基于特低渗砂岩覆压试验渗透率方向性特征分析

由于地质成因的不同，岩石渗透率往往具有不同程度的大小和方向性差异，即呈各向异性的特点。目前，岩石渗透率方向性特征的理论和试验研究仍然不足，缺乏岩石渗透率各向异性量化评估。基于国内CCUS场地储层砂岩，沿正交方向钻取岩芯试件若干，完成瞬态法覆压渗透率测试。试验结果表明，特低渗砂岩正交方向渗透率随围压（上覆岩层压力）加载呈幂律函数递减，当围压加载至20 MPa以后，正交方向渗透率差异性逐步降低，即渗透率由各向异性逐步向各向同性过渡,且储层砂岩竖向渗透率压力敏感系数普遍低于水平侧向。通过建立一种空间坐标系下渗透率计算模型，将岩石内部渗流规律等效为空间正交方向渗透率模型，并借用统计学标准差定义提出渗透率各向异性系数?k，将不同岩性储层岩石渗透率不均匀性作归一化处理，定量地描述了覆压试验过程中试件渗透率各向异性的变化规律。

Analysis of direction-dependent characteristics of permeability of sandstone with ultra-lowly permeability in confining pressure tests

The permeability of rock is apparently dependent on the direction or anisotropic, which is resulted from the rock-forming geological processes. At present, theoretical and experimental studies on directional permeability properties of sandstone remain insufficient, and there is lack of quantitative assessment on the anisotropy of permeability. The transient tests are performed on sandstone from CCUS by orthogonally sampling cores. Experiential results indicate that there exists a power function relation between the permeability and the confining pressure. However, as the confining pressure is increased up to 20 MPa, the difference of permeability inorthogonal direction declines gradually, which shows a convert of permeability from anisotropy to isotropy. The pressure sensitivity of the permeability in the vertical coring samples is generally lower than that in the horizontal ones. Furthermore, a theoretical model is deduced in the Cartesian coordinate for the anisotropic permeability, which can be used to explain the vector properties of rock permeability. The parameter of anisotropy ?k is also proposed on the basis of the standard deviation and then is employed to normalize the heterogeneity of permeability in different reservoir sandstones. It shows that the proposed method is quantitative to describe variations of anisotropy in the seepage field system.