1. In this module you will learn about
Porosity
Press the button to start

2. 3 Measurments of porosity
Topic Overview
1 General Aspects
2 Idealized Models
3 Measurments of porosity

3. Art-micrograph of sandstone with oil
General aspects
One may distinguish between two types of porosity, namely absolute and effective
Absolute and effective porosity are distinguished by their access capabilities to reservoir fluids
Permeable
spaces
contributes
to effective
porosity
Void spaces
contributes
to absolute
porosity
Porosity is the best known physical characteristic of an oil reservoir. It determines the volume of oil and gas present, and all recovery computations must be based on knowledge of its value.
The porosity constitute the part of the total porous rock volume which is not occupied by rock grains or fine mud rock, acting as cement between grain particles
Porosity is the ratio of void volume to the bulk volume (grains plus void space). This void space consists of pore space between grains or crystals, in addition to crack space.
A pore is a discrete void within a rock, which can contain air, water, hydrocarbons or other fluids. In a body of rock, the percentage of pore space is the porosity. In sedimentary rocks, the amount of pore space depends on the degree of compaction of the sediment (with compaction generally increasing with depth of burial), on the packing arrangement and shape of grains, on the amount of cementation, and on the degree of sorting. Typical cements are siliceous, calcareous or carbonate, or iron-bearing minerals.
Porosity determines the storage capacity of the sand and is generally expressed on a percentage basis or as a fraction or a decimal.
In oil reservoir, the porosity represent the percentage of the total space/pore volume or void space or the volume within the rock that is available for occupancy by either liquids or gases/that can contain fluid.
One may distinguish two types of porosity, namely, absolute and effective.
Art-micrograph of sandstone with oil
Back
Next

4. Absolute porosity
Total or absolute porosity is the total void space in the rock whether or not it contributes to fluid flow
Formula:
Vb = bulk volume
VPa = total void volume voids
a = absolute porosity
Back
Next

5. Effective porosity
Effective porosity implies the ratio of the total volume of interconnected voids Vp to the bulk volume Vb of the rock
Effective porosity is the percentage of interconnected void space with respect to the bulk volume.
Formula:
Effective porosity is typically less than total porosity.
Effective porosity is the interconnected pore volume or void space in a rock that contributes to fluid flow or permeability in a reservoir.It excludes isolated pores and pore volume occupied by water absorbed on clay minerals or other grains.
Effective porosity depend on several factors like rock type, heterogeneity of grain sizes and their packing, cementation, weathering, leaching, type of clay, its content and hydration, etc.
 = effective porosity
VP = total volume of interconnected voids
Vb = bulk volume
Back
Next

6. Genetically the following types of porosity can be distinguished:
Intergranular porosity
Fracture porosity
Micro- porosity
Vugular porosity
Intragranular porosity
Rock media having both fracture and intergranular pores are called double-porous or fracture-porous
media.
Back
Next

7. Intergranular porosity
Unfilled interparticle porosity (in Oolite). Porosity is black.
Back
Next

8. Fracture porosity
Fracture porosity is a form of secondary porosity generated by tectonic fracturing of the rock
Such porosity can develop in any rock, allowing the development of productive reservoir in rocks such as granites and gneisses
A strongly fractured chalk from an area of only mild deformation. Such fractures are commonly lte diagenetic, and postdate most other diagenetic features in the rock.
A strongly fractured chalk from an area of only mild deformation. Such fractures are commonly late diagenetic, and postdate most other diagenetic features in the rock.
Back
Next

9. Micro- porosity
Micro-porosity is that part of the pore space that has a characteristic dimension less than 1 micron
In general, this includes not only very small pores but also the porosity associated with surface roughness
The water in this pore space is part of the capillary-bound water and the small-pore water. Water in micropores is not expected to flow on production
The term is also defined as porosity that cannot be seen at magnifications less than 50x
Back
Next

10. Vuggy porosity. Probably solution enlarged. Porosity is black.
Vugular porosity
Vugular porosity is the pore space consisting of cavities or vugs
Vugular porosity can occur in rocks prone to dissolution, such as limestone, in which case is secondary porosity
Vuggy porosity. Probably solution enlarged. Porosity is black.
Vuggy porosity. Probably solution enlarged. Porosity is black.
Back
Next

11. Intragranular porosity
Unfilled intraparticle porosity (within a large coral fragment).
Porosity is black.
Back
Next

12. Photomicrograph of secondary porosity in sandstone
Secondary porosity is the porosity created through alteration of rock, commonly by processes such as dolomitization, dissolution and fracturing
Photomicrograph of secondary porosity in sandstone
Back
Next

13. Photomicrograph of primary porosity in sandstone
Primary porosity is the space between grains that were not compacted together completely
Photomicrograph of primary porosity in sandstone
Back
Next

14. Sandstone with quartz cement and secondary porosity
Consolidated
From the point of view of pores susceptibility to mechanical changes, one should distinguish between consolidated and unconsolidated porous media
Consolidated porous media pertain to sediments that have been compacted and cemented to the degree that they become coherent, relatively solid rock
A typical consequences of consolidation include an increase in density and acoustic velocity, and a decrease in porosity
Sandstone with quartz cement and secondary porosity
Back
Next

15. Photomicrographs of sorting in sandstones
Sorting is the tendency of sedimentary rocks to have grains that are similarly sized--i.e., to have a narrow range of sizes
Poorly sorted sediment displays a wide range of grain sizes and hence has decreased porosity
Well-sorted indicates a grain size distribution that is fairly uniform
Depending on the type of close-packing of the grains, porosity can be substantial.
Photomicrographs of sorting in sandstones
Back
Next

16. Schematic diagram of sediments and Udden-Wentworth scale
Grain size
The term rock refers to the bulk volume of the material, including the grains or crystals as well as the contained void space
The volumetric portion of bulk rock that is not occupied by grains, crystals, or natural cementing material is termed porosity
Schematic diagram of sediments and Udden-Wentworth scale
Back
Next

17. Section 2: Idealised Models
Parallel cylindrical pores
Irregular-packed spheres with different radii
Regular orthorhombic-packed spheres
Regular rhombohedral-packed spheres
Regular cubic-packed spheres
Geometric character of rock’s permeable pore space is in reality quite complicated, and may vary greatly from one rock type to another.
Several idealised models have been developed to approximate porous rock media and their varied characteristics.
The different models may serve as a ”mental image” or idealised concretisation of a rather complex porous rocks.
The advantage of idealised models, is the opportunity they offer for simple quantification and representation of characteristic parameters.
Porous medium can be represented by several types of idealised models:
Parallel Cylindrical Pores
Regular Cubic-Packed Spheres
Regular Orthorhombic-Packed Spheres
Regular Rhombohedral-Packed Spheres
Irregular-Packed Spheres with Different Radii
Back
Next

18. Parallel Cylindrical Pores
Estimation of porosity accounting to this model:
Idealised porous medium represented by a system of parallel cylindrical pores (pipes).
It is rather obvious that rocks do not have pores like this and this model gives a unrealistically high porosity value.
This model may though, be used in some situations where fluid flow under simplified conditions is modelled.
As we can se the porosity is independent of radius.
Back
Next

19. Regular Cubic-Packed Spheres
Estimation of porosity accounting to this model:
Idealised porous medium represented by a regular system of cubic-packed spheres.
Back
Next

20. Regular Orthorhombic-Packed Spheres
Estimation of porosity accounting to this model:
Idealised porous medium represented by a regular system of orthorhombic-paced spheres.
Back
Next

21. Regular Rhombohedral-Packed Spheres
Estimation of porosity accounting to this model:
Idealised porous medium represented by regular system of rhombohedral-packed spheres.
Back
Next

22. Irregular-Packed Spheres with Different Radii
The figure shows an example of an idealised porous medium represented by four populations of spheres (sorted by radii)
The histogram shows the hypothetical grain-size distribution.
Back
Next

23. Example Porous medium blended with three types of sediment fractions:
Fine pebble gravel
with porosity (pebble=0,30)
Sand (sand=0,38)
Fine sand (f.sand=0,33)
The sand fills the pore volume of the fine pebbles and the fine sand fills the pore volume of the sand.
Back
Next

24. Measurement of porosity
Core Analysis
Well Logs
Measurement of Porosity
Uncertainty
Back
Next

25. Core Analysis Full-diameter Core Analysis
Grain-volume measurements based on Boyle`s law
Fluid-Summation Method
Bulk-volume measurements
Pore-volume measurements
Back
Next

26. Section 3.1: Full-diameter Core Analysis
Used to measure the porosity of rocks that are distinctly heterogeneous. (Ex: carbonates and fissured vugular rocks)
The same core-plug is a non-representative elementary volume for this type of rock.
In heterogeneous rocks, the local porosity may be highly variable It may include:
micro-porosity
intergranular porosity
vugues
fractures various combinations of these.
A full-diameter core sample usually has a diameter of 5 inches (12,5 cm) and a length of 10 inches (25 cm)
Does not differentiate between the actual types of porosity involved.
Back
Next

27. Section 3.2: Grain-Volume Measurements Based on Boyle`s Law
Injection and decompression of gas into the pores of a fluid-free (vacuum), dry core sample.
Either the pore volume or the grain volume can be determined, depending upon the instrumentation and procedures.
Porosity measurements based on the Boyle`s law
Back
Next

28. Section 3.2: Grain-Volume Measurements Based on Boyle`s Law
Helium gas is often used due to its following properties:
The small size of helium molecules makes the gas rapidly penetrate small pores
Helium is an inert gas that will not be absorbed on the rock surface and thus yield erroneous results
Alternatives: N2 and CO2
Back
Next

29. Section 3.2: Grain-Volume Measurements Based on Boyle`s Law
Calculation of the grain volume
Ideal gas law:
In case of vacuum inside the sample chamber:
Assuming adiabatic conditions, we obtains:
Back
Next

30. Nomenclature in section 3.2
p – Pressure
p1 – Initial pressure
p2 –Final pressure
n – Total numbers of moles
R – Universal ideal gas constant
T – Temperature
V – Volume
Vref – Reference volume
VS – Volume of the sample chamber
Vg –Grain volume
Back
Next

31. Section 3.3: Bulk-Volume Measurements
This technique uses the Archimedes` principle of mass displacement:
The core sample is first saturated with a wetting fluid and then weighed.
The sample is then submerged in the same fluid and its submerged weight is measured.
The bulk volume is the difference between the two weights divided by the density of the fluid
Back
Next

32. Section 3.3: Bulk-Volume Measurements
Fluids normally used:
Water which can easily be evaporated afterwards.
Mercury which normally not enters the pore space in a core sample due to its non-wetting capability and its large interfacial energy against air.
A very accurate measurement, with a uncertainty of 0,2%.
Back
Next

33. Section 3.3: Bulk-Volume Measurements
Example: Uncertainty analysis in measuring the bulk volume using Archimedes` principle.
The core is measured in two steps:
Weighing the sample in a cup of water; m (Assuming 100% water saturation)
Then weighting the sample in air as it is removed from the cup; m2
The bulk volume is:
Differentiating the equation above gives us:
Back
Next

34. Section 3.3: Bulk-Volume Measurements
If the density measurement as well as the two mass-measurements above, is considered to be independent measurements, the relative uncertainty in the bulk volume is:
It may also be written as:
If the uncertainty in determined the water density is estimated to 0,1% and the weighting accuracy is equal to 0,1g , we find a relative uncertainty in the bulk volume of approximately 0,5%.
Back
Next

35. Nomenclature in section 3.3
Vb – Bulk volume
m1 – mass of the sample in a cup of water
m2 – mass of the sample in air
– water density
Back
Next

36. Section 3.4: Pore-Volume Measurements
A core sample is placed in a rubber sleeve holder that has no voids space around.
This is called a Hassler holder, see fig.
Helium or one of its substitutes is injected into the core plug through the end stem.
Back
Next

37. Section 3.4: Pore-Volume Measurements
Calculations of the pore volume
It is important to notice that the Hassler core holder has to be coupled to a volume of known reference, Vref.
Back
Next

38. Nomenclature in section 3.4
p0 – … pressure
p1 – … pressure
p2 – … pressure
Vp – Pore volume
Vref – Reference volume
n – Total numbers of moles
R – Universal ideal gas constant
T – Temperature
Back
Next

39. Section 3.5: Fluid-Summation Method
Technique is to measure the volume of gas, oil and water present in the pore space of a fresh or preserved core of known bulk volume.
The core sample is divided into two parts:
One part (ca. 100 g) is crushed and placed in a fluid-extraction resort. Vaporised water and oil move down and are collected in a calibrated glassware, where their volumes are measured.
Second part of the rock sample (ca. 30 g) is weighed and then placed in a pycnometer, filled with mercury. The bulk volume is determined, measuring the volume of the displaced mercury.
Then the pressure of the mercury, PHg , is raised to 70 bar. At this pressure mercury are filling the pore space originally occupied with gas. Gas volume can then be calculated
Back
Next

40. Section 3.5: Fluid-Summation Method
The laboratory procedure provides the following information:
First sub sample gives the rock`s weight, WS1 , and the volumes of oil, Vo1 , and water, VW1 , are recorded.
Second sub sample gives the volume of gas, Vg2 , and the rock`s bulk volume, Vb2.
Fraction of the gas-bulk volume:
Also:
Back
Next

41. Section 3.5: Fluid-Summation Method
The formation oil- and water factor are calculated as follow:
The sum of the fluid-volume factor then gives the porosity value:
Back
Next

42. Section 3.5: Fluid-Summation Method
Example: Use of pycnometer in matrix volume calculation.
In order to define the matrix volume, Vm , of a core sample, the following measuring steps are carried out:
The pycnometer cell is fully saturated with mercury.
The pycnometer piston is withdrawn and a gas (air) volume of V0 is measured.
The core sample is placed in the cell, and the cell volume is sealed. The equilibrium condition inside the cell is written:
Mercury is injected into the cell and a new gas volume, V1 , and pressure, is measured.
New equilibrium is reached and we write:
Finally; the matrix volume is found as follows:
Back
Next

43. Nomenclature in section 3.5
Fg,o,w – Fraction of the gas, oil, water-bulk volume
Sg,o,w – Saturation of gas, oil and water
Vg1 – Gas volume of the first subsample
Vb1 – Bulk volume of the first subsample
Vg2 – Gas volume of the second subsample
Vb2 – Bulk volume of the second subsample
– Porosity
Vo1 – Volume of oil in the first subsample
Vw1 – Volume of water in the first subsample
WS1 – Weight of the rock in the first subsample
WS2 – Weight of the rock in the second subsample
– Apparent bulk density of the fluid-saturated rock sample
Back
Next

44. Porosity Estimation from Geophysical Well Logs
Porosity can be estimated from:
Formation resistivity factor
Microresistivity log
Neutron-gamma log
Density (gamma-gamma) log
Acoustic (sonic) log
Sediment porosities can be determined from numerous borehole log measurements (reviewed by Serra, 1984). At Sites 994, 995, and 997 we have attempted to use data from the lithodensity (HLDT), neutron porosity (CNT-G), and electrical resistivity (DITE) logs to calculate sediment porosities. Core-derived physical property data, including porosities (Shipboard Scientific Party, 1996a, 1996b, 1996c), have been used to both calibrate and evaluate the log-derived sediment porosities.
Back
Next

45. Potential Error in Porosity Estimation
Experimental data
Involve a degree of uncertainty related to the possible measurement errors
The measurement of porosity is normally a function of Vp, Vm and/or Vb
Back
Next

46. Potential Error in Porosity Estimation
If the porosity is defined as
The equation can be differentiated
The potential error of prosity measurement is then
Lyd side i boka
Back
Next

47. FAQ
Add Q&A
Back
Next

48. References Figures taken with permission from the authors of
Reservoarteknikk1: A.B. Zolotukhin and J.-R. Ursin
Figures also taken with permission from Ola Ketil Siqveland
Back
Next