تجزیه و تحلیل حساسیت از جاری شدن سیل آب متناوب CO2 برای بازیافت نفت پیشرفته در مخازن نفت کاهش آب

The objective of this work is to investigate the effect of operational schemes, reservoir types and development parameters on both the amount of incremental oil produced and CO2 stored in high water cut oil reservoirs during CO2 water-alternating-gas (WAG) flooding by running compositional numerical simulator. The method used is the orthogonal experimental design method to optimize operation parameters, including CO2 slug size, ratio of CO2 slug size to water slug size (WAG ratio), CO2 injection rate, and voidage replacement ratio. The Net Present Value (NPV) was used as an objective function for economic analysis. Various 3-D heterogeneous reservoir models were built to investigate the impact of reservoir types and development parameters on CO2 flooding efficiency and storage capacity. The results indicate that as compared to inverted nine-spot pattern and inverted seven-spot pattern, five-spot pattern is more suitable for CO2 WAG flooding. The earlier water injection is switched to CO2, the more benefit can be obtained. Compared with CO2 injection cost and tax credit per ton of CO2 stored, oil price is considered as the most influential economic parameter on CO2 WAG flooding.

Carbon dioxide flooding has been recognized as one of the most effective options for oil recovery enhancement in depleted or mature oil reservoirs [1], [2] and [3]. The benefits of injecting CO2 include the expansion of oil volume and the reduction of oil viscosity [4] and [5]. CO2 is able to displace the residual oil that is immobilized by water flooding and therefore improve the microscopic displacement efficiency [6]. The CO2 EOR projects in Weyburn and the North Sea have also proved the great potentials of both oil production increment and CO2 sequestration [7] and [8]. However, if the gas source is located far from a target oil reservoir, considering the cost of CO2 capture, transportation, compression and injection, CO2 EOR projects may not be profitable without economic incentives from the government. Ghomian et al. [9] established the amounts and types of economic incentives for different reservoir types. They found that sandstone reservoirs had higher probability of need for economic incentives than carbonate reservoirs. Using the methodology of NPV, Jahangiri and Zhang [10] determined that a minimum of $40/ton of carbon tax credit is required for immiscible CO2 flooding so as to obtain the same NPV as water flooding, while miscible CO2 flooding is more profitable than water flooding even without any economic incentives. Regarding the optimization of operational scheme, a number of studies have been conducted. Yang et al. [11] developed an integrated model to optimize the production-injection operation systems (PIOS). Taking the NPV as an objective function, the optimum production and injection parameters were achieved in a WAG miscible flooding reservoir. Kovscek and Cakici [12] defined an objective function that combines the ultimate oil recovery and the fraction of reservoir volume filled with CO2. The most effective injection and production scheme was determined which could co-optimize oil recovery and simultaneous CO2 sequestration. Chen et al. [13] developed a hybrid method that integrates orthogonal array and Tabu technique into a genetic algorithm. When conducting a sensitivity analysis on oil recovery and NPV, controlling variables were selected including injection rate, WAG ratio, injection time and bottomhole pressure for the producers. Studies revealed that WAG flooding recovers more oil than continuous injection flooding. That is because WAG flooding can reduce CO2 viscous fingering and provide better vertical sweep efficiency [14] and [15]. Additionally, the horizontal well impacts CO2 flooding greatly for the reason that the displacement provides better sweep efficiency based on both reservoir simulations and laboratory studies [16], [17] and [18]. Despite the potentials of CO2 EOR, this technology is not suitable for all types of hydrocarbon reservoirs [19] and [20]. Based on both field results and oil recovery mechanism study, Taber et al. [21] proposed the screening criteria for CO2 miscible and immiscible flooding, respectively. Shaw and Bachu [22] presented a method for the screening and ranking of oil reservoirs suitable for CO2 EOR. Oil gravity, reservoir temperature and pressure, minimum miscibility pressure and remaining oil saturation were selected as variables. However, most of studies on CO2 flooding described above have been conducted on undeveloped oil reservoirs, and very few results from high water cut oil reservoirs are seen in the literature. The main objective of this study is to investigate the effect of operational schemes, reservoir types and development parameters on WAG flooding in high water cut oil reservoirs by running compositional simulations. By applying orthogonal experimental design, the most effective operational scheme was determined which could maximize the incremental oil produced by WAG flooding. Afterwards, various geological models were constructed by employing different reservoir parameters and development parameters. A technical analysis of five reservoir parameters and two development parameters was conducted. The NPV model was built for economic analysis. The effect of oil price, CO2 injection cost and tax credit on the NPV was investigated in the study.

This paper presented a numerical study on WAG performance with water flooded reservoirs. Orthogonal experimental design was applied to optimize operation parameters of CO2 WAG flooding in a water flooding reservoir. Significant work was done on sensitivity analysis aiming to evaluate the suitable reservoir conditions and development parameters. The following conclusions were achieved. • The optimal scheme was determined with a CO2 slug size of 0.05 PV, a WAG ratio of 1:2, a CO2 injection rate of 40,000 Sm3/d, and a voidage replacement ratio of 1:0.9. The recovery factor can be increased 17.64% as compared to water flooding. The voidage replacement ratio is recognized as the most influential operation parameter. • The technical and economic efficiency of WAG flooding worsen as the average horizontal permeability and the ratio of vertical to horizontal permeability increase. This is attributed to the quicker production of CO2 in the higher permeability reservoir. Even though WAG flooding recovery factor and the improved recovery factor increase as the permeability variation coefficient increase, the highest NPV is achieved when permeability variation coefficient equals 0.6. • WAG flooding offers no technical advantage over water flooding in reverse rhythmic reservoirs, and compound reverse rhythmic reservoirs could benefit the most economically from WAG flooding. The technical efficiency of WAG flooding improves initially as the net thickness of formation increases, and then worsens when it exceeds 30 m. However, the NPV increases significantly as the formation becomes thicker because of the substantial oil production increase. • As compared to inverted nine-spot pattern and inverted seven-spot pattern, five-spot pattern is more suitable for WAG flooding. Appropriately expanding well spacing improves the economic efficiency, even though the recovery factor decreases slightly. The analysis of WAG injection timing reveals that the earlier water injection is switched to CO2 in a water flooding reservoir, the more benefit can be obtained. • Additionally, oil price, rather than CO2 injection cost and tax credit per ton of CO2 stored, is considered as the parameter that impacts the economic efficiency of WAG flooding more significantly.