Integration of the seismic data with rock physics and reservoir modeling in the FRS project

Davood Nowroozi

This thesis is related to a CO₂ injection project from the well logging to seismic modeling and imaging, so many disciplines are involved. The reservoir is at a shallow depth of 300m so it is in a low temperature and low pressure state. A black-oil reservoir simulation was not appropriate for the study, so a compositional method was used for the fluid simulation. The change of phase possible around the anticipated pressure and temperature for CO₂ injection is another limitation for a compositional simulation, so the gas phase injection was selected for the simulation modelling. Results show that the CO₂ injection will decrease the density of formation around 3%, and the P-wave velocity between 7 and 15%. It can also affect the S-wave velocity, and in the seismic studies, there is enough of a change in the S-wave velocity to consider PS and SS-wave data for the reservoir characterization. The rock physics equations solved for the pressure changes by the Equation of State for CO₂ and for the brine and a set of curves related to the fluid mixed type were introduced. After 5 years of injection at bottom-hole pressure of 4.9 MPa, the injected CO₂ plume has a diameter of 185m.

The seismic studies based on the rock physics models show that the fluids mix type is a determinative factor for interpretability of a reservoir. Seismic forward modelling was undertaken using both acoustic and elastic finite difference approaches, and imaging was done using reverse time migration. For patchy or semi-patchy saturation, mixed with a linear (or near linear) converter, the saturation is calculated with an acceptable error by the acoustic, seismic response. In a parabolic converter as Reuss average in a fine mixed type, the time-lapse acoustic response is insufficient to identify saturation explicitly.