Chapter 5: Porosity

1 Formation Evaluation MSc Course Notes PorosityDr. Paul Glover Page 43 Chapter 5: and DefinitionTotal Porosity is defined as the fraction of the bulk rock volume V that is not occupied bysolid matter. If the volume of solids is denoted by Vs, and the pore volume as Vp = V - Vs, wecan write the Porosity as: ===V-VVVVPoreVolumeTotalBulkVolumesp( )The Porosity can be expressed either as a fraction or as a percentage.

2 Two out of the threeterms are required to calculate should be noted that the Porosity does not give any information concerning pore sizes, theirdistribution, and their degree of connectivity. Thus, rocks of the same Porosity can havewidely different physical properties. An example of this might be a carbonate rock and asandstone. Each could have a Porosity of , but carbonate pores are often very unconnectedresulting in its permeability being much lower than that of the range of differently defined porosities are recognised and used within the hydrocarbonindustry.

3 For rocks these are:(i)Total porosityDefined above.(ii)Connected porosityThe ratio of the connected pore volume to the total volume.(iii)Effective porosityThe same as the connected Porosity .(iv)Primary porosityThe Porosity of the rock resulting from its original depositionalstructure.(v)Secondary porosityThe Porosity resulting from diagenesis.(vi)MicroporosityThe Porosity resident in small pores (< 2 m) commonlyassociated with detrital and authigenic clays.(vii)Intergranular porosityThe Porosity due to pore volume between the rock grains.(viii)Intragranular porosityThe Porosity due to voids within the rock grains.

4 (ix)Dissolution porosityThe Porosity resulting from dissolution of rock grains.(x)Fracture porosityThe Porosity resulting from fractures in the rock at all scales.(xi)Intercrystal porosityMicroporosity existing along intercrystalline boundaries usuallyin carbonate rocks.(xii)Moldic porosityA type of dissolution Porosity in carbonate rocks resulting inmolds of original grains or fossil remains.(xiii)Fenestral porosityA holey ( bird s-eye ) Porosity in carbonate rocks usuallyassociated with algal mats.(xiv)Vug porosityPorosity associated with vugs, commonly in carbonate should be noted that if the bulk volume and dry weight, or the bulk volume, saturatedweight and Porosity of a rock sample is known, then the grain density can be calculated.

5 Thisparameter is commonly calculated from the data to compare the results with the known grainFormation Evaluation MSc Course Notes PorosityDr. Paul Glover Page 44densities of minerals as a QA check. For example the density of quartz is g cm-3, and aclean sandstone should have a mean grain density close to this on PorosityThe initial (pre-diagenesis) Porosity is affected by three major microstructural are grain size, grain packing, particle shape, and the distribution of grain , the initial Porosity is rarely that found in real rocks, as these have subsequently beenaffected by secondary controls on Porosity such as compaction and geochemical diageneticprocesses.

6 This section briefly reviews these SizeThe equilibrium Porosity of aporous material composed of arandom packing of sphericalgrains is dependent upon thestability given to the rock byfrictional and cohesive forcesoperating between individualgrains. These forces areproportional to the exposedsurface area of the grains. Thespecific surface area (exposedgrain surface area per unit solidvolume) is inversely proportionalto grain size. This indicates that,when all other factors are equal, agiven weight of coarse grains willbe stabilised at a lower porositythan the same weight of finergrains.

7 For a sedimentary rockcomposed of a given single grainsize this general rule is borne outin Figure (to the left). It canbe seen that the increase in Porosity only becomes significant at grain sizes lower than 100 m, and for some recent sediments porosities up to have been measured. As grain sizeincreases past 100 m, the frictional forces decrease and the Porosity decreases until a limit isreached that represents random frictionless packing, which occurs at Porosity , and isindependent of grain size.

8 No further loss of Porosity is possible for randomly packed spheres,unless the grains undergo irreversible deformation due to dissolution-recrystallisation,fracture, or plastic flow, and all such decreases in Porosity are termed PackingThe theoretical porosities for various grain packing arrangements can be calculated. Thetheoretical maximum Porosity for a cubic packed rock made of spherical grains of a uniformFormation Evaluation MSc Course Notes PorosityDr.

9 Paul Glover Page 45size is , and is independent of grain size. The maximum Porosity of other packingarrangements is shown in Table and Figure Maximum Porosity for different packing arrangementsPackingMaximum Porosity (fractional)Random (dependent on grain size) The porosities of standard packing ShapeThis parameter is not widely understood. Several studies have been carried out on randompackings of non-spherical grains, and in all cases the resulting porosities are larger than thosefor spheres.

10 Table shows data for various shapes, where the Porosity is for the frictionlesslimit. Figure shows data comparing rounded and angular grains, again showing that theporosity for more angular grains is larger than those that are The effect of grain shape on porosityGrain ShapeMaximum Porosity (fractional)Sphere (dependent on grain size) Evaluation MSc Course Notes PorosityDr. Paul Glover Page Size DistributionReal rocks contain a distribution of grain sizes, and often the grain size distribution is multi-modal.