within the boundaries of the coal seam. The deflection member, properly used, enables an operator to maintain the borehole between the upper and lower boundaries of the coal seam.

[0015] The method of drilling a borehole in a mineral bed using the deflection member of this invention will now be described. The drilling apparatus is set up, and after drilling an initial surface hole a short distance into the seam, the apparatus as shown in Figure 1 is inserted in the borehole and oriented with the wall-contacting surface 26 of the deflection member oriented against either the top or the bottom of the borehole. In order to guide the drill bit successfully and contain it in the mineral bed, it is essential to know the position of the bit in relation to the roof and the floor of the mineral bed and the pitch and roll of the borehole. Borehole survey instruments which provide the required information are available. These instruments may be pumped down the drill shaft to a position near the motor where they are positioned by an orienting sub, and information as to the position of the borehole may be obtained. The survey probe is then removed, and if a change in direction of the borehole is indicated, the drill shaft is rotated through a part of a turn, thereby also rotating the deflection member the same amount, and drilling is resumed. Normally, the deflection member is oriented to provide either an upward or downward force to the drill bit, although in some cases a side force may be indicated. If the borehole is moving upward, and the surveying probe indicates that the borehole is near the top surface of the mineral bed, the drill shaft and deflection member are rotated 180. to provide a downward force to the drill bit. Drilling is resumed until the surveying probe indicates that the borehole is approaching the bottom layer of the mineral bed, at which time the drill shaft and deflection member are again rotated 180°. This procedure is repeated until the borehole extends the desired distance into the mineral bed.

[0016] The deflection member in accordance with the invention has proven to be effective in drilling degasification holes in coal seams in advance of mining. Several degasification holes have been drilled through coal beds for distances greater than 300 meters, thereby providing significant reductions in methane concentrations at the time of actual mining of the coal bed.

Claims

1. Drilling apparatus comprising a down-hole motor, a rotatable shaft for connecting said motor to a drill bit, a non-rotating housing extending from said motor over said shaft, and a deflection member carried by said housing, said deflection member extending radially beyond said housing at one side thereof sufficiently to apply a deflection force to the wall of a borehole during drilling.

2. Apparatus as claimed in claim 1, wherein said deflection member is biased outwardly from said housing and movable perpendicularly to the longitudinal axis thereof.

3. Apparatus as claimed in claim 2, wherein said deflection member is spring-biased.

4. Apparatus as claimed in any of claims 1 to 3, wherein said deflection member-is mounted on said housing in a manner to assure rotation of said deflection member corresponding to rotation of said housing.

5. Apparatus as claimed in any of the preceding claims, wherein said deflection member encompasses said rotatable shaft and said housing and is adapted for sliding movement over said housing.

6. Apparatus as claimed in any of the preceding claims, wherein said deflection member is biased outwardly from said housing by a.pair of coil springs,

7. Apparatus as claimed in claim 6, wherein said pair of coil springs provide a deflection force of at least 225 newtons.

8. Apparatus as claimed in claim 7, wherein said deflection force is from 450 to 900 newtons.

9. A method of drilling a substantially horizontal borehole through a subterranean mineral bed with a drilling apparatus having a down-hole motor which provides rotation to a drill bit, wherein:

(a) a deflection member having an outwardly biased wall contacting surface is provided on said drilling apparatus between said motor and drill bit and oriented relative to said borehole;

(b) said borehole is extended for a distance into said mineral bed while maintaining the orientation of said deflection member;

(c) information regarding the trajectory of said borehole is obtaining; and

(d) said deflection member is rotated through part of a turn whereby upon resumption of drilling the trajectory of said borehole is modified.

10. A method as claimed in claim 9, wherein said deflection member is oriented-to cause an upward pitch to said borehole trajectory and said orientation is maintained until said borehole approaches the upper boundary of said mineral bed, at which point said deflection member is re-oriented to provide a downward pitch to said borehole trajectory.

11. A method as claimed in claim 10, wherein said downward pitch is maintained until said borehole approaches the lower boundary of said mineral bed, at which time said deflection member is again oriented to provide an upward trajectory.

12. A method as claimed in claim 11, in which the orientation of said deflection member is sequentially adjusted to provide alternating upward and downward trajectory of said borehole between the upper and lower boundaries of said mineral bed as said borehole progresses therethrough.

Drawing