Microanalysis of natural and engineered CO2 mineralisation at CarbFix2 site, Iceland

二氧化碳捕集与封存（CCS）是唯一能够直接减少化石燃料燃烧或工业过程排放产生的二氧化碳排放的工业规模技术。鉴于全球对化石燃料的能源依赖和工业制造需求，CCS是全球实现二氧化碳净零排放的关键技术。地质封存CO2的成功与否，关键取决于封存地点的长期安全。沉积储层中浮力气相CO2的储存需要长时间的监测。另外，可以通过将注入的二氧化碳永久转化为储存地点内的新碳酸盐矿物来保证安全储存。最近进行了几项将二氧化碳注入活性玄武岩的封存测试，其中最引人注目的是位于冰岛Hellisheidi地热田的CarbFix1项目。对注入二氧化碳的溶解无机碳和示踪剂的测量表明，在两年内，注入的二氧化碳（175吨）中有95%实现了矿化1。然而，该方法在工业规模上的可行性仍然不确定，因为未知的关键问题是可行的注入能力可以维持多长时间。为了解决这个问题，CarbFix2实验于2014年启动，目前每年向地热场注入10,000吨二氧化碳和5,000吨H2S。关键研究问题：-根据雷克雅未克能源公司在钻井和示踪剂测试期间的观察结果，确定在注入之前，该油田发生了天然二氧化碳矿化的地方。-利用世界一流的微束分析和地球化学模型，解决单个晶体生长过程中自然二氧化碳沉淀的条件。在工程注入CO2和h2s之前，建立基线地球化学指纹——从井下样品中识别CarbFix1碳酸盐的同位素特征。对天然类似物的表面暴露进行实地考察。意大利利古里亚（Liguria）侏罗纪蛇绿岩杂岩中含有碳酸盐矿化的海底蛇绿石，这为矿化路径和地球化学指纹随时间的保存提供了空间信息和限制。实验再现井下切割样品的矿化过程，并用同步辐射x射线显微层析成像仪成像。从这些实验中，矿物的溶解和再沉淀将被绘制和量化。-4D微层析成像实验，改变温度和盐度，将评估是否可以通过有目的的注入策略来增强CO2矿化，从而改变盐水盐度。方法和时间表：该项目将利用地球科学学院的世界级离子微探针（SIMS）仪器（Cameca 1270和新的Cameca IMS 7f-geo）。您将从钻孔岩屑和矿化玄武岩岩芯样品中对单个晶体进行微观分析。每个晶体内部的微米级生长带，就像树上的年轮一样，记录了晶体生长过程中孔隙水的同位素和微量元素特征。重建天然碳酸盐的生长期、温度、CO2和水的来源。这些分析将通过在新型x射线透明反应池中的矿化实验进行补充，该反应池允许使用时间分辨同步加速器x射线微断层扫描监测流体-岩石相互作用。这些方法将使用井下岩屑样品直接绘制和量化储层条件下矿物的溶解和再沉淀。所产生的知识将用于确定如何通过不同的注入策略来增强二氧化碳矿化，并告知如何将这些控制措施准确地纳入该油藏的二氧化碳矿化预测模型。

CO2 capture and storage (CCS) is the only industrial scale technology that can directly reduce the CO2 emissions produced by the combustion of fossil fuels or industrial process emissions. Given global reliance on fossil fuels for energy and industrial manufacturing needs, CCS is an essential technology for the global drive to reach net zero CO2 emissions to the atmosphere4. The success of geologic CO2 storage critically depends on the long-term security of the storage site. Buoyant gas phase CO2 stored in sedimentary reservoirs requires long duration monitoring of storage. Alternatively, secure storage can be guaranteed through the permanent conversion of injected CO2 into new carbonate minerals within the storage site.Several tests of the sequestration that occurs when CO2 is injected into reactive basalts have recently been undertaken, notably the CarbFix1 project, located at the Hellisheidi geothermal field in Iceland[1]. Measurements of dissolved inorganic carbon and tracers injected with the CO2 indicate that mineralisation of >95 % of the injected CO2 (175 tonnes) was achieved within two years1. However, feasibility of the method at an industrial scale is still uncertain, as the key question of how long viable injectivity can be maintained is unknown. To address this question, the CarbFix2 experiment was launched in 2014, and currently 10,000 tonnes of CO2 and 5,000 tonnes of H2S are being injected per year into the geothermal field[2]Key research questions:-Identify where natural CO2 mineralisation has occurred in the field prior to injection based on observations made during drilling and tracer tests by Reykjavik Energy.-Resolve the conditions under which natural CO2 precipitation took place during growth of individual crystals using world-class microbeam analysis and geochemical modelling.-Establish baseline geochemical fingerprints prior to engineered injection of CO2 and H2S-Identify isotopic signatures of CarbFix1 carbonates from downhole samples.-Undertake field visit to surface exposures of natural analogues. Well-studied Jurassic ophiolite complexes in Liguria (Italy) host ophicalcites of carbonate mineralised ocean floor, which provide spatial information and constraints on the preservation of mineralisation pathways and geochemical fingerprints over time.-Reproduce the mineralization process on downhole cutting samples experimentally and image it with synchrotron-based X-ray microtomograph. From these experiments, dissolution and re-precipitation of minerals will be mapped and quantified.-4D microtomography experiments, varying temperature and salinity, will assess if CO2 mineralisation can be enhanced through purposeful injection strategies into hotter or colder parts of the reservoir with changed brine salinity.Methodology and timetable:This project will utilise the world-class ion microprobe (SIMS) instruments within the School of GeoSciences (Cameca 1270 and new Cameca IMS 7f-geo). You will make micro-analytical traverses across individual crystals from borehole cuttings and from mineralised basalt core samples. The micrometre growth zones within each crystal, like the rings on a tree, record the isotope and trace element signatures of porewater during the crystal growth. The growth duration, temperature, CO2 and water origins of natural carbonates will be reconstructed.These analyses will be complemented by mineralisation experiments in a novel x-ray transparent reaction cell that allows monitoring fluid-rock interaction with time-resolved synchrotron x-ray microtomography. These will use downhole cuttings samples to directly map and quantify dissolution and re-precipitation of minerals at reservoir conditions[3]. The knowledge generated will be used to ascertain how CO2 mineralisation can be enhanced through varied injection strategies and inform how to accurately incorporate such controls into predictive models of CO2 mineralisation in this reservoir.