A continuum-scale representation of Ostwald ripening in heterogeneous porous media
A continuum-scale representation of Ostwald ripening in heterogeneous porous media
复制标题
非均质多孔介质中奥斯特瓦尔德熟化的连续尺度表示
DOI：
10.1017/jfm.2020.53
复制
发表时间：
2020
影响因子：
3.7
通讯作者：
S. Benson
中科院分区：
文献类型：
--
作者：
Yaxin Li;C. Garing;S. Benson
Ostwald熟化是一种使分散相变粗直到热力学平衡的孔隙尺度现象。基于我们以前的发现，在复杂的多孔介质中，多泡平衡是可能的，我们发展了一个新的连续尺度的非均匀多孔介质中的Ostwald成熟模型。在该模型中，具有两条不同毛细压力曲线的多孔介质接触，只允许通过水相的扩散流重新分配被捕获的气相。结果表明，即使当气相被毛细管力捕获到孔隙空间时，Ostwald熟化也可以提高一种介质中的含气饱和度，而降低另一种介质中的含气饱和度。我们发展了一个类似的延迟因子，表明Ostwald熟化的特征时间大约是单相扩散问题的10^{5}倍，这是因为在建立平衡之前，分离相气体需要更大的传质量。给出了预测两种介质间饱和度再分布的近似解。通过对多种物理参数的数值模拟，验证了该模型的有效性。毫米到厘米尺度的系统在几年内达到平衡，对于米尺度的系统来说，时间跨度最长可达10000年，甚至更长。这些发现对于地质上的$\Text{CO}_{2}$存储尤其相关，其中残留圈闭是固定$\Text{CO}_{2}$的重要机制。我们的工作表明，由于多孔介质中的非均质性导致的Ostwald成熟过程与其他重新分布捕获的过程(如对流混合)相比，在相似的时间尺度上是缓慢的。
Ostwald ripening is a pore-scale phenomenon that coarsens a dispersed phase until thermodynamic equilibrium. Based on our previous finding that multi-bubble equilibrium is possible and likely in complex porous media, we develop a new continuum-scale model for Ostwald ripening in heterogeneous porous media. In this model, porous media with two different capillary pressure curves are put into contact, allowing only diffusive flow through the aqueous phase to redistribute a trapped gas phase. Results show that Ostwald ripening can increase the gas saturation in one medium while decreasing the gas saturation in the other, even when the gas phase is trapped in pore spaces by capillary forces. We develop an analogous retardation factor to show that the characteristic time for Ostwald ripening is about $10^{5}$ times slower than a single-phase diffusion problem due to the fact that separate-phase gas requires a much larger amount of mass transfer before equilibrium is established. An approximate solution has been developed to predict the saturation redistribution between the two media. The model has been validated by numerical simulation over a wide range of physical parameters. Millimetre to centimetre-scale systems come to equilibrium in years, ranging up to 10 000 years and longer for metre-scale systems. These findings are particularly relevant for geological $\text{CO}_{2}$ storage, where residual trapping is an important mechanism for immobilizing $\text{CO}_{2}$. Our work demonstrates that Ostwald ripening due to heterogeneity in porous media is slow and on a similar time scale compared to other processes that redistribute trapped $\text{CO}_{2}$ such as convective mixing.