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 DEFINITION

POROSITY F = (PORE VOLUME)/(TOTAL FORMATION VOLUME)

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.

Source: Integrated Formation Evaluation

Formation Water

Source: Integrated Formation Evaluation

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.

Crossplot Porosity

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

Log Derived Total Porosity Validated with Core Data

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

Porosity From Core

Analysis Perspective

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

Porosity From Service

Company Perspective

Porosity – Why this presentation?

Source: Bob Truman, Baker Atlas

Defining Porosity

Source: Bob Truman, Baker Atlas

Summary of Porosity Issues

Source: Bob Truman, Baker Atlas

Danger

Source: Bob Truman, Baker Atlas

Observations and Comments

Source: Bob Truman, Baker Atlas

Porosity From A

Consultant Perspective

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 Deacon, 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 Deacon, 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 Deacon, Consultant, Petrophysics Pty Ltd

Porosity From Oil Company

Perspective

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.

Rho Fluid = 1.03 g/cc

Calibrating density log

with core porosity, used

in certain companies.

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)

Thank you for your attention!!!