3.4.1. Parameterization and Sensitivity Analysis
The results of the sensitivity analysis below show the effect of each parameter on the two main objectives of this study—cumulative oil recovered, and mass of CO2 stored. It can be inferred from the qq-plot of the simulated cumulative oil recovered versus proxy cumulative oil recovered that the proxy matches the high-physics calculations enough to be used suitably as a substitute in prediction.
Figure 18.
Q-Q Plot of Simulated Cumulative Oil Produced vs. Proxy Predicted Cumulative Oil Produced.
Figure 18.
Q-Q Plot of Simulated Cumulative Oil Produced vs. Proxy Predicted Cumulative Oil Produced.
The results of the Monte Carlo plot demonstrate the robustness of the development strategy with a percentage difference of 0.587% between estimated P10 and estimated P90. With P90 standing at 1.30258 × 107STB and P10 at 1.29494 × 107STB, there is only a difference of 76, 400 STB.
Figure 19.
Monte Carlo Simulation Plot of Cumulative Oil Sensitivity Analysis.
Figure 19.
Monte Carlo Simulation Plot of Cumulative Oil Sensitivity Analysis.
The next plot to be viewed is the results of the Morris Analysis. This plots the standard deviation which represents the non-linearity effect of each sensitive parameter against the Absolute Elementary Effect Mean (AEEM) which is quantifies the impact it has on the objective function. This uses a one-at-a-time approach to determining local sensitivity of the parameter and the impact it has on the cumulative oil produced.
Figure 20.
Plot of Morris Analysis for Cumulative Oil Produced.
Figure 20.
Plot of Morris Analysis for Cumulative Oil Produced.
where;
PMO_Pressure = Pressure on Production Manifold
HPS_Inlet Pressure = High Separator Pressure
Gas_Injection_Rate = Compressor Rate
IG1_GI = Injection Group 1—Gas Injection Cycle Duration
IG1_WI = Injection Group 1—Water Injection Cycle Duration
IG2_GI = Injection Group 2—Gas Injection Cycle Duration
IG2_WI = Injection Group 2—Water Injection Cycle Duration
IG3_GI = Injection Group 3—Gas Injection Cycle Duration
IG3_WI = Injection Group 3—Water Injection Cycle Duration
IG4_GI = Injection Group 4—Gas Injection Cycle Duration
IG4_WI = Injection Group 4—Water Injection Cycle Duration
According to the Morris analysis, the most sensitive parameter is the Inlet Pressure of the high-pressure separator, which exhibits the highest AEEM and standard deviation. This is followed by the gas injection cycle of group 3 (IG3_GI) and water injection cycle of group 2 respectively (IG2_WI). Following at a distance are gas injection cycle of group 2, pressure on production manifold outlet and gas injection rate. However, the effect of pressure on the production manifold outlet exhibits less non-linearity than the other 2.
The Sobol Analysis (
Figure 21) further supports the Morris Analysis findings, showing that the gas injection rate has the highest total effect at approximately 63%, followed by HPSIP at 61%. The distinction between these two is that HPSIP directly impacts cumulative oil produced, whereas the gas injection rate influences other parameters, thereby affecting the cumulative oil.
For CO
2 storage, the qq-plot (
Figure 22) indicates a close match between the proxy and the high-physics model, suggesting the proxy’s reliability for further analysis. Using this proxy, a plot of the Monte Carlo simulation shows a difference of 3.74% between the P90 and P10 of estimated CO
2 stored. This shows a wider range in percentage than in the case of cumulative oil produced. However, this range is quantitatively small and also shows the robustness of the model. With P90 at 4452.31 × 106 and P10 at 4285.95 × 106, a difference of 166MMlb of CO
2 is stored.
The Morris Analysis (
Figure 24) reveals that the injection group cycles of gas and water have a more significant impact on CO
2 storage than surface components such as HPSIP, Production Manifold Outlet Pressure (PMOP) and gas injection rate. This is likely due to the fact that the amount of CO
2 stored depends on how much anthropogenic CO
2 is purchased which in turn depends on the gas production rate. The gas production rate is heavily dependent on pressure distribution in the reservoir, gas channels and pathways and rate of oil production. These are influenced more by the amount of water (for pressure maintenance) and gas injected (for miscible flood but excess injection can lead to early gas breakthrough). An effective miscible flood would slow down the gas production and increase sweep efficiency. This would allow more CO
2 to be stored since more would be purchased.
The Sobol Analysis (
Figure 25) corroborates the Morris Analysis, indicating that injection group cycles have a greater influence on CO
2 storage than surface components operating conditions. This analysis highlights the importance of optimizing injection strategies to maximize CO
2 storage.
Despite this proxy model of the Field NPV showing a greater deviation than the previous two, the maximum absolute error is 3.4%. This affirms that our proxy is suitable for the investigations ahead.
Figure 26.
QQ-Plot of Proxy for Field NPV.
Figure 26.
QQ-Plot of Proxy for Field NPV.
The Monte Carlo plot shows that this proxy estimates a difference of 9.87% between the P90 and P10. This wider range suggests a higher uncertainty in NPV predictions compared to oil production and CO2 storage. With P90 at 1.08612 × 1010 and P10 at 1.20509 × 1010, a resulting in a difference of 1.1897 × 109.
Figure 27.
Monte Carlo Simulation for Field NPV.
Figure 27.
Monte Carlo Simulation for Field NPV.
The Morris analysis clearly shows injection group 4 as being the most impactful despite all parameters having a similar degree of non-linearity. Following closely is the gas injection cycle of injection group 2 and at a distance is the gas injection of injection group 3. Parameters cluster together on the lower left side, including the surface components. This is due to limiting the range for sensitivity to existing field conditions.
Figure 28.
Morris Analysis for Field NPV.
Figure 28.
Morris Analysis for Field NPV.
The Sobol analysis supports the results of the Morris analysis with injection group 4 being the most influential sensitive parameter at 52% and 25%, and gas injection of injection groups 2 and 3 following 12% and 4.2%. The parameters do not seem to show much interaction with each other in the outcome of the Field NPV.
Figure 29.
Sobol Analysis for field NPV.
Figure 29.
Sobol Analysis for field NPV.
Following the results of the sensitivity analysis, 7 of the 11 sensitivity parameters are selected for optimization using the particle swarm optimizer. Since our study focuses on the surface facility, all 3 of the surface component operation conditions are selected. These are the HPSIP, gas injection rate and Production Manifold Outlet Pressure (PMOP). The additional 4 parameters that are selected are the gas and water injection cycles for injection group 4, and gas injection cycles for injection groups 2 and 3.
3.4.2. Optimized Operating Conditions
The application of the particle swarm optimizer yielded optimum operating conditions for the selected sensitive parameters. The results of this optimization are depicted in the plots below, which demonstrate the improvements achieved through the optimization process.
Based on the cumulative oil produced, the optimal experiment ID is 31, as shown in
Figure 30. This experiment resulted in the highest incremental oil production. The optimal strategy produced a cumulative oil volume of 14,043,372 STB, which is an incremental increase of 96,804 STB over the base development strategy, translating to a 0.694% improvement (
Figure 31 and
Table 8).
Table 7.
Percentage Increase in Cumulative Oil Production.
Table 7.
Percentage Increase in Cumulative Oil Production.
Experiment ID |
Cumulative Oil Produced (STB) |
Incremental Oil Recovered (STB) |
Percentage Increment |
Development Case |
13,946,568 |
|
|
Optimal Strategy |
14,043,372 |
96,804 |
0.694 |
The optimized operating conditions not only enhanced oil recovery but also significantly reduced the amount of CO
2 purchased. As illustrated in
Figure 32, the optimal strategy utilized approximately 10% less purchased gas (1,984 MMSCF) to achieve a slightly higher cumulative oil production. This is the benefit of having the right operating conditions in the surface facility components and optimal scheduling of water and gas injection cycles. The savings involved in CO
2 purchased is the driving force in raising NPV of experiment 31 above the others.
The economic benefits of the optimized strategy are evident in the Field NPV results (
Figure 33). The optimal strategy yielded a field NPV of
$114,871,730, which is a 25.84% increase over the base development strategy (
Table 10). The reduced CO
2 purchase costs and the efficient utilization of injected CO
2 are the primary drivers behind this substantial increase in NPV. However, it is important to note that the reduction in CO
2 purchase also results in less CO
2 being stored subsurface.
Figure 33.
Field NPV at the end of forecast period.
Figure 33.
Field NPV at the end of forecast period.
Table 9.
Percentage Increase in Field NPV.
Table 9.
Percentage Increase in Field NPV.
Experiment ID |
Field NPV ($) |
Incremental Field NPV ($) |
Percentage of Incremental Field NPV |
Development Case |
91,281,921 |
|
|
Optimal Strategy |
114,871,730 |
23,589,809 |
25.84 |
Table 10.
Incremental CO2 Stored per Experimental ID.
Table 10.
Incremental CO2 Stored per Experimental ID.
Experiment ID |
Cumulative CO2 stored (MMIbs) |
Incremental Storage (MMIbs) |
Percentage Decrement |
Development Case |
5,061.68 |
|
|
Optimal Strategy |
4,832.18 |
-229.5 |
4.53 |
Figure 34.
Plot of CO2 Stored per Experimental ID.
Figure 34.
Plot of CO2 Stored per Experimental ID.
The cumulative CO
2 stored for the optimal strategy is 4,832.18 MMlbs, compared to 5,061.68 MMIbs in the base development strategy (
Figure 35). This discrepancy is due to the decreased need for external CO
2, as the optimized strategy maximizes the use of recycled CO
2 from produced gas. Based on these results. Experiment 31 is selected as our ideal optimized case.
The amount of CO2 stored is equal to the amount of purchased CO2 because all CO2 remains trapped in the closed system. This would not have huge economic significance since the financial gains and tax reliefs of CO2 storage last only 12 years. This simulation goes for 15 years and thus, the last 3 years of extra purchased CO2 does not contribute economic benefits to the strategy.