Page 45 - Applied Petroleum Geomechanics

36 Applied Petroleum Geomechanics

Porosity can be determined by lab experiments in core samples and by
well logs, such as density, resistivity, neutron, and NMR (Nuclear Mag-
netic Resonance) logs. The densityeporosity equation (Eq. 2.1) can be
rewritten to the following form for calculating porosity purpose:
r r b
m
f ¼ (2.6)
r r f
m
Fig. 2.4 illustrates how to use the density log to calculate porosity from
Eq. (2.6). The calculated porosities are also compared to the measured
porosities from core samples in the Haynesville shale gas formation. In
3
the calculation, the following parameters are used: r m ¼ 2.67 g/cm ,
3
r f ¼ 1.05 g/cm , and bulk density r b was obtained from the density log
shown in the left plot of Fig. 2.4.
For clean sandstones with moderate porosities, porosity is often esti-
mated by the empirical timeeaverage equation (Wyllie et al., 1956):
Dt Dt m
f ¼ (2.7)
Dt f Dt m
where Dt, Dt m , and Dt f are the sonic (or acoustic) compressional transit time
of the formation, rock matrix, and pore fluid, respectively.

Bulk density (g/cm 3 ) Porosity (fraction)
2 2.5 3 0 0.1 0.2 0.3
11500 11500
Core
Rhob
From Rhob
12000 12000

12500 12500
Depth (ft) 13000 Depth (ft) 13000

13500 13500

14000
14000
Figure 2.4 Porosity measured from core samples and calculated from density log
using Eq. (2.6) in the Haynesville shale gas formation.