一种定量评价碎屑岩储层各向异性的新方法

A novel method for quantitative anisotropy interpretation in clastic sediments

储层各向异性分析是低渗透岩性油气藏评价与开发方案优化的一项重要研究内容.引起储层各向异性的因素很多,既包括沉积原因引起的不同方向上储层粒度与层理上的差异,也有构造应力差异导致的各向异性.但无论如何,这些因素都会导致储层电阻率在不同方向上的差异.传统上,交叉偶极声波测井是连续获取井周附近各向异性定量信息的唯一来源,而微电阻率扫描成像测井能够提供井周附近不同方位的地层微电阻率变化信息,是刻画储层各向异性的重要手段.本文提出了一种基于微电阻率扫描成像测井定量计算碎屑岩储层各向异性指数的新方法.该方法通过对微扫成像测井记录的数据矩阵滤波、开窗,然后根据成像测井环井周扫描的特点对行和列分别采用调和平均和极值比较等算法得到反映窗长内不同方位上电阻率的极大值与极小值,利用二者的比值作为反映各向异性强弱的系数.通过对不同盆地的多个实例的对比分析,这种方法在块状弱各向异性地层以及裂缝、强挤压应力和煤层等不同因素导致的各向异性评价中,其计算结果与交叉偶极声波、声波扫描测井完全一致,表明该方法能够用于碎屑岩储层各向异性的定量评价.但应该指出的是,这种方法也存在一些不足之处,特别是与阵列声波测井相比,微扫成像测井探测深度较浅,且受井眼状况影响较为显著.

Anisotropy interpretation is an essential step for lithologic reservoir evaluation and development plan optimization,and also an extended hot spots of geophysics.There are many factors contributing to the formation anisotropy,including azimuthal grain size or texture difference caused by depositional setting,and stress diversity by tectonic actions.In any case,the resistivity of detrital reservoir is affected and change with azimuth.Traditionally,cross dipole sonic logging is the only source for consecutive near-well-bore anisotropy analysis.Meanwhile,abundant reservoir information about the micro-resistivity is provided by scanner imager logging with higher resolution.A new method for determining anisotropy index from micro resistivity scanner imager is suggested.The scanner image log is treated as array datums.As a data matrix,the imager log is filtered over sliding window firstly,and then harmonic average and extremum comparing are applied on row and column respectively.By this,the resistivity maximum and minimum is determined,which represents electrical contrast.Then anisotropy index is defined as their ratio for electrical inhomogeneity characterization.Several cases study from different basins are presented to verify it＇s applicability in these formations with different extent of anisotropy caused by fractures,strong squeeze stress or coal bed.The response regularity of anisotropy index from scanner image is concordant with that from dipole sonic logs over these cases,including time-based and energy-based anisotropy.It shows that this method is applicable for quantitative anisotropy analysis in clastic reservoirs.Finally,it＇s indicates that this new method still leaves some to be desired,particularly the investigation depth and borehole-sensitivity of imager logging.