Characteristics of horizontal wellbore Stoneley and S-wave responses to faults in unconventional shale reservoir

ABSTRACT Understanding and identifying faults is crucial in revealing the structure and dynamic mechanisms of the earth’s interior, predicting seismic activity, and extracting petroleum resources. In unconventional shale reservoir exploration and development, the faults developed through horizontal wells can increase rock permeability. Still, they also pose a risk of casing deformation and interwell pressure interference during hydraulic fracturing processes. Most of these faults have a small fault throw (<5 m), making it difficult to determine them using seismic exploration methods accurately. Therefore, this study investigates fault identification based on high-frequency sonic logging data. Three-dimensional numerical models, such as shale reservoir, horizontal well, and fault, are designed, and forward simulations are conducted. The study investigates the response characteristics of the Stoneley wave and the dipole S wave to small- and large-scale faults. In addition, the types of fault-filling media and fault width are analyzed. The results indicate that Stoneley wave and dipole S wave are sensitive to faults. A thin fluid layer within the fault significantly increases the attenuation of both wave amplitudes. However, fault width is not a sensitive parameter. Based on the forward simulation studies, we develop a method to identify faults using borehole mode waves and further determine key parameters such as fault throw and fault-filling media types. The effectiveness of the fault identification method is verified through a field example. This technique offers noteworthy benefits in detecting small-scale faults and in the subsequent assessment of vital fault parameters.