Page 44 - Applied Petroleum Geomechanics

Rock physical and mechanical properties 35

2.6

2.4
2.2
Bulk density (g/cm 3 ) 1.8 2

1.6
1.4
Density fit
ODP sand/silt/clay
1.2 ODP silty clay
Ostermeier 2001
1
0 500 1000 1500 2000 2500 3000 3500 4000
Depth (ft bml)
Figure 2.3 Density curve (the line) obtained from Eqs. (2.3) and (2.4) compared to the
measured shallow bulk density from the Ocean Drilling Program (ODP) and Ostermeier
et al. (2001) (Zhang et al., 2008).

In engineering practice, porosity has several descriptions, but the two
most common terms are total porosity (defined as the above) and effective
porosity. Effective porosity represents the ratio of the interconnected pore
space to the total bulk volume of the rock. Therefore, effective porosity is
also called interconnected porosity. The fluids in the interconnected pores
contribute to fluid flow. Porosity is not only primarily controlled by the
shapes, sizes, and arrangements of the rock grains but also dependent on
rock mechanical processes (such as compaction, deformation, fracture
evolution) and geochemical processes (e.g., dissolution, precipitation,
mineralogical changes). Rock grain packing types have important effects on
porosity, and Table 2.3 lists the effects of several ideal packing types on
porosity, assuming that the rock consists of identical spherical rock grains.

Table 2.3 Porosity in different packing types of spherical rock grains.
Packing type f
Cubic 0.48
Orthorhombic 0.40
Tetragonal 0.30
Rhombohedral 0.26