Monte Carlo Simulations of the Spectrometric n-Gamma Well Logging. Comparison of Simulations and Measurements for the SO-5-90 Logging Tool

Abstract

The results of the Monte Carlo simulations (MCNP code) of the spectrometric neutron-gamma well logging (sNGL) are presented. The sNGL can be very useful for the precise lithology determination, what is especially important for oil and gas prospecting in shally–sand formations. Possibility of determination of the Si, Ca and Fe content in rocks by means of the analysis of gamma ray spectra induced by the captured neutrons is well known for 40 years at least [Larionov, 1969]. Preliminary sNGL experiments performed at the Polish Calibration Station in Zielona Góra, using a simple borehole spectrometer equipped with the isotopic Am-Be neutron source and the BGO detector, fully confirmed this possibility. MCNP simulations of the sNGL experiments have been done for the SO-5-90 neutron-gamma spectrometer and for the lithology models (standards) of the Calibration Station. The concentrations of H, Si, Ca and Fe in these models have been determined from the real and simulated experiments using the simulated (“MCNP”) and measured (“meas”) spectra of gamma-rays from neutron interactions with the rock nuclei. The H, Si and Ca concentrations obtained from the simulated experiment are well correlated with their reference values, known from the chemical (“chem”) analyses - regarded as true values. Squares of the correlation coefficients (R2) amount to 0.992, 0.988 and 0.984, respectively. For Fe, a lower R2 has occurred for the linear dependence FeMCNP(Fechem) than for Femeas(Fechem) amounting to 0.834 and 0.923 respectively. Although the established correlations between results of the real and simulated sNGL experiments show the high values of correlation coefficients, the noticeable improvement of these correlations is expected for the fully verified chemical composition of the Zielona Góra standards and for improved parameters of the n-gamma spectrometer. The usefulness of the MCNP code for modeling the spectrometric neutron-gamma logging tool responses is confirmed. The possibility of the fast and accurate computer simulations opens a new field for development of the spectrometric n-gamma logging. Especially the influence of such disturbing factors as borehole conditions, borehole and formation salinities or presence of the additional rock elements (like B, S, K, Gd ) must be precisely known when the quantitative determination of Si, Ca and Fe is required.