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
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
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