鄂尔多斯盆地延长组页岩裂缝特征与地应力研究

Study on the characteristics of shale fracture and in-situ stress in the Yanchang Formation of the Ordos Basin

鄂尔多斯盆地延长组是我国近些年陆相页岩气的重点勘探区之一。本文从页岩的露头观测、岩石学特征、成像测井分析、地应力特征及可压裂性等方面，研究了鄂尔多斯盆地延长组泥页岩层段的裂缝发育特征，以及地应力对人工压裂诱导缝的影响。基于岩石学分析，延长组长7和长9段的泥页岩脆性矿物平均含量较高，分别为57．2％和44％。根据泥页岩露头观测和成像测井分析，长7段泥页岩发育有北东东向为主的垂直裂缝组，同时可见北北东向和南北向的垂直裂缝组。此外，压裂诱导缝的优势方位和成像测井分析结果表明，现今最大水平主应力方向为70°左右（即北东东向）。裂缝开启性分析表明，北东东向至近东西向裂缝组与现今最大水平地应力方向一致，开启性较好；而北北东裂缝组与现今最大水平地应力交角相对较大，开启性相对较差。因为现今地应力的水平应力比值（σH/σh=1．54）相对较小，在后期人工压裂中延长组泥页岩中的诱导裂缝更倾向形成复杂的裂缝网格系统，这样会使裂缝与储层之间获得最大的接触，从而更有利于提高产能。

In recent years, Yanchang Formation of Ordos Basin is one of the of continental shale gas in China. Through the shale outcrop observation, logging analysis, in-situ stress characteristics and fracturing, key exploration areas petrology, imaging etc., we study the characteristics of shale fracture of Yanchang Formation in the Ordos Basin, and the influence of in-situ stress on the hydraulic fracture. Based on the analysis of petrology, the average content of shale brittle mineral of Chang-7 and Chang-9 Members in Yanchang Formation are higher, 57.2% and 44%, respectively. According to the shale outcrop observation and imaging logging analysis, there is a main set of vertical fractures, which strike is the NEE. Meanwhile, there are NNE-trending and N-S-trending vertical fractures. In addition, the predominant orientation of induced fractures and imaging logging analysis show that the orientation of maximum horizontal principal stress is about 70 degrees （i.e. NEE）. Crackopening analysis shows that the NEE-trending to E-W-trending fractures are consistent with the orientation of present maximum horizontal principal stress, and the crack opening condition is better. However, the angle between NNE-trending fracture and present maximum horizontal stress is large, so the condition of crack opening is poor. Because that the horizontal stress ratio （σH/σh = 1.54） of present in-situ stress is low, the induced fractures of shale in Yanchang Formation is more tend to form complex fractures networks during the artificial fracturing stage, this can enable induced fractures and reservoirs to get the maximum contact, and thus more conducive to increase production capacity.