POROELASTIC AND SEISMIC CHARACTERIZATION OF HETEROGENEOUS RESERVOIR ROCKS

The primary focus of this dissertation is to link the poroelastic model that couples rock’s elastic and hydraulic properties to seismic characterization of the heterogeneous reservoirs. I have presented how to incorporate the dynamic poroelastic responses of microscopic and mesoscopic flow into the classical Biot theory. The resulting effective Biot media can capture the characteristics of velocity dispersion and wave attenuation in heterogeneous media. On the basis of this effective Biot media, I developed an approach to quantify the impact of both global flow and local flow simultaneously on the signatures of seismic reflectivity. The computed poroelastic reflections not only depend on the elastic properties contrast and incident angle, but also rely on the fluid mobility and observational frequency. For a typical shale-sand reflector, we find that the effect of local flow causes reflection amplitude variations in frequency to be as high as 40%, and a maximum phase shift as high as 12 degrees at the seismic exploration frequency band. However, the global flow effect on reflectivity is almost trivial (<1.5%) and occurs mainly at ultrasonic frequency band.
Poroelastic seismic analysis shows that ignoring the dispersion behavior of seismic reflection can lead to inaccurate seismic imaging and misleading interpretation of reservoir properties. I further demonstrate that the AVO response at the interface is strongly impacted by the reflection dispersion behavior: the bright spot (Class III AVO) gets brighter at lower frequency, the dim spot (Class I AVO) gets dimmer at lower frequency, and the Class II AVO reservoir exhibits significant phase distortion in the frequency domain. It is found that, for certain permeability ranges (about 2 orders of magnitude), seismic amplitude can exhibit an almost linear relationship with permeability variation. For Class III AVO reservoir scenario, high fluid mobility zones usually enhance the seismic amplitude; while for Class I AVO reservoir scenario, high fluid mobility zones weaken the seismic amplitude.
Finally, a field case study on the Offshore Brazil data set shows that poroelastic reflection from the interface of underlain carbonate with overburden marlstone exhibits considerably different frequency behavior at two nearby wells. Based on the poroelastic modeling, this discrepancy is likely to be caused by the fact that the fluid mobility in the underlain grainstone at well A is remarkably greater than that in the underlain packstone at well B.