Study on Seismic Data Sensitivity on Pressure-Saturation Discrimination

Abstract

The effects of pore pressure and fluid saturation on the seismic velocity changes are coupled and important to discriminate one from the other in the context of time-lapse monitoring of fluid injection into the subsurface such as the CO2 storage case. The purpose of this thesis is to compare the use of conventional streamer data to multi-component seismic data. Explicit linear expressions for pressure and saturation discrimination that are used for petroleum application are tested for a CO2 injection scenario. The explicit linear expressions are found to be oversimplified for CO2 injection; hence the problem is solved by the use of non-linear systems of equations. The multi-component method is demonstrated to give a more sturdy pressure-saturation discrimination compared to conventional streamer data. Several limitations need to be taken into account when using the methods, as several factors affect the result. These include the reliability on a priori information of spatially variant parameters, a reasonable certainty in measured seismic parameters, and good initial information about the in situ pressure and saturation. Typical uncertainties in the measured seismic parameters are tested. The results show the use of multi-component seismic data to be superior to the conventional streamer data. The methods in this thesis utilize an existing reservoir simulation for the Smeaheia area in order to investigate the issues mentioned above (i.e. better discrimination). In this course, there are some uncertainties related to the existing reservoir simulation. The uncertainties are explored through a simple reservoir simulation to deduce the certainty in discrimination for other scenarios than the provided Smeaheia fluid flow simulation. It is learned from this study that the conventional amplitude versus offset (AVO) method is applicable for CO2 saturations less than 50 %. For increasing saturation and pressure, which case is more important for the integrity of reservoir and the cap rock, the multi-component seismic data seem necessary for pressure-saturation discrimination.