Porosity

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

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

3. 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 Art-micrograph of sandstone with oil Back Next

4. 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

5. 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

6. Sorting • 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

7. 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 Back Next

8. Parallel Cylindrical Pores • Estimation of porosity accounting to this model: Back Next

9. Regular Cubic-Packed Spheres • Estimation of porosity accounting to this model: Back Next

10. Regular Orthorhombic-Packed Spheres • Estimation of porosity accounting to this model: Back Next

11. Regular Rhombohedral-Packed Spheres • Estimation of porosity accounting to this model: Back Next

12. 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

13. 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) Back Next

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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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; m1 (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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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 Back Next

31. 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

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

33. FAQ • Add Q&A Back Next

34. 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