Page 259 - Advanced Mine Ventilation

236 Advanced Mine Ventilation

that gives accurate permeability data with minimal expenses. It also yields the coal reser-
voir pressure and fracture extension pressure for the design of hydraulic fracturing [1].

14.2.2 Minifrac Injection Testing
In this test, a small volume (1000e2000 gallons) of 2% KCl water is injected into the
coal formation at a low rate of 3e5 bbl/min. Normal fracking of coal is done at a much
higher rate of injection, 30e35 bbl/min. Bottom-hole pressure (BHP) is continuously
measured as the minifrac progresses. The buildup of pressure until the coal minimally
fracks is recorded. The injection is stopped as soon as about 1000e2000 gallons have
been pumped in. The BHP at this point is immediately recorded. This pressure is called
the instantaneous shut-in pressure (ISIP). The sum of the ISIP and the hydrostatic head
divided by the depth of the borehole is called the “frac gradient”. It can be used to pre-
dict permeability by history matching. A better estimate of matrix permeability is ob-
p ffiffi
tained by plotting the BHP against square root of time, t. The point where the two
straight lines intersect is the closure pressure, P C , as shown in Fig. 14.5. Like the
frac gradient, closure pressure can be used to estimate permeability.
Fig. 14.6 shows a relationship between the permeability and the closure pressure for
a US coal field. In general, the higher the closure pressure, the lower the coal
permeability.
The ISIP is 1250 psi. The depth of gas well is 1781 ft.
ISIP Hydrostatic head
Frac gradient; F.G. ¼ þ
Depth Depth

1250
or F.G. ¼ þ 0.434
1781
¼ 1.14 psi=ft

Figure 14.5 A plot of bottom-hole pressure against time. ISIP, instantaneous shut-in pressure.