INTRODUCTION OF ENGINEERING GEOLOGY FOR UNDERGROUND ROCKS :
Density : Rock density (U ) is a measure of mass of the rock contained in a given unit volume (density = mass/volume). It is usually expressed in g/cm3
Rock density is also called bulk density. Since most of rocks are porous
media, bulk density of porous rocks depends on not only the density of the
solid matrix material, but also the density of pore fluids as well as saturation, as shown in the following equation:
where Ub is the rock or bulk density; Uma is the matrix or solid density, Uf is the fluid density, I is the porosity.
Porosity : Porosity (I ) is an important property to analyze oil and gas reservoir and aquifer storage. Porosity is defined to be the ratio of a volume of void spaces within a rock to the total bulk volume of the rock, i.e.:
In engineering practice, porosity has several descriptions, but, the two
most common 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 interconnected pores contribute to fluid flow.
Porosity is primarily controlled by the shape, size and arrangement of
the rock grains (Peng and Meng 2002). It also depends on rock mechanical
processes (such as compaction, deformation, fracture evaluation) and geochemical processes (e.g. dissolution, precipitation, mineralogical changes). different packing effects on porosity for identical spherical rock grains.