Determination of porosity and saturation using seismic waveform inversion

Abstract

Characterization of a reservoir model requires determination of its petrophysical parameters, such as porosity and saturation. We propose a new method to determine these parameters directly from seismic data. The method consists of the computation and inversion of seismic waveforms. A high frequency method is presented to model wave propagation through an attenuative and dispersive poroelastic medium. The high frequency approximation makes it possible to efficiently compute sensitivity functions. This enables the inversion of seismic waveforms for porosity and saturation. The waveform inversion algorithm is applied to two laboratory crosswell datasets of a water saturated sand. The starting models were obtained using travel time tomography. The first dataset is inverted for porosity. The misfit reduction for this dataset is approximately 50%. The second dataset was obtained after injection of a nonaqueous-phase liquid (NAPL), possibly with some air, which made the medium more heterogeneous. This dataset was inverted for NAPL and air saturation using the porosity model obtained from the first inversion. The misfit reduction of the second experiment was 70%. Regions of high NAPL and high air saturation were found at the same location. These areas correlate well with the position of one of the injection points as well as regions of higher NAPL concentrations found after excavation of the sand. It is therefore possible to directly invert waveforms for pore fluid saturation by taking into account the attenuation and dispersion caused by the poroelasticity.