Nuclear logs have unique characteristics of the penetrating capability of the neutrons and gamma photons since the logs can detect radioactivity through casings and regardless of fluid types in the borehole. That is why these loggings are widely used for the development of petroleum, geothermal energy, groundwater, and mineral resources. The nuclear logs can be separated into two types: one type (density log or neutron log) that uses artificial radioactivity sources and the other type (natural gamma-ray log or spectral gamma-ray log) that uses natural radiation.
When we carry out nuclear logs at a borehole, the logs are strongly affected by borehole conditions (casing, fluid in the borehole, stand-off, density of fluid in the formation and so on). This study examined the effects of borehole conditions when we perform the density log and the spectral gamma-ray log. The effects of borehole conditions can be evaluated by using laboratory experiment or numerical modeling. The laboratory experiment has a risk of radiation exposure, while numerical modeling is safe from radiation. This study used the MCNP (Monte Carlo N-Particle) method that uses probabilistic method. The density and spectral gamma-ray logs were simulated by using the MCNP method for different values of the borehole conditions such as increment of borehole size, fluid in borehole and casing effects. The calibration curve made by the MCNP numerical modeling of density log nearly matched with the calibration curve provided by the geophysical company. This suggests that the MCNP method can be applicable for calibration instead of the laboratory experiment. For different borehole sizes, the count rates of both density log and spectral gamma-ray log increased with increment of borehole size because the low density of water increased the counting rate. For different fluid types in the borehole, the count rates in case of air in the borehole is larger than the count rates in case of water because air has low density that water. For different casing materials, the count rates in case of steel casing is lower than the count rates in case of PVC casing due to different density values.
As a result, the construction of calibration curve of the density and spectral gamma-ray sondes by the MCNP method can be applied to different borehole conditions.