Joint well-placement and well-control optimization for energy-efficient water flooding of oil fields

Abstract

Hydrocarbon production driven by water flooding is energy-intensive. This paper presents well-placement and joint well-placement-and-control optimization studies with objective function taking into account CO2 emissions, pursuing more energy-efficient water flooding. To assess the effect of emission cost on the optimal drainage strategy, multiple optimization problems implementing different CO2 tax levels are solved. The solutions for higher tax levels usually inject less water and proportionally emit less CO2. The trade-off between emissions and oil production is non-linear: For lower tax levels, we obtain solutions providing large emission reductions with limited reductions in production. Increasing the CO2 tax further, any additional emission reduction leads to a larger reduction in oil production due to reduced opportunities for energy efficiency. Enforcement of higher tax levels usually produces optimal drainage strategies with higher injection effectiveness and lower emission intensity. However, some increases in CO2 tax, particularly above the critical level, have negligible impact on the optimal solution, hence they only affect the profitability negatively without further reducing emissions or improving emission intensity. Compared to the well-placement solutions, the solutions for our joint optimization emit less CO2 and provide higher profit. They also have lower emission intensity due to lower injection pressure. The adjustable well-control enables our joint optimization to stop field production after some years which could be more profitable than continuing the production at high CO2 tax. These clearly demonstrate the advantages of incorporating the well-control variables, giving more freedom to our joint optimization. In well-placement optimization, reductions in water injection and emissions are obtained by decreasing well productivity and injectivity and by increasing inter-well distance. In joint optimization, water injection can also be reduced by adjusting the well-control, hence there is no clear trend in well productivity and inter-well distance with changes in injection volume. The trends in well characteristics concluded from the well-placement solutions could mislead the search for more energy-efficient drainage as they are different from the trends observed in the joint optimization solutions, indicating the importance of joint optimization in this study.