4. The many definitions of Porosity*
* from Eslinger, E. and Pevear, D. "Clay Minerals for Petroleum Geologists and
Engineers", SEPM Short Course No. 22, 1988
5. • PHIT - The total porosity includes isolated pores and the
space occupied by clay-bound water*
• PHIE - Effective porosity excludes isolated pores and pore
volume occupied by water adsorbed on clay minerals*
• PHIT = PHIE + Clay Bound Water
*Schlumberger Oilfield Glossary
The Petrophysicist’s Porosity
Matrix Silt Clay
Clay
Bound
Water
Hydrocarbon
Capillary
Bound
Water
F Total
F Core
F Core (Humidity dried)
F Effective
V Shale
V Clay
*Schlumberger Oilfield Glossary
IsolatedPores
Isolated pores can be neglected in most clastic formations
6. Where Total Porosity and Effective
Porosity are used
• Different water saturation equations use different
porosities:
– Archie equation assumes clean formation PHIT = PHIE
– Waxman-Smits, Juhasz and Dual Water use PHIT
– Simandoux and Indonesia use PHIE
7. Should petrophysicists use Total or Effective Porosity?
• Using PHIT or PHIE should give you the same
hydrocarbon in place (HCIP)
• The question is which is most useful and gives the
most accurate determination of HCIP
8. Porosity from core
Matrix
Reservoir Fluids
(Hydrocarbon, Capillary Bound
Water, Mud filtrate etc.)
PHIT
PHIE
Silt Clay
Clay
Bound
Water
F Core
PHID
• Density Porosity PHID is not PHIT
– PHID is somewhere between PHIT and PHIE
– It is necessary to calibrate PHID to the Core Porosity
• PHID = Rhoma - Rhob
Rhoma - Rhofl
Where:
PHID Density derived porosity
Rhob Measured bulk density
Rhoma Matrix density
Rhofl Fluid density
Rhob
9. Porosity from density log vs. core regression
• Density Porosity PHID is not PHIT
Core Porosity PHIT (v/v)
DensityPorosityPHID(v/v)
– PHID is somewhere between PHIT and PHIE
– It is therefore necessary to calibrate PHID
to the Core Porosity
• PHIT = PHID * Constant
• PHIE = PHIT(1-0.6425+0.22)Qv
√S
where:
S = salinity in g/l NaCl
Qv = CEC/PV (meq/ml)
(from Hill, Shirley & Klein)
or
• PHIE = PHIT – VSH*PHISH
where:
PHISH = Shale porosity
(See later slide)
10. Porosity from electrical logs
• Density Porosity is not PHIT
– Somewhere between PHIT and PHIE
– Without core it is necessary to calculate PHIE before PHIT
• The density response equation:
• Rhob = ( 1 - PHIE - VSH ) * Rhoma + VSH * Rhosh + PHIE * Rhofl)
Where:
Rhob Measured bulk density
Rhoma Matrix density
Rhosh Shale density
Matrix
(Rhoma)
Reservoir Fluids
(Rhofl)
PHIT
Rhob
PHIEVSH1-PHIE-VSH
Silt Clay
Clay
Bound
Water
Rhofl Fluid density
PHIE Effective porosity
VSH Volume of Shale
11. Porosity from electrical logs
Solving for PHIE:
PHIE = Rhoma – Rhob - VSH * Rhoma - Rhosh
Rhoma - Rhofl Rhoma – Rhofl
PHIT is then calculated from PHIE
PHIT = PHIE + VSH * PHISH
Where:
PHISH = Shale porosity
Matrix
(Rhoma)
Reservoir Fluids
(Rhofl)
PHIT
Rhob
PHIEVSH1-PHIE-VSH
Silt Clay
Clay
Bound
Water
12. Calculation of shale porosity PHISH
PHISH = Shale porosity due to Clay Bound Water
• PHISH is often assumed to be 10% if the shale contains an equal
mixture of illite, chlorite and kaolinite clay minerals
– but only for non-smectite formations
• PHISH can be calculated from:
PHISH = Rhodsh - Rhosh
Rhodsh - Rhow
Where:
Rhodsh Dry shale density
Rhosh Shale density
Rhow Shale water density
• Rhodsh can be determined from the density vs. neutron
porosity crossplot
13. Calculation of shale porosity PHISH
DensityPorosity(p.u)
0 Neutron Porosity (p.u) 100
-20
0
100
Dry
Clay
Wet
Clay
Dry
Shale
Matrix
Water
Wet
Shale
PHISH = Rhodsh - Rhosh
Rhodsh - Rhow
The dry shale density,
Rhodsh can be determined
from density vs. neutron
porosity crossplot
• The wet shale point is at
the end of the data
boomerang
• The dry shale is the
extrapolation of the shale
line to the dry line
Colour
Gamma-Ray
14. In Conclusion
• Petrophysics should calculate both PHIT and PHIE
• Density Porosity PHID is not PHIT
• Density derived porosity is between PHIT and PHIE
• If core data is available calculate PHIT, then PHIE
• If no core data is available calculate PHIE, then PHIT
