A New Lab Device and Processing Method to Quantify Vuggy Porosity in OBM Logging Environments

Abstract

Abstract With the popularity of nonconductive drilling fluid (NCM), a new generation electrical borehole imaging tool that uses megahertz logging frequencies is developed to decrease the capacitance of the NCM for acquisition of high-resolution images. At this frequency range, both electrical conductivity and dielectric permittivity of the subsurface dictate the logging measurement. This challenges our understanding of the tool response in terms of resistivity contrast and affects the algorithms in the geologic interpretation software that use the resistivity image value. For example, open vugs filled with NCM can appear resistive on images and invalidate the basic assumptions of existing secondary porosity quantification software that open vugs are filled with conductive mud (since they were developed for water-based mud). A new laboratory device that features the same logging frequency, as the logging tool has been developed to assess the new algorithms for quantitative interpretation. The device was used to investigate images of secondary porosity features acquired in NCM in controlled laboratory conditions. It is demonstrated that for effective analysis of fluid-filled vugs, a resistivity image is not sufficient to count the NCM-filled vuggy area. A new post processing method is introduced by combining the effects of resistivity and dielectric permittivity and generating a new image called the rock Hayman factor image. On the Hayman factor image, it is possible to differentiate fluid-filled vugs from cemented vugs. Based on this analysis, a new NCM vuggy formation characterization workflow is proposed. The workflow was applied to a downhole case study for a vuggy carbonate reservoir. The Hayman factor image agrees with the resistivity log/image in identifying oil, transition, and water zones in the well, and it shows enhanced heterogeneous texture patterns in different zones. Software incorporating Otsu's method was capable of discriminating between the continuous rock background phase and heterogeneous phase by varying input parameters and was used to test the image feature contrast of a resistivity image logged in oil-base mud (OBM) by using the conventional heterogeneity analysis method and the new Hayman factor image. Interestingly, when the OBM-logged resistivity image is input, no vugs were found in the area where core photographs indicate vugs are present. However, running the software on the Hayman factor image can characterize vugs with a frequency that matches well with the core photographs. This shows that the Hayman factor image has improved feature contrast compared with the original resistivity image. The new postprocessing Hayman factor image is designed to quantify rock resistivity and the dielectric permittivity effect. A new vug characterization workflow using this new image is proposed for NCM environment secondary porosity quantification.