formation evaluation pete 663 - college of · pdf filepete 663. summer 2010. ... laminated...
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SHALY SAND EVALUATION - A
FORMATION EVALUATION
PETE 663
Summer 2010
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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
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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
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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)
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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
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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
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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
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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)
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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
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Relative Abundances
Mudstone(Siltstoneand shale)
(clastic)~75%
Sandstone andConglomerate
(clastic)~11%
Limestone andDolomite
(carbonate)~14%
SEDIMENTARY ROCK TYPES:
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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
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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
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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
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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
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SHALE / CLAY ORIGINSIN SANDSTONES
EXPANDED DISCUSSION ON NEXT SLIDES
DETRITAL AUTHIGENIC
From Halliburton EL-1007
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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
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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
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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
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(Whole Core Photograph, MisoaSandstone, Venezuela), W. Ayers
Whole Core
Laminated Ss-Sh Reservoir Rock
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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SHALY SANDS -Vshale Estimation
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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
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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
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Porosity Estimation Using Vsh – 1
shshappcorr V φφφ −=
• Effective porosity = φcorr
• Apparent porosity, matrix adjusted = φapp
• Apparent porosity in shale = φsh
• Example...
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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
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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
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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+
=
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SHALY SANDS ARE COMPLICATED!
Oil orGas
Free Water
BoundWater
Sandstone Matrix (Solids)
SbSwSh
Swt
VSh Vma
φe
φ t
φz
DryClay
ShaleSandstonePore- Filling
Fluids
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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