裂缝性低渗透油藏各向异性的尺度效应

在对低渗透储集层中发育的裂缝进行分组的基础上，以分形几何为工具描述单组裂缝分布，建立裂缝长度的幂律关系式。根据裂缝渗透率的定义，推导得到相互正交方向上的裂缝渗透率与考察尺度的关系表达式，再加上基质渗透率的影响，获得了储集层综合渗透率各向异性强度随考察尺度变化的关系式。研究表明，裂缝性低渗透储集层中各向异性强度随考察尺度的不同而变化，同时说明，通常所认为的各向异性强度为常值这一观点仅是这一表达式的某种特例。以发育2组裂缝的储集层为例，详细讨论了各向异性强度随考察尺度变化的主要影响因素，结果表明，分形维数、夹角及长度初值比对各向异性强度的变化影响显著，基质裂缝渗透率比和基质各向异性强度对其的影响仅表现在某一尺度范围内。图5表1参21

Scale effects of anisotropy in fractured low permeability reservoirs

Based on fracture strike grouping and the fractal geometry which was utilized to describe a single group of fractures, a power law expression of fracture length was established. According to the definition of fracture permeability, an analytical expression of fracture permeability in the orthogonal orientation was obtained. Then, a scale transformation formula of anisotropy was derived with the matrix influences combined. The results show, in fractured low permeability reservoirs, anisotropy varies with scale transformation; accordingly only under very few special conditions does the conclusion of anisotropy being a constant come into existence. With two groups of fractures as examples, the factors which affect anisotropy intensity are discussed in detail: fractal dimensionality, strike angle and the ratio of initial length are primary factors; the permeability ratio of matrix to fracture and the matrix anisotropy intensity are the secondary factors.