ABSTRACT
The petroleum reservoir rocks, for example, shown in Figure 3.1, are a sandstone sample that appear to be solid but are often not so solid. The sandstone such as the one in Figure 3.1 is basically a result of sand grains of varying sizes coming together as part of the depositional process and forming the consolidated sandstone rock, with open spaces remaining between the grains. Therefore, even though a reservoir rock looks solid to the naked eye, a microscopic examination reveals the existence of tiny openings in the rock. According to Tissot and Welte,1 most sedimentary rocks have grain diameters in the range of 0.05-0.25 mm, resulting in average radii of the void spaces or pores or tiny openings between 20 and 200 μm. These pores in petroleum reservoir rocks are the ones in which petroleum reservoir fluids are present or stored, much like a sponge soaked with water. A schematic representation of a pore space is shown in Figure 3.2. This particular storage capacity (see Equation 1.1) of reservoir rocks is called porosity. The more porous a reservoir rock material is, the greater the amount of open space or voids it contains, hence the greater the capacity to store petroleum reservoir fluids. From a reservoir engineering perspective, porosity is probably one of the most important reservoir rock properties.
