| Abstract: | Although carbonates hold more than 60% of the world's oil reserves, they, nevertheless, exhibit much lower average recovery factor values than terrigenous sandstone reservoirs. Thus, utilization of advanced enhanced oil recovery (EOR) techniques such as high pressure CO₂ injection may normally be required to recover oil in place in carbonate reservoirs. This study addresses how different rock types can influence the seismic monitoring of CO₂ sequestration in carbonates. This research utilizes an elastic parameter, defined in a rock physics model of poroelasticity and so-called as the frame flexibility factor, to successfully quantify the carbonate pore types in core samples available from the Great Bahama Bank (GBB). This study shows that for carbonate samples of a given porosity the lower the frame flexibility factors the higher is the sonic wave velocity. Generally, samples with frame flexibility values of <4 are either rocks with visible moldic pores or intraframe porosity; whereas, samples with frame flexibility values of >4 are rocks with intercrystalline and microporosity. Hence, different carbonate pore geometries can be quantitatively predicted using the elastic parameters capable of characterizing the porous media with a representation of their internal structure on the basis of the flexibility of the frame and pore connectivity. In this research, different fluid substitution scenarios of liquid and gaseous CO₂ saturations are demonstrated to characterize the variations in velocity for carbonate-specific pore types. The results suggest that the elastic response of CO₂ flooded rocks is mostly governed by pore pressure conditions and carbonate rock types. Ultrasonic P-wave velocities in the liquid-phase CO₂ flooded samples show a marked decrease in the order of 0.6 to 16%. On the contrary, samples flooded with gaseous-phase CO₂ constitute an increase in P-wave velocities for moldic and intraframe porosities, while establishing a significant decrease for samples with intercrystalline and micro-porosities. Such velocity variations are explained by the stronger effect of density versus compressibility, accounting for the profound effect of pore geometries on the acoustic properties in carbonates. The theoretical results from this research could be a useful guide for interpreting the response of time-lapse seismic monitoring of carbonate formations following CO₂ injection at depth. In particular, an effective rock-physics model can aid in better discrimination of the profound effects of different pore geometries on seismic monitoring of CO₂ sequestration in carbonates. |
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