Joint Use of PP and PS AVOA Data to Estimate Fluid Indicator in Vertically Fractured Medium

Abstract:

The existence of fractures induces anisotropy in medium. This anisotropy might be a comprehensive result of fractures' properties, such as the direction, spacing, apertures, intensity, microstructure, fluid infill, and so on. Among these properties, the preferential orientation of fracture networks makes the medium azimuthally anisotropic with respect to seismic wave propagation. To the medium containing a set of vertical fractures, the tangential weakness does not vary with the fluid content, however on which the normal weakness shows great dependence. Based on the theory of linear slip model and the sensitivity to fracture weakness of PP- and PS-reflection coefficients which can be derived by a Born formula, we did both the PP-AVOA and PS-AVOA numerical experiment and also the joint inversion of fluid indicator. Results show that when the fractures have low saturation of gas, the fluid indicator estimated from PP-AVOA data is precise enough; when gas saturation goes up to 70%, joint inversion can help to improve the poor quality of PP-AVOA data inversion. Under high gas-saturated case, both PP inversion and joint inversion are sensitive to the errors in g, where g is the square of the ratio of S- and P- wave velocity in the unfractured medium. This dependency can be reduced by adding a different weight to PP and PS data during the inversion.Based on the result of numerical experiment, we processed field data in Sichuan Basin in China. The inversion result is consistent with the well interpretation.

The first column in figure represents the PP- and PS-reflectivity computed by matrix method(Fryer and Frazer,1984). The second column is the result of Born linearized method. In the last column, upper one shows the estimated fluid indicator in different gas saturation case and the below one consider the effect of error in g on the inversion results.