SHALY SAND EVALUATION - B

FORMATION EVALUATION

PETE 663

Summer 2010

SHALY FORMATION ISSUES –Water Saturation

Obstacles with Shaly Sands

• Fresh formation water can cause conventional analysis to overestimate water saturation

• Salty formation water can cause low resistivity, meaning pay zones can be bypassed

• Thin beds may lead conventional log analysis to underestimate porosity and overestimate water saturation

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 FORMATION ISSUES

WELL LOG EFFECTS - 1• Well “X”• Water leg

– OWC @ 150 ft• Shaly interval

– 220 - 230 ft.– Resist. increase– Sonic Δ t

increase– Density ρ b

increase

WELL LOG EFFECTS - 2

• HC zone– OWC @ 150 ft

• Shaly interval– 115 - 130 ft.– Resist. decrease– Sonic Δt increase– Density ρb

increase– Neutron φN

increase

FCC

FRR

wo

wo

=

=or

shaleWithout

wC

oCF1

XF

CC wo +=

shaleWith

The factor X is the excess conductivity caused by the fact that the clays and shales are conductors of current

wC

oCF1

X

Grain Brine

Pore

Pore Throat

Current

ELECTRICAL CURRENT FLOWING THROUGH BRINE – CLEAN SANDSTONE

SCHEMATIC - ELECTRICAL CURRENT FLOWING THROUGH DISPERSED CLAY COATING AND BRINE, SHALY SANDSTONE

Grain Brine Clay Coat

Total Conductivity =

What is the effect of dispersed clayon resistivity when pores arefilled with brine?

ELECTRICAL CURRENT FLOWING THROUGH DISPERSED CLAY COATING, SHALY SANDSTONE

Grain Oil Clay Coat

Electrical Current

What is the effect of dispersed clay on resistivity when the pore-filling fluid is oil rather thansaline water?

SILICATE TETRAHEDRON SILICATE

MINERALS

Modified from Grim, 1968

SiO2

• Silicates are the most abundantminerals

• Basic building block is thesilicate tetrahedron (SiO4)

Si+ 4 O- 2,Oblique View

Map View

MONTMORILLONITE STRUCTURE

9.7 – 17.2

n – H2O & Mg, Na, Ca

n– H2O & Mg, Na, CaModified from Halliburton, EL-1007

Montmorillonite

From Grim, 1968

MUSCOVITESTRUCTURE

(Similar to Illite)

Electrical CurrentAluminum replacessilicon

• Mobile cations; • Includes Helmholtz

Planes

Illite

WHAT IS SHALE?

• Clay + silt + other• Clays

– Plate-like form– Large surface area– Contain Al+3 and

Si+4

– Substitution by Mg+2

– Negative charge results

– Attraction by water and cations

AbsorbedWater

HydrationWater

Sodium Ion

Water

SchematicWater

MoleculeOuterHelmholtz

Plane

xH

ClayCrystal

DIFFERENT MODELS OF DIFFUSE LAYER

AbsorbedWater

HydrationWater

Sodium Ion

xH

ClayCrystal

H+

H+O

SalineWater

Na+

Cl-

0 Xd

Distance FromClay Surface

Ionic ConcentrationIn NaCl Solution

)(106.3n

X d =

Gouy profile of diffuse layer,

thickness

increases as salinity decreases.

Model of exclusion layer(Helmholtz Plane)sodium ionsexcluded from surface layer bydielectric properties of water

SalineWater

(XH / 6.18 A)

ClayCrystal

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

SURFACE AREA vs CEC

WATER SATURATION –SHALY SANDS

• Shales/clays have several origins and forms• Vshale

– Estimation– Assumptions– Log responses

• Shales/clays affect formation– Permeability– Porosity

• Shales conduct electricity– Problems with Archie-based methods– Rwa problem– Sw errors

Shaly Formation Issues –Water Saturation

WATER SATURATION EQUATIONS

• Many different water saturation equationshave been developed

• Archie’s model for a clean formation is:

• All other models are for shaly formationswhere the rock is not a perfect insulator

t

wnw R

RFS =

XF

CC wo +=

shaleWith

The factor X is the excess conductivity caused by the fact that the clays and shales are conductors of current

wC

oCF1

X

Commonly used formulas to account forshale:

“Vsh” Models– Simandeaux (better with saline fm water)

– Indonesia (developed for fresher fm water)

“Double-layer” model (Attempt to avoid using Vsh)

– Waxman-Smits

– Dual water

CO vs CW in Shaly and Clean Sands

Slope = 1 / F

CO

CW

Non-LinearRegion

LinearRegion

(IndonesianEquation)

(Simandoux Equation)

Modified from Halliburton, EL-1007

CW

CW

CO

Clean Sand, F = aφ-m

Cw/Co Variation with Cw and Vcl

Modified from Halliburton, EL-1007

• Archienw

me

wt SA

CC φ=

( ) shshnw

sh

nw

mew

t CVSVASCC 1

1−+

−=

φ• Simandeaux (better with saline water)

– All Vsh models are similar: total C = clean C + shale C

• Indonesia (better with fresh water)

2/)2/(12/ nwsh

Vsh

nw

wt SCVS

FCC sh−+=

Waxman-Smits model

Where:

Note independent conduction paths by free water and bound water

nwt

mt

wt SA

CC φ'=

wt

vww S

QBCC +='

New terms– BQV: conductivity of bound water– QV: cation exchange capacity (meq/gm dry clay)

• 1 meq = 6E20 atoms• measures how many cations are present• different clays have different CEC’s

– kaolinite 0.03 to 0.06– chlorite 0 to 0.1– illite 0.1 to 0.4– montmorillonite 0.8 to 1.5

– B is: specific counterion conductivity (mho/m per meq/cc)

• Counterions are the charge-balancing Na cations• B is a per unit measure• Measures how effectively cations conduct electricity

Waxman-Smits Swt obtained by iteration

where Swt0 is the initial guess, Swt1 is the next guess, etc., and

Note: Rw in B equation is at 75°F.

( )

n

wtvw

twt

i

iSBQ

R

RF

S

1

11

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

+

⎟⎟⎠

⎞⎜⎜⎝

⎛

=+

( )[ ] max5.083.01 BeB

AF

wR

mt

−−=

=φ

Maximum Equivalent Conductance of Sodium-Exchange Ions, λNA′ or Bmax vs Temperature

020 40 60 80 100 120 140 160 180 200

Temperature, °C

0.05

0.10

0.15

0.20

0.25

λ°N

Aor

Bm

ax, m

ho -

cm-2

mca

-1

• Graphing values of Bmax vs. temperature(°R and/or °F) on various types of graphpaper, one finds that Bmax vs. log (T°R) ismore or less linear:

( ) 2.317ln)31.51(max −°= RTB

• Approximate values for Qv are:

Very shaly Qv = 1.5Moderate shale Qv = 1.0Medium shale Qv = 0.5Low shale Qv = 0.25No shale Qv = 0

• CEC or Qv should, however, be lab measured

• Qv may correlate with logs (e.g., GR)

• Dual water model

where

The dual-water model is a more general form of the Waxman-Smits model.

– The “free water” salinity can be different than the salinity of the “claybound” water.

– To determine Sw, use iterative method, like W-S

nwt

mt

wt SA

CC φ″=

wfwt

bwb

wt

bw C

SSC

SSC ⎟⎟

⎠

⎞⎜⎜⎝

⎛−+=″ 1

The Cotton Grove is identified as the interval from 6542’ to 6620’. Sample shows calcareous shaly sandstone. Calculate Sw at 6566 and 6680 ft.

Conventional Analysis

Rw = 0.05 @ Tf

F = 0.81/ф2

Rmf = 0.47@Tf

ρρρρ

φ−

−=

ma fl

ma bD

2

22ND

eφφφ +=

ρf = 1.0 gm/cc

ρma = 2.68 gm/cc

Case study 1. Pennsylvanian Cottage Grove Sandstone

Shaly sand

At depths 6566’ and 6580’

6566’

6680’

Depth φd φn φxp ρb φd Sw 6566 .135 .145 .14 2.47 .125 0.66 6580 .15 .145 .14 2.45 .137 0.61

• Conventional Analysis (Archie’s method)

gives the results below

• Sw Results are high (60%), which could

make one cautious about developing the well

Calculate Sw at 6566 and 6680 ft.

shshappcorr V φφφ −=

2

22NcorrDcorr

corrφφφ +=

ρρρρ

φ−

−=

ma fl

ma bD

MINMAX

MINSH GRGR

GRGRI−

−= )12(33.0 *2 −= shI

shV

Shaly Sand Analysis -Steps

1.

2.

3.

4.

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛+=

SH

SH

SH

SH

tw

e

e

wwe R

VRV

RRRS

22

2 **5*4.0 φ

φ5.

Simandoux Equation

Using Simandoux Equation for shaly sand analysis we have:

Depth φd ρb GR IGR Vcl

6566 .125 2.47 45 0.33 0.196580 .137 2.45 45 0.33 0.19

Depth φnc φdc φe Swe 6566 0.092 .119 .106 0.51 6580 .067 .135 .107 0.45

Comparison of Sw values:

with shaly sand analysis Sw = 0.45 - 0.51

without shaly sand analysis Sw = 0.61 – 0.66

Case Study 2. Permian Basin, Spraberry Sandstone, Midland Basin

Deep Spraberry sandstone was encountered at a depth of 7720’ to 7750’.

The formation is not very clean, as is evident from the log sandstone

Log Analysis of Depths 7724’,7732’,7738’

7724’

7238’7732’

Conventional log analysis produced the following results:

ρ f = 1.0 gm/cc

ρ ma = 2.68 gm/cc

Depth φd φn φxp ρb φd Rt Sw 7724 0.18 0.22 0.20 2.39 0.17 2.6 0.66 7732 0.23 0.23 0.23 2.32 0.21 2.9 0.50 7738 0.24 0.25 0.245 2.30 0.23 2.0 0.55

The saturations obtained are 0.50 to 0.66, which is fairly high

Shaly sand analysis produced the following:Depth ρb φd GRLOG IGR VSH

7724 2.39 0.17 75 0.48 0.317732 2.32 0.21 68 0.42 0.267738 2.30 0.23 65 0.39 0.24 Depth φnc φdc φe Swe 7724 0.136 0.127 0.132 0.477 7732 0.160 0.186 0.173 0.383 7738 0.185 0.199 0.192 0.459

Comparison of saturation values:

Without shaly sand analysis - range 0.50 - 0.66

With shaly sand analysis - range 0.38 - 0.48

Detection of Secondary Porosity

Vertical, Mineralized Fracture: 1U Payzone Shackelford 1-38A

Mineralized Fracture: 1U Payzone Shackelford 1-38A

Shackelford 1-38A (1-U in the Upper Spraberry) water saturation with different m & n compared with measured water saturation from whole core analysis. Sharp contrast between pay and non-pay is observed, by fluorescence, at a

depth of 7092 ft.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

7083 7084 7085 7086 7087 7088 7089 7090 7091 7092 7093 7094 7095 7096Depth, ft.

Wat

er s

atur

atio

n (S

w)

Pay Non-pay

Sw (a=1, m=1.66, n=1.46)

Sw (a=0.81, m=2, n=2)

Sw (core)

Fractured Zone Identification

0

0.1

0.2

0.3

7083 7084 7085 7086 7087 7088 7089 7090 7091 7092 7093 7094 7095 7096Depth, ft.

Poro

sity

Fractured Zone (pay) Non-fractured zone (Non-pay)

Porosity (neutron)

Porosity (sonic)Porosity (core)

Fracture Detection

Sponge Core -5U Zone O’Daniel #37

FMIO’Daniel #37 5U Zone

Detection of Secondary Porosity

SHALY SAND ANALYSIS - INDONESIA EQUATIONWELL “X” EXAMPLE

• Shale layer : 0 - 12 feet

• Clean layer: Approx from 190 – 220 feet

• Required : Water saturation at 225 feet and 47 feet using the Indonesia equation

• The matrix is sandstone

WELL “X” EXAMPLE

2/)2/(12/ nwsh

Vsh

nw

wt SCVS

FCC sh−+=

INDONESIA EQUATION

Porosity Estimation Using Vsh – 1

shshappcorr V φφφ −=

• Effective porosity = φcorr

• Apparent porosity, matrix adjusted = φapp

• Apparent porosity in shale = φsh

• Example...

Depth Rhob ØD ØNLS ØNSS GR Vsh ØD

(corr)ØN

(corr)Rt Sw

010 2.39 15.5 37.5 41.5 88 100 - - 2.7 -

225 2.27 23 23 27.5 20 12.8 20 19.6 0.5 0.95

047 2.17 29 24 28 20 12.8 26.2 20.5 30 0.1

SHALY SAND ANALYSIS - INDONESIA EQUATION

WELL “X” EXAMPLE

SHALY SAND ANALYSIS - INDONESIA EQUATION

Depth ρb φD φNLS φNSS CGR VSH φD corr φN corr Rt Sw

010 2.39 15.5 37.5 41.5 88 100% 2.7225 2.27 23.0 23 27.5 20 19% 20.0 19.6 0.5 0.95047 2.17 29.0 24.0 28.0 20 19% 26.2 20.5 30 0.10

WELL “X” EXAMPLE

• New terms– W-S and Dual Water models depend

on CEC

– Without CEC, have to use nearby shale

• Sb - bound water saturation– Sb = f(CEC, Cwf)

– Sb = Vshφsh/φt

• Cwb- bound water conductivity– Cwb = g(CEC, Sb)

• Clays are conductive – complex resistivity responses, Sw determination

• Two types of Sw models for shaly sand

– Vsh models (e.g., Simandeaux)

– Double-layer models

• Vsh models– Empirical

– Assume shale properties same as nearby shale

– All shales have same effect

• Double-layer models– Do not use Vsh

– Use electrical properties of clays (CEC)

SUMMARY