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Ko Ko Kyi
Retired Principal Petrophysicist
12 April 2019
TOTAL VS EFFECTIVE POROSITY
A PETROPHYSICAL DILEMMA
– Porosity Definition
– Porosity Concepts
– Porosity From Logs
– Choice of Porosity System
– Current Company Practice
– Prevailing Industry Practices
– Conclusions and Discussions
OUTLINE OF PRESENTATION
POROSITY CONCEPTS
TOTAL POROSITY (PETROPHYSICAL DEFINITION)• INCLUDES ALL FLUIDS
FT = FW + FH + FWB
WHERE, FT = TOTAL POROSITY
FW = POROSITY OCCUPIED BY WATER (BOTH FREE AND
IRREDUCIBLE)
FH = POROSITY OCCUPIED BY HYDROCARBONS
FWB = POROSITYOCCUPIED BY CLAY BOUND WATER
TOTAL POROSITY CAN BE COMPUTED DIRECTLY FROM LOGS.
EFFECTIVE POROSITY (PETROPHYSICAL DEFINITION)• EXCLUDES CLAY BOUND WATER
FE = FT – FWB
WHERE, FE = EFFECTIVE POROSITY
EFFECTIVE POROSITY IS DERIVED FROM TOTAL POROSITY BY CORRECTING FOR
CLAY VOLUME. IN A CLEAN CLAY-FREE FORMATION, EFFECTIVE POROSITY IS THE
SAME AS TOTAL POROSITY.
Log Derived Porosity
Density Log
FD = (rma – rb)/(rma – rf)
Where, FD = Total porosity derived from density log
rma = Matrix (grain) density, gm/cc
rb = Bulk density of formation, gm/cc
rf = Fluid density, gm/cc
• Porosity derived from the density log is adversely affected
by borehole rugosity, washouts and light hydrocarbons.
• The porosity derived from density log needs to be
corrected for light hydrocarbon or gas effects.
Log Derived Porosity
Neutron Porosity Log
• The neutron porosity tool measures the Hydrogen Index of
the formation.
• In clean formations, where the pores are filled with water or
oil, the neutron log reflects the formation porosity.
• The neutron porosity needs to be corrected for proper
lithology type to determine the formation porosity.
• In shaly formations, the neutron log gives overly high
porosities due to the presence of water in the clays.
• The neutron log is greatly affected by the presence of gas
in the formation. In a gas bearing zone, the neutron log gives
very low porosities.
Log Derived Porosity
Sonic Log
FS = (DT – DTma)/(DTf – DTma) (Wyllie’s time average equation)
Where, FS = Total porosity from sonic log
DT = Formation transit time, ms/ft
DTma = Matrix transit time, ms/ft
DTf = Fluid transit time, ms/ft
• The sonic log is the most unpredictable log to be used for
deriving formation porosity.
• The sonic log is affected by shales (distribution), formation
compaction and the presence of gas in the formation.
Log Derived Porosity
Sonic Log
FS = C*(DT – DTma)/DT (Raymer, Hunt, Gardner equation)
Where, FS = Total porosity from sonic log
DT = Formation transit time, ms/ft
DTma = Matrix transit time, ms/ft
DTf = Fluid transit time, ms/ft
C = Constant (0.62 to 0.7)
Log Derived Porosity
Crossplot Porosity
• Derived from the crossplot of neutron and density logs,
using the appropriate matrix density.
• The apparent total porosity thus obtained is then corrected
for shale effects to derive the effective porosity.
• In clean water bearing formations, the crossplot porosity
(total porosity) is similar to that derived from the density log.
• In shaly formations, the crossplot porosity (total porosity) is
overly optimistic due to the effect of shales on neutron log.
Log Derived Porosity
Effective Porosity
FE = FT – Vsh*FTsh
Where, FE = Effective porosity
FT = Total porosity
Vsh = Shale volume
FTsh = Total porosity of shale
• Log derived effective porosity is subject to uncertainties in
the determination of shale volume and total porosity of shale.
• Effective porosity cannot be calibrated against core data.
Log to Core Calibration
Log derived total porosity is
calibrated against core
porosity, which has been
corrected for net
overburden stress
Choice of Porosity System
• It has been demonstrated that the total porosity derived from
the density log matches the porosity from core (oven dried
and corrected for net overburden stress).
• Effective porosity derived from logs has many uncertainties
due to different ways in which the shale volume is computed.
• The total porosity of shale FTsh used in the computation of
effective porosity may not be representative of the clays
present in the reservoirs, thus leading to uncertainties.
• Effective porosity cannot be calibrated to core data, as the
measurement of FE from core is prone to uncertainties.
• Due to its robustness and proper validation with core data,
Total Porosity should be used in petrophysical evaluations.
Current Company Practice
• Total porosity derived from density log, using appropriate
matrix density and fluid density
• Matrix or grain density derived from a mixture of three main
lithological components namely sand, silt and clay
• Light hydrocarbon correction implemented using applicable
equations and crossplots
• Total water saturation determined using total porosity and
appropriate saturation equation
• Effective porosity and effective water saturation determined
using the material balance equation
• Both total and effective parameters provided to end users
This figure represents the components of the gross rock (bulk) volume
as a strip. The individual components are not to scale. For example,
porosity and pore volume are over-emphasised for illustrative purposes.
Source: Tony Kennaird, Core Laboratories
Porosity Types – Core Analysis Perspective
Source: Tony Kennaird, Core Laboratories
Porosity Types – Core Analysis Perspective
• Total Porosity: that volume of the reservoir rock which is fluid
(oil, water, gas) filled, expressed as a percentage or a fraction of
the gross (bulk) rock volume.
• Effective Porosity Φe1: The sum of all the interconnected pore
space. In the vast majority of cases, this core analysis and
Petroleum Engineering definition of effective porosity equates
to total porosity.
• Effective Porosity Φe2: Effective porosity measured on core
samples which are dried in a humidity oven so that clays retain
one or two molecular layers of bound water—however, this
CBW tends to a minimum and is likely not reservoir
representative.
• Effective Porosity Φe3: Total porosity minus clay-bound water
(CBW).
Source: Tony Kennaird, Core Laboratories
Definitions of Porosity
• Effective Porosity Φe4: Log effective porosity. In essence,
total porosity minus shale water, where solid minerals and the
volume of shale (Vsh) constitute the matrix (non-effective
porosity) and the remaining volume constitutes the effective
porosity. For practical purposes, Vsh includes solid clays and
the clay-sized and silt-sized fraction of non-clay minerals plus
CBW and capillary bound water associated with shale
micropores.
• Effective Porosity Φe5: In a hydrocarbon-bearing reservoir
above the transition zone, only that pore space which is filled
with hydrocarbons. From the NMR log, this equates to the
Free Fluid Index (FFI), in other words, all pore space above
the T2 cut-off.
• Effective porosity Φe6: That volume of pore space which
contains only producible hydrocarbons.
Source: Tony Kennaird, Core Laboratories
Definitions of Porosity
• Clay Bound Water (CBW): Total porosity × SF × Qv
Where:
CBW = Clay bound water
SF = Salinity Factor (0.6425 * S-0.5 + 0.22)
Qv = Cation Exchange Capacity, meq/ml pore space
S = Salinity, g/l
Source: Tony Kennaird, Core Laboratories
Definitions of Porosity
Total or Effective Porosity?
• The detail of this porosity evaluation and the generally
consistent results between numerous on-screen different core
and log inter-well comparisons suggest a high degree of
certainty in the evaluated total porosity
• This porosity is based on core oven dried helium porosities
and is therefore very close to "total porosity" despite core
companies use of the term "effective porosity" to describe oven
dried porosity.
• The confusion regarding total vs. effective porosity is due
primarily to the longstanding inconsistency between core
companies use of the term "effective porosity" meaning
"interconnected pores", vs. mainstream petrophysicists use of
"effective porosity" to mean (total porosity - clay bound water).
Source: Mark Deakin, Consultant, Petrophysics Pty Ltd
• ’Total’ porosity is useful for the calculation of porosity and
water saturation. Core porosity measurements usually give total
porosity.
• E&P managers need to know something different. They need
estimates of potential reservoir thickness, ’effective’ porosity,
permeability and the volumes of producible hydrocarbon and
water.
• ‘Total’ and ‘effective’ porosity are equal in non-shaly reservoirs,
and may be nearly equal in shaly sandstones containing clays
(other than smectites) with little clay-bound water in the clay
structure.
• In shaly sandstones containing hydrated clays (smectites),
‘effective’ porosity may be much less than ‘total’.
Effective Porosity - the Practical Result
Source: Mark Deakin, Consultant, Petrophysics Pty Ltd
• To obtain practical results, petrophysicists and log analysts
certainly need to understand and evaluate ‘total’ porosity and
the volumes of water bound in clays.
• However, for the end users of the petrophysical results it is
more informative to be provided with ‘effective’ porosity.
• It reduces confusion and give a more practical evaluation of
the reservoir.
• In depth plots of the results, ‘Total’ porosity should be de-
emphasised and ‘Effective’ porosity emphasized.”
Dick Woodhouse, Consultant
Effective Porosity - the Practical Result
Source: Mark Deakin, Consultant, Petrophysics Pty Ltd
Prevailing Industry Practices
• There are several companies, including major ones, which use
Total Porosity and Total Water Saturation equations.
• Likewise, there are also companies which use Effective Porosity
and Effective Water Saturation equations.
• The choice of the porosity system to be used is according to the
company policy and preference.
• However, it is important to note that if Total Porosity is used, a
corresponding Total Water Saturation should be used.
• Similarly, if Effective Porosity is the preferred choice, a suitable
Effective Water Saturation equation should be used with it.
• Mixing of the two different porosity systems can result in over or
underestimation of water saturation values.
An Oil Company Perspective
• Density log is the best tool for porosity measurement.
• If mineralogy is variable, use the density log in combination with
the neutron log.
• The density log is not affected by shaliness as much as other
porosity logs.
• The Sonic Log is the best tool if the hole is irregular, secondary
porosity is important, or heavy minerals (such as pyrite) are
present.
• The Neutron Log can be combined with the density log to
determine porosity in oil and gas bearing zones.
• Combined neutron-density log porosity is closer to core porosity
in mixed lithologies.
PHIT from CPOR-DENB
X-Plot
Comparison of
results with
PHIT from
triple lithology
component log
analysis
Comparison of
porosity from
calibrated
density log with
core porosity
Conclusions and Discussions
• Impressive magnitude of laboratory work and amount of support
in technical literature have advocated the use of Total Porosity
system in formation evaluation.
• This has led to a great majority of the petrophysical community
in accepting the Total Porosity concept as the petrophysical
model of choice in their work.
• There may be a number of companies in the industry, which
prefer to use the Effective Porosity system. However, it is believed
that they are probably in the minority.
• It is recommended to use Total Porosity and Total Water
Saturation models in performing petrophysical evaluation.
• Total porosity derived from logs should be calibrated with total
porosity from core (oven dried, net overburden stress corrected)