Well Logging

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MCNP Simulation of LWD Neutron and Gammadensity Logs
Libai Xu, Dan Speaker, and Ashraf Shehata Center for Engineering Applications of Radioisotopes North Carolina State Univeristy September 6th 2006

Outline
 Introduction
 GR-Density

tool simulations  Neutron Porosity tool simulations  Analysis and Discussions  Conclusions

Introduction
shale


Formation Environments
 

Sandstone bed (4ft) sandwiched by shale Thinly laminated sand-shale • Sand or shale thickness is 3” • Sand or shale thickness is 6”

sand



Variable dip angle Generic GR-Density Tool Generic Neutron Porosity Tool Variable azimuth angle and position Symmetrical or non-symmetrical response and impact on dip estimation Thin bed response in vertical well vs. high angle and horizontal well α: dip angle



Tool information
  



Objectives


β: azimuth angle



Computation Environments
 NC


State Univ. Cluster Resources



175 dual Xeon computer nodes with 2.8-3.2 GHz Intel Xeon Processors Each node has two Xeon processors, 4 GB of memory, and a 40 GB disk.

 CEAR
 

Cluster Resources

10 SunBlade100 nodes Each node has 1GB of memory, and a 20 GB disk.

LWD Density Tool Simulation
 

The borehole diameter was 8.5 inches and filled with water The generic LWD GR-Density tool was 7.5 inches in diameter   

Far detector

Collimated Cs-137 source Collimated NaI(TI) dual detectors. The near spacing was 18cm, and the far spacing was 40cm. 41 cm Sand: Quartz+water, 2.24 g/cc Shale: Illite+Quartz+Water, 2.55 g/cc 18 cm



Formation:
 

Near detector



Each case took 500min computer time, providing results with a statistical accuracy of ±0.5% for both near and far detectors

Cs 137 source

Density Tool Comparisons

Far detector

41 cm

18 cm

Near detector

Cs 137 source

Vertical and * Alberto Mendoza et. al. (2005), “Monte Carlo Modeling of Nuclear Measurement in June 26-29 Horizontal Wells in the Presence of Mud-filtrate Invasion and Salt Mixing”, SPWLA 46 Annual Logging Symposium, ** Robin P. Gardner et al. (1991), “Monte Carlo Nuclear Well Logging Benchmark Problems with Preliminary th

*

**

Intercomparison Results”, Nucl. Geophys. Vol. 5, No. 4, pp. 429-438

GR-Density

Sandstone Density Calibration Curves

GR-Density

Sandwiched Formation

GR-Density

Far Detector dip = 20
2.6 2.5

Far detector dip = 40
2.6 2.55
2.6 2.55 2.5

Far Detector dip = 80

Density (g/cc)

Density (g/cc)

2.4 2.3 2.2 2.1 2 1 3 5 7 9 11 13 15 17 19 21 23 25 27 Depth Position 0 45 90 135 180

Density (g/cc)

2.5 2.45 2.4 2.35 2.3 2.25 2.2 1 3 5 7 9 11 13 15 17 19 21 23 25 27 Depth Position 0 45 90 135 180

2.45 2.4 2.35 2.3 2.25 2.2 1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 Depth Position 0 45 90 135 180

Near detector dip = 20
2.7 2.6

Near Detector dip=40
2.6 2.55

Near detector dip = 80 2.6 2.55

Density (g/cc)

Density (g/cc)

2.5 2.4 2.3 2.2 2.1 2 1 3 5 7 9 11 13 15 17 19 21 23 25 27 Depth Position 0 45 90 135 180

Density (g/cc)

2.5 2.45 2.4 2.35 2.3 2.25 2.2 1 3 5 7 9 11 13 15 17 19 21 23 25 27 Depth Position 0 45 90 135 180

2.5 2.45 2.4 2.35 2.3 2.25 2.2 1 5 9 13 17 21 25 29 33 37 41 45 49 Depth Position 0 45 90 135 180

GR-Density

2-D Density Images

GR-Density

Laminated sand-shale
Thickness=3”
2.54

2.49

2.44

Density (g/cc)

2.39

2.34
Far Detector Near Detector

2.29

2.24 0 1 2 3 Position 4 5 6

GR-Density

Far Detector dip=20
0 2.6 2.55 2.5 45 90 135 180
Density (g/cc)

Far detector dip=40 thickness=3"
2.6 2.55 2.5 2.45 2.4 2.35 2.3 2.25 0 45 90 135 180

Far Detector dip=80
0 2.6 2.55 2.5 45 90 135 180

Density

2.45 2.4 2.35 2.3 2.25 2.2 1 2 3 4 Depth Position

Density (g/cc)

2.45 2.4 2.35 2.3 2.25 2.2

2.2 0 1 2 Depth Position 3 4 5

1

2

3

4

Depth Position

Near Detector dip=20
2.6 2.55 2.5
2.6

Near...
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