Implication of Fast Shear Wave Azimuth and Critically Stressed Fractures in Coalbed Methane Reservoir

Abstract

The orientation of fractures and stresses within coal seams plays a critical role in gas production from coalbed methane reservoirs. In this study, we utilized resistivity images and sonic logs to investigate these parameters, aiming to (i) establish the relationship between fracture orientations and the polarization angles of fast shear waves, and (ii) detect active fractures in the coal seam. Shear waves in anisotropic formations split into fast and slow components, with Alford’s rotation method used to determine the polarization angles of the fast shear wave. We found that the fast shear wave aligns with the direction of higher fracture intensity. Subsequently, we incorporated the poroelastic strain model to estimate vertical (Sv), maximum horizontal (SH), and minimum horizontal (Sh) stresses in the wellbore. These stress magnitudes aided in identifying the faulting regime and were corroborated by vertical opening mode fractures. Validation of SH and Sh involved comparison with breakout width in image logs and closure pressure observed during hydraulic fracturing treatments. Applying Mohr’s Coulomb criteria, the stress model discerned the state of fractures, transforming stress magnitudes into shear and effective normal stress on each fracture plane. Our observations indicated that identified fractures existed in a non-critically stressed condition, suggesting a lack of interconnectivity among them. These findings correspond to the absence of gas production to date, providing insights into the dynamics of fractures and their impact on production behavior.