Eliminating the Rugosity Effect From Compensated Density Logs by Geometrical Response Matching

Abstract

Summary A theoretical and experimental effort to understand the effects of borehole rugosity on individual detector responses yielded an improved method of processing compensated density logs. Historically, the spine/ribs technique for obtaining borehole and mudcake compensation of dual-detector, gamma-gamma density logs has been very successful as long as the borehole and other environmental effects vary slowly with depth and the interest is limited to vertical features broader than several feet. With the increased interest in higher vertical resolution, a more detailed analysis of the effect of such quickly varying environmental effects as rugosity was required. A laboratory setup simulating the effect of rugosity on Schlumberger Litho-DensitySM tools (LDT) was used to study vertical response functions. These functions were used to derive matching filters, enabling a significant improvement in the compensated log response in the presence of rugosity. The data served as a benchmark for the Monte Carlo models used to generate synthetic density logs in the presence of more complex rugosity patterns. The results show that proper matching of the two detector responses before application of conventional compensation methods can eliminate rugosity effects without degrading the measurement's vertical resolution. The accuracy of the results is as good as that obtained in a parallel mudcake or standoff with the conventional method. Application to both field and synthetic logs confirmed the validity of these results.