Pressure and saturation estimation from<i>P</i>and<i>S</i>impedances: a theoretical study

Abstract

This work represents the first step of a study to integrate time lapse seismic and reservoir engineering data where a petro-elastic inversion from seismic data to pressure and saturation is presented. This inversion is made through an optimization procedure. In order to better understand and validate the initial step of the methodology, synthetic data (initially free of noise and errors) have been used. Through this ideal set of data, it was possible to show that pressure and saturation can be extracted from P and S impedances using only one seismic survey (3D inversion). It is also shown that this 3D approach is not robust when errors are assumed in reservoir data and it fails when, for instance, uncertainty in porosity data occurs. Thus, an improvement is made and the algorithm is rewritten based on 4D differences that diminish the wrong reservoir data effect. For both algorithms (3D and 4D), we have presented a discussion of the objective function behaviour concerning the use of P and S impedances simultaneously, the initial guess and the solution space. A sensitivity analysis discussing the influence of porosity and the dynamic properties on P and S impedances for 3D and 4D approaches is also presented. After understanding the inversion process behaviour for an ideal data set, an analysis of its results assuming different combinations of pressure and saturation variations and including some errors in the data set used is presented in the last subsections.