Connectivity of Pore Space as a Control on Two-Phase Flow Properties of Tight-Gas Sandstones
Connectivity of Pore Space as a Control on Two-Phase Flow Properties of Tight-Gas Sandstones
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DOI：
10.1007/s11242-012-0017-x
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发表时间：
2012-05
影响因子：
2.7
通讯作者：
M. Mousavi;S. Bryant
中科院分区：
文献类型：
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作者：
M. Mousavi;S. Bryant
我们预测毛细管压力（排水）曲线在致密气砂岩，几乎没有基质或微孔隙度使用定量粒度模型。该模型考虑了一些沉积和成岩过程的几何结果，这些过程对致密气砂岩的孔隙度和渗透率降低很重要，例如埋藏和石英胶结过程中韧性颗粒的变形。该模型将原始沉积物表示为致密、无序的球体堆积。我们模拟了该模型沉积物的演变成低孔隙度砂岩，通过应用不同数量的韧性颗粒和石英沉淀。模拟成岩蚀变封闭了沉积物中相当一部分原始孔喉。由于逾渗阈值对应于一半孔喉的闭合，因此这种类型的致密气砂岩中的孔隙空间连通性差，通常接近于完全断开。不同的模型岩石的排水曲线计算使用侵入渗流直接从颗粒尺度的几何形状和拓扑结构的模型中的网络。一些总的趋势遵循经典的预期，并通过实验测量得到证实：增加水泥量使排水曲线向更大的压力移动。这与喉道闭合导致孔隙空间连通性降低有关。韧性颗粒模型中韧性颗粒的存在也降低了孔隙空间的连通性，但它对待喉道分布不同，导致排水曲线被转移到较大的束缚水饱和度时，水泥加入到模型中。这些连通性效应很重要的孔隙度范围对应于致密气砂岩常见的孔隙度范围。因此，即使在大的气体饱和度下，这些岩石对气体的有效渗透率也很小。这个问题发生在岩石中具有大量韧性颗粒的较大孔隙中，因为韧性变形甚至在胶结作用开始之前就阻塞了相当一部分孔喉。预测排水曲线同意测量两个样品的微孔很少，一个主要由刚性颗粒，其他包含一个显着的部分韧性颗粒。我们的结论是，基质孔隙空间的连通性是一个重要的因素，为了解流动性质的致密气砂岩。
We predict capillary-pressure (drainage) curves in tight-gas sandstones which have little matrix or microporosity using a quantitative grain-scale model. The model accounts for the geometric results of some depositional and diagenetic processes important for porosity and permeability reduction in tight-gas sandstones, such as deformation of ductile grains during burial and quartz cementation. The model represents the original sediment as a dense, disordered packing of spheres. We simulated the evolution of this model sediment into a low-porosity sandstone by applying different amounts of ductile grains and quartz precipitation. A substantial fraction of original pore throats in the sediment is closed by the simulated diagenetic alteration. Because the percolation threshold corresponds to closure of half of the pore throats, the pore space in this type of tight-gas sandstone is poorly connected and is often close to being completely disconnected. The drainage curve for different model rocks was computed using invasion percolation in a network taken directly from the grain-scale geometry and topology of the model. Some general trends follow classical expectations and were confirmed by experimental measurements: increasing the amount of cement shifts the drainage curve to larger pressures. This is related to reduction of the connectivity of pore space resulting from closure of throats. Existence of ductile grains in the ductile grain model also reduces the connectivity of pore space but it treats the throats distribution differently causing the drainage curves to be shifted to larger irreducible water saturation when cement is added to the model. The range of porosities in which these connectivity effects are important corresponds to the range of porosities common for tight gas sandstones. Consequently these rocks can exhibit small effective permeability to gas even at large gas saturations. This problem occurs at larger porosities in rocks with significant content of ductile grains because ductile deformation blocks a significant fraction of pore throats even before cementation begins. Predicted drainage curves agree with measurements on two samples with little microporosity, one dominated by rigid grains, the other containing a significant fraction of ductile grains. We conclude that connectivity of the matrix pore space is an important factor for an understanding of flow properties of tight-gas sandstones.