Porosity

definition

The porosity definition is the void spaces in the reservoir rocks are, for the most part, the intergranular spaces between the sedimentary particles. Porosity is defined as a percentage or fraction of void to the bulk volume of the rock. While the proportion of void can be calculated from regular arrangements or uniform spheres, the arrangements within actual reservoirs is a much more complex picture and effected by many different parameters. In this case, measurements are either done in the laboratory on core samples whereby actual conditions are simulated as closely as possible prior to measurement, or in-situ via suites of electric logs such as Neutron, Density and Sonic Logs.

Processes after sedimentation (cementation, re-crystallization, weathering, fracturing etc.) can modify substantially the proportion and distribution of pore space. In reservoir engineering, only the interconnected or effective porosity is of interest since this is the only capacity which can make a contribution to flow. Pore spaces initially present but subsequently sealed off by cementation or recrystallization effects are of no interest.

Primary Porosity refers to the void spaces remaining after sedimentation of the granules in the
matrix and hence is a matrix porosity.

Secondary Porosity is the contribution from pits, bugs, fractures and other discontinuities in the bulk volume of the matrix. The contribution of secondary porosity to the overall bulk porosity is generally small yet it can lead to dramatic increase in the ease with which hydrocarbons flow through the rock.

From the reservoir engineering point of view, the distinguishing factor between primary and secondary porosity is not the mode of occurrence but the very different flow capacity where an interconnected secondary porosity system is present. This is known as a dual porosity system. In the real world of the reservoir this is often the case and one can easily see how quickly our simulated models can be made complex. Fortunately in the world of mathematical modeling certain practical assumptions are made to help unbundle this complex approach and best fit the real world to a
workable model.

The measurement of porosity

Porosity is the ratio of the pore volume to the bulk volume of the reservoir rock on percentage basis. That is

Percentage porosity =(porevolume/bulkvolume)×100

Bulk volume = the total volume of the rock
Pore volume = the volume of the pores between the grains

The measurement of porosity is important to the petroleum engineer since the porosity determines the storage capacity of the reservoir for oil and gas. It is necessary to distinguish between the absolute porosity of a porous medium and its effective porosity. In porous rocks there will always be a number of blind or unconnected pores.
Absolute porosity includes these pores as well as those open to the flow of fluids whereas the effective porosity measures only that part of the pore space that is available to fluid flow(as discussed later). The figure below shows the arrangement of pores in a piece of rock.