Page 266 - Advanced Mine Ventilation

Reservoir Properties of Coal Seams 243

are overpressurized and highly productive. The Fairway region in the San Juan basin
of the United States is a good example. Many vertical wells completed in thick, over-
pressurized coal seams have had a production of 2e10 MMCFD. The coal seam thick-
ness is 40e60 ft. Similarly, there are a few coal seams that are seriously
underpressurized and are poor producers.

14.4.1 Measurement of Reservoir Pressure
The simplest and perhaps the most reliable technique is to use a pressure gage, such as
an RPG gage from Halliburton Services.
A vertical well is drilled into the coal seam and extended into the floor for
100e200 ft. A 4.5 in. casing is set in the well just above the coal seam using a forma-
tion packer shoe and cemented to the top. The coal seam is hydrojetted with high pres-
sure water at about 3000 psi. Next, an RPG gage is lowered into the coal seam and a
packer is set just above the coal seam. The well is kept shut for 72e96 h. The gage
shows the pressure buildup on a graph paper. The asymptotic pressure on the graph
is the reservoir pressure of the coal seam. Depending on depth, most of the world’s
coal seams have pressures in the range of 100e800 psi.

14.4.2 The Vertical Pressure, sv

It is a commonly accepted fact that

s v ¼ 1.1 D psi (14.23)

where D is the depth in feet.
This is valid at least to a depth of 10,000 ft. Most coal seams occur above this depth.

14.4.3 Horner’s Plot for Reservoir Pressure Measurement
As discussed earlier, the minifrac is done to determine coal seam permeability, but it
can also be used to determine the reservoir pressure, P*. A plot of BHP against log t o þt si
t si
gives a straight line, and its intercept of “y” axis is the reservoir pressure, as shown in
Fig. 14.11.
Let us assume the minifrac was accomplished in 21 min of fluid pumping. The well
is shut and ISIP is noted. The BHP is recorded at regular time intervals of 1 min or so.
A typical data set is shown in Table 14.4.
Here t o ¼ 21 min; t si is the time after shut-in.
It is popularly called Horner’s plot.
Ground stress has three components: vertical stress, major horizontal stress, and mi-
nor horizontal stress. A reference should be made to a book published by Thakur [1]
for details.