Experimental study on CO2 plugging effect and instability conditions of hydrates in fractures in seafloor sediments
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摘要:
海底二氧化碳（CO2）地质封存技术已成为目前碳封存、碳中和研究的热点，我国南海北部海底沉积层中存在CO2水合物形成的有利空间和温压条件，在裂隙和孔隙中形成CO2水合物，能够阻挡CO2进一步向上运移而产生自封堵能力。然而，目前裂隙中CO2水合物封堵效果与失稳条件仍不明确。采用高压低温水合物生长失稳可视化实验平台，模拟2℃、3～4 MPa的海底沉积盖层条件，观测裂隙中CO2水合物的形成与注水增压失稳过程，利用突破压力、突破压差、持续时间、失稳起始阶段渗透率及封堵率为指标对水合物失稳条件与封堵效果进行了研究。研究结果表明，裂隙中水合物的形成经历成核、扩张、成型与聚集4个阶段；裂隙中形成的水合物能够高效地封堵水和CO2等流体的迁移，但在流体压力逐渐增加达到临界突破压力时开始出现失稳现象，水合物的失稳经历粒度退化与表面摩擦破坏2个阶段，水合物团块内核部先失稳，在水合物表面未破裂前可保持封堵状态；裂隙中水合物失稳的突破压力为6.414～6.966 MPa，突破压差为2.403～3.203 MPa，决定水合物突破压力的关键因素是温度和压力条件，而流速的效应则主要体现在调节水合物进入失稳状态的具体时间；在3～4 MPa的海底沉积盖层条件下，裂隙模型中水合物封堵率为99.0%～99.6%，失稳起始阶段渗透率为0.555×10−3～1.260×10−3 μm2。需要保证封盖CO2水合物层之下压力与海底实际压力之差小于2 MPa，以保持水合物的封堵效果。研究结果为南海类似条件下CO2海底地质封存上覆盖层风险性评价提供参考。
Abstract:Objective Seabed carbon dioxide (CO2) geological sequestration technology has gained popularity in the fields of carbon sequestration and carbon neutralization. In the northern South China Sea, favorable conditions for the formation of CO2 hydrates exist in seabed sediments, where hydrates forming in cracks and pores may block the further upward migration of CO2 and generate self-sealing capacities. However, the effects of CO2 leakage in fractures and the instability of hydrate plugging remain unclear.
Methods A visualization experimental platform for hydrate growth and the instability process under water injection supercharging at high pressure and low temperature was used to observe CO2 hydrate formation. The platform simulated the conditions of seafloor sedimentary cover at 2℃ and 3～4 MPa, and evaluated hydrate instability and plugging effects using parameters of breakthrough pressure, breakthrough pressure difference, duration, permeability coefficient at the initial instability stage, and plugging rate.
Results The experimental results show that hydrate formation proceeds through four stages: Nucleation, expansion, formation, and aggregation. Hydrates formed in cracks effectively block the migration of fluids such as water and CO2; however, instability begins when fluid pressure increases and reaches the critical breakthrough pressure. The instability process can be divided into two stages: Particle size degradation and surface friction failure. The hydrate mass core becomes unstable first, while the sealing state persists until the surface of the hydrate fails due to friction. The breakthrough pressure ranges from 6.414 to 6.966 MPa, and the breakthrough pressure difference is between 2.403 and 3.203 MPa. Hydrate instability is primarily influenced by flow rate, with the flow rate affecting the instability rate through interface effects. The breakthrough pressure of hydrates is mainly determined by temperature and pressure, while flow rate influences the exact time when hydrates enter instability. At 3～4 MPa seafloor sedimentary cover, the sealing rate is 99.0%～99.6%, and the permeability coefficient at the initial instability stage ranges from 0.555×10−3 to 1.260×10−3 μm2.
Conclusion The experimental results offer a reference for risk assessments of overlying layers in similar conditions in the South China Sea for CO2 seabed geological sequestration. To maintain the sealing effect of CO2 hydrates, the pressure difference between the capped hydrate layer and the actual seabed pressure should be less than 2 MPa.
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Key words:
- carbon dioxide(CO2) /
- seafloor sediment /
- hydrate /
- plugging effect /
- instability condition /
- fracture
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图 1 海底地层内CO2水合物封存物理模型[3]
Figure 1. Physical model of CO2 hydrate storage on the seafloor
图 3 实验装置示意图
CP. 平流泵；GC. 气瓶；RV. 减压阀；CV. 背压阀；PS. 压力传感器；TS. 温度传感器；PP. 加压泵；CB. 恒温水浴；PA. 摄像设备；RS. 反应釜；PC. 计算机
Figure 3. Schematic diagram of experimental device
图 6 4 MPa，2℃条件下水合物形成过程图
Figure 6. Diagram of hydrate formation process at 4 MPa and 2°C
图 8 不同流速注入过程中水合物层前后端压力变化曲线
Figure 8. Pressure curves at front and rear ends of hydrate layer during injection at different flow rates
表 1 失稳条件与封堵效果评价
Table 1. Instability conditions and evaluation of plugging effect
驱替速度/
(mL·min−1)初始饱
和度/%失稳起始阶段
渗透率/10−3 μm2封堵
率/%突破
压力/MPa突破
压差/MPa失稳
时间/s持续
时间/s0.005 85.3 0.771 99.4 6.703 2.403 6311 50633 0.010 80.1 0.555 99.6 6.966 3.131 6054 25999 0.020 70.3 0.828 99.3 6.414 3.203 1983 13149 0.030 75.2 0.938 99.2 6.593 3.004 1659 8759 0.050 78.9 1.260 99.0 6.497 2.572 1082 5165 
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