Abstract Safe subsurface sequestration of carbon dioxide (CO2) is becoming increasingly important to meet climate goals and curb atmospheric CO2 concentrations. The world-wide CO2 storage capacity in carbonate formations is significant; within deep, saline aquifers and through several CO2-enhanced oil recovery projects, with associated CO2 storage. Carbonates are complex, both in terms of heterogeneity and reactivity, and improved core scale and sub core-scale analysis of CO2 flow phenomena is necessary input to simulators, aiming to establish large-scale behavior. This paper presents a recent advancement in in-situ imaging of CO2 flow, utilizing high-resolution micro-Positron Emission Tomography and radioactive tracer [11C]arbon dioxide to explicitly track CO2 during dynamic flow and subsequent trapping at the core scale. Unsteady state water injection (imbibition) and CO2 injection (drainage) were performed in a low-permeable chalk core at elevated pressure conditions. Short-lived radioisotopes were used to label water and CO2, respectively, and facilitated explicit tracking of each phase separately during single phase injection. Local flow patterns and dynamic spatial fluid saturations were determined from in-situ imaging during each experimental step. Initial miscible displacement revealed displacement heterogeneities in the chalk core, and dynamic image data was used to disclose and quantify local permeability variations. Radial permeability variations influenced subsequent flow patterns, where CO2 predominantly flooded the higher-permeability outer part of the core, leaving a higher water saturation in the inner core volume. Injection of water after CO2 flooding is proposed to be the most rapid and effective way to ensure safe storage, by promoting capillary trapping of CO2. PET imaging showed that presence of CO2 reduced the flow of water in higher-permeability areas, improving sweep efficiency and promoting a nearly ideal core-scale displacement. Alternate injections of water and gas is also expected to improve sweep efficiency and contribute to improved oil recovery and CO2 storage on larger scales. Sub-core analysis showed that residually trapped CO2 was evenly distributed in the chalk core, occupying 40% of the pore volume after ended water injection. Micro-Positron Emission Tomography yielded excellent small-scale resolution of both water and CO2 flow, and may contribute to unlocking fluid flow dynamics and determining mechanisms on the millimeter scale; presenting a unique opportunity in experimental core-scale evaluations of CO2 storage and security.

中文翻译：

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使用微正电子发射断层扫描 (μPET) 在核心尺度上显式跟踪 CO2 流

摘要 二氧化碳 (CO2) 的安全地下封存对于实现气候目标和抑制大气 CO2 浓度变得越来越重要。碳酸盐岩地层中的全球 CO2 储存能力非常重要；在深层咸水含水层中，并通过几个二氧化碳强化石油回收项目，以及相关的二氧化碳储存。碳酸盐在异质性和反应性方面都很复杂，改进的核心尺度和亚核心尺度的 CO2 流动现象分析是模拟器的必要输入，旨在建立大规模行为。本文介绍了 CO2 流原位成像的最新进展，利用高分辨率微正电子发射断层扫描和放射性示踪剂 [11C] 二氧化碳在动态流和随后的核心尺度捕获期间明确跟踪 CO2。在高压条件下在低渗透性白垩岩芯中进行非稳态注水（渗吸）和 CO2 注入（排水）。短寿命放射性同位素分别用于标记水和 CO2，并有助于在单相注入过程中分别显式跟踪每个相。在每个实验步骤中，局部流动模式和动态空间流体饱和度由原位成像确定。初始混相位移揭示了白垩岩芯中的位移非均质性，动态图像数据用于揭示和量化局部渗透率变化。径向渗透率的变化影响了随后的流动模式，其中 CO2 主要淹没了渗透率较高的岩心外部，在岩心内部留下了更高的水饱和度。通过促进 CO2 的毛细管捕集，在 CO2 驱替后注入水被认为是确保安全储存的最快速和有效的方法。PET 成像显示 CO2 的存在减少了高渗透率区域的水流，提高了波及效率并促进了近乎理想的岩心位移。水和气的交替注入也有望提高波及效率，并有助于更大规模地提高石油采收率和二氧化碳封存。子岩心分析表明，残余捕集的 CO2 均匀分布在白垩岩心，占注水结束后孔隙体积的 40%。微正电子发射断层扫描对水流和二氧化碳流都产生了出色的小尺度分辨率，可能有助于在毫米尺度上解锁流体流动动力学和确定机制；