Petrophysical aspects for a constrained and joint inversion of deep reading LWD-data

Rock physics templates are commonly created in the industry to characterize reservoirs based on acoustic impedance and velocity relationships on different measurement scales (laboratory – well logging - seismic). Plotting acoustic impedance versus compressional-shear velocity ratios provide basin-dependent information on porosity, lithology and fluid saturation. To precisely land and navigate wells, extra-deep reading-while-drilling resistivity tools were developed. To integrate these measurements with acoustic and seismic measurements for even better navigation, a thorough understanding of tool responses at lithology boundaries and fluid contacts is required. This study expanded the idea of rock physics templates by including resistivity as an additional parameter. Logging-while-drilling data from 33 vertical or slightly deviated wells from the Norwegian Continental Shelf were used to investigate and evaluate resistivity and acoustic impedance contrast at more than 50 caprock-reservoir boundaries and various fluid contacts in a wide range of lithologies (in various stratigraphic levels, from Permian to Paleogene). The outcome is a new kind of ratio-based rock physics template. The influence of factors such as shale content, porosity, water saturation, and rock anisotropy is taken into account for the analysis of the resistivity and acoustic impedance data sets and to generate rock physics templates. Boundaries between caprock and reservoir can be defined through resistivity and acoustic impedance contrast. Fluid contacts in the reservoir can be detected by either resistivity ratios or acoustic impedance ratios or both depending on type of fluid. Compaction, fluid pressure, temperature, and fluid type play an important role for interpreting the data and templates. Thus, understanding of local and regional geological history, e.g. burial history has to be taken into account. Considering these, the created templates provide a useful tool for estimating lithology boundary and fluid contact conditions for better evaluating deep reading while drilling resistivity tools.