# Four-Dimension Seismic Analysis in Carbonate: A Closed Loop Study
## 3 Results & Discussion
### Matching of 4D Seismic Signal with Reservoir Performance  
Viewing reservoir performance in 4D time-lapse between the 1994 baseline survey and the 2014 monitor survey, we created maps of reservoir saturation properties from the dynamic reservoir simulation model to match the variation in these properties with the change in seismic response over 20 years. The simulator had embedded computational equations with several theories that could upscale the static model. It was found that water injection was about 4% in 1994 and less than 2% in 2014 (Figure 10a). Gas injection, on the other hand, was near zero Million Standard Cubic Feet (Mscf)/day in 1994, then increased to 800 K Mscf/day and decreased to 400 K Mscf/day in 2014 (Figure 10b).  
Figure 8: A subline over the predicted seismic density volume with the actual density log of the intersected well (**a**) 13 wells used to train and validate the data; (**b**) 6 wells used to train and validate the data, and (**c**) 6 wells with the machine learning approach. The black rectangle encloses the reservoir interval, and the black arrow indicates the resolution differences between the different paths.  
Figure 9: Reservoir horizon slice. (**a**) DV velocity variation between baseline and monitor survey: (**b**) P impedance of seismic monitor survey: (**c**) monitor-predicted density, and (**d**) monitor-predicted porosity. The decrease in DV, impedance, and density and the increase in porosity are indicated by the black arrow.  
Oil and gas saturation increased simultaneously throughout the production period; gas saturation reached about 120,000 Mscf/day in 1994 and increased to about 450,000 Mscf/day, while oil saturation increased from about 200,000 Mscf/day in 1994 to 240,000 Mscf/day, However, in 1994, water saturation was near zero Mscf/day and then gradually increased to less than 40,000 Mscf/day (Figure 11a). In addition, the water cut was zero in 1994 and increased to 12% by 2009, then decreased to 4% by 2014 (Figure 11b).  
The average map of reservoir pressure changes over time shows that the pressure was about 3600 PSI in 1994 and increased to about 3900 PSI in 2014. The average pressure difference over 20 years shows a change of less than 300 PSI to maintain reservoir pressure and production (Figure 12).  
Figure 11: Reservoir performance over time. (**a**) Oil, gas, and water saturation in Mscf/day and (**b**) water cut %.  
Figure 10: Reservoir performance over time. (**a**) Water injection, (**b**) gas injection. The white arrow shows the 4D time-lapse between the base year 1994 and the monitoring year 2014, showing a decrease in water injection with a simultaneous increase in gas injection.  
Moreover, the maps of average oil and water saturation show that average oil saturation had almost decreased due to production, while water saturation had increased by about 30%, which can be clearly seen on the map of average difference (Figure 13c-f).  
Based on what has been shown so far in this study, we can easily relate the pressure changes to the difference in seismic amplitude of the final migrated stack of the baseline and monitor survey when trying to understand the structure of the predicted 4D signal. In the polygon of the noticeable changes, we saw in pressure saturation could be overlaid the seismic amplitude difference between the 1994 baseline and the 2020 seismic monitor survey (Figure 14). As gas takes the place of oil, velocity, and density decrease, resulting in acoustic attenuation that is reflected in an increase in seismic amplitude and a downward shift in time.