Schlumberger
(05/96)
Contents
F1.0 SHALY SAND POROSITY............................................................................................................1F1.1 CALCULATING φ
t, φ
e, AND S
W IN SHALY SANDS ...................................................................1
F1.2 GRAPHICAL CALCULATION...................................................................................................6F1.3 DIRECT CALCULATION OF EFFECTIVE POROSITY..............................................................6
F2.0 EXAMPLE CALCULATION...........................................................................................................7
F3.0 WORK SESSION .......................................................................................................................13
(05/96)
Introduction to Openhole Logging
Schlumberger
(05/96) F-1
F1.0 Shaly Sand Porosity
F1.1 CALCULATING φT,
φ
e AND S
w
IN SHALY SANDS
To this point our calculations have beenfairly straightforward in evaluating porosityand hence water saturation. As indicated inSection E, shale presence complicates inter-pretation considerably. To arrive at the bestpossible value for S
w, we must develop a qual-
ity value for porosity. This means we mustcorrect φ
T for the volume of shales and obtain
φe (effective porosity, shale free). This correc-
tion can be done graphically for all cases orusing an average assumption for neutron anddensity porosity, through equations. Boththese methods are outlined in this section.
Before giving the methodologies, let's developthe basis for the graphical correction for whichthe direct calculation approximates. Shalyclastics are generally modelled with the com-position of silt-shale-sand in which the shalescan be laminated, dispersed or structural. Thebasic model is suggested by the groupings ofthe plotted points on the neutron-densitycrossplot of Figures F1 and F2. These plotsrepresent a typical crossplot through a se-quence of sands, shales and shaly sands. Mostof the data belong to two groups: Group A,identified as sands and shaly sands, and GroupB, identified as shales.
To explain the spread of points in Group Balong the line from Point Q through Point Sh
o
to Point Cl, the shales are considered mixturesof clay minerals, water and silt in varying pro-portions. Silt is fine grained and is assumed toconsist predominantly of quartz, but it mayalso contain feldspars, calcite and other miner-als. Silt has, on the average, nearly the sameneutron and density log properties as the ma-trix quartz; pure quartz silt would plot at thequartz point, Q. Silt, like quartz, is electricallynonconductive. Points near the "wet clay"point, Point Cl, correspond to shales that arerelatively silt free. Point Sh
o corresponds to
shale containing a maximum amount of silt.
The shaly sands in Group A grade fromshales, on Line Sh
o-Cl, to sands at Point Sd, on
Line Q-Sd. The shale in these shaly sandsmay be distributed in various ways. When allthe shale is laminar shale, the point falls on theSd-Sh
o line. Dispersed shale causes the point
to plot to the left of the line. Structural shalecauses the point to plot to the right of the line.
(05/96) F-2
Introduction to Openhole Logging
Figure F1: Neutron-Density Frequency Crossplot Illustrating the Shaly Sand Model
Schlumberger
(05/96) F-3
0.2 0.4 0.6 0.8 1
0.8
0.6
0.4
0.2
– 0.2
Figure F2: Expanded φN - φD Crossplot for Shaly Sand Showing All End Points
(05/96) F-4
Introduction to Openhole Logging
Typically, few points plot in Area C. Whenthey do, they usually represent levels wherelog readings have been affected by boreholerugosity, or where shale properties have beenaffected by hydration of the clay in contactwith the mud, or where matrix lithology nolonger corresponds to a shale-sand sequence(e.g., porous carbonates, lignite).
Once Points Sd, Sho and Cl have been de-
termined from inspection of the crossplot, theplot can be scaled for water-bearing sands andshales in terms of φ and V
cl, as shown in Fig-
ure F3. The lines of constant φe are parallel to
the shale line, Q-Cl. They range from φe = 0
on the shale line to φ = φmax
on the line throughPoint Sd (Figure F3a). The lines of constantV
cl are parallel to the clean sand line, Q-Water
Point; they range from Vcl = 0 on the clean
sand line to Vcl = 100% at Point Cl. A similar
scaling of Vsh
is possible if the location of thelaminar shale point, Point Sh
o, is fixed; the
scaling ranges from Vsh
= 0 on the clean sandline to V
sh = 100% at Point Sh
o.
With a grid so established, the location of apoint on the neutron-density crossplot definesits shale volume V
sh; breaks down the total
shale volume into clay volume Vcl and silt vol-
ume or silt index, Isl (where I
sl = [V
sh – V
cl]/V
sh);
and defines effective porosity φ for waterbearing formations.
Because hydrocarbons, particularly gas andlight hydrocarbons, can significantly affect theneutron and density log responses, hydrocar-bon-bearing zones must be handled differently.Zone shaliness is first evaluated using a shaleindicator (SP, GR, R
t, R
xo, etc.). The neutron
and density logs are corrected for shaliness andthen used to determine porosity and hydrocar-bon density.
With φ, Vsh
and Rw now defined, water satu-
ration in the noninvaded, virgin formation canbe determined using the true resistivity from adeep resistivity log.
Schlumberger
(05/96) F-5
φDφ0.5
φe φ
t
φN
0.5
Figure F3a: φN – φD Crossplot Scaled for φt and φe
φD
0.5
φN
0.5
Figure F3b: φN – φD Crossplot Scaled for Vcl
(05/96) F-6
Introduction to Openhole Logging
F1.2 GRAPHICAL CALCULATION
φt and φ
e can be found graphically on a φ
N –
φD crossplot; the steps are outlined in the fol-
lowing. This method helps identify gas-bearing zones with the resistivity input (seeFigure F4).
1. Calculate Vsh
from gamma ray oppositezone of interest.
2. Determine φD shale and φ
N shale from
average responses above the zone of in-terest.
3. Plot φD shale and φ
N shale on the cross-
plot (shale point).4. Draw shale line from shale point to clean
matrix line at zero porosity.5. Plot φ
D and φ
N for zone of interest (Point
A).6. Move the shaly sand point parallel to the
shale line a distance proportional to Vsh
(Point B).7. If the corrected point falls above the clean
matrix line, gas is present.8. Gas-correct the point (if necessary) by
moving to the clean matrix in the directionof the approximate gas correction arrows(Point E).
9. Once the shale and gas corrections havebeen made, you have graphically calcu-lated φ
e (Point E).
10. If a gas correction of total porosity is re-quired, shifting the original point in anidentical manner will produce φ
t (Point
C).11. Using φ
e, therefore
FRw
Swe
2 =
R
t
F1.3 DIRECT CALCULATION(APPROXIMATELY) OFEFFECTIVE POROSITY
φN + φ
D
a)φt ≅
2
b) φe = φ
t(1 - V
sh)
therefore,
FRw
Swe
2 = R
t
Figure F4: Graphical Solution of φt and φ
e
1. Shale Correction2. Gas Correction - φ Effective3. Gas Correction - φ Total
Schlumberger
(05/96) F-7
F2.0 EXAMPLE CALCULATION: Using the log in Figure F5 for the zone from 444 to 447 m calculate:1) Vsh 2) φt 3) φe
450
1/240
1 20-MAY-1992 16:36 INPUT FILE(S) CREATION DATE
CP 32.6 FILE 7 20-MAY-1992 11:40
.60000 0.0
DPHI(V/V )
.60000 0.0
NPHI(V/V )
125.00 375.00
CALI(MM )
0.0 150.00
GR(GAPI)
125.00 375.00
BS(MM )
---BS
GR---
---NPHI
DPHI---
---CALI
SANDSTONE
SHALE POINT
GRSHALE115GR
CLEAN23
63 API
Φ = 17DΦ = 49N
Φ = 31N Φ = 27D
Figure F5
(05/96) F-8
Introduction to Openhole Logging
EXAMPLE CALCULATION (continued)
GR - GRCL 63 - 231. Calculate V
sh. X = = = 0.435 Using V
sh-1: V
sh = 25%
GRSH - GRCL 115 - 23
2. Plot the shale point on Figure F6.
φD
φN
Figure F6
Schlumberger
(05/96) F-9
EXAMPLE CALCULATION (continued)
3. Plot the shale-sand point on Figure F7.4. Draw the shale line.
φD
φN
Figure F7
(05/96) F-10
Introduction to Openhole Logging
EXAMPLE CALCULATION (continued)
5. Make the shale correction on Figure F8.
Figure F8
Schlumberger
(05/96) F-11
EXAMPLE CALCULATION (continued):
6. Make the gas correction and read φe.
7. Gas correct the log value and read φt.
φe
φt
Figure F9
(05/96) F-12
Introduction to Openhole Logging
Schlumberger
(05/96) F-13
F3.0 Work Session
1. Shaly Sand Problem (Figures F10 – F13)
Given: BHT = 24oCR
mf = 3.08 at 14.4oC
Rm = 2.86 at 18.8oC
Rmf = 2.435 at 24oC
Gel Chem Mud; Mud Weight = 1090 kg/m3
Viscosity = 585pH = 8.5Fluid loss = 7.0 cm3
a. Find hydrocarbon zones.b.R
w - Calculate R
w for this interval.
c. φe - Determine effective porosity.
d.φt - Determine total porosity.
0.62 Rw
e. SWT
- From SWT
2 = φ
t2.15 R
t
Note: When φe has been determined, R
t must also be corrected for effect of shale to properly calcu-
late Swe
. This is discussed in the next section.
(05/96) F-14
Introduction to Openhole Logging
SP---
425
---ILM
400
1/240
1 20-MAY-1992 15:48 INPUT FILE(S) CREATION DATE
CP 32.6 FILE 16 20-MAY-1992 12:10
.20000 2000.0
SFL(OHMM)
.20000 2000.0
ILD(OHMM)
.20000 2000.0
ILM(OHMM)
-120.0 30.000
SP(MV )
DUAL INDUCTION - SFL
---ILM
---ILD
---SFL
Figure F10
Schlumberger
(05/96) F-15
425
---BS
GR---
---NPHI
DPHI---
400
---CALI
1/240
1 20-MAY-1992 17:09 INPUT FILE(S) CREATION DATE
CP 32.6 FILE 8 20-MAY-1992 11:42
.60000 0.0
DPHI(V/V )
.60000 0.0
NPHI(V/V )
125.00 375.00
CALI(MM )
0.0 150.00
GR(GAPI)
125.00 375.00
BS(MM )BS(MM )
COMPENSATED NEUTRON - LITHO DENSITY (NO PEF CURVE)
SANDSTONE
Figure F11
(05/96) F-16
Introduction to Openhole Logging
425
400
---DT
---BS
GR---
---CALI
1/240
1 20-MAY-1992 17:37 INPUT FILE(S) CREATION DATE
CP 32.6 FILE 9 20-MAY-1992 11:51
500.00 100.00
DT(US/M)
125.00 375.00
CALI(MM )
0.0 150.00
GR(GAPI)
125.00 375.00
BS(MM )BS(MM )
BOREHOLE COMPENSATED SONIC
Figure F12
Schlumberger
(05/96) F-17
400
1/240
1 20-MAY-1992 17:55 1 20-MAY-1992 17:37 INPUT FILE(S) CREATION DATE
CP 32.6 FILE 11 20-MAY-1992 11:56
.60000 0.0
NPHI(V/V )
500.00 100.00
DT(US/M)
125.00 375.00
CALI(MM )
0.0 150.00
GR(GAPI)
125.00 375.00125.00 375.00
BS(MM )
425
---DT
---BS
---GR
---NPHI
---CALI
COMPENSATED NEUTRON - BHC SONIC
Figure F13
(05/96) F-18
Introduction to Openhole Logging
Top Related