Coal Fractures and Present-Day Stress: Integration of Borehole Images and Sonic Anisotropy, Bowen and Surat Basins, East Australia

摘要

Operators in the Bowen and Surat basins commonly use borehole images and to a lesser degree acoustic anisotropy to infer permeability anisotropy for use in field development; however it is not until now that other information from these technologies are fully unified to have potential to predict permeability. An ongoing challenge in these wells is to understand coal fractures and their relationship to present-day stress, as these geological factors define the permeability and ultimately production. Borehole images are used to identify near-wellbore fracture discontinuities / fractures (coal cleats, tectonic fractures, and drilling-induced fractures), which can show a high density with a wide variation in azimuth. The polarization direction of the fast shear from borehole sonic logs will be a result of the dominant mechanism of stress and fractures in the far field; which is a net result of both fractures and present-day stress. It is uncertain, however whether the anisotropy is dominated by coal fractures or in-situ stress. An improved method to characterize the complex relationship between fracture discontinuities and stress anisotropy has been applied to define the dominant mechanism. Orientations of coal fractures (cleats, tectonic fractures, and drilling-induced features) from near-wellbore imaging are combined and used to predict the far-field shear anisotropy direction from borehole sonic, and a comparison is made to the fast shear azimuth. Discrepancies between the direction of the fast shear and the fracture directions are then used to understand the net effect of fractures and present-day stress. Preliminary results from comparing the anisotropy direction with the directions of stress and fractures in the study have shown that where anisotropy direction matches the fracture direction, there was lower permeability compared to when the anisotropy matches the maximum stress direction. Stress-dominated anisotropy, and in particular coals with a net contribution from both stress and fractures, showed higher permeability.