If we are to continue to use acoustic logging as a viable method for characterizing wellbore formation characteristics, such as natural fractures, then logging techniques developed for conventional reservoirs and vertical wells should be updated and adapted for use in today's unconventional and directional drilling applications. Unlike seismic processes, which have undergone research and changes to adapt to the more complex characterization needs of today's wells, acoustic logging techniques have stayed relatively stagnant. In order to stay relevant, acoustic logging processes must adapt for use in unconventionals as well. The following discussion focuses on four seismic techniques adapted for acoustic logging processes, the process for choosing these particular techniques to adapt, and the results achieved.
We have based new acoustic logging algorithms and routines on these four seismic techniques and adapted the research into proprietary software for processing raw, acoustic waveform data. First, we have adapted the use of compressional waves to map drops in energy across natural fractures which allows industry to move away from many of the limitations imposed by more widely used techniques. Second, as part of the workflow presented in the compressional fracture identification methodology, we have adapted stacking to improve results and amplify anomalies. Third, in order to provide a broad available toolsets and usable data, we have incorporated advanced filtering routines based on predictive deconvolution, which can be used to process acoustic data from alternative tools or to enhance data collected using measurement-while-drilling or logging-while-drilling tools. Fourth and last, applicable to both shear and compressional waveform processing, we have chosen to move away from Alford-based azimuth computations, or at least correct for assumptions in Alford-based azimuth computations that are not relevant in unconventionals, so that borehole deviation does not adversely affect the processing.
The use of compressional waves to map natural fracture patterns has found great success in both vertical and lateral wells and has been used in the field in the Permian, Eagle Ford, the Bakken, and in various international fields. Comparisons against image and core results demonstrate an 80-85% correlation rate. The following discussion will go over two case studies, the evaluation method used to study the correlation, briefly discuss the workflow used, and also provide an example of how the advanced filtering produces much cleaner waveforms for processing than relying solely on frequency filtered waveforms.
These new processes allow acoustic processing to enter the modern age of exploration and create a new, more direct process for finding the fracture density in the well bore. Furthermore, they may be incorporated in to new and old processing techniques, making the most of the modern tools available.