Well Logging

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