1.2 Total Porosity and Effective Porosity
The porosity of a rock is a measure of its capacity to contain or store fluids. Porosity is calculated as the pore volume of the rock divided by its bulk volume.
Expressed in terms of symbols, Eq. (1.1) is represented as:
In Eq. (1.2), f = porosity; Vp = pore volume; and VB = bulk volume. Pore volume is the total volume of pore spaces in the rock, and bulk volume is physical volume of the rock, which includes the pore spaces and matrix materials (sand and shale, etc.) that compose the rock.
Two types of porosities can exist in a rock. These are termed primary porosity and secondary porosity. Primary porosity is described as the porosity of the rock that formed at the time of its deposition. Secondary porosity develops after deposition of the rock. Secondary porosity includes vugular spaces in carbonate rocks created by the chemical process of leaching, or fracture spaces formed in fractured reservoirs. Porosity is further classified as total porosity and effective porosity. Total porosity is defined as the ratio of the entire pore space in a rock to its bulk volume. Effective porosity is the total porosity less the fraction of the pore space occupied by shale or clay. In very clean sands, total porosity is equal to effective porosity. As shown in Figure 1.1, effective porosity represents pore space that contains hydrocarbon and non-clay water.1 Free formation water that is neither bound to clay nor to shale is called non-clay water. An accurate definition of effective porosity is total porosity minus volume of clay-bound water (Figure 1.1). The relationship between total porosity and effective porosity can be represented for a shaly sand model as:
Figure 1.1 Porosity model for a shaly sand reservoir (from Al-Ruwaili et al. 1 © 2004 SPE, Reproduced with permission).
In Eq. (1.3), f t = total porosity, fraction; f e = effective porosity, fraction; Vsh = volume of shale, fraction; and f sh = shale porosity, fraction. The determination of shale porosity from well logs can be difficult and erroneous because the selection of the 100% shale section can be wrong and subjective.1 For this reason, an approximate form of Eq. (1.3) is obtained by replacing shale porosity f sh with total porosity f t to get:
For a clay model, effective porosity is represented as:
In Eq. (1.5), Vcbw = volume of clay-bound water, fraction. The application of Eq. (1.5) for calculation of accurate effective porosity depends on accurate quantification of the volume of clay-bound water. This can be determined from an elemental capture spectroscopy (ECS) well logs.1