A case study in modeling a nuclear formation evaluation sub

Measurement-while-drilling (MWD) requires extremely rugged tools. These tools are called subs. The modular neutron porosity (MNP) sub is one of several modular formation evaluation MWD tools that can be joined together in various combinations at the wellsite. The design of this sub was based heavily on the results of Monte Carlo modeling. Modeling efforts for the MNP sub are discussed. All modeling for the MNP tool was performed using the MCNP code. Once the model has been developed, its credibility must be established by comparing model predictions with experimental results. This requires that experiments meet stringent criteria. Standard experiments use large tanks filled with carefully prepared rock matrices. Eleven such tanks were initially available. The model results are compared with measurements taken in these tanks. The model can be used to predict tool response to many situations not reproducible in a laboratory. High temperatures, high pressures, and pore fluids composed of liquid-gas mixtures are typical of conditions encountered in real oil wells that are difficult to reproduce in a laboratory. Tool design and the effects of design modifications can also be evaluated efficiently using the model