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Petrophysics/Rheology > 03 - Porosity > Flashcards

03 - Porosity Flashcards

1
Q

Definition

Porosiy

A

fraction of rock bulk volume occupied by pore space

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2
Q

Formula

phi

A

phi = V_pore / V_bulk

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3
Q

Formula

V_pore

A

V_pore = V_bulk - V_solidmatrix

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4
Q

What is needed to calculated porosity?

! important to know this card

A

2 of the 3 volumetric parameters need to be measured:

  1. Bulk volume: V_bulk
  2. Pore volume: V_pore
  3. Matrix/Solid volume: V_solid
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5
Q

Definition

Total porosity

A

ratio of the total void space in the rock to the bulk volume of the rock

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6
Q

Definition

Effective porosity

A

ratio of the interconnected void space in the rock to the bulk volume of the rock

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7
Q

Porosity can be determined by

A
  • direct measurements
    • -> V_bulk & V_solid
    • -> gas expansion or displacement tequniques
  • indirect measurements
    –> correlation between porosity & properties
    (like density, neutron response, seimic wave velocity)
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8
Q

Definition

Primary porosity

A

Void volume of a sediment when it was deposited

pre-diagenetic in clastic sediments

grain size distribution, grain packing & particle shape

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9
Q

Definition

Secondary porosity

A

Void volume that is created by diagenetic processes

result of mechanical or geochemical processes

compaction, deformation, fracturing
dissolution, precipitation, mineralogical changes

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10
Q

Pore Size classification

d = diameter

A

Rough pore
d > 2mm

macrocapillary
2 mm > d > 50 μm

capillary
50 μm > d > 2 μm

microcapillary
2 μm > d > 50 nm

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11
Q

Pore Size classification

d = diameter

A

rough pore
d > 2mm

macrocapillary
2 mm > d > 50 μm

capillary
50 μm > d > 2 μm

microcapillary
2 μm > d > 50 nm

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12
Q

Definition

Bulk volume:
- can be calculated for samples with ..

A

V_bulk

.. the exact cylindrical shape -> measurement of diameter and length

or measurement of the volume of mercury displaced (mercury does not penetrate the pores)

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13
Q

sample types for methods

A

core plugs -> most methods

multiple samples -> statistically representative

technique imortant !

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14
Q

which volume will be determined with He-pycnometer?

A

matrix/solid volume V_m

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15
Q

Bulk Volume

direct calculation
for cylindrical

A

for regulary shaped cores or core plugs

V_bulk = (pid^2L)/4

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16
Q

Bulk Volume

fluid displacement methods (2)

A

Gravimetric

Volumetric (mercury pycnometer)

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17
Q

Pore Volume

  1. Gravimetric
A

Archimedes principle

weight dry W_dry
weight saturated W_sat
weight submerged in water W_sub

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18
Q

Pore Volume

  1. Boyle’s Law
A

p1 * V1 = p2 * V2

Gas expansion

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19
Q

Archimedes principle

Problems?

A

complete fluid saturation of pore space

depending on better and poorer quality rocks

reactions of fluid with rocks

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20
Q

Archimedes principle

V_pore

A

V_pore = (W_sat-W_dry) / rho_fluid

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21
Q

Archimedes principle

V_matrix

A

V_matrix = (W_dry - W_sub) / rho_fluid

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22
Q

Archimedes principle

V_bulk

A

V_bulk = (W_sat - W_sub) / rho_fluid

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23
Q

Archimedes principle

porosity phi

A

phi = (W_sat - W_dry) / (W_sat - W_sub)

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24
Q

A core sample coated with paraffin immersed in a container of
liquid displaced 10.9 cm3 of the liquid. The weight of the dry
core sample was 20.0 g, while the weight of the dry sample
coated with paraffin was 20.9 g. Assume the density of the
solid paraffin is 0.9 g/cm3.

Calculate the bulk volume of the sample.

A

W_paraffin = W_dry(coated with paraffin) - W_dry(core sample)

= 20.9 g - 20 g = 0.9 g

Vol_paraffin = W_paraffin / rho_paraffin

= 0.9 g / 0.9 g/cm^3 = 1 cm^3

V_bulk = V_bulk(coated core) - V_paraffin

V_bulk = 10.9 cm^3 - 1 cm^3 = 9.9 cm^3

25
Calculate porosity using Archimedes method: calculate the pore and bulk volumes and the porosity. Is this porosity total or effective? Dry weight of sample, Wdry = 427.3 g Weight of sample saturated with water, Wsat = 448.6 g Density of water (f ) = 1.0 g/cm3 Weight of saturated sample submerged in water, Wsub = 269.6 g
V_pore = (W_sat - Wdry) / rho_fluid = 448.6-427.3 / 1 = 21.3 cm^3 V_bulk = (W_sat-W_sub)/rho_fluid = 448.6 - 269.6 / 1 = 179.0 cm^3 phi = V_pore / V_bulk = 21.3/179.0 = 0.12 = 12 %
26
V_matrix calculated over matrix density accuracy? problems?
V_matrix = W_dry / rho_matrix Accurate only if matrix density is known density measured, because Core = polymineralic
27
V_matrix displacement method how measured
sample reduced to particle size measured by - volumetric method - archimedes method (gravimetric)
28
The volume of liquid displaced by the grains was 7.7 cm3 Bulk Volume, Vb = 9.9 cm3 Matrix Volume, Vma = 7.7 cm3 effective or total porosity?
phi = (V_bulk-V_matrix) / V_bulk = (9.9-7.7) / 9.9 = 22 %
29
Matrix volume determination method
- assume matrix (grain) density - displacement method - boyles law (gas expansion)
30
correlation of porosity and grain sorting
positively correlation well sortet -> poorly sorted high porosity -> low porosity the better it is sorted the higher is the porosity
31
correlation of porosity and compaction
eff. pressure due to overburden sediments - > compressibility of rock skeleton drainage of pore fluids -> permeability, time grain rearrangement -> packing porosity decreases with more compaction (more in shale than in sand)
32
initial porosity of shale and sand
initial porosity of shale is higher than for sand
33
porosity depending on Coordination Number
idelized .. .. cubic CN=6: 0.48 porosity ..Orthorhombic CN=8: 0.4 porosity .. hexagonal,rhomb. CN=12: 0.26 porosity
34
Diagenetic processes
- postdiagenetic: dissolution, cementation, recrystallization, dolomitizalion - leaching of grains (meteoric pore fluids): enhance (dissolution) or decrease (cementation) reservoir quality - burial compaction, fracturing, stylolithification: highly prermeable and barrier zones
35
postdiagenetic process: Dolomitization what is it? correlation with porosity?
replacement of calcite by dolomite -> increase porosity of 13 %
36
secondary porosity Fractures
porosity increase or creates permeability for fluids mechanical strength decreases changes elastic wave velocity, electrical resistivity & thermal conductivity
37
Specific Internal Surface S_total
S_total = surface area of pores / TOTAL VOLUME
38
Specific Internal Surface S_pore
S_pore = surface area of pores / PORE VOLUME
39
Specific Internal Surface S_mass
S_mass = surface area of pores / TOTAL MASS
40
Fluids in Porespace Types
- free movalble water - capillary-bound water (connected with the grain surface) -clay-bound water (strong clay-water effects)
41
Fluids in Porespace Determination of saturation (3)
1. directly from cores, plogs - > by fluid extraction & capillary pressure measurements 2. indirectly from logs - > resistivity, dielectric, neutron measurements 3. by NMR measurements
42
Fluids in Porespace Saturation S_i
S_i = volume of fluid i / pore volume
43
Fluids in Porespace Saturation of reservoir with different fluids: water,oil,gas
S_water + S_oil + S_gas = 1
44
Porosity determination in well logging what is measured? by which logs?
- percentage of pore volume | - acoustic, nuclear or electrical logs (or combination)
45
Porosity determination in well logging Acoustic logs
measure characteristics of sound waves propagated through well-bore environment
46
Porosity determination in well logging Nuclear technology
emitted neutrons are being scattered by the hydrogen atoms
47
Porosity determination in well logging Electrical resistivity
measures conductive formation fluid
48
Porosity determination in well logging What can indicate the presence of hydrocarbons?
difference between neutron and electrical porosity
49
Porosity determination in well logging Neutron log - measures .. - Logging device ? - Method?
- measures hydrogen concentration - noncontact tool that emits neutrons from a source - Maximum energy loss occurs when emitted neutrons collide with hydrogen atoms because both have almost same mass
50
Porosity determination in well logging Neutron log - correlation with porosity?
- in porous formation, hydrogen is concentrated in fluid filling pores
51
Classification of Neutrons using their energy
- fast neutrons (> 500keV) - intermediate neutrons (1 - 500 keV) - slow neutrons (<1 keV) - > epithermal neutrons (0.1 eV - 1 keV) - > thermal neutrons ( < 0.1 eV)
52
Source-detector system of Neutron measurements Sources?
- chemical or alpha-n sources: - > e.g. AmBe - Neutron generator - > deuterium-tritium reaction
53
Neutron Interactions with .. interaction types (2)
with nuclei -> transfer & loose enegy 1. mederating or sowing-down interactions - > inelastic & elastic scattering 2. Absorptive interactions if they reach thermal energy - > capturing & activation
54
Neutron Interactions Inelastic scattering results in ...
- neutron of lower energy - gamma ray of characteristic energy - > used for carbon/oxygen log
55
Neutron Interactions Elastic scattering - how is the interaction? - what controls it?
- neutron collides with atomic nucleus -> loses kinetic energy -Y energy converted to combined kinetic energy = scattering process - > biliard ball interaction - no induced radiation - hyrogen content controls the strength of elastic scattering water content or neutron porosity
56
what can be read in a | DENSITY - NEUTRON log ?
- matrix density -> calculate porosity - gas present in formation pore spacce - shale/clay in present in formation --> low intensity in neutron porosity = gas bearing strata (bulk density low) --> Bulk density & porosity medium -> oil or brine
57
Correlation of bulk density and neutron porosity e.g. of Sandstone, Limestone, Dolomite
bulk density increasing with increasing neutron porosity
58
Obtaining porosities from density log
bulk density is function of matrix density, porosity & fluids (in pore space) porosity phi = (rho_matrix - rho_bulk) / (rho_matrix - rho_fluid)

Petrophysics/Rheology (10 decks)

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