Experimental on CO2 plugging effect and instability condition of hydrate in fracture of 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；实验探究裂隙中水合物失稳条件，其突破压力为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，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 become a hot spot in carbon sequestration and carbon neutralization. There are favorable space and temperature and pressure conditions for the formation of CO2 hydrate in the seabed sediments in the northern part of the South China Sea, and the formation of CO2 hydrate in the cracks and pores can block the further upward migration of CO2 and generate self-sealing capacity. However, the CO2 leakage in the fracture and the effect of hydrate plugging and instability conditions are still unclear.
Methods In this paper, the visualization experiment platform of hydrate growth and the instability process of water injection supercharging at high pressure and low temperature was used to observe the formation of CO2 hydrate and experiment platform was used to simulate the conditions of seafloor sedimentary cover under the conditions of 2℃ and 3～4 MPa, and the hydrate instability condition and plugging effect were evaluated with the breakthrough pressure, breakthrough pressure difference, duration, permeability coefficient of initial instability stage and plugging rate as indicators.
Results The experimental results show that hydrate formation can be simplified into four processes: nucleation, expansion, forming and aggregation. Hydrate formed in cracks can efficiently block the migration of fluids such as water and CO2, but the instability phenomenon begins when the fluid pressure gradually increases and reaches the critical breakthrough pressure. The instability process of hydrate can be simplified into two parts: particle size degradation and surface friction failure. The core of hydrate mass is unstable first, and the sealing state can be maintained before the surface of hydrate fails to friction fracture. The instability conditions of hydrate in the fracture are investigated experimentally. The breakthrough pressure is 6.414～6.966 MPa and the breakthrough pressure difference is 2.403～3.203 MPa. The instability rate of hydrate is mainly affected by the flow rate, followed by the saturation of hydrate, and the flow rate affects the instability rate through the interface effect. The key factors determining the breakthrough pressure of hydrate are temperature and pressure conditions, while the effect of flow rate is mainly reflected in regulating the specific time when hydrate enters the instability state. Under the condition of 3～4 MPa seafloor sedimentary cover, the sealing rate is 99.0～99.6% and the permeability coefficient of initial instability stage is 0.555～1.260 md.
Conclusion The experimental results provide a reference for the risk assessment of the overlying layer under similar conditions in the South China Sea for CO2 seabed geological sequestration. The difference between the pressure under the capped CO2 hydrate layer and the actual pressure on the seabed should be ensured to be less than 2 MPa to maintain the sealing effect of the hydrate.
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Key words:
- carbon dioxide(CO2) /
- sub-sea sediments /
- hydrate /
- plugged effect
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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 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 curve at front and rear end of hydrate layer during injection with different flow rates
表 1 失稳条件与封堵效果评价
Table 1. Instability condition and evaluation of plugged effect
驱替速度/
(mL·min)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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