COPYRIGHTcloud1.activelearner.com/contentcloud/portals/hosted3/PetroAcademy/PPH-PEC/PPH-PEC-1...What Is Petrophysical Evaluation? Petrophysical Evaluation Core Learning Objectives

What Is Petrophysical Evaluation?

Petrophysical Evaluation Core

Learning Objectives

By the end of this lesson, you will be able to:

List three or more tasks that petrophysicists perform

Discuss the basic elements of a petrophysical evaluation

Describe the principles of log interpretation

List five or more parameters on a petrophysical spreadsheet summary

Explain how to calculate hydrocarbon volume

Petrophysical Evaluation Core ═══════════════════════════════════════════════════════════════════════════════════

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What Do Petrophysicists Do?

Develop and document Evaluation Plan for acquiring wellbore data

Supervise execution of Evaluation Plan

Edit and process log data

Prepare the Pore-Pressure & Fracture Gradient plot




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Performing The Petrophysical Evaluation

VCLAYPOROSITY

Selected Porosity FLUIDS

BULK

MD



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The Petrophysical “Detective”




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Basic Concepts of Log Analysis

Determine lithology and porosity

Evaluate Hydrocarbons and/or wet zones

Apply Archie relationshipsFind Rw and Sw

Pay summaries

Determine porous, “permeable” zones (Net/Gross)

Estimate permeability



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Principle of Log Interpretation

Reservoir

Non-reservoir

Hydrocarbonbearing

Waterbearing

Gas bearing

Oil bearing

Principle of Log Interpretation

Sample Petrophysical Evaluation Summary

TOP-TV

FT-T VD

TOPFT-MD

BA-SEFT-MD

GROSSFT-MD

NET SANDFT-MD

N/GNET

Pay FT-MD

Hole Angle

TVNet PAY

HC TYPE

POR % SWA %Perm Insitu

16053 16414 16470 56 15.5 0.28 15.5 14 15.0 OIL 25 44 110

16107 16470 16540 70 47.5 0.68 47.5 14 46.0 OIL 31 35 432

16175 16540 16553 13 7 0.54 7 14 6.8 OIL 31 38 369

16187 16553 16567 14 10.6 0.76 10.6 14 10.3 OIL 29 35 342

1.Locate reservoirs

2.Detect hydrocarbons

3.Distinguish oil and gas

4.Evaluate: Shc, , h, k



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Let’s calculate some hydrocarbons!

hA h*(N/G) * φ (1 - Sw)

hh * (N/G)

HCVOLBo

HCVOL h

Note: STOIIP means “Stock Tank Oil Originally in Place”

Basic Petrophysical Evaluation Questions

What kind of rock? What kind are they?

Are hydrocarbons present?

How much is there?

Quick-Look

AdvancedTechniquesAND

Petrophysical Evaluation Core═══════════════════════════════════════════════════════════════════════════════════

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= STOIIP

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Parameters Needed

Lithology and Porosity

Resistivity of Rocks and Fluids

Formation Water Resistivity – Rw

Water Saturation – Sw

Hydrocarbon Indicator(s)

End Product or “Answer Log”

Delineates Lithology, Fluids• SS, LS, DOL, etc. (Reservoir)• SH, ANY, etc. (Non-reservoir)• Oil, Gas, Water

Quantifies• Porosity• Water, Oil saturation• Net Feet of Pay

Depths of cores, sidewall samples, pressure tests and fluid samples are marked on this log along with brief summaries of the respective data.



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How Does Petrophysics Integrate?

Static model(Geologic model)

Dynamic model(Reservoir Simulation model)

(Borehole) Seismic

Core data

Mudlog data

LWD Wireline Logs

Reservoir monitoring

Open hole logs• Resistivity• Nuclear• Acoustic• Other

Cased hole logs• Nuclear• Production

logs• Other

Field studies

Corrections:• Invasion• Layering• Deviation

Interpretation models incl. QC & Uncertainty

Static model(Geologic model)

Dynamic model(Reservoir Simulation model)

Learning Objectives

You are now able to:

List three or more tasks that Petrophysicists perform

Discuss the basic elements of a Petrophysical evaluation

Describe the principles of log interpretation

List five or more parameters on a petrophysical spreadsheet summary

Explain how to calculate hydrocarbon volume



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Understanding Reservoir Saturation


Learning Objectives


Discuss the requirements for hydrocarbons to migrate and saturate a reservoir rock

Describe the relationship between pore throat size and hydrocarbon saturation

Tell how hydrocarbon and formation water fluid gradient plots can be used to determine the free water level (FWL) of a reservoir

Discuss the impact of permeability on the height of the capillary transition zone

Explain how to identify the best reservoir rock sample from a capillary pressure plot with multiple curves



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Oil generated from Woodford Shale, Texas,

at 350C under laboratory conditions

Oil Generation from Shale: Migration Onset

Amalgamated oil droplets forming in a cavity

Shale + Kerogen

Shale Trap or Seal(“Caprock”)

Source Rock

Potential Reservoir Rock

(Water saturated)Sw = 100%

Reservoir Saturation at Deposition



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Water Wet System

water saturated reservoir rock

Migrating oil

Seal

Oil

Rock

Pw

Po

r1

r2

WaterWater

Capillary Pressure … Drainage



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Upward migration of hydrocarbon droplets through water column

At any given structural height, the amount of HC accumulation is controlled by the pore throat size (for any given droplet size)

Pore throats too narrow.No passage of HC

Wide pore throats allow passage of HC

HC

Pore Geometry and Hydrocarbon Accumulation

OIL

Fluid Distribution in the Reservoir



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Water Leg or Aquifer

Oil/Water Contact

Trap or Seal(“Caprock”)

Source Rock

Transition Zone

Irreducible Zone

Shale + Kerogen

Shale

Sw

ReservoirSaturation

Profile

0 1

OIL

WATER

Reservoir Saturation Distribution after Hydrocarbon Migration



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Transition Zone Height: Key Points

high perm

low Swirreducible

short transition zone

low perm

high Swirreducible

long transition zone

Large Pore Throats

Large Pore Throats

Small Pore Throats

Small Pore Throats

Transition Zone Height: Key Points

Short transition at gas/water contact

Long transition at oil/water contact

Large density difference between

water and hydrocarbon

Large density difference between


Small density difference between


Small density difference between




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Reservoir Quality and Capillary Pressure Curves

water saturation0 100

h

A

CB


water saturation0 100

h

A

CB

The three main regions of a capillary pressure curve shown for sample “C”:

the region of Swirr

the transition zone between initial entry and Swirr (for sample “C”)

The initial non-wetting fluid entry into the pore system



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Permeability and Capcurves

0

200

400

600

800

1000

0 20 40 60 80 100

Wetting phase saturation (%)

Cap

illar

y p

ress

ure

(p

sia) 3 mD

40 mD

170 mD

370 mD

=12.7% =12.6%

=15.7% =15.8%

Sample (p.u.) K (mD)

1 12.6 402 12.7 33 15.7 1704 15.8 370

Non-wetting phase saturation (%)0100

Cap psi = 0 = FWL

1st entry of non-wetting phase or OWC


1st entry of non-wetting phase at 75 psia.

Swirr

approx. 30%



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1st entry of non-wetting phase at very low psia.

Swirr

approx. 20%


0 20 40 80 100

20

15

10

5

0

Sw (%pv)

Ca

pil

lary

pre

ss

ure

Sandy shale or slightly permeablelimestone

Sh

ale or

den

se lim

eston

e

Shaly sandstone orlow permeable

limestone

Clean sandstone orpermeable limestone Entry height, OWC

Swirr

FWL




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The Link between Capcurves and Reservoir Quality

When k (mD) is high:

Transition zone is short

Entry Height is low

Sw is low

Learning Objectives


Discuss the requirements for hydrocarbons to migrate and saturate a reservoir rock

Describe the relationship between pore throat size and hydrocarbon saturation

Tell how hydrocarbon and formation water fluid gradient plots can be used to determine the free water level (FWL) of a reservoir

Discuss the impact of permeability on the height of the capillary transition zone

Explain how to identify the best reservoir rock sample from a capillary pressure plot with multiple curves



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Net Reservoir, Net Pay, and N/G


Learning Objectives


Tell about picking bed boundaries to determine zone thickness

Explain the terms gross thickness, net pay and net/gross ratio

Discuss how to calculate net reservoir rock from the Gamma Ray log

Describe how to calculate shale volume, Vsh, from the Gamma Ray log



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Basic Log Interpretation

Depth of investigation and resolution depends on the principles of interpretation specific to each tool used.

Different tools give us the basic elements of petrophysical evaluation, which are:

Petrophysical Evaluation Summary :

TOP-TV

FT-T VD

TOPFT-MD

BA-SEFT-MD

GROSSFT-MD

NET SANDFT-MD

N/GNET

Pay FT-MD

Hole Angle

TVNet PAY

HC TYPE

POR % SWA %Perm Insitu

16053 16414 16470 56 15.5 0.28 15.5 14 15.0 OIL 25 44 110

16107 16470 16540 70 47.5 0.68 47.5 14 46.0 OIL 31 35 432

16175 16540 16553 13 7 0.54 7 14 6.8 OIL 31 38 369

16187 16553 16567 14 10.6 0.76 10.6 14 10.3 OIL 29 35 342

• Saturation

• Permeability

• N/G

• Porosity

• Resistivity

Reading Log Responses

How the Petrophysicist proceeds:

Considers vertical resolution limits of logging tools.

Considers effect on logs of contrasting lithology in adjacent beds.

Quickly evaluates bed boundaries, which should be at the same depth on all log curves for a well. Adjusts if necessary.



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Reading Log Responses

Strategies for determining bed boundaries: All log curves help, but the density log and the

gamma ray log define bed boundaries best in many cases.

Mudlog data and sidewall cores can be very useful for picking bed boundaries.

Lith description, hydrocarbon shows, and saturations can confirm evaluation.

Curve Blocking to Highlight Zones

In thin beds, the log response cannot

resolve the true value.

In thick beds, the log response is

approximately correct.



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Net/Gross – GR Interpretation in a Sand Shale Sequence

Exercise

Identify the sandstone and estimate N/G.

Draw a shale line in green.

Draw a clean sand line in yellow.

Draw the 50% line in blue.



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Exercise

Determine sandstone N/G from the GR curve.

630 /1500





630/1500 = .42 or 42%

Exercise

Determine sandstone N/G from the GR curve.







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Net pay – Net reservoir interval that contains hydrocarbons and can flow them (i.e., porosity, hydrocarbon saturation, AND permeability)

Verify Definitions with Your Sub-surface Team

Gross sand – Sand portion of gross interval (not shale)

Net reservoir – Net sand that has sufficient porosity to store hydrocarbons

Net sand – Sand that meets some lithologic criteria (e.g., Vsh < some critical value)

Gross interval – Entire section regardless of lithology

Be Careful! – Always make sure all are using the same language.

Reminder: Which “Net” Do You Need?

GR

Top of Sand

Base of Sand

Top Porosity

HC/Water Contact or Sw cut-off

Net

San

d

Net

Res

ervo

ir

Net

Pay



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Shale Volume Determination from the GR log

First, calculate the gamma ray index:

(GR – GRcleansd)

(GRsh – GRcleansd)

Vsh – shale volume

GR – reading at a specific depth or in an interval of interest

GRcleansd – average reading in nearby clean sands

GRsh – average reading in nearby 100% shale intervals

VshGR =

Shale Volume Determination from the GR log

Cautions!

• Avoid taking the GRsh reading from very thin, highly radioactive streaks. These can be enriched in radioactive minerals owing to unusual geological conditions.

• Be careful of the assumption that surrounding shales are similar in nature to those within the sand intervals. Remember detrital (deposited) and authigenic (formed after deposition) clays?



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Steiber Vshale =

Quick-Look Vsh from Gamma Index

II

Iwhere = gamma ray index

gr x 0.5 1.5 - gr

Relationship Equation

Linear Vshale = X

Clavier Vshale = 1.7 – (3.38- (X + .7)2)1/2

Bateman Vshale = X(X + GR factor)

Steiber Vshale =

GR Index vs. Vshale



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And (from previous chart)

for the interval 11,600’–11,640’,

Vsh (Steiber) = 8% shale

Determine Vsh From Gamma Ray

Between 11,600’ and 11,640’

1. Estimate Vsh (linear)

2. Estimate Vsh (Steiber)

(GRlog – GRcleansd)

(GRsh – GRcleansd)Vsh(linear) =

= = .20 or 20%(40 – 20)

(120 – 20)

40 API units

Learning Objectives


Tell about picking bed boundaries to determine zone thickness

Explain the terms gross thickness, net pay and net/gross ratio

Discuss how to calculate net reservoir rock from the Gamma Ray log

Describe how to calculate shale volume, Vsh, from the Gamma Ray log




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Porosity and Quick-Look Petrophysics


Learning Objectives


Explain how to calculate porosity from the Density log

Describe how to recognize gas effect on a Density – Neutron combination log

Discuss how to correct for the effect of free gas on the Density –Neutron combination log using the 1/3 – 2/3 “rule of thumb”

Tell what parameters are required to calculate shale volume, Vsh, from the Density – Neutron combination log



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A Porosity Refresher

The total bulk volume (v) comprised of grains and fluid-filled pores (v = A • h)

Scanning Electron Microscopy (SEM) photograph of quartz sand.

A Porosity Refresher

Porosity—φ—indicates how much fluid can be held.

Porosity is pore volume per unit volume of the formation. φ =

pore volume

unit volume



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Density Values in Formation Evaluation (g/cc)

m - b =

m - f

Density log porosity,

ρb Log curve density

ρm Formation matrix density

Quartz (sandstone) 2.65 g/cc

Calcite (limestone) 2.71

Dolomite 2.87

ρf Pore fluid density

Fresh water 1.00

Salt water (200 g/l) 1.13

Fresh water with 30% residual oil 0.90 - 0.94

Fresh water with 30% residual gas 0.73 - 0.74

Rho (ρ) = g/cc = gm/cc = gm/cm3

Calculate Φ at 12,440 and 13,000 Feet

Determine Porosities using the equation

Porosity,

Calculate Φ at 12,440’ and 13,000’

m - b =

m - f

At 12,440’, ρb = 2.10 g/cc

At 13.000’, ρb = 2.20 g/cc

See the answers on the next slide.

Assume:12,440' is an oil sand13,000' is water sand



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Calculate Φ at 12,440 and 13,000 Feet

Determine Porosities using the equation

Porosity,

Calculate Φ at 12,440’ & 13,000’

At 12,440’, ρb = 2.10 g/ccAt 13.000’, ρb = 2.20 g/ccAssume:

• 12,440' is an oil sand

• 13,000' is water sand

For these calculations:• Rho-m is 2.65 since this is a sand.

• At 12,440’, rho-f is 0.9 gm/cc since this is an oil sand.

• At 13,000’, rho-f is 1.0 since this is a wet sand.

m - b =

m - f

Remember Nuclear Device Porosities!

The Density Tool The Neutron Tool

The Density tool responds to the electron density of the

formation in front of the tool.

The Neutron tool responds to the hydrogen index of the

formation in front of the tool.



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Quick-Look Porosity Determination in Gas Bearing Zones

The 1/3 to 2/3 “Rule of Thumb”

Recognizing Gas on Density/Neutron Logs



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Recognition of Gas

A Few More Words on Net Sand Selection

Density/Neutron Crossplot



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Determining Vshale from Density/Neutron

D = + Vsh ϕDsh

N = + Vsh ϕNsh

ϕNsh ϕDsh

N Dsh

Nsh Dsh

( - )V =

( - )

Solving for Vshale,

Remember to Use Gas Corrections

A Few More Words on Net Sand Selection

Example of DensityNeutron Xplot: Clastic Reservoir



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A Few More Words on Porosity

Gas and shale increase the matrix composition uncertainty and thus porosity uncertainty.

Clean sand line

Learning Objectives

You should now:

Explain how to calculate porosity from the Density log

Describe how to recognize gas effect on a Density – Neutron combination log

Discuss how to correct for the effect of free gas on the Density –Neutron combination log using the 1/3 – 2/3 “rule of thumb”

Tell what parameters are required to calculate shale volume, Vsh, from the Density – Neutron combination log




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Resistivity, Archie, and Saturation Determination Part 1


Learning Objectives


List two or more ways to approach water saturation, Sw,determination

Discuss why formation resistivity is required to use the ArchieEquations

Tell about three resistivity parameters required to apply theArchie Equations to calculate water saturation

Explain how hydrocarbon saturation is determined after watersaturation is known

Describe a method of Quick Look evaluation when a “TripleCombo” log is available (Resistivity/Density/Neutron andGamma Ray)



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Sw Estimation Processes

Core analysis

Nuclear Magnetic Resonance Imaging (MRI)

Archie relationships – resistivity, porosity saturationmodel

• Principles of resistivity logs• Derivation of required parameters – (Rw, “m”, “n”)• Archie Crossplot – resistivity, porosity

Sw Estimation Processes

Other Quick-Look approaches• Ratio method – resistivity and SP• F overlays or Synthetic Ro – excellent method

using porosity

Special cases – shaly sands



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Resistivity Definitions

• Rw: Resistivity of formation water• A function of salinity and temperature

• The higher these two variables, the lower theresistivity of the water (the water will be moreconductive).

• Ro: Resistivity of water-bearing formation• Ro > Rw (more rock, less water)

• Rt: Resistivity of hydrocarbon-bearing formation• Rt > Ro (rock, hydrocarbon are non-conductive)

How to Measure Resistivity?

Dual Laterolog Measurement Induction Measurement



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Criteria for Resistivity Tool Selection: Induction versus Laterolog

Which resistivity tool?

Laterolog Induction

Oil Based Mud No Yes

Salt Water Mud Yes Possible 1

Fresh Mud Possible 2 Yes

Air Filled Holes No Yes

High Rt Yes No

Low Rt Possible 3 Yes

Rt > Rxo Preferred

Rt <Rxo Preferred

1. Possible if the following conditions are met:• Hole is small and in gauge.• Rt/Rm is low.• Tool position in the borehole is well known.

2. Possible if Rt/Rm is high.3. Possible if tool string length correction is applied.

Hydrocarbon Saturation and the Combined Archie Equation

True resistivity, Rt, of HC-bearing rock depends on:

Rw—Resistivity offormation brine

—Porosity

Sw—Water saturation

m, n—Lithology

S—Saturation, is a fraction of pore volume

Shc = 1 – Sw

nw

mw

tS

RR

1



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Combined Archie Equations

nw

mw

tS

RR

1

The combined Archie equation

can be expressed as two equations:

Archie I is for wet formations: F = = -m

Archie II is for hydrocarbon zones: I = = Sw-n

If you solve Archie I for Ro, and substitute that into Archie II, the result is the combined equation.

Also, mathematically, Sw-n = 1/Sw

n

Ro

Rw

Rt

Ro

Archie I

F = Formation resistivity factor (FRF or F)

Ro = Resistivity of 100 % brine saturated rock

Rw = Brine resistivity (m)

= Porosity

m = Cementation factor

F

1.00.11

100

10m

Each point representsa separate rock sample 100% saturated with water of Rw

Archie I: The foundation of quantitative petrophysics:

F = = -mRo

RwF

m



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F versus Chart (for 100% water saturated rock )

F = = 1 Ro

m Rw

Humble Equation

Easy Square Root

Cementation Factor – m

Formation Factor, F =

m is cementation exponent

m has large effect on saturation calculation

m is a function of poresystem tortuosity (the morecomplex the pore system,the higher the formationfactor F.

1 m

L

L



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m-Values, Common Reservoir Rocks

Lithology m

Sandstones Unconsolidated 1.4

Loosely consolidated 1.6

Friable 1.7

Average 1.8

Very hard 2.0

Carbonates 2.0-2.2

Archie II

I = = Sw-n

I = Resistivity index

Rt = Resistivity of partly brine saturated rock

R0 = Resistivity of fully brine saturated rock

Sw = Water saturation

n = Saturation exponent

All points aremeasured onthe samecore sample

I

Sw 1.00.11

100

10n

Saturation exponent

Rt

Ro



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Water saturation (Sw) isour variable.

Porosity () and waterresistivity (Rw) are fixed

Rock resistivity (Rt) ismeasured

n = saturation exponent

Resistivity Index (I) =

Sw = ( ) (1/n)

Archie II, Sw & Resistivity Index Summarized

Rt

Ro

Saturation Exponent “n”

Rw

m x Rt Sw = ( ) (1/n)

Rw

m x Rt

Quick-Look Evaluation

1. Identify sands in GR log andidentify water-saturated zonein sand.

2. Evaluate porosity, phi, fromdensity/neutron logs.

3. From deep-resistivity log,identify water zone based onlow resistivity; calculateporosity in hydrocarbonzones.

4. In density/neutron log,calculate water (closecurves) or gas (separation ofcurves) in hydrocarbonzones.

5. In deep resistivity log identifycontacts and hydrocarbonfluid types.

1 2 3

4 5



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Quick-look Evaluation – Uncertain Fluid Contacts

Sources of Rw

Rw catalogs

Rw

Rwa

Direct measurement of a water sample

Ratio technique

F overlays Chemical analysis and conversion to resistivity

Estimation of spontaneous potential curve (SP)

Crossplots



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Gas zone

Oil zone

Wet zone

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Resistivity, Archie, and Saturation Determination

Part 2

Example:

Ro = 5 Ohm.m = 0.25m = 2Rw = 0.252 • 5 = 0.0625 • 5 = 0.3125 Ohm.m

Use of Archie I Equation to Determine Rw

Rw – Resistivity of formation brine

Archie I, solved for Rw = m • Ro



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Calculate Rw at 13000'

Archie I – Rw = m • Ro

At 13,000’:

Rwa Exercise

Rwa = Rt • m = Rw in 100% water saturated formation

Level Porosity % Rt1 23 19.52 17 36.0

3 19 18.04 22 6.55 18 5.56 20 4.07 16 6.5

Let m = 2 to start

Rwa1.031.040.650.310.180.160.17



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Rwa Exercise

Rwa = Rt • m = Rw in 100% water saturated formation

Level Porosity % Rt Rwa1 23 19.5 1.032 17 36.0 1.04

3 19 18.0 0.654 22 6.5 0.315 18 5.5 0.186 20 4.0 0.167 16 6.5 0.17

Let m = 2 to start

Direct Measurement of Rw

Drill stem test

Wireline formation test tool

Produced sample at the separator

What are the limitations to each?



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Rw from Chemical Analysis of a water sample

Ion SymbolConcentration

ppmMultiplier*

Effective Concentration

**Sodium & Chloride Na+ & Cl - 25,000 1.00 25,000

Carbonate CO3-- 2,000 0.40 800

Sulfate SO4++ 5,000 0.37 1,850

Bicarbonate HCO3- 10,000 0.20 2,000

Magnesium Mg ++ 4,000 0.95 3,800

Total 46,000 33,450**

**Effective Concentration in NaCl equivalents.* “Multiplier” is a function of both type of ion and temperature.

Conversion to Resistivity

Function of concentration and temperature

Chart for conversion

Example: 33,450 ppm TDS (the red line), assume the formation temperature is 200 deg F (94 degC) in the objective formation.

Then the Rw at 200 degF is 0.065 ohm.m = Rw

Chart can be approximated by Arps equation

R2 = R1[(T1 + 6.77)/(T2 + 6.77)] oF

R2 = R1 [(T1 + 21.5)/(T2 + 21.5)] oC

constant salinity line of 33,450 ppm



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Saturation Determination – Archie II Equation

Sw

Matrix

Oil

Water

1 -

(1 - Sw)

Bulk Volume Water (BVW) = Sw

Sw = { (Rw) / (m • Rt) } (1/n)

Sw Determination in Hydrocarbon Bearing Sand

Shc (red)

Swirr (blue)

water saturation coating the grains

Resistivity:



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Sw Determination – Archie Equations Combined

Sw Determination – Archie Equations

Archie I

Archie II



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nw

wt

FRS

R

Archie Saturation Equations

0nw

t

RS

R

n

ww m

t

aRS

R

Exercise

Calculate Sw

at 12,440'



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A Quick-Look Sw Chart

Nomogram for clean sandstones

Sw Determination Porosity – Resistivity Crossplots

Provide overall picture of relative potential of all zones.

Errors or suspect data become obvious as a pattern

Pickett Plot• Widely used • Available in most petrophysical software

Hingle Plot• Somewhat antiquated – Pickett more common



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Sw Determination Porosity – Resistivity Crossplots

Made on log-log paper

Rt versus

Demands little prior knowledge of fixed parameters Rw and m and may allow their determination

A “bird’s-eye view” of all zones on one display

A better perspective than looking at a long log print.

An easy, comprehensive comparison of multiple zones

Requires special graph paper

Reciprocal root of Rt versus linear

Somewhat antiquated— Pickett more common

versusPickett Plot Hingle Plot

Sw Determination – Pickett Plot

Slope = -1/m

Rw = Resistivity at 100% porosity!



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when Sw = 1, log Sw = 0 and y = mx + b,

log Rt = - m log + log Rw + log a - n log Sw


n

ww

mt

aRS

R

log Rt = - m log + log Rw

Pickett Plot Summary

Provides estimation of Sw with minimum of pre-determined information

Can help determine m and Rw by rewriting Archie equation

Can plot porosity on the y axis and resistivity on the x axis



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%

P

RW

slope = -1/m

F Overlay Quick-Look Technique: Ro versus Rt

Formation Factor or F Curve is calculated from a porosity log, usually the density log

F curve is readily converted to a synthetic Ro curve by multiplying it by Rw (from wet sand data or as assumed)

F overlay quick-look is similar to Rwa technique

Ro

Rw

1m

F = =



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F Overlay Quick-Look Technique: Ro versus Rt

1. From the density log, B, create a porosity log, D

2. From the density porosity log, D, compute F curve (where F = 1/ D

m) and let m = 1.8 (sands) or 2.0 (carbonates)

3. On the logarithmic resistivity scale, overlay the F curve to match the resistivity of any available wet zone. At this point, the F overlay curve becomes a synthetic Ro curve, since Ro = F • Rw.

4. Or, if no wet zones available in a sand-shale environment, overlay the F curve on the shale resistivity.

5. Ro – Rt separation → Sw

Finally, shade in all intervals where the deep resistivity curve reads higher than the synthetic Ro curve. This shaded area represents probable hydrocarbon saturation.

• Ro is calculated from the F overlay curve.

Sw

F Overlay Quick-Look : Ro versus Rt

• If Rw is unknown, the F curve is normalized to Ro by overlaying it on wet sand, or a barren (non-source rock) shale.

• The separation between Ro and Rt on this log scale can be scaled into Sw.

• In water-bearing zones, Ro and Rt are approximately the same.



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Discriminate reservoir from non-reservoir GR, SP, D/N logs

Discriminate between water-bearing and hydrocarbon-bearing reservoir intervals

resistivity

Discriminate between gas-bearing and oil-bearing reservoir intervals

D/N separation, sonic

Evaluate reservoir porosity D/N, sonic

Calculate Rw Archie I or Pickett plot

Determine reservoir water saturation Archie 2, Pickett plot, quick-look



resistivity




Calculate Rw Archie I or Pickett plot



resistivity






resistivity





resistivity

Evaluation Checklist


Learning Objectives

You should now:

List two or more ways to approach water saturation, Sw, determination

Discuss why formation resistivity is required to use the Archie Equations

Tell about three resistivity parameters required to apply the Archie Equations to calculate water saturation

Explain how hydrocarbon saturation is determined after water saturation is known

Describe a method of Quick Look evaluation when a “Triple Combo” log is available (Resistivity/Density/Neutron and Gamma Ray)




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Petrophysical Evaluation Approach and Shaly Sands Evaluation


Learning Objectives


Discuss how to approach a petrophysical evaluation including data sources and required parameters

Explain why, at a minimum, we need a porosity log and a resistivity log to calculate water saturation

Tell about the difference in traditional log analysis and statistical log analysis



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Further Steps in Evaluation

Introduce Petrophysical Evaluation Process Flow

Re-visit “net” terminology

Discuss porosity and statistical models

Introduce saturation models for shaly sands• Influence of shale on logs and production

• Shaly sand evaluation: models based on Vshale and Rshale

• Saturation models based on electrical double layer theory

Objective: To make you aware of some petrophysicalevaluation considerations beyond quick-look.

Level of Detail One point from

one zone in a well

A few zones in one well

An entire well

Many wells in a field

Interpretation Detail versus Application?

Application Pick top, marker bed,

casing, test or core point

Completion/testing decisions

Reserves, well to well correlations

Field study, mapping, ϕ, OOIP, etc.

Petrophysicistworks closely with geologist and reservoir engineer to tailor approach.



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Data Gathering – How Much?

Sources of DataLWDMudlogsWireline logsCore analysesWireline testingProduced fluid analysisTest results

Reconnoitering

• Use a 1"– 2" per 100' log scales

(1/500 or 1/1000 scale for log depths in meters)

• Visit with other disciplines.

• Does the story make sense?

• Shales versus sands

• Porous zones

• Permeable zones

Start with overall view

Look for trends

Quick-look key areas



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Model Selection Considerations

Use Archie equations … including thin

beds, and clay mineral effects:

Use resistivity and ϕ readings

… including carbonates and evaporites

Use statistical models

For clean formations For shaly

sandstones For multi-mineral, mixed lithologies

Determining Calculation Parameters

Bare minimum m, n, Rw

Sometimes useful Rmf

Lithology parameters rma, Dtma, etc.

Shale parameters GRsh, fNsh, rsh, Rsh, etc.

Preliminary cutoffs for net pay f, Sw, Vsh, Bvw, etc.



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Calculations

Computer analysis packages (petrophysical software)

Spreadsheets for petrophysicalevaluations and pay summaries

Quick-look methods: Cross-plots, Rwa, etc

Service company statistical analysis

Provide petrophysical answer products

• Understand the process and inputs

Scientific calculators and nomograms

Petrophysical Evaluations: Process Flow (1)

Petrophysicsis all about

integration of data and its

interpretation!



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Petrophysical Evaluations: Process Flow (2)

In the case of a field study, be ready to

iterate.

Interpretation Reminders

Log Used Objective Parameters Required

SP, GR Find reservoir rocks

Density, Neutron,Sonic, Pe

ØFind Porosity & Lithology

Neutron Matrix Settingma, t, btma, tf, t

LaterlogInduction

SwFind Water Saturation

Rt, Rw, Ø“a”, “m” & “n”

SFL or MSFL Sx0Find Sx0

Rx0, Rmf, Ø‘a”, “m” & “n”

Density, Neutron, hyDetermine hydrocarbon type

ØN, ØD

hFind pay thickness

Sw maximumØ minimum

OIPFind Oil in Place

H, Ø, Sw

NRecoverable volumes

OIP, Area Recovery Factor o or g



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A Few More Words on Porosity

Due to presence of mixed mineral lithologies, porosity and lithology often determined with Statistical Log Analysis software.

Traditional Log Analysis Statistical Log Analysis

Learning Objectives

You should now:

Discuss how to approach a petrophysical evaluation including data sources and required parameters

Explain why, at a minimum, we need a porosity log and a resistivity log to calculate water saturation

Tell about the difference in traditional log analysis and statistical log analysis




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Saturation Models in Shaly Sands


Learning Objectives


Explain what a shale is and how to distinguish between a shaly sand and a sandy shale

Describe the effect of shale on resistivity logs and Neutron logs

Tell about the difference in the Archie Equations and Shaly Sand models

Discuss why CEC (Cation Exchange Capacity) is important

List three types of clay minerals and tell which has the highest CEC

Describe three modes of shale distribution



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What Are Shaly Sands?

Shaly sands:

Reservoir sands containing 10% to 35% Vshale

(clay minerals plus other components)

Shale is defined by grain size. Clay size particles are those less than 4 microns

Shaly Sands: What Are Effects?

Alters calculated Sw Due to excess conductivity. The use of Archie’s equation can underestimate HC: use one of the shaly sand equations for evaluation

Alters porosity calculated from Neutron log

Due to H+ & OH- ions and CBW in clay minerals. CEC, Do not use Neutron log – use Density

Increase actual Sw of pay sand

Allows sand to produce water-free at higher Sw (e.g.50-70%)

Can significantly decrease the effective porosity and permeability

When clay minerals occur in pores and pore throats

Can increase sensitivity of reservoir to commonly used well-bore fluids

Drilling filtrateCompletion/workover fluids



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Shaly Sand Resistivity

Archie equation assumes rock framework has no electrical conductivity

True for clean sandstones and clean carbonates

Not true for rocks that contain shale/clay mineral

Clay minerals are conductive in water and brine

The presence of clay minerals (common in sandstones, less common in carbonates) adds a conductivity contribution

Cause Sw over-estimation in shaly rock using Archie equation —a major problem in shaly sandstone reservoir evaluation

Additional Conductance Model

More lanes less traffic jam

Equally:

More conductance paths less resistivity

Hence, clay conductance leads to decrease in resistivity

Additional conductance is expressed with Qv (or CEC). Qv is related to the amount and type of clay



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Shaly Sand Resistivity Equations

All shaly sand models are based on Archie for limit of no shale content

All are constructed with the concept of two parallel resistances of:

• The pore brine

• The shale components

The general equation takes the form:

= + X,

where X is the conductivity contribution of the shale element

1Rt

Sw2

F • Rw

Shaly Sand Resistivity Equations

Two families of equations:

Traditional Family: Shale is considered a homogeneous conductive medium and resistivity equations are based on Vsh. These equations use an Rsh term for shale resistivity

Technically Correct Family: The conductivity of the shale component is a function of the cation exchange capacity (CEC) of the various types and abundances of clay minerals



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Major Shaly Sandstone Equations

2wsh

sh

shwt

1- V S V1 = + R F R R

22shw

shwt

VS1 = + R F R R

2shw

shwt

eVS1 = + R F R R

2w v w

wt

S BQ S1 = + R F * R F *

2wshw

shwt

V SS1 = + R F R R

2-Vsh 2-Vsh2w wsh shw

w sh shwt

V S V SS1 = + +R F R FR R R

sh1-V

2wshw

shwt

V SS1 = + R RF R

2w q v wbww

o owt

C - C V Q SS1 = + R F R F

wsh sh

1 1-

F R R

2wsh Shw

wt

V SS1 = + +R F R

Poupon et al (1954)

Hossin (1960)

Simandoux (1963)

Waxman and Smits (1968)

Barton and Pied (1969)(modified Simandoux)

Poupon and Leveaux (1971)

Schlumberger (1972)

Clavier et al (1977)

Juhasz (1981)

Shaly Sandstone Analysis

Archie versus Waxman Smits

None

Clay

Solids Wat

er HC

Sw

None

Clay

Solids

clay

C

B

W

HC

Swt

t

Wat

er

Log Ro

Rw

**

**

* **

* *

Log

Rw

* * * * ** ***

*

Log

Log Ro

Points plot straight

Shaly sand points plot off straight line

Shale Effect

d

Total porosity from density log

n

Total porosity from neutron log



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Older Equations Based on Vsh, Rsh.

Physical basis for these equations is incorrect

• These log-based Vshale models don’t account for shale/clay minerals existing in differing morphologies

• Note that clay mineralogy may differ between sandstones and associated shales

However, equations can provide reasonable approximations of Sw especially when equation parameters are adjusted to conform with local data from Sw measured in cores or production tests

These equations are widely used today because of:

• Simplicity

• Limited requirement for input parameters

Simandoux (1963) is most commonly used.

Gamma Ray Resistivity

Schematic for Estimation of Vsh and Rsh

x

2 wshwwt sh

V x SS1 = + R F R R

= + X1Rt

Sw2

F • Rw

Basic form of the shalysandstone equation has two components:

Archie equation

Shale component, X

The two parameters of X are:

Vsh (volume of shale)

Rsh (shale resistivity)– Both are problematic and

result in uncertainty



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To overcome the problem related to Rsh and Vsh

determination, the Waxman Smits equation use a solution based on ionic double layer theory.

CEC is a function of the various clay mineral species and the abundance of each.

Shale Conductivity

x

2w w

wt

S BQvS1 = + R F * R F *

Clay-rich sands with dispersed shale have excess conductivity due to the presence of cations bound at the clay surface

CEC is the most important property of clay in shalysand evaluation

The concentration of Na+ can be measuredby chemical means and is called:

Cation Exchange Capacity (CEC)

CEC Definition



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Shaly Sands/Sandstones

Different clay mineral species have different measured values of CEC. Each species will have a different affect on logs

Smectite(fine grained clay)

80 to 150meq/100gm

Illite 10 to 40meq/100gm

Chlorite 0 to 20meq/100gmKaolinite(coarse grained clay) 3 to 5meq/100gm

CEC Lab: Measurement Methods

Membrane potential(recommended)

Wet Chemistry method: Conductometric titration is usual contractor method

Sample is crushed, placed in barium solution, and

titrated

Multiple salinity Absorbed water(not recommended)



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Understanding Clay Chemistry: Bound/Free Water

Positiveions movingalong claysurface

Gouylayer

Freeions

Negatively charged clay surface creates additional conductance path

Negatively charged clay surface

Qv Determination: Wet Chemistry Titration Method

CEC 1 - Phi GDQV =

100 Phi

Crushed sample

BaCI2 wash

Ba + clay titrated with MgSO4

(BaSo4 + Mg + clay) titrated

Gives CEC



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Problems in Use of Waxman Smits Model for Sw

2 wsh shww wt sh sh

V SS1 1 1 = + - + R F x R F R R

Waxman Smits model requires representative core samples in a laboratory, on which to measure CEC

Requires actual rock samples from reservoir

Difficult to obtain reliable values of CEC from rock samples using standard laboratory methods

For these reasons double ionic layer models are often considered to be impractical

Consequently, Waxman-Smitts model modified to use substitute data determined from logs using surrogate variables

Juhasz equation (1981 technical paper):

Saturation Models: Different Shale Distributions

Clean Sand Structural Shale

Laminar Shale Dispersed Shale



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Saturation Models: Recognizing Shale Distributions

Sand with a pore volume 100% filled with dispersed clay

End point for laminated clean sand-shale,100% shale.

Saturation Models: Recognizing Shale Distributions

Density/Neutron Crossplot



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Example of a “Petrophysical Evaluation Summary Log”

Net (TV)Kb

15’110 md

46’432 md

7’369 md

10.3’342 md

16495’ MD(16131’ TV)9366 psia (11.2 ppg)

171o F27.9o API1135 GOR

MDT Spls.

16563’ MD(16197’ TV)9386 psia (11.2 ppg)

172o F26.7o API1150 GOR

M1 Sand 16414’-16567’ MD

Gross 153’ MDNet 81’ MD (78’ TV)N/G 0.53Por. 29.6%Sw 37%Kb 350 mdHole Angle 14o

NET NET TV

SAND Top-TV TOP BASE GROSS SAND N/G PAY Hole Net HC POR SWA Perm

FT-TVD FT-MD FT-MD FT-MD FT-MD FT-MD Angle PAY TYPE % % insitu

H9 16053 16414 16470 56 15.5 0.28 15.5 14 15.0 OIL 25 44 110H9 16107 16470 16540 70 47.5 0.68 47.5 14 46.0 OIL 31 35 432H9 16175 16540 16553 13 7 0.54 7 14 6.8 OIL 31 38 369H9 16187 16553 16567 14 10.6 0.76 10.6 14 10.3 OIL 29 35 342

Model: Thomas Steiber (N/G & Sand Porosity)Waxman Smits SaturationH-C LAM Resistivity ModelPermeability Correlation w/sidewalls KaRw = .03 (from wet sand)m, n = 1.75

Learning Objectives

You should now:

Explain what a shale is and how to distinguish between a shaly sand and a sandy shale

Describe the effect of shale on resistivity logs and Neutron logs

Tell about the difference in the Archie Equations and Shaly Sand models

Discuss why CEC (Cation Exchange Capacity) is important

List three types of clay minerals and tell which has the highest CEC

Describe three modes of shale distribution




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PetroAcademyTM Foundations of Petrophysics

Petrophysical Data and Open Hole Logging Operations Core

Mud Logging, Coring and Cased Hole Logging Operations Core

Gamma Ray and SP Logging Core

Porosity Logging (Density, Neutron and Sonic) Core

Formation Testing Core

Resistivity Logging Tools and Interpretation Core


Core Analysis Core Knowledge

Special Petrophysical Tools: NMR and Image Logs Core



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