## 3 Numerical Analysis
### Oil Production Profile  
Figure 5 compares the oil production rate and cumulative oil production between bottom-up assisted pressure drive and SAGD processes. In order to be comparable between the two simulation models, the oil rate in the bottom-up assisted pressure drive process is the total oil production of producer 1 and producer 2 (as shown in Figure 3b), while the oil rate in the SAGD process is directly obtained from the producer (as shown in Figure 3a). This is a very critical point that must be clarified.  
An obvious difference is observed in the oil production curves. The peak oil rate in the bottom-up assisted pressure drive is approximately 600 m\({}^{3}\)/day, which is three times that in the SAGD process (i.e., 200 m\({}^{3}\)/day). In addition, the peak rate in the SAGD process is observed after 1500 days of operation due to the presence of shale barriers. The timing of the peak rate in the bottom-up assisted pressure drive is 180 days of the operation. The timing of the peak rate is of great importance in the economic evaluation, which will be discussed in the following section.  
As shown in Figure 5a, the oil production rate obtained from the bottom-up assisted pressure drive at the early stage is much greater than in the SAGD process. This observation is consistent with the steam chamber development discussed in the previous section. The high production rate is mainly attributed to the homogeneous reservoir properties near the bottom of the formation. The effect of shale barriers on oil production has not started. Once the steam chamber encounters shale barriers (shale barriers I and II), oil production reduces significantly. Oil production begins to decline after 1200 days of operation.  
Figure 5: Comparison of (**a**) oil production rate and (**b**) cumulative oil production between bottom-up assisted pressure drive and SAGD processes indicating that bottom-up assisted pressure drive improves oil production.  
Oil production in the early stage is greatly affected by the height of the shale barrier in the SAGD process. In the bottom-up process, the presence of the shale barrier seldom affects oil production in the early stage. Because of the effect of shale barriers, the oil production curve in the SAGD process (as shown in Figure 5a) is also different from the typical type curve, which includes ramp-up, plateau, and depletion. A short ramp-up process exists, and the plateau is prolonged. The peak oil rate is not observed in the early stage due to the presence of shale barriers. The peak rate is postponed to 1600 days.  
The cumulative oil production is compared in Figure 5b. The cumulative oil production in the bottom-up assisted pressure drive is 20% higher than in the SAGD process. This is mainly attributed to two aspects. In the typical SAGD process, the region between two well pairs is not swept by steam (as seen in Figure 6a). In the field application, the infill well is usually introduced to recover the oil in this region. The producer in the bottom-up assisted pressure drive process is located in the infill well location. Therefore, the oil near the bottom of the formation can be recovered effectively and efficiently without drilling extra infill wells. In addition, by comparing the oil saturation of the two processes, it is obvious that considerable oil is still trapped above the shale barriers in the SAGD process. This oil cannot be effectively drained by gravity because shale barriers have low permeability and low heat conductivity. Although there is a small amount of oil trapped above the shale barriers in the bottom-up assisted pressure drive process, most of the oil is migrated to the producers by the force of the pressure drive. This reveals two advantages of bottom-up assisted pressure drive over the SAGD process.