Correcting Errors Due to Borehole and Formation Factors during Azimuthal Gamma Spectrum Logging While Drilling

Under different borehole and formation conditions, there are some differences in the response of azimuthal gamma spectrum logging while drilling which can affect the subsequent logging interpretation and geosteering results. Therefore, it is necessary to study the influence of borehole and formation factors on the LWD azimuthal gamma spectrum, so as to eliminate the adverse effects and errors. First of all, the instrument structure of SAGR tool was compared and analyzed, and one of the structures was selected to be the study objective to establish the corresponding MCNP model. The response of LWD SAGR tool under different wellbore and formation conditions was simulated with the Monte Carlo method. The degree of influence for each of the following factors was obtained: mud density, KCl content, formation matrix and the dip angle, azimuth and thickness of inclined radioactive formation on the LWD SAGR Tool. Using the derived results it was possible to develop an appropriate, correction method for the influence of each of the non-stratigraphic factors. The results showed that the counting rate was positively correlated with KCl, dip angle and the formation thickness, and negatively correlated with mud density, the formation matrix density and azimuth. Only KCl rather than other factors can change the shape of energy spectrum. The study found that the counting rate after being corrected by the borehole influencing factors or the shale content, K, U, Th calculated by energy spectrum were closer to the true values, which could provide more accurate guidance for logging interpretation and geosteering.