## 3 Results and Discussion
### Effects of the Nitrogen Volume  
The maximum _STEP_ values and the producible period with energy efficiency are observed from the changing of the mole fraction of the nitrogen from 0 mol % to 10 mol %. Nine cases were examined, as follows: 0 mol %, 0.1 mol %, 0.5 mol %, 1 mol %, 2 mol %, 3 mol %, 5 mol %, 7 mol %, and 10 mol %. As demonstrated in Figure 3c, the _STEP_ value increases and then decreases due to the problematic effects of the top water-bearing thief zone. The maximum value here, obtained at an inflection point of the Figure 3c curve, represents the maximum profitable point with energy efficiency. Figure 6 shows, according to the nitrogen concentration, the maximum _STEP_ and the arrival time when _SOR_ is 8, i.e., the inflection point in Figure 3a that means the producible period with energy efficiency; Figure 6a  
Figure 5: Oil saturation at the end of production in the cases of: (**a**) the finite aquifer; and (**b**) the infinite aquifer. draws the maximum \(STEP\) and Figure 6b shows the arrival time to \(SOR\) = 8. Table 4 summarizes these results with the maximum values. Figure 6a confirms that the optimum nitrogen mole fraction is 0.1 mol % in the finite aquifer, while SAGD without nitrogen injection is most efficient in the case of the infinite aquifer with continuous mass flux. Since significant and continuous water influx into the steam chamber is unavoidable, if we have the infinite aquifer overlying the oil sands reservoir, the fast thermal process would be desirable until the chamber arrives at the top water-bearing zone, shutting in the wells, and then a new infill drilling would be an alternative. If the operators want to the larger producible time, 2 mol % of nitrogen would be a solution in the finite aquifer. For the case of the infinite aquifer, Figure 6b would suggest the large amount of nitrogen injection, but the nitrogen interrupts the chamber growth and thereby results in ineffective SAGP operation. Figure 6: Effects of the nitrogen mole fraction on \(STEP\) and the production time with energy efficiency: plot of **(a)** the maximum \(STEP\) and **(b)** the arrival time to \(SOR\) = 8 vs. the nitrogen to steam ratio at the injector. This paper concentrates on the flow characteristics between the top water zone and the steam chamber, i.e., the water influx from the top aquifer and the fluids loss from steam chambers in the homogeneous reservoir. To ensure practical applicability, additional discussions are required, e.g., reservoir heterogeneity, various flow directions in 3D reservoirs, the chamber interferences, and the optimum wellpad system consisting of several wellpairs [4; 5; 6]. The reservoir heterogeneity, i.e., the variation of the rock property and the facies models according to the location, should be considered since it is able to influence the flowing characteristics of the displacing fluid, as well as the entire performance [24; 25; 26; 27]. An impermeable barrier, e.g., the shale layer that exists in the bitumen deposit, is able to impede the water influx into the steam chamber, while it also interrupts the optimum enlargement of the steam chamber in terms of the energy efficiency. This study assumes the functional relationship of the gas-liquid \(K\) value as defined in Equation (1) and neglects the gas solubility and the originally-dissolved amount of non-condensable gas in the oil. The experimental validation of thermodynamic data, e.g., the gas solubility and \(K\) value, would be helpful to explain the accurate flow of gas phases. The regular SAGD produces the non-condensable gas, e.g., methane, and thereby the thermal simulation considered the originally-dissolved non-condensable gas could increase the reliability [14; 15; 18; 28; 29; 30]. The non-condensable gas is not viscous, and its density in relation to the diluted fluid mixture is vastly different; therefore, it can release into the aquifer without difficulty and form the flow channel between the thief zone and the steam chamber.