Porosity Volumetrics and Pore Typing

The primary objective of porosity estimations based on measurements made either from petrophysical logs or core is the volume of pore space within the rock, given simply by the equation: . . . Φ = Vp/Vb . . . The Greek letter, phi, is the standard symbol for porosity and is expressed in this equation as the ratio of the volume of void space (Vp) to the bulk volume of the rock (Vb). The simplest concepts of porosity are generally explained in terms of the packing of spheres as the sum of the pore volume of the space between the spheres. There are five basic arrangements of uniform-sized spheres that can be constructed: simple cubic, orthorhombic, double-nested, face-centered cubic, and rhombohedral packing (Hook, 2003). Each has a geometrically defined pore volume that represents an upper limit for granular rocks whose constituent grains have a variety of sizes and shapes and whose pore volumes have been reduced by compaction and diagenetic cements. This intergranular model is a useful starting point for the characterization of pores in clastic rocks and will be considered first, before reviewing the additional complexities of pore geometry introduced by dissolution in carbonate rocks. The solid framework of a sandstone consists of a nonconductive “matrix” dominated by quartz, but commonly with accessory nonconductive minerals, and conductive clay minerals, whose electrical properties are caused by cation exchange with ions in saline formation water. It is important to distinguish between connected and unconnected pores, as well as larger pores that sustain fluid movement in contrast to smaller pores filled with capillary-bound water. A graphic presentation of these components is widely used in the petrophysical literature as a reference basis to disentangle terminology that can be confusing and contradictory. In particular, the term “effective porosity” has different meanings that vary from one technical discipline to another. In their review of porosity terms, Wu and Berg (2003) concluded that many core analysts considered all porosity to be effective, log analysts excluded clay-bound water, while petroleum engineers excluded both clay-bound and capillary-bound from porosity consideration, thereby restricting effective porosity to pores occupied by mobile fluids.