Triassic Dolomitic Sand
This example points out several pitfalls that can trap inexperienced analysts. First, the high resistivity at the top of the log segment suggests a great hydrocarbon show. But this is an anhydrite cap rock with zero porosity. The density neutron log separation quickly show this. You need resistive AND porous to have a reservoir.

Second, the pay zones underneath the cap consists of three thin beds (each about 1.5 to 2 meters thick). It is tempting to average the porosity over the entire 5 meters. While this would give a reasonable average porosity and reserves estimate, it would underestimate the productive capacity of the well by a factor of about 10. That's because permeability is a function of porosity to the power 6 and the high porosity streak in the middle of the reservoir must be analyzed separately to see the tremendous impact it has on productivity.

Third, this is not a pure quartz sand and the density porosity by itself underestimates the effective porosity by 2 to 4% porosity, depending on the amount of heavy minerals in the sandstone. The quicklook rules given elsewhere in this Handbook can handle all these problems, but you have to apply them carefully. Traditional, over-simplified methods will give extremely pessimistic results, so don't use them.

Finally, there is no low resistivity on this log segment, so there is no obvious water zone. RW must come from another well down dip or from a catalog value.


 Raw log  data depth plots for Dolomitic Sand Example. Note how porosity is picked on the peaks because the beds are thin (2 meters – 6 feet or less). Even then, the log analysis will underestimate porosity compared to core.

 

This example is sandstone with complex mineralogy. The porosity resistivity overlay will help find the zone of interest but the high resistivity of the anhydrite cap and lower shale may be confusing at first. Some people like simple rules for defining pay zones, such as a resistivity cutoff. This would not work in this case, or in many other situations either. The SP also suffers badly from the high resistivity.

 

Crossplots for Dolomitic Sand Example

 

Crossplots help define the lithology and the lack of a water or transition zone.

 

1.       Porosity vs Resistivity - shows no water.

 

2.       Porosity vs Saturation - shows constant water volume lines. Data follows one hyperbolic line – no transition or water zones. Shale data falls to the bottom of the graph.

 

3.       Density vs Neutron - shows all data below sandstone line, indicating either no perfectly clean and or mixed lithology sand (GR suggests clean sand). Shale data falls towards bottom and right.

 

4.       Core porosity vs core permeability - shows a data distribution that can be used to derive a regression line mathematically.

 

A plot of the computed results is shown below. Notice the complex lithology throughout the interval. Using a simplified quick look analysis can result in a porosity that is 4% too low – a 25% error in this low porosity environment.

 


 Depth plot for Dolomitic Sand Example. Black dots are core analysis data. The log does not match
the core as well as we would like, but this is mainly due to very thin porosity laminations,
thinner than the logging tool resolution.

 

The listings for final results and hydrocarbon summary are shown below. The log analysis porosity is a little low and could be increased to match core analysis better by adjusting the target DENSMA parameter in the META/LOG program.

 


 Results and raw data for Dolomitic Sand Example


 Summary results of log and core data for Dolomitic Sand Example


Notice the very close comparison between the core porosity (black dots) and log analysis porosity (smooth line) on the computed log analysis depth plot shown above.


This is a thin bed problem as much as a heavy mineral problem, so data in the porous interval must be picked at very close intervals. The “peaks and valleys” rule applies. Any form of averaging the data will mask the contribution of the central porous “hot spot”. Ninety percent of the productivity comes from this 1.5 meter interval (2056.5 –2058.0 meters), but it contains only half the oil in place.

 

Notice the excellent permeability versus porosity relationship shown in the graph at the right.

 

Below are the well history and core data for this well. Note that the perforated interval listed on the well history database was reported 10 meters too deep and the Halfway top was 2 meters too deep, probably due to clerical errors. Database contents are not perfect either.

 

WELL HISTORY INFO

 

Pancanadian Beaverlodge 11-36-72-08W6

KB Elev: 689.4 m      Logs: DIL-SP, FDC-CNL-GR, BHCS-GR

Log depths in METERS.


   
 

Testing Record 11-36

 

DST #2: 2044.0 – 2070.0 m  Inflate Straddle 
VO: 10.0/45.0 SI: 150.0/249.0 min

FP: 1937.0/2877.0 psi  SIP: 3124.4/3101.5 psi  HP: 3614.2/3614.2 psi

PMAX: 21686 kPa; BHTEMP: 72 C; Relatively high permeability; no formation damage;

Blow description: test run with closed chamber, oil to surface in 30 minutes after blowdown on final flow.  Recovery: 323.6 m clean uncontaminated oil; maximum gas to surface 42793 m3/d (1.511 mmcf/d)

 Well history listing for Dolomitic Sand Example

 

Compare your results with the core data on the next page, especially in the thin high porosity streak 2056.8 to 2058 meters. You will also have to adjust the CPERM parameter in the permeability equation to get a good match to core. Note that a conventional shaly sand analysis will under-estimate porosity by as much as 4% porosity, so the only correct approach is the shale corrected complex lithology model recommended in this book. If heavy minerals are not a problem, the complex lithology model works just as well as the shaly sand model, so there is no reason to ever use the shaly sand approach..

 

   CORE ANALYSIS DATA FOR 11-36-72-08W6

11367208W6

 

 

 

 

 

 

 

 

 

 

 

S#

Top

Base

Len

Kmax

K90

Kvert

Porosi

GrDen

BkDen

Soil

Swtr

Lithology

 

meters

meters

meter

mD

mD

mD

frac

Kg/m3

Kg/m3

frac

frac

 

1

2054.35

2054.54

0.19

0.32

0.30

0.08

0.045

2881

2796

0.138

0.138

DOL INTRANHYARGL

2

2054.54

2054.74

0.20

2.45

2.38

0.41

0.100

2737

2563

0.152

0.237

SS  F   DOL

3

2054.74

2054.91

0.17

20.40

20.00

0.34

0.116

2689

2493

0.147

0.118

SS  F   CALC

4

2054.91

2055.07

0.16

16.40

16.40

0.83

0.103

2706

2530

0.136

0.136

SS  F   CALC

5

2055.07

2055.26

0.19

64.50

57.70

40.30

0.145

2683

2439

0.117

0.183

SS  F   CALC

6

2055.26

2055.48

0.22

60.30

58.80

37.30

0.148

2679

2431

0.124

0.198

SS  F

7

2055.48

2055.58

0.10

84.20

80.00

0.01

0.145

2700

2454

0.116

0.206

SS  F

8

2055.58

2055.74

0.16

1.77

0.31

0.03

0.037

2736

2672

0.104

0.363

SS  F   DOL

9

2055.74

2055.89

0.15

10.00

10.00

4.86

0.124

2694

2484

0.156

0.208

SS  F   DOL

10

2055.89

2056.02

0.13

15.00

14.20

0.36

0.119

2695

2493

0.145

0.232

SS  F   DOL

11

2056.02

2056.21

0.19

25.40

19.10

0.07

0.099

2721

2551

0.000

0.142

SS  F   CALC

12

2056.21

2056.30

0.09

15.00

0.01

0.01

0.107

2700

2518

0.188

0.263

SS  F

13

2056.30

2056.47

0.17

99.80

98.60

54.70

0.147

2696

2447

0.107

0.246

SS  F

14

2056.47

2056.75

0.28

230.00

225.00

164.00

0.158

2679

2414

0.101

0.251

SS  F

15

2056.75

2056.93

0.18

189.00

170.00

67.00

0.168

2691

2407

0.098

0.245

SS  F   CALC

16

2056.93

2057.13

0.20

206.00

198.00

175.00

0.171

2678

2391

0.088

0.296

SS  F

17

2057.13

2057.37

0.24

108.00

104.00

94.10

0.166

2658

2383

0.120

0.361

SS  F

18

2057.37

2057.55

0.18

152.00

141.20

82.00

0.196

2663

2337

0.115

0.298

SS  F

19

2057.55

2057.73

0.18

135.00

135.00

80.00

0.191

2672

2353

0.162

0.246

SS  F   V/F

20

2057.73

2058.01

0.28

186.00

186.00

80.00

0.207

2659

2316

0.099

0.262

SS  F   V/F

21

2058.01

2058.26

0.25

37.10

36.50

0.55

0.129

2701

2482

0.095

0.219

SS  F   DOL

22

2058.26

2058.44

0.18

207.00

181.00

28.60

0.197

2683

2351

0.086

0.224

SS  F

23

2058.44

2058.62

0.18

0.90

0.29

0.01

0.022

2737

2699

0.276

0.331

SS  F   CALC

24

2058.62

2058.77

0.15

271.00

237.00

5.88

0.150

2678

2426

0.081

0.359

SS  F   CALC

25

2058.77

2058.99

0.22

7.45

7.33

0.12

0.091

2701

2546

0.109

0.146

SS  F   DOL

26

2058.99

2059.20

0.21

15.70

14.00

0.06

0.098

2700

2533

0.163

0.163

SS  F   DOL

27

2059.20

2059.42

0.22

27.80

18.89

4.35

0.139

2697

2461

0.162

0.223

SS  F   DOL

28

2059.42

2059.59

0.17

12.80

12.80

0.05

0.104

2710

2532

0.183

0.160

SS  F   DOL

29

2059.59

2059.76

0.17

30.90

29.60

0.01

0.075

2720

2591

0.145

0.181

SS  F   DOL

30

2059.76

2059.88

0.12

77.90

77.10

68.10

0.145

2647

2408

0.086

0.205

SS  F

31

2059.88

2060.14

0.26

76.20

72.90

25.50

0.160

2666

2399

0.096

0.221

SS  F

32

2060.14

2060.34

0.20

21.50

20.30

0.10

0.185

2777

2448

0.132

0.205

SS  F   DOL

33

2060.34

2060.47

0.13

12.60

11.80

0.38

0.102

2719

2544

0.177

0.155

SS  F   DOL

34

2060.47

2060.77

0.30

0.08

0.08

0.01

0.047

2716

2635

0.138

0.255

SS  F   DOL ARGL

35

2060.77

2060.95

0.18

0.13

0.07

0.01

0.055

2712

2618

0.000

0.535

SS  F   DOL

36

2060.95

2061.10

0.15

 

0.01

0.01

 

 

 

 

 

SHALE

37

2061.10

2061.26

0.16

0.02

0.01

0.01

0.031

2752

2698

0.000

0.504

SS  F   DOL ARGL

38

2061.26

2061.52

0.26

0.03

0.01

0.01

0.047

2743

2661

0.000

0.505

SS  F   DOL

39

2061.52

2061.71

0.19

0.05

0.01

0.01

0.012

2720

2699

0.000

0.622

SS  F   DOL

40

2061.71

2061.94

0.23

 

0.01

0.01

 

 

 

 

 

SHALE

41

2061.94

2062.07

0.13

0.02

0.01

0.01

0.027

2773

2725

0.000

0.534

SS  F   DOL

42

2062.07

2062.31

0.24

3.31

3.20

1.32

0.084

2707

2564

0.155

0.217

SS  F   DOL

43

2062.31

2062.54

0.23

8.80

8.38

3.62

0.121

2735

2525

0.096

0.119

SS  F   DOL

44

2062.54

2062.67

0.13

12.90

11.30

6.41

0.132

2800

2562

0.103

0.144

SS  F   DOL ANHY

45

2062.67

2062.92

0.25

0.90

0.80

0.01

0.077

2737

2603

0.108

0.108

SS  F   DOL

46

2062.92

2063.09

0.17

0.37

0.37

0.04

0.057

2726

2628

0.167

0.232

SS  F   DOL

47

2063.09

2063.26

0.17

0.54

0.53

0.13

0.080

2731

2593

0.107

0.215

SS  F   DOL

48

2063.26

2063.39

0.13

1.51

1.40

0.62

0.077

2721

2588

0.134

0.205

SS  F   DOL

49

2063.39

2063.64

0.25

0.42

0.37

0.17

0.074

2717

2590

0.116

0.143

SS  F   DOL

50

2063.64

2063.89

0.25

0.29

0.28

0.03

0.074

2741

2612

0.149

0.275

SS  F   DOL

51

2063.89

2064.00

0.11

0.18

0.16

0.02

0.050

2701

2616

0.084

0.189

SS  F   DOL

52

2064.00

2064.17

0.17

0.26

0.23

0.09

0.061

2720

2615

0.048

0.240

SS  F   DOL

53

2064.17

2064.31

0.14

0.16

0.04

0.01

0.031

2772

2717

0.123

0.197

SS  F   DOL ARGL

54

2064.31

2064.46

0.15

0.13

0.09

0.03

0.059

2743

2640

0.138

0.277

SS  F   DOL

55

2064.46

2064.64

0.18

0.02

0.02

0.01

0.039

2755

2687

0.000

0.314

SS  F   DOL ARGL

56

2064.64

2064.80

0.16

0.06

0.05

0.01

0.028

2736

2687

0.000

0.406

SS  F   DOL

57

2064.80

2064.96

0.16

0.12

0.09

0.01

0.046

2740

2660

0.000

0.270

SS  F   DOL

58

2064.96

2065.19

0.23

 

0.01

0.01

 

 

 

 

 

SHALE

59

2065.19

2065.39

0.20

0.08

0.05

0.01

0.061

2782

2673

0.000

0.622

DOL P/S ANHYSD

60

2065.39

2065.53

0.14

1.27

1.18

0.10

0.081

2783

2639

0.089

0.221

DOL P/S ANHYSD

61

2065.53

2065.71

0.18

1.78

1.56

0.29

0.074

2802

2669

0.087

0.134

DOL P/S ANHYSD

62

2065.71

2065.87

0.16

0.32

0.32

0.02

0.060

2783

2676

0.000

0.565

DOL P/S ARGLSD

63

2065.87

2066.08

0.21

0.28

0.24

0.04

0.065

2791

2675

0.054

0.108

DOL P/S ANHYSD

64

2066.08

2066.15

0.07

0.18

0.01

0.01

0.077

2780

2643

0.000

0.054

DOL P/S SD  CHT

65

2066.15

2066.54

0.39

0.18

0.15

0.01

0.041

2849

2773

0.100

0.125

DOL P/S ANHYSD

66

2066.54

2066.67

0.13

30.90

28.10

2.28

0.156

2815

2532

0.119

0.089

DOL P/S ANHYSD

67

2066.67

2066.82

0.15

13.30

10.20

2.18

0.108

2774

2582

0.115

0.064

DOL P/S SD

68

2066.82

2067.10

0.28

4.62

2.34

1.52

0.092

2787

2623

0.108

0.108

DOL P/S ANHYSD

69

2067.10

2067.34

0.24

0.36

0.19

0.01

0.039

2831

2760

0.047

0.047

DOL P/S H/F ANHYSD

70

2067.34

2067.57

0.23

0.06

0.04

0.01

0.033

2804

2744

0.096

0.479

DOL PP/VANHYSD

71

2067.57

2067.59

0.02

 

0.01

0.01

 

 

 

 

 

SHALE

72

2067.59

2067.86

0.27

0.07

0.07

0.01

0.040

2758

2688

0.139

0.312

SS  VF  DOL ARGL

73

2067.86

2067.87

0.01

 

0.01

0.01

 

 

 

 

 

SHALE

74

2067.87

2068.04

0.17

0.05

0.05

0.01

0.033

2752

2694

0.092

0.459

SS  VF  DOL ARGL

75

2068.04

2068.29

0.25

0.16

0.15

0.01

0.067

2761

2643

0.000

0.610

SS  F   DOL

76

2068.29

2068.54

0.25

0.04

0.04

0.01

0.025

2782

2737

0.000

0.396

SS  F   DOL

77

2068.54

2068.76

0.22

0.05

0.04

0.01

0.031

2754

2700

0.000

0.284

SS  F   DOL

78

2068.76

2068.91

0.15

0.02

0.02

0.01

0.027

2795

2747

0.000

0.360

SS  F   DOL

79

2068.91

2069.10

0.19

0.11

0.03

0.01

0.016

2753

2725

0.000

0.427

SS  F   DOL

80

2069.10

2069.39

0.29

0.04

0.02

0.01

0.024

2803

2760

0.000

0.600

SS  F   DOL ARGLPRY

81

2069.39

2069.68

0.29

0.02

0.02

0.01

0.019

2794

2760

0.000

0.382

SS  F   DOL ANHY

 

 

 

 

 

 

 

 

 

 

 

 

 

Arithmetic Averages

0.19

33.0

28.8

12.8

0.089

2735

2582

0.091

0.268

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     Core data listing for Dolomitic Sand Example

This well had an initial rate of 200 bbl/day and declined gracefully until year 22 when pump rate

was increased, causing water breakthrough.

 

Cumulative production was 429 000 bbl oil and

Over 1 million bbl water. On a logarithmic production

scale, the exponential decline is a straight line. The
log analysis predictions are a little high on
recoverable oil and a little low on initial production
rate. Only a minor amount of tuning is required to get both results to match ground truth.

 

 

 

 

Linear production scale (above) shows exponential decline, logarithmic scale (right) shows a straight line, also demonstrating exponential decline
 

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