Porosity estimation based on the shear modulus inversion of seismic shear wave

Abstract

Reliable estimation of subsurface porosity is necessary for hydrocarbon reservoir characterization and fluid identification. Porosity estimations from seismic data can provide the lateral distribution of subsurface porosity, but the results may be highly nonunique because subsurface elastic properties (such as velocity and density) can be affected by both porosity and pore fluids. Since shear modulus is insensitive to pore fluid content, it can be effectively used to estimate porosity. We present a novel porosity estimation method using the reflected SH-wave (SH-SH wave), whose propagation characteristics depend mainly on shear modulus and S-wave velocity. We derive a new analytical expression for the SH-SH wave reflection coefficient, which acts as a function of shear modulus and S-wave velocity in its natural logarithm form. This new approximation has high accuracy, and both coefficients corresponding to shear modulus and S-wave velocity are “model-parameter independent”; thus, there is no need for prior estimation of any model parameter during inversion. Numerical analysis shows shear modulus inverted from the SH-SH wave has lower uncertainty, lower ill-conditioning, and lower requirements of data incident angle than that of the PP-wave. Furthermore, the highly correlated rock physics relationships between porosity and shear modulus facilitate accurate porosity estimation. A field data application shows that high-resolution porosities of fine structures can be reliably recovered using the proposed method.