Simulation of carbon dioxide mineralization and its effect on fault leakage rates in the South Georgia rift basin, southeastern U.S
在本研究中，我们评估了CO2与地下材料之间的常见化学反应在原地以及CO2羽流分布与造成矿化的封隔区内的CO2泄漏之间的关系。使用地下地震数据和井记录信息，创建了一个由储层和封隔区组成的三维模型，并对美国东南部的南乔治亚裂谷(SGR)盆地进行了评估。使用CMG 2017对CO2矿化对断层渗透率最优值的影响进行了模拟，这是由于地层水和CO2之间的流体替代。
该模型模拟了二氧化碳与镁铁矿物质之间的化学反应，产生稳定的碳酸盐岩矿物形成在断层中。初步结果表明，CO2迁移在0.1-1 mD的断层渗透率范围内可以有效控制。在这个范围内，矿化有效地减少了封隔区内的CO2泄漏。
Abstract
Over the past few decades, measured levels of atmospheric carbon dioxide have substantially increased. One of the ways to limit the adverse impacts of increased carbon dioxide concentrations is to capture and store it inside Earth’s subsurface, a process known as CO2 sequestration. The success of this method is critically dependent on the ability to confine injected CO2 for up to thousands of years. Establishing effective maintenance of sealing systems of reservoirs is of importance to prevent CO2 leakage. In addition, understanding the nature and rate of potential CO2 leakage related to this injection process is essential to evaluating seal effectiveness and ultimately mitigating global warming.
In this study, we evaluated the impact of common chemical reactions between CO2 and subsurface materials in situ as well as the relationship between CO2 plume distribution and the CO2 leakage within the seal zone that cause mineralization. Using subsurface seismic data and well log information, a three-dimensional model consisting of a reservoir and seal zones was created and evaluated for the South Georgia Rift (SGR) basin in the southeastern U.S. The Computer Modeling Group (CMG, 2017), was used to model the effect of CO2 mineralization on the optimal values of fault permeability permeabilitydue to fluid substitution between the formation water and CO2. The model simulated the chemical reactions between carbon dioxide and mafic minerals to produce stable minerals of carbonate rock that form in the fault. Preliminary results show that CO2 migration can be controlled effectively for fault permeability values between 0.1-1 mD. Within this range, mineralization effectively reduced CO2 leakage within the seal zone.
Keywords:
Carbon dioxide sequestration; Fault; Leakage; Mineralization; Permeability; Plume migration; Porosity.
结论：
了解注入二氧化碳与化学反应相关的迁移和反应是确定矿化对SGR盆地泄漏影响的关键。断层渗透性的变化极大影响了碳酸与周围岩石的相互作用以及形成的多相二氧化碳在地层中的分布。
考虑到相对短暂的20年内注入的每年1000万立方米CO2，关键范围的断层渗透性对于减少羽流的垂直迁移和最大化与矿化相关的化学反应时间有效。
在断层渗透性值为100和1000 mD时，高断层渗透性值支持CO2向模型顶部的迁移，这段时间不足以允许矿化发生，特别是在100和1000 mD的断层渗透性值情况下。较高断层渗透性值的低阻力也允许溶解态二氧化碳在模拟开始时穿过密封带进入。10 mD的渗透性是允许成功存储的最大值。因为在渗透率更高的情况下，CO2羽流的迁移可能会穿过密封层，一些CO2羽流会逃到大气中。
关键断层渗透性值（0.1-1 mD）迫使溶解态CO2保持在储层区（D-E）内，并允许羽流充足的时间在断层带内形成碳酸盐矿物。因此，落在关键范围内的渗透性值对应于密封层内有效的矿化。溶解态CO2通常位于注入区，而矿化则通常发生在密封层。密封层的阻力使CO2尽可能地被包裹，并增强了溶解溶解态CO2的能力。
方解石在注入期间溶解达到平衡点，其溶解率等于沉积速率。在达到平衡之前，羽流朝向密封层迁移，运载着来自注入区的方解石和高岭土矿物到达正常分布的矿物线。在此过程中，高岭土开始在注入区矿化，直到模拟结束，在所有区域内都持续发生。此过程中成功矿化的高岭土数量取决于地层的断层渗透性。
该研究表明，在没有化学反应发生的情况下，低断层渗透性允许二氧化碳的迁移，并且这种迁移可以通过矿化被控制。这可以通过观察到，当在断层带中生成新的矿物质（如方解石和高岭土）时，低断层渗透性值相关的泄漏显著减少。因此，CO2羽流通过断层的机会被减少，从而为在带有断层的储层中隔离注入的CO2提供了机会。
鉴于目前认为CO2矿化是安全存储二氧化碳的最安全方法，该研究提供了更多有关该存储机制在储层内的上下文资料，同时还建议如何通过确保地层中的密封带来进一步稳定存储。本研究中描述的方法可用于各种复杂的地质环境，特别是那些受地壳运动影响的需要二氧化碳存储的地质环境。
8. Conclusions
Understanding the migration and chemical reactions associated with injected CO2 was key to determining the effect of mineralization on leakage in the SGR basin. Variations in fault permeability greatly impacted the interaction of carbonic acid with the surrounding rock and the resulting multi-phase CO2 distribution in the formation. When considering the relatively small amount of 10 million m3/year CO2 injected over the short period of 20 years, the critical range of fault permeability was effective for both reducing the vertical migration of the plume and maximizing the time for chemical reactions associated with mineralization. High fault permeability values supported the migration of CO2 to the top of the model in a time period that was not long enough to allow for mineralization, especially at fault permeability values of 100 and 1000 mD. The low resistance of higher fault permeability values also allowed aqueous phase CO2 to penetrate seal zones at the beginning of the simulation.
A permeability of 10 mD was the maximum value that would allow for a successful storage system. This is because in a scenario with a greater permeability, the migration of the CO2 plume could penetrate the seal and some of the CO2 plume would escape into the atmosphere. The critical values of fault permeability (0.1–1 mD) forced CO2 in the aqueous phase to remain within the reservoir zone (D-E) and allow ample time for the plume to form the carbonate minerals within the fault zone. Due to this, permeability values that fell within the critical range corresponded to effective mineralization occurring within the seal. CO2 in the aqueous phase was located generally in the injection zone, while mineralization typically occurred in the seal zone. The resistance of the seal zones contained CO2 as long as possible and enhanced the dissolution of aqueous CO2.
Calcite dissolved at the beginning of the injection period then arrived at the equilibrium point where its dissolution rate was equal to the rate of deposition. Before equilibrium was reached, the plume migrated towards the seal, carrying calcite and kaolinite from the injection zone to the fault zone. During this process, kaolinite began to mineralize in the injection zone at the time of injection and continued to do so until the end of the simulation throughout all of the zones. The amount of kaolinite that mineralized during this process was dependent on fault permeability in the formation.
This study suggests that low fault permeabilities allowed for CO2 migration in cases where no chemical reactions occurred, and that this migration could be contained by mineralization. This was evidenced by the observation that leakage associated with low fault permeability values decreased significantly when new minerals such as calcite and kaolinite were generated in the fault zone. Consequently, the opportunity for the CO2 plume to migrate through the fault was reduced which provided an opportunity to sequester injected CO2 in a reservoir with faults.
Given that CO2 mineralization is currently considered the safest way to store CO2, this research provides more context for this safe storage mechanism within the reservoir but also suggests how to further stabilize storage through securing seal zones within a formation. The methods described in this study could be used for a variety of complex geological settings, particularly those impacted by tectonic activity, for which CO2 storage is desired.
