深层页岩储层天然裂缝连通性表征及力学有效性分析——以川东南盆缘丁山—东溪地区五峰组—龙马溪组为例

Analysis of connectivity characterization and mechanical effectiveness of natural fracture in deep shale reservoirs:A case study of the Wufeng-Longmaxi formations in the DingshanDongxi area,southeastern margin of Sichuan Basin

表征页岩储层天然裂缝连通性以及分析原位地应力控制下的力学有效性对于页岩气勘探及增产具有重要意义。以川东南盆缘丁山—东溪五峰组—龙马溪组为例,综合钻井岩心观察、成像测井、三维激光扫描,明确了天然裂缝特征,表征和分析了其连通性和力学有效性。结果表明:天然裂缝以高角度剪切缝、层间滑移缝和层理缝为主,优势走向为NE和近EW向,未充填率约为9.7%,未充填缝主要存在于层间滑移缝和高角度剪切缝(>45°)中。通过成像测井的影像特征可以较好地区分丁山—东溪地区的天然裂缝类型及相互连通关系。裂缝网络的拓扑分析显示裂缝的平均连接数(CL)及分支平均连接数(CB)分别普遍低于2和1.5,表明缝网连通性较差,连通缝网主要存在于1~3小层且纵向的CL和CB值相差1~3倍,平面上丁山鼻状褶皱叠合区裂缝组系较多,连通性相对较好。研究区天然裂缝面的节理粗糙度系数(JRC)介于2.2~14.1之间,随黏土含量增加呈现为降低趋势。在原位地应力条件下,所有的天然裂缝均处于力学无效状态,其中在最小主应力较小和两向应力差较大的含黏土—黏土质页岩段(黏土含量>30%),与最大主应力方向夹角在30°~75°之间的低角度—高角度天然裂缝具有更好的力学有效性。研究成果可为页岩储层裂缝精细建模和“甜点”预测提供借鉴。

It is greatly beneficial for shale gas development and increased production by characterizing the con⁃nectivity of natural fractures in shale reservoirs and evaluating the mechanical effectiveness under in-situ stress condition.Taking the Wufeng-Longmaxi formations in the Dingshan-Dongxi area,southeastern margin of Sich⁃uan Basin as an example,the characteristics of natural fracture were identified by core observation,imaging log⁃ging,and 3D laser scanning,and their connectivity and mechanical effectiveness were characterized and ana⁃lyzed.The results show that the natural fractures are mainly medium-high dip angle shear fractures,interlayer slip fractures,and bedding fractures with dominant NE and near EW strike,and the unfilled percentage is about 9.7%.Unfilled fractures mainly exist in interlayer slip fractures and high dip angle(>45°)shear fractures.The natural fracture types in the Dingshan-Dongxi area can be well distinguished by the image characteristics from imaging loggings.The topological analysis of the fracture network shows that the number of lines(CL)and the number of branches(CB)are generally lower than 2 and 1.5,respectively,indicating that fracture network is poorly connected.Furthermore,there are many fractures with more sets and good connectivity in the overlap⁃ping areas in Dingshan nose shaped folds.The connected fractures network mainly exists in the 1-3 sub-mem⁃bers and the longitudinal CL and CB are 1-3 times different.The joint roughness coefficient(JRC)of the natural fracture surface in the study area is between 2.2 and 14.1,which shows a decreasing trend with the increase of clay content.Under in-situ stress,all natural fractures are in mechanical ineffectiveness state.Among them,in the section of argillaceous and argillaceous-rich shale(clay content>30%)with small minimum principal stress and large two-directional stress difference,the low-high dip angle natural fracture with 30°-75°to the maximum principal stress has a better mechanical effectiveness.Our research can provide a reference for natural fracture fine model and“sweet pot”prediction of shale reservoirs.