Page 88 - Applied Petroleum Geomechanics

Rock physical and mechanical properties 79

For the gas-filled low porosity formation, (1 f) >> fr f /r m . Thus,
Eq. (2.92) can be simplified as:
ð1 fÞV 2
a ¼ 1 S (2.93)
V Sma
2
where the formation shear velocity V S can be obtained from downhole
sonic log (shear transit time). The shear velocity of the matrix (V Sma ) can
be obtained from the shear velocity in the minerals that the rock is
composed of. If expressed in shear transit time, Eq. (2.93) can be rewritten
in the following form:

ð1 fÞDt 2
a ¼ 1 s ma (2.94)
Dt 2 s

Figure 2.31 Biot’s coefficient computed from downhole well log data in the Bossier
and Haynesville shales. The left track shows the porosities from well log and core tests
with lithology column; the middle track presents the sonic log of the compressional
and shear transit time; the right track is the calculated Biot’s coefficient from Eq. (2.94).