SHALY SAND EVALUATION - A

FORMATION EVALUATION

PETE 663

Summer 2010

Shaly Formations

• Archie’s Sw equations assume aclean formation with a non-conductivematrix

• Shales are conductive

• Shaly sand conductivity varies with:• Clay type (mineralogy)• Shale origin (dispersed, structural laminated)• Fluid composition

Shaly Sand ObstaclesIn Shaly Formations:• Fresh formation waters can cause

conventional log analysis to overestimate water saturation

• Relatively salty formation waters lead to low resistivity, which may cause pay zones to be bypassed

• Thinly bedded zones may cause conventional log analysis to underestimate porosity and overestimate water saturation

Shaly Formations• In Formation Evaluation “shale” and

“clay” are used synonymously• In fact, shale is a rock

– Comprised dominantly of clay minerals– May be mineralogically complex (many minerals)– May have variable properties

• And clay– May refer to a grain size (diameter < 0.004 mm), or– May refer to a class of minerals (e.g., illite,

smectite, montmorillonite, chlorite, kaolinite)

LECTURE A• Shales/clays have several origins and forms• Shales/clays affect:

– Porosity– Permeability– Vshale

• Estimations• Assumptions• Log responses

LECTURE B• Shales conduct electricity• Problems with Archie-based methods

– Rwa problem– Sw errors

• Shaly sand analysis of Rwa and Sw

SHALY FORMATION ISSUES

WHAT IS SHALE1. Shale is type of clastic sedimentary rock

- Comprised dominantly of clay mineralsand other clay-size fragments

- Contains some silt-sized grains of- Quartz- Feldspar- Other minerals- May contain organic frags. (source rock)

2. Shale is a fissile rock – it splits along bedding planes3. Claystone – massive appearance – not fissile

WHAT IS CLAY?1. Clay is the name for a family of alumino-

silicate minerals including:- Kaolinite- Illite- Smectite- Montmorillonite- Chlorite- others

2. Clay is a class of clastic sediments with a grain size(diameter) < 0.004 mm (< 4 microns)

- May contain clay minerals, quartz, feldsparminerals, etc.

3. Clays form approximately 40% of the sedimentaryrocks

Grain-Size Classification, Clastic Sediments

Name Millimeters Micrometers

BoulderCobblePebbleGranuleVery Coarse SandCoarse SandMedium SandFine SandVery Fine SandCoarse SiltMedium SiltFine SiltVery Fine SiltClay

4,096256644210.50.250.1250.0620.0310.0160.0080.004

500250125

623116

84

(modified from Blatt, 1982)

SPECIFIC SURFACE AREAS OF SOME MINERALS

Mineral Ft^2/ft^3

Sand 4.3-8.7 thousand

Kaolinite 15.2 million

Illite 85.4 million

Montmorollinite 274 million

• Clays have extremely large surface areas

• Surface area varies greatly among clay minerals

Relative Abundances

Mudstone(Siltstoneand shale)

(clastic)~75%

Sandstone andConglomerate

(clastic)~11%

Limestone andDolomite

(carbonate)~14%

SEDIMENTARY ROCK TYPES:

AVERAGE DETRITAL MINERAL COMPOSITIONOF SHALE AND SANDSTONE

Mineral Composition Sandstone

Clay Minerals

Quartz

Feldspar

Rock Fragments

Carbonate

Organic Matter,Hematite, andOther Minerals

5 (%)

65

10-15

15

<1

<1

(modified from Blatt, 1982)

Shale

60 (%)

30

4

<5

3

<3

Framework

Matrix

Cement

Pores

- Sand- and silt-size detrital grains (load-bearing)

- Silt and clay-size detrital material

- Minerals precipitated post-depositionally,during burial and diagenesis

- Cements fill pores and may replaceframework grains

- Voids Among the Above Components

FOUR MAJOR COMPONENTS OF SANDSTONE

FOUR COMPONENTS OF SANDSTONE

MATRIXFRAMEWORK

(QUARTZ)

FRAMEWORK(FELDSPAR)

CEMENT

PORE

Note different use of “matrix”by geologists and engineers

0.25 mm

1. Framework2. Matrix3. Cement4. Pores

Engineering“matrix”

Geologist’s Classification

SANDSTONE COMPOSITION,Framework Grains

Norphlet Sandstone, Offshore Alabama, USAGrains ~0.25 mm in diameter/length

PRF KF

P

Q

KF = PotassiumFeldspar

PRF = Plutonic RockFragment

P = Pore

Potassium feldspar isstained yellow with achemical dye

Pores are impregnated withblue-dyed epoxy

Q = Quartz

Photo by R. Kugler

SHALE / CLAY ORIGINSIN SANDSTONES

EXPANDED DISCUSSION ON NEXT SLIDES

DETRITAL AUTHIGENIC

From Halliburton EL-1007

SHALE / CLAY OCCURRENCESEXPANDED DISCUSSION ON NEXT SLIDES

Dispersed Clay

Clay Lamination

Structural Clay(Rock Fragments,

Rip-Up Clasts,Clay-Replaced Grains)

φe

φe

φe

ClayMinerals

Detrital QuartzGrains

1

2

3

Order of discussion

D

E

T

R

I

T

A

L

AUTH

HOW DO SHALES/CLAYS OCCUR? - 1

Structural Shale– Replaces matrix (e.g., or feldspar) or occurs

as detrital grains– May not affect por., perm,– Example – clast lag in channels deposits– Clay composition may differ from nearby

shales

Structural Clay(Rock Fragments,

Rip-Up Clasts,Clay-Replaced Grains)

φe

ClayMinerals

Detrital QuartzGrains

HOW DO SHALES/CLAYS OCCUR? - 2

Laminated Shale– Interlayered with sand– Reduces poro., perm.– Common– Example – shale

laminae– Assume composition

similar to nearby shale

Clay Lamination

φe ClayMinerals

Detrital QuartzGrains

(Whole Core Photograph, MisoaSandstone, Venezuela), W. Ayers

Whole Core

Laminated Ss-Sh Reservoir Rock

STS61A-42-0051 Mississippi River Delta, Louisiana, U.S.A. October 1985

STS084-721-029 Selenga River Delta, Lake Baykal, Russia May 1997

Which environments are most likelyto result in:- Structural shales/clays? - Laminated shales/clays?- Clean sands?

Log patterns reflect grain size andmineral composition, which in turn,are related to depositional environmentand sediment transport energy.

Ayers, 2004

HOW SHALES/CLAYS OCCUR? - 3Dispersed Clay/Shale

– Pore-filling clays– Very common– Forms in situ (authigenic clay - diagenesis)– Mineral composition may differ greatly from

nearby shales– Por. and perm. reduction depend on clay minerals

Dispersed Clayφe

ClayMinerals

Detrital QuartzGrains

DIAGENESISDiagenesis:• Post-depositional chemical andmechanical changes that occur insedimentary rocks

Diagenetic Effects include:- Compaction- Precipitation of cement-Dissolution of frameworkgrains and cement

Diagenesis may:-Enhance or degrade reservoirquality

Whole core, Misoa Formation, Venezuela

CarbonateCemented

OilStained

Photo by W. Ayers

POROSITY IN SANDSTONE

QuartzGrain

Pore

Scanning Electron MicrographNorphlet Sandstone, Offshore Alabama, USA

Porosity in sandstonetypically is lower than

spheres owing to:

- Variation in grain size

- Variation in grain shape

- Cementation

- Mechanical and chemicalcompaction

Photomicrograph by R.L. Kugler

that of idealized packed

POROSITY IN SANDSTONE

Scanning Electron MicrographNorphlet Formation, Offshore Alabama, USA

Pores provide thevolume to storehydrocarbons

Pore throats connectpores; may restrictflow

PoreThroat

Photomicrograph by R.L. Kugler

POROSITY IN SANDSTONE

Scanning Electron MicrographTordillo Sandstone, Neuquen Basin, Argentina

Pore throats insandstone maybe lined witha variety ofcement mineralsthat affectpetrophysicalproperties

Photomicrograph by R.L. Kugler

DISPERSED CLAY TYPES AND FORMS• Kaolinite – booklets, particles

– Moderate perm effects– May dislodge, block

throats• Chlorite – linings, coatings

– Significant perm loss,sensitive to acid treat.

– Trap water• Illite – pore-bridging tangles

– Choke pores and throats– Drastic perm reduction– Collapse if dried,

giving anomalous labvalues

Clay Minerals in Sandstone Reservoirs,Authigenic Kaolinite

Secondary electron micrographCarter SandstoneNorth Blowhorn Creek Oil UnitBlack Warrior Basin, Alabama, USA

• Significant permeabilityreduction

• High irreducible water saturation

• Migration of finesproblem

(Photograph by R.L. Kugler)

• Not recognized bygamma ray

Clay Minerals in Sandstone Reservoirs,Authigenic Chlorite

Electron photomicrographJurassic Norphlet SandstoneOffshore Alabama, USA (Photograph by R.L. Kugler)

• Occurs as thincoats on detritalgrain surfaces

• Occurs in severaldeeply buriedsandstones withhigh reservoir quality

• Iron-rich varieties reactwith acid

~ 10 μm

Clay Minerals in Sandstone Reservoirs,Fibrous, Authigenic Illite

Electron PhotomicrographJurassic Norphlet SandstoneHatters Pond Field, Alabama, USA

(Photograph by R.L. Kugler)

Illite

•Significantpermeabilityreduction

• Negligible porosityreduction

• Migration offines problem

• High irreduciblewater saturation

SHALY FORMATIONS - 2

• Shales affect por., perm. - Illite

• Reduces porosity• Changes permeability

– Reduces perm.– Reduces variability– Reduces anisotropy

0.001

0.01

0.1

1

10

100

1000

10000

Perm

eabi

lity

(mD

)

0 5 10 15 20 25 30Porosity (%)

Illite-affectedIllite-free

0.001

0.01

0.1

1

10

100

1000

10000

Ver

tical

Per

mea

bilit

y (m

D)

0.001 0.1 10 1000

Horizontal Permeability (mD)

Illite-affectedIllite-free

1

0.1

0.01

0.001kv/k h

INTERGRANULAR PORE AND MICROPOROSITY

• Intergranular porescontain hydrocarbonfluids

• Micropores containirreducible water

Backscattered Electron MicrographCarter Sandstone, Black Warrior Basin,Alabama, USA

IntergranularPore

Microporosity

KaoliniteQuartzDetritalGrain

(Photograph by R.L. Kugler)

SHALY SANDS -Vshale Estimation

Vshale Estimation• Several estimators

– (Vsh)GR

– (Vsh)SP

– (Vsh)DS

• All depend on defining– Clean point e.g., GRmin– Shale point e.g., GRmax

• Set Vsh = min{(Vsh)GR,(Vsh)SP,(Vsh)DS}– Each estimator has flaws e.g., GR and mica, SP and

HC’s– Assumes smallest estimate is accurate

Vshale Assumptions

• Response in nearby shale gives 100% shale

• Some interval has 0% shale

• Shale in formation same as nearby shale

• The minimum is best estimate

Porosity Estimation Using Vsh – 1

shshappcorr V φφφ −=

• Effective porosity = φcorr

• Apparent porosity, matrix adjusted = φapp

• Apparent porosity in shale = φsh

• Example...

EXAMPLE - WELL “X”

• In a 10-ft shale interval,– RHOB = 2.39

150.165.239.265.2 =

−−

=

−−

=flma

bmash ρρ

ρρφ

23=−−

=flma

bmaapp ρρ

ρρφ

20)15(19.023 =−=Dcorrφ

• 224ft shaly sand– RHOB = 2.27– Vsh = 19%

Using φcorr eq.: (prev. Slide)

Determine CorrectDensity Porosity

EXAMPLE - WELL “X”

• 10ft shale– PHIN = 36 (LS)– PHIN = 40 (SS)

19)40(19.027 =−=Ncorrφ

• 224ft shaly sand– PHIN = 23 (LS)– PHIN = 27 (SS)– Vsh = 19%

• Density & neutron agree within 1 pu

Determine CorrectNeutron Porosity

Porosity Estimation using Vsh - 2

shshappcorr V φφφ −=

• In water, φcorr for each tool will agree• In HC’s, φcorr may still differ• For the density-neutron,

2

22NcorrDcorr

corrφφ

φ+

=

SHALY SANDS ARE COMPLICATED!

Oil orGas

Free Water

BoundWater

Sandstone Matrix (Solids)

SbSwSh

Swt

VSh Vma

φe

φ t

φz

DryClay

ShaleSandstonePore- Filling

Fluids

LECTURE A• Shales/clays have several origins and forms• Shales/clays affect:

– Porosity– Permeability– Vshale

• Estimation• Assumptions• Log responses

LECTURE B• Shales conduct electricity• Problems with Archie-based methods

– Rwa problem– Sw errors

• Shaly sand analysis of Rwa and Sw

SHALY FORMATION ISSUES