POROSITY

1
POROSITY
Many slides contain more detailed notes that may
be shown using the Notes Page View
2
Acknowledgments

• Dr. Walt Ayers, PETE 311, Fall 2001
• NExT PERF Short Course Notes, 1999
• Note that many of the NExT slides appears to have
  been obtained from other primary sources that are
  not cited

3
RESERVOIR POROSITY

• Definition Porosity is the fraction of a rock
  that is occupied by voids (pores).
• Discussion Topics
• Origins and descriptions
• Factors that effect porosity
• Methods of determination

4
ROCK MATRIX AND PORE SPACE
Note different use of matrix by geologists and
engineers
5
POROSITY DEFINITION

• Porosity The fraction of a rock that is
  occupied by pores

• Porosity is an intensive property describing the
  fluid storage capacity of rock

6
ROCK MATRIX AND PORE SPACE
7
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

8
CLASSIFICATION OF ROCKS
IGNEOUS
SEDIMENTARY
METAMORPHIC
Molten materials in deep crust and upper mantle
Weathering and erosion of rocks exposed at
surface
Rocks under high temperatures and pressures in
deep crust
Crystallization (Solidification of melt)
Sedimentation, burial and lithification
Recrystallization due to heat, pressure,
or chemically active fluids
9
SEDIMENTARY ROCKS

• Clastics
• Carbonates
• Evaporites

10
CLASTIC AND CARBONATE ROCKS
Clastic Rocks
Consist Primarily of Silicate Minerals
Are Classified on the Basis of
- Grain Size
- Mineral Composition

Carbonate Rocks
Consist Primarily of Carbonate Minerals
-2
(i.e. Minerals With a CO Anion Group)
3
Limestone - Predominately Calcite (Calcium
Carbonate, CaCO3)
Dolomite - Predominately Dolostone (Calcium
Magnesium Carbonate, CaMg(CO3)2 )

11
SEDIMENTARY ROCK TYPES,
Relative Abundances
12
(No Transcript)
13
(No Transcript)
14
Average Detrital Mineral Composition
of Shale and Sandstone
Shale
Sandstone
Mineral Composition
Clay Minerals
60 ()
5 ()
Quartz
30
65
4
10-15
Feldspar
Rock Fragments
lt5
15
Carbonate
3
lt1
Organic Matter,
lt3
lt1
Hematite, and
Other Minerals
(modified from Blatt, 1982)
15
SANDSTONE CLASSIFICATION
16
FOUR MAJOR COMPONENTS OF SANDSTONE
17
FOUR COMPONENTS OF SANDSTONE
Geologists Classification
18
ORIGINS OF POROSITY IN CLASTICS AND
CARBONATES(Genetic Classification)

• Primary (original)
• Secondary (induced)
• (Generally more complex than
• primary porosity)

19
PRIMARY (ORIGINAL) POROSITY

• Developed at deposition
• Typified by
• Intergranular pores of clastics or
• carbonates
• Intercrystalline and fenestral pores of
  carbonates
• Usually more uniform than induced porosity

20
SECONDARY (INDUCED) POROSITY

• Developed by geologic processes after deposition
  (diagenetic processes)
• Examples
• Grain dissolution in sandstones or carbonates
• Vugs and solution cavities in carbonates
• Fracture development in some sandstones, shales,
  and carbonates

21
(No Transcript)
22
FACTORS THAT AFFECT POROSITY
PRIMARY

• Particle sphericity and angularity
• Packing
• Sorting (variable grain sizes)
• Cementing materials
• Overburden stress (compaction)
• Vugs, dissolution, and fractures

SECONDARY (diagenetic)
23
ROUNDNESS AND SPHERICITY OF CLASTIC GRAINS
Porosity
Porosity
24
FACTORS THAT AFFECT POROSITY
PRIMARY

• Particle sphericity and angularity
• Packing
• Sorting (variable grain sizes)
• Cementing materials
• Overburden stress (compaction)
• Vugs, dissolution, and fractures

SECONDARY (DIAGENETIC)
25
GRAIN PACKING IN SANDSTONE
Line of Traverse
4 Types of Grain Contacts
(using microscope)
Packing Proximity
A measure of the extent to
Tangential Contact
which sedimentary particles
are in contact with their
neighbors
Sutured Contact
Long Contact
Packing Density
Cement
A measure of the extent to
which sedimentary particles
occupy the rock volume
Matrix
Concavo-Convex
(clays, etc.)
Contact
This Example
Packing Proximity 40
Packing Density 0.8
(modified from Blatt, 1982)
26
CUBIC PACKING OF SPHERES
27
Porosity Calculations - Uniform Spheres

• Bulk volume (2r)3 8r3
• Matrix volume
• Pore volume bulk volume - matrix volume

28
(No Transcript)
29
RHOMBIC PACKING OF SPHERES
30
FACTORS THAT AFFECT POROSITY
PRIMARY

• Particle sphericity and angularity
• Packing
• Sorting (variable grain sizes)
• Cementing materials
• Overburden stress (compaction)
• Vugs, dissolution, and fractures

SECONDARY (DIAGENETIC)
31
(No Transcript)
32
Grain-Size Sorting in Sandstone
33
(No Transcript)
34
(No Transcript)
35
STS61A-42-0051 Mississippi River Delta,
Louisiana, U.S.A. October 1985
36
STS084-721-029 Selenga River Delta, Lake Baykal,
Russia May 1997
37
FACTORS THAT AFFECT POROSITY
PRIMARY

• Particle sphericity and angularity
• Packing
• Sorting (variable grain sizes)
• Cementing materials
• Overburden stress (compaction)
• Vugs, dissolution, and fractures

SECONDARY (DIAGENETIC)
38
Photo by W. Ayers
39
DUAL POROSITY IN SANDSTONE
40
SANDSTONE COMPOSITION,
Framework Grains
KF Potassium
Feldspar
PRF Plutonic Rock
Q
Fragment
Q Quartz
PRF
KF Q
KF
P Pore
Potassium Feldspar is
Stained Yellow With a
Chemical Dye
P
Pores are Impregnated With
Blue-Dyed Epoxy
Norphlet Sandstone, Offshore Alabama, USA
Grains 0.25 mm in Diameter/Length
Photo by R. Kugler
41
POROSITY IN SANDSTONE
Photomicrograph by R.L. Kugler
42
POROSITY IN SANDSTONE
Photomicrograph by R.L. Kugler
43
POROSITY IN SANDSTONE
44
(No Transcript)
45
(No Transcript)
46
INTERGRANULAR PORE AND MICROPOROSITY
47
(No Transcript)
48
DISSOLUTION POROSITY
Photo by R.L. Kugler
49
DISSOLUTION POROSITY
50
(No Transcript)
51
Iles GambierTuamotu Archipelago
52
Maldive Islands
53
FOLK CARBONATE ROCK CLASSIFICATION
54
DUNHAM CARBONATE ROCK CLASSIFICATION
55
CARBONATES POROSITY TYPES
56
(No Transcript)
57
CARBONATE POROSITY - EXAMPLE
58
CARBONATE POROSITY - EXAMPLE
59
PORE SPACE CLASSIFICATION(In Terms of Fluid
Properties)
60
PORE-SPACE CLASSIFICATION

• Total porosity, ?t
• Effective porosity, ?e

• Effective porosity of great importance
• contains the mobile fluid

61
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
  lt ?t

62
MEASUREMENT OF POROSITY

• Core samples (Laboratory)
• Openhole wireline logs

63
SANDSTONE POROSITY MEASURED
BY VARIOUS TECHNIQUES
64
INFORMATION FROM CORES
Standard Analysis
Special Core Analysis

• Porosity
• Horizontal permeability to air
• Grain density

• Vertical permeability to air
• Relative permeability
• Capillary pressure
• Cementation exponent (m) and saturation exponent
  (n)

Allows calibration of wireline log results
65
CORING ASSEMBLYAND CORE BIT
66
COMING OUT OF HOLEWITH CORE BARREL
67
WHOLE CORE

Whole Core Photograph, Misoa C Sandstone,
Venezuela



Photo by W. Ayers
68
SIDEWALL SAMPLING GUN
69
SIDEWALL CORING TOOL
70
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

71
WHOLE CORE ANALYSIS vs. PLUGS OR SIDEWALL CORES
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

72
CORE PLUG
Sparks and Ayers, unpublished
73
NEXT
LABORATORY DETERMINATIONOF POROSITY
74
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.