Joint elastic-electrical effective medium models of reservoir sandstones

Improvements in the joint inversion of seismic and marine controlled source electromagnetic data sets will require better constrained models of the joint elastic-electrical properties of reservoir rocks. Various effective medium models were compared to a novel laboratory data set of elastic velocity and electrical resistivity (obtained on 67 reservoir sandstone samples saturated with 35 g/l brine at a differential pressure of 8 MPa) with mixed results. Hence, we developed a new three-phase effective medium model for sandstones with pore-filling clay minerals based on the combined self-consistent approximation and differential effective medium model. We found that using a critical porosity of 0.5 and an aspect ratio of 1 for all three components, the proposed model gave accurate model predictions of the observed magnitudes of P-wave velocity and electrical resistivity and of the divergent trends of clean and clay-rich sandstones at higher porosities. Using only a few well-constrained input parameters, the new model offers a practical way to predict in situ porosity and clay content in brine saturated sandstones from co-located P-wave velocity and electrical resistivity data sets.