Stress And Stress Release Effects On Acoustic Velocities From Cores, Logs And Seismics

A technique for producing synthetic sandstone under controlled stress conditions has been used to study stress and stress release effects on acoustic velocities. Laboratory experiments were performed to simulate seismic, log and core measurements. The results show that seismic studies may to some extent be used to monitor stress changes, however the relations between stress changes and changes in the seismic velocities will not be linear and will depend on the stress path. In the vicinity of a wellbore there will be a zone with reduced sonic velocities. This zone is thin enough that conventional long-spaced sonic tools will not be affected. If specially designed tools are used to log velocities of the damaged zone, the data may be interpreted to yield stress information. Unloaded cores will show a velocity anisotropy that reflects the forming stress state. Cores that have been loaded back to the in situ stress state will not have frilly recovered velocities, and the stress dependency of the velocities will be larger than in situ.