06_Shaly Sand Porosity

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Contents F1.0 F1.0 SHALY SHALY SAND SAND POROSI POROSITY................................. TY........................................................ .............................................. .............................................. .............................1 ......1

F1.1 CALCULATING φt , φe , AND S W  IN SHALY SANDS .............................................. ...................................................................1 .....................1 F1.2 GRAPHICA GRAPHICAL L CALCULATI CALCULATION......... ON................................ .............................................. .............................................. ............................................6 .....................6 F1.3 DIRECT CALCULATION OF EFFECTIVE POROSITY..............................................................6 F2.0 F2.0 EXAMPL EXAMPLE E CALCUL CALCULATI ATION................................ ON....................................................... .............................................. .............................................. .............................7 ......7 F3.0 WORK WORK SESSION SESSION .............................................. ..................................................................... .............................................. .............................................. ...........................1 ....1 3

  (05/96)

Introduction to Openhole Logging

(05/96)

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F1.0

Shaly Sand Porosity

F1.1 CALCULATING φT , φe   AND IN SHALY SANDS

Sw

To this point our calculations have been fairly straightforward in evaluating porosity and hence water saturation. As indicated in Section E, shale presence complicates interpretation considerably. To arrive at the best possible value for S w, we must develop a quality value for porosity. This means we must correct φT  for the volume of shales and obtain φ e  (effective porosity, shale free). This correction can be done graphically for all cases or using an average assumption for neutron and density porosity, through equations. Both these methods are outlined in this section. Before giving the methodologies, let's develop the basis for the graphical correction for which the direct calculation approximates. Shaly clastics are generally modelled with the composition of silt-shale-sand in which the shales can be laminated, dispersed or structural. The basic model is suggested by the groupings of  the plotted points on the neutron-density crossplot of Figures F1 and F2. These plots represent a typical crossplot through a sequence of sands, shales and shaly sands. Most of the data belong to two groups: Group A, identified as sands and shaly sands, and Group B, identified as shales.

To explain the spread of points in Group B along the line from Point Q through Point Sh o to Point Cl, the shales are considered mixtures of clay minerals, water and silt in varying proportions. Silt is fine grained and is assumed to consist predominantly of quartz, but it may also contain feldspars, calcite and other minerals. Silt has, on the average, nearly the same neutron and density log properties as the matrix quartz; pure quartz silt would plot at the quartz point, Q. Silt, like quartz, is electrically nonconductive. Points near the "wet clay" point, Point Cl, correspond to shales that are relatively silt free. Point Sho  corresponds to shale containing a maximum amount of silt. The shaly sands in Group A grade from shales, on Line Sh o-Cl, to sands at Point Sd, on Line Q-Sd. The shale in these shaly sands may be distributed in various ways. When all the shale is laminar shale, the point falls on the Sd-Sho  line. Dispersed shale causes the point to plot to the left of the line. Structural shale causes the point to plot to the right of the line.

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Introduction to Openhole Logging

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

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0.8

0.6

0.4

0.2

0.2

0.4

0.6

0.8

1

 – 0.2

Figure F2: Expanded φ  N  - φ  D Crossplot for Shaly Sand Showing All End Points

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Introduction to Openhole Logging

Typically, few points plot in Area C. When they do, they usually represent levels where log readings have been affected by borehole rugosity, or where shale properties have been affected by hydration of the clay in contact with the mud, or where matrix lithology no longer corresponds to a shale-sand sequence (e.g., porous carbonates, lignite).

With a grid so established, the location of a point on the neutron-density crossplot defines its shale volume V sh ; breaks down the total shale volume into clay volume V cl and silt volume or silt index, I sl (where I sl = [V sh – V cl]/ Vsh  ); and defines effective porosity φ   for water bearing formations.

Once Points Sd, Sh o  and Cl have been determined from inspection of the crossplot, the plot can be scaled for water-bearing sands and shales in terms of φ   and V cl, as shown in Figure F3. The lines of constant φ e  are parallel to the shale line, Q-Cl. They range from φ e = 0 on the shale line to φ = φmax on the line through Point Sd (Figure F3a). The lines of constant V cl are parallel to the clean sand line, Q-Water Point; they range from V cl  = 0 on the clean sand line to V cl = 100% at Point Cl. A similar scaling of V sh  is possible if the location of the laminar shale point, Point Sho, is fixed; the scaling ranges from V sh  = 0 on the clean sand line to V sh  = 100% at Point Sh o.

Because hydrocarbons, particularly gas and light hydrocarbons, can significantly affect the neutron and density log responses, hydrocarbon-bearing zones must be handled differently. Zone shaliness is first evaluated using a shale indicator (SP, GR,  Rt ,  R xo , etc.). The neutron and density logs are corrected for shaliness and then used to determine porosity and hydrocarbon density.

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With φ, V sh  and  R w now defined, water saturation in the noninvaded, virgin formation can be determined using the true resistivity from a deep resistivity log.

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φ

φ

0.5

D 

φ

φ

e 

t 

0.5

φ

N 

Figure F3a: φ  N  – φ  D  Crossplot Scaled for φ t   and φ e

φ

0.5

D 

0.5

φ

N 

Figure F3b: φ  N  – φ  D Crossplot Scaled for V cl

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Introduction to Openhole Logging

F1.2

GRAPHICAL CALCULATION

φ t  and φe can be found graphically on a φ N  – φ D  crossplot; the steps are outlined in the following. This method helps identify gasbearing zones with the resistivity input (see Figure F4). 1. Calculate V sh  from gamma ray opposite zone of interest. 2. Determine φ D  shale and φ N  shale from average responses above the zone of interest. 3. Plot φ D  shale and φ N  shale on the crossplot (shale point). 4. Draw shale line from shale point to clean matrix line at zero porosity. 5. Plot φ D  and φ  for zone of interest (Point A). 6. Move the shaly sand point parallel to the shale line a distance proportional to V sh (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 direction of the approximate gas correction arrows (Point E). 9. Once the shale and gas corrections have been made, you have graphically calculated φe  (Point E). 10. If a gas correction of total porosity is required, shifting the original point in an identical manner will produce φt  (Point C). 11. Using φe , therefore FRw 2 S we =  Rt 

F1.3

DIRECT CALCULATION (APPROXIMATELY) OF EFFECTIVE POROSITY

φ N  + φ D a)φ t  ≅ 2 b) φ e  = φt (1 - V sh ) therefore, FRw S we 2 =  Rt 

 N 

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Figure F4: Graphical Solution of  φ t   and φ e 1. Shale Correction 2. Gas Correction - φ Effective 3. 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) V sh

2) φt 

3) φe

BS(MM ) 125.00

375.00 GR(GAPI)

NPHI(V/V )

0.0

150.00

.60000

0.0

CALI(MM )

DPHI(V/V )

125.00

375.00

.60000

0.0

SANDSTONE CP 32.6

FILE

7

20-MAY-1992 11:40

INPUT FILE(S) CREATION DATE 1 20-MAY-1992 16:36  

1/240 ΦN = 49

Φ D = 17

SHALE POINT

ΦN = 31

ΦD = 27

63 API

450

 ---BS GR-- ---NPHI DPHI-- ---CALI GR CLEAN 23

GR SHALE 115

Figure F5

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Introduction to Openhole Logging

EXAMPLE CALCULATION (continued) GR - GRCL

1. Calculate V sh . X =

63 - 23 =

GRSH - GRCL

= 0.435

Using V sh-1 : V sh  = 25%

115 - 23

2. Plot the shale point on Figure F6.

φN 

φD 

Figure F6 

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EXAMPLE CALCULATION (continued)

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

φN 

φD 

Figure F7 

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Introduction to Openhole Logging

EXAMPLE CALCULATION (continued)

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 .

φt 

φe 

Figure F9

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Introduction to Openhole Logging

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Schlumberger

F3.0

Work Session

1. Shaly Sand Problem (Figures F10 – F13)

Given:

BHT = 24oC  Rmf  = 3.08 at 14.4 oC  Rm  = 2.86 at 18.8 oC  Rmf  = 2.435 at 24 oC Gel Chem Mud; Mud Weight = 1090 kg/m3 Viscosity = 585 pH = 8.5 Fluid loss = 7.0 cm 3 a. Find hydrocarbon zones. b. Rw - Calculate Rw  for this interval. c. φ e  - Determine effective porosity. d.φ t  - Determine total porosity. 0.62 Rw e. S WT  - From S WT 2 = φt 2.15 Rt 

Note: When φe has been determined,  Rt  must also be corrected for effect of shale to properly calculate S we . This is discussed in the next section.

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Introduction to Openhole Logging

DUAL INDUCTION - SFL ILM(OHMM) .20000

2000.0 ILD(OHMM)

.20000

2000.0

SP(MV ) -120.0

SFL(OHMM) 30.000

CP 32.6

FILE

16

.20000

2000.0

20-MAY-1992 12:10

INPUT FILE(S ) CREATION DATE 1 20-MAY-1992 15:48  

1/240

400

425

SP---

 ---ILM  ---ILD  ---SFL

Figure F10

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Schlumberger COMPENSATED NEUTRON - LITHO DENSITY (NO PEF CURVE) BS(MM ) 125.00

375.00 GR(GAPI)

NPHI(V/V )

0.0

150.00

.60000

0.0

CALI(MM )

DPHI(V/V )

125.00

375.00

.60000

0.0

SANDSTONE

CP 32.6

FILE

8

20-MAY-1992 11:42

INPUT FILE(S) CREATION DATE 1 20-MAY-1992 17:09  

1/240

 ---BS GR-- ---NPHI DPHI-- ---CALI 400

425

Figure F11

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Introduction to Openhole Logging

BOREHOLE COMPENSATED SONIC BS(MM ) 125.00

375.00 GR(GAPI)

0.0

150.00 CALI(MM )

DT(US/M)

125.00

375.00

CP 32.6

FILE

9

500.00

100.00

20-MAY-1992 11:51

INPUT FILE( S) CREATION DATE 1 20-MAY-1992 17:37  

1/240

 ---DT  ---BS GR-- ---CALI

400

425

Figure F12

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Schlumberger COMPENSATED NEUTRON - BHC SONIC BS(MM ) 125.00

375.00 GR(GAPI)

0.0

DT(US/M) 150.00

500.00

100.00

CALI(MM ) 125.00

CP 32.6

NPHI(V/V ) 375.00

FILE 11

.60000

0.0

20-MAY-1992 11:56

INPUT FILE(S) CREATION DATE 1 20-MAY-1992 17:37 1 20-MAY-1992 17:55  

1/240

400

 ---DT  ---BS  ---GR  ---NPHI  ---CALI 425

Figure F13

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Introduction to Openhole Logging

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