Page 279 - Advanced Mine Ventilation

256 Advanced Mine Ventilation

display unit. The collected data are digitized and transmitted acoustically through the
drill string. Outside the borehole, a magnetic pickup located on the borehole wellhead
(or the drill string) receives the signal and displays data sequentially on the display
unit. This system has a depth limit of 3000 ft. Recently, a hard-wired communication
system was put into use. All drill rods have an insert.
When put together, it provides a solid conductor to transmit data, with a range of
well over 5000 ft. It works so well that it has totally replaced acoustic transmission-
type instruments.
All downhole electronic parts are housed in approved explosion-proof aluminum
tubing that is watertight and rugged enough to withstand the rigorous downhole envi-
ronment. The DDM can read pitch, roll, and azimuth with a resolution of 0.1, 1, and
1 degrees, respectively. The ranges for pitch, roll, and azimuth are 0e90, 0e360, and
0e360 degrees, respectively.
The portable data collection and display unit is a battery-powered, intrinsically safe,
MSHA-approved unit for use in return airways of underground mines. The display unit
functions as a real-time analyzer to serve the operator in deciding how to orient the
bent housing for subsequent drilling and to store various parameters of the borehole
being drilled. The storage section of the display unit consists of solid-state memory
components with the capability of retaining borehole data which can be taken to the
surface and transferred to a larger and more powerful computer. This data can then
be used to plot horizontal and vertical profiles of the boreholes. The horizontal profile
(plan view) is plotted on mine maps for later use during mine development. The
display unit can also be used by the operator to check vertical deviation, horizontal de-
viation, and drilling parameters such as water pressure and rotary speed if the drilling is
done in the rotary mode (i.e., the drilling string is rotated from outside). Data are
received by the display unit via a magnetically coupled piezoelectric crystal attached
to the wellhead which converts small acoustic signals into electrical signals that are
stored in the display unit memory or a hard disc. Each data set received includes the
pitch, roll, azimuth, and gamma ray counts per minute. After the operator enters a
value corresponding to the depth of the borehole, other parameters can be calculated,
such as vertical deviation and horizontal deviation with respect to the wellhead. The
internal memory of the display unit can store up to 200 sets of borehole data. Any
particular data set can be recalled for the operator’s review.

15.2.2 Gas Production From Horizontal and Vertical Wells
The initial time-dependent flow, steady-state flow, and eventual production decline
from vertical and horizontal wells have been discussed in details by Thakur [1].
Only steady-state flow will be discussed here.

15.2.2.1 Production From Horizontal Boreholes
The most reliable production estimation for horizontal boreholes/wells is done by the
“specific gas production” rates as defined earlier. The data for some important coal
seams are presented in Table 15.2.