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Reservoir porosity

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Reservoir Porosity; Porosity Definition; Types Porosity; Origins of Porosity in Clastics and Carbonates; Primary (Original) Porosity; Secondary (Induced) Porosity; Pore Space Porosity Classification; Absolute (or Total) Porosity; Effective Porosity; Porosity Calculated; Porosity Values; Porosity in Sandstone; Sandstones Porosity Types; Factors That Affect Porosity in Sandstones ; Grain Packing in Sandstone; Progressive Destruction of Bedding Through Bioturbation; Dual Porosity in Sandstone; Dissolution Porosity in Sandstone; Porosity in Carbonate; Carbonates Porosity Types; Idealized Carbonate Porosity Types; Comparison of Total and Effective Porosities; Reservoir Average Porosity; MEASUREMENT OF POROSITY

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Reservoir porosity

  1. 1. @Hassan Z. Harraz 2019 Reservoir Porosity Prof. Dr. Hassan Z. Harraz Geology Department, Faculty of Science, Tanta University hharraz2006@yahoo.com Spring 2019 Many slides contain more detailed notes that may be shown using the “Notes Page View” 1
  2. 2. OBJECTIVES ❑To provide an understanding of ❖The concepts of rock matrix and porosity ❖The difference between original (primary) and induced (secondary) porosity ❖The difference between Total and Effective Porosity ❖Laboratory methods of porosity determination ❖Determination of porosity from well logs ❖Discussion Topics: • Origins and descriptions • Factors that effect porosity • Methods of determination 2
  3. 3. Outlines 1) Porosity in Sandstone: ▪ Major Components of Sandstone ▪ Sandstone Classification ▪ Sandstones Porosity Types ▪ Factors That Affect Porosity in Sandstones ▪ Grain-Size Sorting in Sandstone ▪ Grain Packing in Sandstone ▪ Types of Textural Changes Sensed by the Naked Eye as Bedding ▪ Progressive Destruction of Bedding Through Bioturbation ▪ Diagenesis ▪ Dual Porosity in Sandstone ▪ Dissolution Porosity in Sandstone ▪ The role of Rock Texture 2) Porosity in Carbonate: ▪ Carbonate Rock Classification ▪ Carbonates Porosity Types ▪ Idealized Carbonate Porosity Types ❑ Comparison of Total and Effective Porosities ❑ Reservoir Average Porosity ❑ MEASUREMENT OF POROSITY: ▪ Core samples (Laboratory) ▪ Openhole wireline logs 3 ❑ Introduction ❑ Reservoir Porosity ❑ Porosity Definition ❑ A Pore space ❑ Rock Matrix and Pore Space ❑ Classification of Rocks ❑ Sedimentary Rock Types ❑ Grain-Size Classification for Clastic Sediments ❑ Comparison of Compositions of Clastic and Carbonate Rocks ❑ Types Porosity: I) Origins of Porosity in Clastics and Carbonates: 1) Primary (Original) Porosity 2) Secondary (Induced) Porosity II) Pore Space Porosity Classification: 1) Absolute (or Total) Porosity 2) Effective Porosity ❑ Porosity Calculated ❑ Porosity Values
  4. 4. Introduction ❑Reservoir rocks may range from very loose and unconsolidated sand to a very hard and dense sandstone, limestone, and dolomite. ❑Knowing the physical properties of the rock and the existing interactions between the hydrocarbon system and the formation is essential in understanding and evaluating the performance of a given reservoir ❑Rock properties are determined by performing laboratory analysis on cores from the reservoirs. ❑These laboratory core analysis are divided in to: ➢routine (porosity, permeability, and saturation) and ➢special tests (overburden pressure, capillary pressure, relative permeability, wettability, and surface and interfacial tension) ❑The rock properties data are essential for reservoir engineering calculations as they affect both the quantity and the distribution of HC and with the fluid properties, control flow of existing phases within the reservoir. 4
  5. 5. Definition: ▪ Porosity is the fraction of a rock that is occupied by voids (pores). ▪ Ratio of the volume of space to the total volume of a rock. ▪ Porosity is expressed as a percentage (%) of the total rock which is taken up by pore space. ▪ Porosity units are fraction or % ▪ Quantitatively ➢ Example: a sandstone may have 8% porosity……This means 92 % is solid rock and 8 % is open space containing oil, gas, or water. ❖ The porosity is a measure of the storage capacity (pore volume) that is capable of holding fluids ❖ Porosity of a rock is a measure of its ability to hold a fluid. ❖ Porosity is an intensive property describing the fluid storage capacity of rock. Reservoir Porosity 5
  6. 6. A Pore • The texture of a rock consists of it's grain or mineral crystal size, the arrangement of the grains or crystals, and the degree of uniformity of the grains or crystals. 6 Elements of Pore Throat: • Size & frequency distribution-uncorrelated, Correlated • Connectivity of pores and throat-No of pore throat connecting to pores • Spatial arrangement-Arrangement of pores of different sizes w.r.t each other
  7. 7. Rock Matrix and Pore Space Note different use of “matrix” by geologists and engineers 7 • Water often exists as a thin film coating the rock grain surface. • Oil and gas occupy the larger pore spaces with a film of water between the rock surface and the hydrocarbons. • Geologist - Rock matrix is the grains of sandstone, limestone, dolomite, and/or shale that do not make up the supporting structure. • Engineer - Matrix is the non-pore space • Pore space is filled with fluids (water, oil, natural gas)
  8. 8. Classification of Rocks Sedimentary Rock-forming process Sourceof material Igneous Metamorphic Molten materials in deep crust and upper mantle Crystallization (Solidification of melt) Weathering and erosion of rocks exposed at surface Sedimentation, burial and lithification Rocks under high temperatures and pressures in deep crust Recrystallization due to heat, pressure, or chemically active fluids 8
  9. 9. 9
  10. 10. Relative Abundances Sedimentary Rock Types Clastic and Carbonate Rocks 10
  11. 11. Grain-Size Classification for Clastic Sediments 11
  12. 12. Comparison of Compositions of Clastic and Carbonate Rocks 12
  13. 13. 13 Genetic Porosity Classification (Origins of Porosity in Clastics and Carbonates) 1) Primary (Original) Porosity 2) Secondary (Induced) Porosity Pore Space Porosity Classification (In Terms of Fluid Properties) 1) Absolute (or Total) Porosity 2) Effective Porosity Types Porosity
  14. 14. 1) Primary (Original) Porosity ❑ Porosity of the rock that formed at the time of its deposition. ❑ Primary porosity of a sediment or rock consists of the spaces between the grains. ❑ Primary porosity decrease due compaction and packing of grains. ❑ Primary porosity less than one percent in crystalline rocks like granite. ❑ Developed during the deposition of the rock material (e.g., intergranular porosity of sandstone and intercrystalline porosity of limestone). ❑ Typified by: ➢ Intergranular pores of clastics or carbonates ➢ Intercrystalline and fenestral pores of carbonates ❑ Rocks with the original porosity are more uniform in their characteristics ❑ Usually more uniform than induced porosity ❑ more than 55% in some soils I) Origins of Porosity in Clastics and Carbonates (Genetic Porosity Classification) 14
  15. 15. 2) Secondary (Induced) Porosity ❑ develops after deposition of the rock. ❑Developed by some geologic processes after deposition of the rock (diagenetic processes) ❑Examples: ➢Grain dissolution in sandstones or carbonates ❑ Vugs and solution cavities in carbonate rocks created by the chemical process of leaching. ❑ Fracture: Fracture development in some sandstones, shales, and carbonates (Examples of geologic processes include faulting and uplifting). 15
  16. 16. II) Pore Space Porosity Classification (In Terms of Fluid Properties) ❑Some void spaces become isolated due to excessive cementation, thus many void spaces are interconnected and others are isolated. This leads to the following classification: 1) Absolute (abs) [or Total (t)] Porosity is the ratio between the total pore volume (interconnected pores and isolated ones) and the bulk volume of material (i.e., the ratio of the entire pore space in a rock to its bulk volume). 2) Effective Porosity (e) ➢is the ratio between the interconnected pore space volume and the bulk volume, ➢indicates the percentage of the total volume of reservoir rock where the void space is connected by flow channels. 16
  17. 17. Porosity Calculated Vp is pore volume, ft3 Vb is bulk volume (Vb= Vg + Vp) or Bulk volume of reservoir rock, ft3 Vg is grain volume, ft3 17
  18. 18. Absolute (φabs ) or Total (t) Porosity VolumeBulk Pore VolumeTotal VolumeBulk Pore SpacectedInterconne ❖ Effective porosity: of great importance; contains the mobile fluid. ❖ Production only occurs from the interconnected pore space Effective Porosity (e) = = 18
  19. 19. Porosity Values ❑Porosity Values: • 0-5% - Negligible • 5-10%- Poor • 10-15%- Fair • 15-20%- Good • 20 -25% - Very good • >25% Excellent 19 What is good porosity? ❑Practical Scale for cut-off for Oil: • Sandstone ~8% • Limestone ~5% ❑For Natural Gas the cut off is lower Net pay: the fraction of the reservoir that has porosity above a minimum threshold (this is the sum of the productive zones)
  20. 20. 1) Porosity in Sandstone • Sandstone usually has regular grains; and is referred to as a grain-stone. • Porosity (): Determined mainly by the packing and mixing of grains. • Fractures may be present. 20 ➢ The porosity of a sandstone depends on the packing arrangement of its grains.
  21. 21. Sandstone Classification 21
  22. 22. 1) Framework: Sand (and Silt) Size Detrital Grains 2) Matrix: Silt and Clay Size Detrital Material 3) Cement: Material Precipitated Post-depositionally, During Burial. Cements Fill Pores and Replace Framework Grains 4) Pores: Voids Among the Above Components Four Major Components of Sandstone  Note different use of “matrix” by geologists and engineers 1. Framework 2. Matrix 3. Cement 4. Pores Engineering “matrix” Geologist’s Classification 22
  23. 23. Sandstones Porosity Types i) Intergranular (Primary): Interstitial Void Space between Framework Grains. ii) Micropores: Small Pores Mainly Between detrital Framework Grains or Cement. iii) Dissolution: Partial or Complete Dissolution of or Authigenic Grains (Can Also Occur Within Grains) iv) Fractures: Breakage Due to Earth Stresses. 23
  24. 24. Factors That Affect Porosity in Sandstones: i) Primary Factors: ➢ Particle sphericity and angularity ➢ Packing ➢ Sorting (variable grain sizes) ii) Secondary (Diagenetic): ➢ Cementing materials ➢ Overburden stress (compaction) ➢ Vugs, dissolution, and fractures 24
  25. 25. Factors which influence clastic depositional systems 25
  26. 26. Grain-Size Sorting in Sandstone Roundness and Sphericity of Clastic Grains 26
  27. 27. Line of Traverse (using microscope) Cement Matrix (clays, etc.) Tangential Contact Sutured Contact Long Contact Concavo-Convex Contact Grain Packing in Sandstone (modified from Blatt, 1982) This Example Packing Proximity = 40% Packing Density = 0.8 4 Types of Grain Contacts Packing Proximity Packing Density A measure of the extent to which sedimentary particles are in contact with their neighbors A measure of the extent to which sedimentary particles occupy the rock volume 27
  28. 28. CUBIC PACKING OF SPHERES Porosity = 48% RHOMBIC PACKING OF SPHERES Porosity = 27 % Note that for uniform-sized spheres with cubic packing, porosity is independent of grain size. ( ) %6.47 32 1 r8 r3/4r8 VolumeBulk VolumeMatrixVolumeBulk VolumeBulk VolumePore Porosity 3 33 =  −= − = − = = 28
  29. 29. Porosity Calculations - Uniform Spheres • Bulk volume = (2r)3 = 8r3 • Matrix volume = • Pore volume = bulk volume - matrix volume 3 r4 3  29
  30. 30. Packing of Two Sizes of Spheres Porosity = 14% Porosity increases as the range of particle size decreases Mixing of larger and smaller particles clearly has a significant affect on porosity, reducing the original porosity of 47.6% to 14%. Real formations do not consist of these perfectly shaped spheres, but these theoretical packing models help us understand the effects of particle size and distribution on porosity. 30
  31. 31. Types of Textural Changes Sensed by the Naked Eye as Bedding 31
  32. 32. Progressive Destruction of Bedding Through Bioturbation 32
  33. 33. Diagenesis Diagenesis is the Post- Depositional Chemical and Mechanical Changes that Occur in Sedimentary Rocks Some Diagenetic Effects Include Compaction Precipitation of Cement Dissolution of Framework Grains and Cement The Effects of Diagenesis May Enhance or Degrade Reservoir Quality Whole Core Misoa Formation, Venezuela Photo by W. Ayers 33
  34. 34. Dual Porosity in Sandstone Note different use of “matrix” by geologists and engineers Dual porosity is comprised of two entirely different types of void space, intergranular pore space, and voids in fractures: 1) Primary and secondary “matrix” porosity system 2) Fracture porosity system1. Framework 2. Matrix 3. Cement 4. Pores Engineering “matrix” Geologist’s Classification 34
  35. 35. Sandstone Composition, Framework Grains Average Detrital Mineral Composition of Shale and Sandstone 35
  36. 36. Porosity in Sandstone Photomicrograph by R.L. Kugler 36
  37. 37. Secondary Electron Micrograph Clay Minerals in Sandstone Reservoirs, Authigenic Chlorite Jurassic Norphlet Sandstone Offshore Alabama, USA (Photograph by R.L. Kugler) Occurs as Thin Coats on Detrital Grain Surfaces Occurs in Several Deeply Buried Sandstones With High Reservoir Quality Iron-Rich Varieties React With Acid ~ 10 mm 37
  38. 38. Electron Photomicrograph Clay Minerals in Sandstone Reservoirs, Fibrous Authigenic Illite Jurassic Norphlet Sandstone Hatters Pond Field, Alabama, USA (Photograph by R.L. Kugler) Illite Significant Permeability Reduction Negligible Porosity Reduction Migration of Fines Problem High Irreducible Water Saturation 38
  39. 39. Intergranular Pore and Microporosity Intergranular Pores Contain Hydrocarbon Fluids Micropores Contain Irreducible Water Backscattered Electron Micrograph Carter Sandstone, Black Warrior Basin, Alabama, USA (Photograph by R.L. Kugler) 39
  40. 40. Clay Minerals in Sandstone Reservoirs, Authigenic Kaolinite Secondary Electron Micrograph Carter Sandstone North Blowhorn Creek Oil Unit Black Warrior Basin, Alabama, USA Significant Permeability Reduction High Irreducible Water Saturation Migration of Fines Problem (Photograph by R.L. Kugler) 40
  41. 41. Dissolution Porosity in Sandstones 41
  42. 42. The role of Rock Texture… Soi=(1-Swi) high Soi=(1-Swi) low 42
  43. 43. Poor Reservoir Rock (Isolated Void Space) • This sandstone would not be an acceptable reservoir rock, regardless of the value of its porosity and the hydrocarbon saturations, because each void space is isolated from the other void spaces. • This sandstone has a high absolute porosity but a zero effective porosity Good Reservoir Rock (Interconnected Void Space) • This is sandstone would be an acceptable reservoir rock because of the interconnected pore spaces and hydrocarbon saturation. • This sandstone has a high absolute porosity and a high effective porosity 43
  44. 44. Pore Network-Reconstructed using thin section IMAGE Analysis Porosity intergranular- 0.23 Porosity total- 0.28 Absolute Permeability- 426md Porosity intergranular- 0.37 Porosity total- 0.39 Absolute Permeability- 5600md 44
  45. 45. 2) Porosity in Carbonates 45 Folk Carbonate Rock Classification
  46. 46. Dunham Carbonate Rock Classification • Carbonate rocks are often subjected to early cementation, so reservoir quality depends very strongly on dissolution, fracturing and dolomitization. ➢Most carbonate reservoirs are due to secondary porosity. • Reefs sometimes preserve primary porosity. 46
  47. 47. Carbonates Porosity Types i) Interparticle porosity: Each grain is separated, giving a similar pore space arrangement as sandstone. ii) Intergranular porosity: Pore space is created inside the individual grains which are interconnected. iii) Intercrystalline porosity: Produced by spaces between carbonate crystals. iv) Mouldic porosity: Pores created by the dissolution of shells, etc. v) Fractured porosity: Pore spacing created by the cracking of the rock fabric. vi) Channel porosity: Similar to fracture porosity but larger. vii) Vuggy porosity: Created by the dissolution of fragments, but unconnected. 47
  48. 48. Carbonates Porosity Types Interparticle Intraparticle Intercrystal Moldic Pores Between Particles or Grains Pores Within Individual Particles or Grains Pores Between Crystals Pores Formed by Dissolution of an Individual Grain or Crystal in the Rock Fenestral Fracture Vug Primary Pores Larger Than Grain-Supported Interstices Formed by a Planar Break in the Rock Large Pores Formed by Indiscriminate Dissolution of Cements and Grains 48
  49. 49. Idealized Carbonate Porosity Types (modified from Choquette and Pray, 1970) 49
  50. 50. Carbonate Porosity - Example 50
  51. 51. 51
  52. 52. Comparison of Total and Effective Porosities • Very clean sandstones : e → t • Poorly to moderately well -cemented intergranular materials: t  e • Highly cemented materials and most carbonates: e < t ▪ Effective porosity (e) → of great importance; contains the mobile fluid ➢Production only occurs from the interconnected pore space. 53
  53. 53. In the geology section, we show core photographs with examples of porosity. For now, it is useful to note these effects: ❖ Secondary (induced) porosity are more complex than primary (Original) porosity. ❖ Porosity increases as angularity of particles increases. ❖ Porosity increases as the range of particle size decreases. ❖ In contrast, porosity decreases as the volume of interstitial and cementing material increases. ❖ Porosity decreases as the compaction increases (greater depth generally means higher overburden stresses, higher compaction forces, and lower porosity) ❖ Vugs and fractures will contribute to porosity, but to understand their affect on effective porosity requires careful study of cores and special logging measurements. ❖ A Total Porosity less the fraction of the pore space occupied by shale or clay ❖ In very clean sands, Total Porosity is equal to Effective Porosity. ❖ Effective porosity – of great importance; contains the mobile fluid ❖ Production only occurs from the interconnected pore space. 54
  54. 54. Reservoir Average Porosity • In case of large variation in the porosity vertically and no or small variation horizontally or parallel to the planes, then the arithmetic average or thickness-weighted average porosity is used: 55 Due to the change in sedimentation or depositional conditions can cause porosity in one portion of the reservoir to be greatly different from that in another area, so the areal-weighted average or the volume-weight average can be used:
  55. 55. MEASUREMENT OF POROSITY • Core samples (Laboratory) • Openhole wireline logs 56
  56. 56. Sandstone Porosity Measured by Various Techniques 57
  57. 57. Information From Cores* • Porosity • Horizontal permeability to air • Grain density • Vertical permeability to air • Relative permeability • Capillary pressure • Cementation exponent (m) and saturation exponent (n) Standard Analysis Special Core Analysis *Allows calibration of wireline log results 58
  58. 58. Coring Assembly and Core Bit 59
  59. 59. Coming Out of Hole With Core Barrel 60
  60. 60. Whole Core Photograph, Misoa “C” Sandstone, Venezuela Whole Core Photo by W. Ayers 61
  61. 61. Sidewall Sampling Gun Core bullets Core sample Formation rock 62
  62. 62. Sidewall Coring Tool Coring bit Samples 63
  63. 63. Whole Core Analysis vs. Plugs or Sidewall Cores Whole Core • Provides larger samples • Better and more consistent representation of formation • Better for heterogeneous rocks or for more complex lithologies 64 Plugs or Sidewall Cores • Smaller samples • Less representative of heterogeneous formations • Within 1 to 2% of whole cores for medium-to high-porosity formation • In low-porosity formations,  from core plugs tends to be much greater than  from whole cores • Scalar effects in fractured reservoirs
  64. 64. Sparks and Ayers, unpublished CORE PLUG 65
  65. 65. Student Questions / Answers • intraparticle porosity in carbonates (JC1): • vugs and fractures • why are clays important (JC1): • one major reason is that clays conduct electricity, this can effect water saturation calculations if not accounted for • fines (ABW): • solid particles so small that they can flow with fluids through pores - but they can also plug pore throats • tortuousity (ABW): • the indirect curvy flow path through the pore system to get from point A to point B • holocene: • referring to the Holocene Epoch (geology) or in general meaning about the last 10,000 years. 66
  66. 66. REFERENCES: • Bradley, H.: “Petroleum engineering handbook-chapter 26 properties of reservoir rocks”, 1987 • Ursin, J. and Zolotukhin, A.B.: “Introduction to reservoir engineering-Fundamentals-4-fundamentals of rock properties”, Stavanger,1997. • Folk, R.L. (1974). Petrology of Sedimentary Rocks, 2nd edn., Hemphill Publication Company, Texas, 182pp. ISBN: 0914696033, 9780914696032 • Folk, R.L., Ward, W.C. (1957). Brazos River bar: A study in the significance of grain size parameters. J • Pettijohn, F.J. (1975). Sedimentary Rocks. 2nd edition, Harper and Row, New York, 183 pp. • Pettijohn, F.J.; Potter, P.E., Siever, R. (1987). Sand and Sandstones. Springer, New York, 553 pp. 67