333. 南非Bredasdorp盆地油藏二氧化碳地质封存的动态模拟

Dynamic modeling of geological carbon storage in an oil reservoir, Bredasdorp Basin, South Africa

地质碳储存是一种高效技术，能够大规模减少大气中的碳含量。为了实现零排放的目标，今后可能需要利用油藏进行二氧化碳（CO2）储存项目而不进行石油开采。因此，本研究对南非布雷达斯多普盆地中的一处油藏进行了CO2储存的动态模拟研究。研究考虑了注入CO2到油藏的时间范围，分别为20年（2030年至2050年）和100年（2050年至2150年），并对CO2-盐水-油相互作用和油藏边界条件进行了敏感性分析。

在闭合边界情景下，目标注入速率为每年0.5 Mt，注入速率逐渐减少以避免超过油藏的破裂压力，从而防止压力积聚现象的发生。由于注入的CO2体积减少以及约束压力的存在，CO2浓度的迁移并不迅速。模拟结果显示，该系统受重力主导，在模拟结束时尚未达到重力稳定状态，因为流体界面尚未平坦。

相反，开放边界油藏则具有供应CO2的能力，因为所有边界都是开放的，目标注入速率得到实现。在这种情况下，系统受粘性主导，没有出现压力积聚现象。

在两种情况下，CO2在油和盐水中的溶解是活跃的，水和油中溶解的CO2分数逐渐增加。在2050年至2150年期间，气态CO2相的浓度下降，主要固定机制包括构造固定、溶解在油和水中，以及残留固定。

研究结果显示，边界条件对项目的成功至关重要，直接影响注入速率和压力。这项研究开创性地探索了CO2注入到油藏以及CO2-盐水-油的相互作用，并对南非一个封闭的和一个开放的油气系统的油藏边界条件进行了敏感性分析。

Abstract

Geological carbon storage provides an efficient technology for the large-scale reduction of atmospheric carbon, and the drive for net-zero emissions may necessitate the future usage of oil reservoirs for CO₂ projects (without oil production), hence, dynamic modeling of an oil reservoir for CO₂ storage in the Bredasdorp basin, South Africa, was therefore conducted. Injection into the reservoir was for 20 years (2030–2050), and 100 years (2050–2150) to study the CO₂–brine–oil interactions, with sensitivities carried out on reservoir boundary conditions. The closed boundary scenario experienced pressure buildup with a target injection rate of 0.5 Mt/year, and a cutback on injection rate progressively until 2050 to not exceed the fracture pressure of the reservoir. The CO₂ plume migration was not rapid due to the reduced volume of CO₂ injected and the confining pressure. The system was gravity dominated, and gravity stability was not attained at the end of the simulation as fluid interfaces were not yet flat. The open boundary reservoir did not experience a pressure buildup because all boundaries were open, the target injection rate was achieved, and it was a viscous-dominated system. In both cases, the dissolution of CO₂ in oil and brine was active, and there was a growing increase of CO₂ fraction dissolved in water and oil, a decline in gaseous mobile CO₂ phase between 2050 and 2150, and active trapping mechanisms were structural trapping, dissolution in oil and water, and residual trapping. The study showed that boundary condition was very crucial to the success of the project, with direct impacts on injection rate and pressure. This pioneering study has opened a vista on the injection of CO₂ into an oil reservoir_, and CO₂–brine–oil interactions, with sensitivities carried out on reservoir boundary conditions in a closed and an open hydrocarbon system in South Africa.

Figure 1

Conclusion

In this study, an oil reservoir in the offshore lying Bredasdorp basin, South Africa, has been considered for CO₂ storage, without enhanced oil recovery (EOR). The closed boundary scenario experienced a pressure buildup with a target injection rate of 0.5 Mt/year, and therefore a cutback on injection rate progressively until 2050 to ensure the reservoir and overlying seal were not damaged. Migration of the CO₂ plume was not rapid, due to the reduced volume of CO₂ that was injected and confining state of the reservoir, the system was gravity dominated but did not attain gravity stability at the end of the simulation. There was a growing increase of CO₂ fraction dissolved in water and oil and a decline in the gaseous mobile CO₂ phase between 2050 and 2150. In 2150, 4.8% had dissolved in water, 35.9% dissolved in oil, 3.6% was trapped residually and 55.5% was in the gaseous mobile phase. The open boundary state experienced no pressure buildup in the reservoir and the target injection rate of 0.5 Mt/year was achieved, and 10.4 Mt of CO₂ had been successfully injected into the reservoir. CO₂ plume migrated up-dip without getting to the reservoir flanks, it was a viscous-dominated system attended with gravity movement and segregation. With an increase in the density of formation fluids, the dissolution of CO₂ in brine and oil was active, active trapping mechanisms were structural trapping, dissolution in oil and water and residual trapping. There was a decline in the gaseous mobile CO₂ phase and an increase in CO₂ fraction dissolved in oil and water between 2050 and 2150.

With further residence time, fractions of CO₂ dissolved in the oil and brine phases would increase, as well as residually trapped fractions, with the CO₂ gaseous mobile phase experiencing a continuous decline. Therefore, this study showed that boundary condition was key to the success of the project, as it impacts injection rate and pressure.