| Popis: |
We have measured the stress, stress-path, and pore-pressure dependence of p-wave velocities of different shales. In good approximation, for small deformations, vertical velocity changes are proportional to changes in vertical effective stress. For low-permeability, undrained formations such as shales, it has been well established that pore pressures change as a function of mean-total stress changes and deviatoric stress changes. Analytical and finite-element geomechanical modeling, for a linear-elastic, isotropic half space, demonstrate that a ubiquitous porepressure increase in the low-permeability non-producing formations accompanies depletion of an adjacent reservoir. This pore pressure increase results in positive seismic timelapse time shifts in the overand underburden and may nearly cancel the negative time shifts due to archinginduced total vertical stress increase in the sideburden. Thus, poroelastic effects might offer an alternative explanation for the observation of mostly positive time shifts in time-lapse seismic, which was previously attributed to an asymmetric velocity-strain dependence for loading and unloading. |
