摘要
天然气水合物是一种重要的替代能源,被国务院列为新矿种,但是现阶段的开采技术仍然难以达到商业开采标准.渗透率是判断水合物矿体是否具有开发潜力的重要指标,是水合物开采流程优化等工作的基础参数.然而,现有的渗透率理论模型在定量描述沉积物有效孔隙结构演化过程时仍有不足.因此,本文从定量描述沉积物有效孔隙结构演化过程出发,采用分形分析的方法,提出了一个含水合物沉积物渗透率理论模型;将模型预测结果与前人实验数据进行对比,以验证模型的适用性;最后分析了模型参数对渗透率演化过程的影响关系.结果表明,提出的渗透率分形模型较好地重现了水合物含量及其赋存形式对含水合物沉积物渗透率的影响过程;水合物赋存形式以及由此决定的最大孔径演化关系是影响沉积物渗透率演化过程的关键因素;本模型具有良好的工程应用潜力,为今后含水合物沉积物渗透率研究提供了新的思路.
Natural gas hydrates, ice-like crystals composed of water and natural gas, are widely distributed in marine sediments along the continental margin and permafrost regions. Natural gas hydrates are of great significance as a future energy resource, and have been officially authorized as a new kind of mineral in China. For now, however, the gas production rate of all the existing technologies is far below the commercial criterion. The permeability of hydrate-bearing sediments is a critical parameter that determines the economic feasibility of gas production from hydrate deposits, and it is one of the basic parameters needed for varieties of numerical simulators. However, most of the existing theoretical models for the permeability prediction are lack of quantitative descriptions of the pore space for fluids flow. In this work, a fractal theory based theoretical model is proposed to predict the permeability of hydrate-bearing sediments. In the proposed model, the pore space for fluids flow is equivalent to a bundle of capillary tubes with their diameters obeying the fractal scaling law. A hydrate saturation dependent area fractal dimension is applied to describe how the capillary tube diameters evolve during hydrate dissociation. The theoretically predicted curves are compared with published experimental data to verify its feasibility, and sensitivity analyses on model parameters are performed. The results suggest that the proposed theoretical model can describe the hydrate saturation and pore-scale behavior dependent permeability of hydrate-bearing sediments. The maximum diameter of the pore for fluids flow which is affected by the pore-scale behavior of gas hydrate is the key influence parameter determining the permeability of hydrate-bearing sediments. The proposed model is of great potential for engineering applications.
引文
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