Page 169 - Fundamentals of Gas Shale Reservoirs

OVERPRESSURE ESTIMATION METHODS 149
Well#2 Well#2
DT, NCT_Son ( s/ft) DT, NCT_Son ( s/ft)
140 120 100 80 60 40 140 120 100 80 60 40
2500 2500

2600 NCT_Son 2600 NCT_Son
DTCO DTCO
2700 2700 PPG_Son
OBGrad
2800 2800 OBGrad

2900 2900

3000 3000
TVD (m) 3100 Departure from NCT TVD (m) 3100

3200 3200

3300 3300

3400 3400

3500 3500

3600 3600

3700 3700
0.57
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Overburden stress grad (psi/ft) Pore press/overburden gradients (psi/ft)
FigurE 7.12 The relationship between sonic transit time, over- FigurE 7.13 Estimated pore pressure profit using Eaton
burden stress gradient, and NCT versus depth for Well #2. method (sonic) for Well #2.

7.3.3.1 Hints for Effective Stress Method Referring to
7.3.3 Effective Stress Method
Figure 7.10, to apply the effective stress method, the follow-
This method is also called equivalent depth method, and ing steps are followed:
it further stresses the importance of the NCTs in shales. • The sonic transit time at point A is the same as the sonic
The principle of the effective stress method is that the transit time at point B, which has a normal pressure
overpressured shale has the same effective stress value gradient.
with normally pressured shale that has the same porosity • The effective stress at point B is computed by subtract-
(Mouchet and Mitchell, 1989). Hints for using this method ing the normal pressure at point B from the overburden
are provided in Section 7.3.3.1, and Figure 7.10 illus- stress value at the same point.
trates the application for using the effective stress method. • Implementing the principle of the effective stress
It is important to note that the method is only applicable method, the effective stress at point B is as same as the
when the overpressure‐generating mechanism is the dis- effective stress at point A.
equilibrium compaction and the lithology at the two
points of interest is the same. If overpressure is generated
by unloading mechanisms, then the effective stress will v(A) v(B)
be overestimated, and the pore pressure will be underesti-
mated. Thus, equivalent depth method will fail due to since v ob P p then
deviation of the porosity–effective stress relationship ( P ) ( P )
from the normal trends. ob pA ob nB (7.13)