Page 277 - Advanced Mine Ventilation

254 Advanced Mine Ventilation

wear, and it is very difficult to forecast the rate of angle build precisely. For a given
type of bit, usually a reasonable rotation speed is selected to yield a penetration rate
of 3e5 ft/min, and the thrust is varied to make the bit go up and down. At low-
thrust values the bit pitches down, but at high thrust it will go up. Thrust values and
corresponding rates of angle build for a 4-in. diameter drag bit collected for a typical
500 ft of drilling were analyzed using a computer program. A straight-line relationship
between the rate of angle build, Dq, and thrust, T, exists as given below:

Dq ¼ 6 10 5 T 0.30121 (15.1)

where Dq is in degrees per 10 ft and T is thrust in lb. In this particular case, the rotary
speed was kept steady at 250 rpm and thrust varied from 1000 to 8000 lb.
The three-cone roller and Stratapax bits were also used. They showed similar
trends, but the actual rate of angle build varied from bit to bit. With careful selection
of drilling parameters, such as the rotary speed and thrust, different kinds of bits can be
guided successfully. The drag bit is the easiest to guide but cannot drill through hard
rock inclusions in coal. Three-cone roller bits are a little more difficult to guide but will
cut through most materials. The life of roller bits is generally less than 1000 ft. Even if
the teeth remain sharp, the bearings develop some play and guidance of the bit be-
comes very poor. Stratapax bits (made of poly crystalline diamonds) need higher tor-
que but appear to be most suitable for drilling holes deeper than 3000 ft. The biggest
drawback of rotary borehole assembly is that it cannot be guided in a horizontal plane.
It therefore has a very limited use.

15.2.1.3.2 Guidance of Nonrotary Borehole Assembly
To overcome the deficiencies of a rotary drilling assembly, a nonrotary assembly was
designed. It basically consists of a bit, a deflection device immediately behind the bit,
and a downhole motor which runs on the drilling water or mud, as shown in Fig. 15.5.
The deflection device was a spring-loaded eccentric sub which exerts a constant force
on the side of the bit. The direction of this applied force depends on the orientation of
the device and determines whether the bit will be deflected up, down, left, or right. The
magnitude of this force and hence the rate of angle build are controlled by the size of
the spring. Ideally, the rate of angle build is kept below 0.5 degrees per 10 ft. In coal
seams, a side force of 50e100 lb is generally adequate. This device had a tendency to
get plugged with coal fines, and it was replaced by a “bent housing” of one degree. The
drill bit is forced to go up, down, left, or right depending on the orientation of the bent
housing.

15.2.1.4 The Downhole Drill Monitor

To guide the drill bit successfully and contain it in the coal seam, it is essential to know
both the position of the bit in relation to the roof and the floor of the coal seam and the
pitch of the bit. In the case of nonrotary columns, the roll of the bit and azimuth must