06_Shaly Sand Porosity

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(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

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Introduction to Openhole Logging

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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.

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Figure F1: Neutron-Density Frequency Crossplot Illustrating the Shaly Sand Model

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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

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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.

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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

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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

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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

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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

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3. Plot the shale-sand point on Figure F7.4. Draw the shale line.

φD

φN

Figure F7

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5. Make the shale correction on Figure F8.

Figure F8

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EXAMPLE CALCULATION (continued):

6. Make the gas correction and read φe.

7. Gas correct the log value and read φt.

φe

φt

Figure F9

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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.

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SP---

425

---ILM

400

1/240


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

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425

---BS

GR---

---NPHI

DPHI---

400

---CALI

1/240


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

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425

400

---DT

---BS

GR---

---CALI

1/240


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

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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

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06_Shaly Sand Porosity

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