Monte Carlo simulation of the upgraded, neutron - gamma logging tool, SO-5-90-SN type. Comparison of the simulations and measurements

Abstract

For gas prospecting in the shaly-sand Miocene formations of the Carpathian Foredeep many complementary geophysical techniques are being used. The spectrometric Neutron-Gamma well Logging (sNGL) is one of the nuclear techniques being useful for the formation mineralogy identification. The results of the real and simulated sNGL experiments are compared in this report. These benchmark experiments were performed for determination of the main rock constituents: Si, Ca, Fe and H in the rock models which belong to the Polish calibration station in Zielona Góra (BGW). The upgraded n-gamma spectrometer, SO-5-90-SN type, has been applied in the measurements and was simulated using the Monte Carlo code (MCNP). The results of the real and simulated sNGL experiments are in good conformity. The H, Ca and Fe concentrations obtained from the simulations (“MCNP”) have been highly correlated with their reference (“chem”) concentrations. Squares of the correlation coefficients (R2) amount to 0.931, 0.969, 0.973, respectively. For the Si concentration (CSi), dependence CSiMCNP(CSichem) shows slightly lower R2 (0.972) than for dependence CSimeas(CSichem). This is mainly attributed to the unavailable B, Cl and Rare Earth Elements (REE) contents, of the BGW Zielona Góra rock models and to their geological inhomogeneities. Additional improvement of correlation between results of both experiments is also presented. It has been obtained when data from preliminary analyses for B, Cl and (REE) of two rock models were included in the simulations. The Fe concentration can be used as one of the indicators of the formation clay content, therefore an accurate quantification of this element in the rocks is of high importance. The BGW calibration station in Zielona Góra has limited number of the rock models, and Fe content is poorly diversified in them. Therefore the artificial rocks of different Fe contents have been simulated and spectrometer sensitivity, to the changes of the Fe contents, has been estimated.