03 - Porosity Flashcards
Definition
Porosiy
fraction of rock bulk volume occupied by pore space
Formula
phi
phi = V_pore / V_bulk
Formula
V_pore
V_pore = V_bulk - V_solidmatrix
What is needed to calculated porosity?
! important to know this card
2 of the 3 volumetric parameters need to be measured:
- Bulk volume: V_bulk
- Pore volume: V_pore
- Matrix/Solid volume: V_solid
Definition
Total porosity
ratio of the total void space in the rock to the bulk volume of the rock
Definition
Effective porosity
ratio of the interconnected void space in the rock to the bulk volume of the rock
Porosity can be determined by
- direct measurements
- -> V_bulk & V_solid
- -> gas expansion or displacement tequniques
- indirect measurements
–> correlation between porosity & properties
(like density, neutron response, seimic wave velocity)
Definition
Primary porosity
Void volume of a sediment when it was deposited
pre-diagenetic in clastic sediments
grain size distribution, grain packing & particle shape
Definition
Secondary porosity
Void volume that is created by diagenetic processes
result of mechanical or geochemical processes
compaction, deformation, fracturing
dissolution, precipitation, mineralogical changes
Pore Size classification
d = diameter
Rough pore
d > 2mm
macrocapillary
2 mm > d > 50 μm
capillary
50 μm > d > 2 μm
microcapillary
2 μm > d > 50 nm
Pore Size classification
d = diameter
rough pore
d > 2mm
macrocapillary
2 mm > d > 50 μm
capillary
50 μm > d > 2 μm
microcapillary
2 μm > d > 50 nm
Definition
Bulk volume:
- can be calculated for samples with ..
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)
sample types for methods
core plugs -> most methods
multiple samples -> statistically representative
technique imortant !
which volume will be determined with He-pycnometer?
matrix/solid volume V_m
Bulk Volume
direct calculation
for cylindrical
for regulary shaped cores or core plugs
V_bulk = (pid^2L)/4
Bulk Volume
fluid displacement methods (2)
Gravimetric
Volumetric (mercury pycnometer)
Pore Volume
- Gravimetric
Archimedes principle
weight dry W_dry
weight saturated W_sat
weight submerged in water W_sub
Pore Volume
- Boyle’s Law
p1 * V1 = p2 * V2
Gas expansion
Archimedes principle
Problems?
complete fluid saturation of pore space
depending on better and poorer quality rocks
reactions of fluid with rocks
Archimedes principle
V_pore
V_pore = (W_sat-W_dry) / rho_fluid
Archimedes principle
V_matrix
V_matrix = (W_dry - W_sub) / rho_fluid
Archimedes principle
V_bulk
V_bulk = (W_sat - W_sub) / rho_fluid
Archimedes principle
porosity phi
phi = (W_sat - W_dry) / (W_sat - W_sub)
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.
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
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 %
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
V_matrix
displacement method
how measured
sample reduced to particle size
measured by
- volumetric method
- archimedes method (gravimetric)
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 %
Matrix volume
determination method
- assume matrix (grain) density
- displacement method
- boyles law (gas expansion)
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
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)
initial porosity of shale and sand
initial porosity of shale is higher than for sand
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
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
postdiagenetic process:
Dolomitization
what is it?
correlation with porosity?
replacement of calcite by dolomite
-> increase porosity of 13 %
secondary porosity
Fractures
porosity increase
or
creates permeability for fluids
mechanical strength decreases
changes elastic wave velocity, electrical resistivity & thermal conductivity
Specific Internal Surface
S_total
S_total = surface area of pores / TOTAL VOLUME
Specific Internal Surface
S_pore
S_pore = surface area of pores / PORE VOLUME
Specific Internal Surface
S_mass
S_mass = surface area of pores / TOTAL MASS
Fluids in Porespace
Types
- free movalble water
- capillary-bound water (connected with the grain surface)
-clay-bound water
(strong clay-water effects)
Fluids in Porespace
Determination of saturation (3)
- directly from cores, plogs
- > by fluid extraction & capillary pressure measurements - indirectly from logs
- > resistivity, dielectric, neutron measurements - by NMR measurements
Fluids in Porespace
Saturation S_i
S_i
= volume of fluid i / pore volume
Fluids in Porespace
Saturation of reservoir with different fluids:
water,oil,gas
S_water + S_oil + S_gas = 1
Porosity determination in well logging
what is measured?
by which logs?
- percentage of pore volume
- acoustic, nuclear or electrical logs (or combination)
Porosity determination in well logging
Acoustic logs
measure characteristics of sound waves propagated through well-bore environment
Porosity determination in well logging
Nuclear technology
emitted neutrons are being scattered by the hydrogen atoms
Porosity determination in well logging
Electrical resistivity
measures conductive formation fluid
Porosity determination in well logging
What can indicate the presence of hydrocarbons?
difference between neutron and electrical porosity
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
Porosity determination in well logging
Neutron log
- correlation with porosity?
- in porous formation, hydrogen is concentrated in fluid filling pores
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)
Source-detector system of Neutron measurements
Sources?
- chemical or alpha-n sources:
- > e.g. AmBe
- Neutron generator
- > deuterium-tritium reaction
Neutron Interactions
with ..
interaction types (2)
with nuclei -> transfer & loose enegy
- mederating or sowing-down interactions
- > inelastic & elastic scattering - Absorptive interactions if they reach thermal energy
- > capturing & activation
Neutron Interactions
Inelastic scattering
results in …
- neutron of lower energy
- gamma ray of characteristic energy
- > used for carbon/oxygen log
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
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
Correlation of bulk density and neutron porosity
e.g. of Sandstone, Limestone, Dolomite
bulk density increasing with increasing neutron porosity
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)
