Research on Environmental Correction Method of Measurement Results from Near-Bit Gamma Imagers

It is difficult to maintain absolute centering in the wellbore under the actual working conditions of near-bit gamma imagers. For that reason, the environment associated with the original measurement values of gamma sectors in different directions should be corrected according to the eccentricity. Using the principle of gamma imaging and the structural characteristics of the near-bit gamma imager, the influences of the imager eccentricity on the measurement results under different working conditions were studied. The results were used to develop correction charts and methods for the main factors such as attenuation in drilling fluids and compensation in potassium-based drilling fluids when the near-bit gamma imagers were under eccentric condition. The results revealed that the formation dip calculated by the original gamma imaging analysis had a large error due to the influence of drilling fluid in the annulus between the wellbore and the instrument under such condition. In contrast, after environmental correction of near-bit gamma imaging under eccentric condition, the formation dip calculated by the correction method was essentially consistent with the real one. The research results showed that the gamma spectra obtained using near-bit gamma imager under eccentric condition could reflect the real formation after environmental correction, and could calculate the formation dip more accurately.