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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
Q

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

A

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
Q

V_matrix calculated over matrix density

accuracy? problems?

A

V_matrix = W_dry / rho_matrix

Accurate only if matrix density is known

density measured, because Core = polymineralic

27
Q

V_matrix

displacement method
how measured

A

sample reduced to particle size

measured by

  • volumetric method
  • archimedes method (gravimetric)
28
Q

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?

A

phi = (V_bulk-V_matrix) / V_bulk

= (9.9-7.7) / 9.9 = 22 %

29
Q

Matrix volume

determination method

A
  • assume matrix (grain) density
  • displacement method
  • boyles law (gas expansion)
30
Q

correlation of porosity and grain sorting

A

positively correlation

well sortet -> poorly sorted
high porosity -> low porosity

the better it is sorted the higher is the porosity

31
Q

correlation of porosity and compaction

A

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
Q

initial porosity of shale and sand

A

initial porosity of shale is higher than for sand

33
Q

porosity depending on Coordination Number

A

idelized ..

.. cubic CN=6:
0.48 porosity

..Orthorhombic CN=8:
0.4 porosity

.. hexagonal,rhomb. CN=12:
0.26 porosity

34
Q

Diagenetic processes

A
  • 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
Q

postdiagenetic process:

Dolomitization
what is it?
correlation with porosity?

A

replacement of calcite by dolomite

-> increase porosity of 13 %

36
Q

secondary porosity

Fractures

A

porosity increase
or
creates permeability for fluids

mechanical strength decreases

changes elastic wave velocity, electrical resistivity & thermal conductivity

37
Q

Specific Internal Surface

S_total

A

S_total = surface area of pores / TOTAL VOLUME

38
Q

Specific Internal Surface

S_pore

A

S_pore = surface area of pores / PORE VOLUME

39
Q

Specific Internal Surface

S_mass

A

S_mass = surface area of pores / TOTAL MASS

40
Q

Fluids in Porespace

Types

A
  • free movalble water
  • capillary-bound water (connected with the grain surface)

-clay-bound water
(strong clay-water effects)

41
Q

Fluids in Porespace

Determination of saturation (3)

A
  1. directly from cores, plogs
    - > by fluid extraction & capillary pressure measurements
  2. indirectly from logs
    - > resistivity, dielectric, neutron measurements
  3. by NMR measurements
42
Q

Fluids in Porespace

Saturation S_i

A

S_i

= volume of fluid i / pore volume

43
Q

Fluids in Porespace

Saturation of reservoir with different fluids:

water,oil,gas

A

S_water + S_oil + S_gas = 1

44
Q

Porosity determination in well logging

what is measured?

by which logs?

A
  • percentage of pore volume

- acoustic, nuclear or electrical logs (or combination)

45
Q

Porosity determination in well logging

Acoustic logs

A

measure characteristics of sound waves propagated through well-bore environment

46
Q

Porosity determination in well logging

Nuclear technology

A

emitted neutrons are being scattered by the hydrogen atoms

47
Q

Porosity determination in well logging

Electrical resistivity

A

measures conductive formation fluid

48
Q

Porosity determination in well logging

What can indicate the presence of hydrocarbons?

A

difference between neutron and electrical porosity

49
Q

Porosity determination in well logging

Neutron log

  • measures ..
  • Logging device ?
  • Method?
A
  • 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
Q

Porosity determination in well logging

Neutron log
- correlation with porosity?

A
  • in porous formation, hydrogen is concentrated in fluid filling pores
51
Q

Classification of Neutrons using their energy

A
  • 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
Q

Source-detector system of Neutron measurements

Sources?

A
  • chemical or alpha-n sources:
    • > e.g. AmBe
  • Neutron generator
    • > deuterium-tritium reaction
53
Q

Neutron Interactions

with ..

interaction types (2)

A

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
Q

Neutron Interactions

Inelastic scattering
results in …

A
  • neutron of lower energy
  • gamma ray of characteristic energy
    • > used for carbon/oxygen log
55
Q

Neutron Interactions

Elastic scattering

  • how is the interaction?
  • what controls it?
A
  • 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
Q

what can be read in a

DENSITY - NEUTRON log ?

A
  • 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
Q

Correlation of bulk density and neutron porosity

e.g. of Sandstone, Limestone, Dolomite

A

bulk density increasing with increasing neutron porosity

58
Q

Obtaining porosities from density log

A

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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