# Four-Dimension Seismic Analysis in Carbonate: A Closed Loop Study
## 2 Materials and Methods
### 1D Well-Based Time-Lapse Feasibility Study
Moreover, the synthetic gather angles were generated with different percentages of water flooding (Figure 2a), showing a 4D difference between the baseline (in-situ) and the different production scenarios that were much stronger at 50 to 85% saturation (Figure 2b). CO\({}_{2}\) sweeping was not optimal in the gas-producing well because it is a gas-bearing reservoir. Therefore, we decided to omit this scenario. However, in the water injection well, successfully modeled synthetic angular logging of water flooding was measured with the corresponding log traces (Figure 2c). The 4D effects are evident from the difference between the angle of in-situ and the different water saturation scenarios of the water injection well (Figure 2d). The 4D difference was stronger at 85% flowing water than at 50%, while there was little difference at 10%. There was a sharp increase in density and the ratio VP/VS, which reflects an increase in acoustic impedance of more than 10% at the reservoir level. The 4D effects were visible in the differences between the in-situ angle gather and the angle gather of the different scenarios of CO\({}_{2}\) gas displacement of the water in the injection well. The 4D difference for gas injection was generally stronger than for the water saturation scenarios. While 85% water saturation data was close to that of 50%, it was slightly stronger than 10% for CO\({}_{2}\) gas displacement (Figure 3b). Comparison of P impedance (IP) and shear impedance (IS) for different production scenarios for gas-producing wells or for wells with water injection showed that a water sweep with different scenarios for both wells can change P impedance by about 4% and shear impedance by 3%, while a CO\({}_{2}\) sweep can change P impedance by about 8% and shear impedance by 2% (Figure 4a,b). On the other hand, in the water sweep region, the P impedance was expected to change by about 4% and the VP/VS ratio by 3%, and the CO\({}_{2}\) sweep led to a change in the P impedance by about 8% and the VP/VS ratio by 8% (Figure 4c,d). Thus, the expected changes were greater than 4%, which is the value usually considered detectable by experience and the literature [27]. The petro-elastic model and 1D time-lapse pave the way to link the production data with the current seismic data and measure the similarity between them. The petro-elastic model and the 1D time-lapse model led to the following conclusions:  
Figure 2: (**a**) Gas production well with different water saturation scenarios—angle of synthetic gathers with corresponding log tracks (**b**) Gas production well: angle stack of differences between in-situ and different SW (**c**) Water injection well with different water saturation scenarios—angle of synthetic gathers with corresponding log tracks, 4D difference of three water saturation scenarios and (**d**) water injection well: angle stack of differences between in-situ and different SW.  
For water injection wells:
* Matching with the in-situ situation showed the validity of the rock physics model used, i.e., the Gassmann fluid substitution performed, and the accuracy of the measured logs. * The water-sweep scenarios showed minimal changes in the logs, indicating that water flooding in an oil reservoir is relatively difficult to quantify on a time-lapse seismic image. * For the same oil reservoir, on the other hand, CO2 flooding showed stronger changes, especially in P-wave velocity and thus impedance, making CO2 sweep scenarios more observable on time-lapse seismic. * It is quite clear that CO2 showed pronounced signatures in the form of acoustic impedance changes. For the producing well:
* The matching with the in-situ situation showed some discrepancy between the measured log and the log produced by the rock physics, especially in the very porous sections of the reservoir. * The water sweep scenarios showed some degree of change in the logs, suggesting that a water flood may be observable in the gas reservoir on a time-lapse seismic image.