Factors governing the magnitude of porosity
Fraser and Graton determined the porosity of various packing arrangements of uniform spheres. They have shown that the cubic or wide-packed system has a porosity of 25.9%. the porosity for such a system is independent of the grain size (sphere diameter). However, if smaller spheres are mixed among the spheres of either system, the ratio of pore space to the solid framework becomes lower and porosity is reduced. Fig. shows a three-grain-size cubic packing. The porosity of this cubic packing is now approximately 26.5%.
Fig. Collection of (a) different sized and shaped sand grains; (b) spheres illustrating a cubic packing of three grain sizes
The porosities of petroleum reservoirs range from 55 TO 40% but more frequently are between 10% to 20%. The factors governing the magnitude of porosity in clastic sediments are:
- uniformity of grain size( sorting): is the gradation of grains. If small particles of silt or clay are mixed with larger sand grains, the effective (intercommunicating) porosity will be considerably reduced. These reservoir rocks are referred to as dirty or shaly. Sorting depends on at least four major factors: size range of the material, type of deposition, current characteristics, and the duration of the sedimentary process;
- degree of cementation (consolidation): highly cemented sandstones have low porosities, whereas soft unconsolidated rocks have high porosities. Cementation takes place both at the time of lithification and during rock alteration by circulating groundwater. Cementing materials include: calcium carbonate, iron sulfides, dolomite, clays, including any combination of these materials;
- amount of compaction during and after deposition: compaction tends to close voids and squeeze fluid out to bring the material particles closer together, especially fine-grained sedimentary rocks. The expulsion of fluids by compaction at an increased temperature is the basic mechanism for primary migration of petroleum from the source to reservoir rocks. Whereas compaction is an important lithifying process in claystones, shales and fine-grained carbonates, it is negligible in closely packed sandstones or conglomerates. Generally, porosity is lower in deeper, older rocks. Many carbonate rocks show little evidence of physical compaction;
- methods of packing: with increasing overburden pressure, poorly sorted angular sand grains show a progressive change from random packing to a closer packing.
Engineering classification of porosity
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During sedimentation and lithification, some of the pore spaces initially developed became isolated from the other pores by various diagenetic and catagenetic processes such as cementation and compaction. Many of the pores will be interconnected, whereas others will be completely isolated. This leads to two distinct categories of porosity: total (absolute) and effective, depending upon which pore spaces are measured in determining the volume of these pore spaces. The difference between the total and effective porosities is the isolated or non-effective porosity.
Absolute porosity is the ratio of the total void space in the sample to the bulk volume of that sample, regardless of whether or not those void spaces are interconnected. A rock may have considerable absolute porosity and yet have no fluid conductivity for lack of poor interconnection. Effective porosity is the ratio of the interconnected pore volume to the bulk volume. This porosity is an indication of the ability of a rock to conduct fluids. Effective porosity is affected by a number of lithological factors including type, content and hydration of clays present in the rock, heterogeneity of grain sizes, packing and cementation of the grains and any weathering and leaching that may have affected the rock. Many of the pores may be dead-ends with only one entry to the main channel system. Depending on wettability, these dead-end pores may be filled with water or oil, which are irreducible fluids.
In order to recover oil and gas from reservoirs, hydrocarbons must flow several hundred feet through pore channels in the rock before they reach the producing wellbore. If the petroleum occupies non-connected void spaces, it cannot be produced and is of little interest to the petroleum engineer. Therefore, effective porosity is the value used in all reservoir engineering calculations.
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