Summary
In this paper a method is described to update the low frequencies of the elastic parameters coming from pre-stack time lapse inversion in a data driven manner. In the process no assumptions are made regarding the nature of the reservoir changes. Also no assumptions are made regarding the relationships between elastic parameters or reservoir parameters. The method described here can be seen as a 4D extension to a 3D updating method described by Mesdag et.al. 2010. In this way independent measures are obtained of time lapse absolute P-Impedance and time lapse absolute Vp/Vs (or ?r and µr). These can then be compared with propagation parameters derived from 4D time shifts or velocities.
Introduction
In heavy oil Steam Assisted Gravity Drainage (SAGD) the steam chambers can vary in size up to 40-50 meters in thickness. Time lapse seismic is often used to quantify production parameters such as temperature and pressure or steam chamber development. The wavelet extracted from conventional seismic data imaging such shallow reservoirs is typically a short period one with a dominant wavelength around 30 m (20 ms). It is clear that the conventional seismic signal will only contain part of the information necessary to characterize the reservoir. Much of the information for full bandwidth inversion will have to come from the time lapse low frequency response of the subsurface, i.e. the low frequency difference in the elastic parameter models between monitor and base. Rock physics models (RPM) can be used to update the low frequency model at well control. Production data will provide the information we need to update the petrophysical well curves to the time of the monitor survey and the RPM allows us to convert this to the elastic parameters. Away from wells the seismic data may be the only hard data available.
