Understand the Unconventional Reservoir Heterogeneity via Far-Field Sonic Imaging and Chemical Tracer Evaluations: A Case Study from Wulalike Shale Gas Exploration in Ordos Basin

One of the most challenging aspects of unconventional reservoirs is their heterogeneity. Natural fissures, faults, or laminations, which are the main causes for the heterogeneity, are either measured only on the wellbore or not measured at all. In our recent work on a Wulalike shale gas exploration well, we visualized the heterogeneity with three-dimensional far-field sonic imaging technology during logging, and we evaluated the uneven production profile in the horizontal laterals with chemical tracer during production test. The result of the two methods is highly consistent, which indicated the far-field sonic imaging technology is an effective way to understand reservoir heterogeneity, particularly for unconventional resources. High-resolution image logging and three-dimensional far-field sonic imaging technology were introduced to a newly drilled horizontal well in Wulalike Shale gas. Multilevel multi-azimuthal acoustic waveforms were acquired with long enough record length to capture the reflections from the mentioned microscopic features. The innovative ray tracing inversion and three-dimensional slowness-time coherence (3D STC) methods were utilized on the filtered waveform to quantitatively determine the true dip and azimuth as well as the distance of the features from the wellbore. And as a result, only four depth intervals were identified by 3D far-field sonic where fractures extended away from the wellbore to far field. The distance of the fractures from the wellbore was in a range of 8 to 28 m. Chemical tracers were deployed during the fracturing and completion phase to evaluate the new findings from the logging. The uneven production profile from the chemical tracer indicated that 4 out of the 17 stages were the major contributors for more than half of the gas production. Each lateral for those four stages were accompanied with the observation of fissure or lamination signature from the far-field sonic images and resistivity image logging data. This showed that this sonic imaging technology was effective to link the reservoir heterogeneity to its production, making it a valuable tool for the future unconventional reservoir evaluations.