致密低渗透储层多尺度裂缝及其形成地质条件

根据限制致密低渗透砂岩储层天然裂缝扩展的岩石力学层及其界面特征,提出了多尺度裂缝的划分标准,探讨了多尺度裂缝形成的地质条件。根据油藏内天然裂缝规模及其控制界面,将天然裂缝分为大尺度裂缝、中尺度裂缝、小尺度裂缝和微尺度裂缝4级。大尺度裂缝在砂层组内发育,切割夹层,受隔层控制,延伸长度通常为百米级;中尺度裂缝在复合砂体内发育,切割层面,受夹层控制,延伸长度通常为数十米级;小尺度裂缝在单砂体内发育,受层面控制,延伸长度一般为米级至十米级;微尺度裂缝需要借助微观分析手段进行观察和识别,延伸长度一般为厘米级以下。不同尺度裂缝具有幂律分布的特点。裂缝尺度越大,数量越少;裂缝尺度越小,数量越多。多尺度裂缝的形成主要受岩石力学性质差异、岩石力学边界厚度和构造应力的控制。构造应力越大,越容易形成穿层的大尺度裂缝,较大的岩石力学性质差异和较厚的岩石力学边界才能够限制裂缝扩展;构造应力越小,越容易形成小尺度裂缝,较小的岩石力学性质差异和较薄的岩石力学边界就能够限制裂缝扩展。不同尺度裂缝对致密低渗透油藏的作用不同。微尺度裂缝主要起储集空间作用,改善了致密低渗透储层的储渗性能,是致密低渗透油藏稳产的重要因素;小...

Multi-scale fractures in tight sandstone reservoirs with low permeability and geological conditions of their development

Given the mechanical stratigraphic units and their interface properties constraining natural fracture propagation in tight sandstone reservoirs with low permeability,we propose the division criterion of multi-scale fractures,and discuss the geological conditions of multi-scale fracture formation.According to the scale of natural fracture and mechanical stratigraphy interfaces controlling their development,the natural fractures within reservoirs can be grouped into 4 levels,namely the large-scale,meso-scale,small-scale and micro-scale fractures.Large-scale fractures were developed within sandstone layer groups with a propagation of hundreds of meters,cutting through interlayers and constrained by barriers;meso-scale fractures were developed within composite sand bodies with a propagation of tens of meters,cutting through beddings and constrained by interlayers;small-scale fractures were developed within single sand bodies with a propagation ranging from several meters to over ten meters,and constrained by beddings;micro-scale fractures are to be observed and identified under microscopes,usually with a propagation of less than several centimeters.These fractures feature power-law distribution:the fracture size is in negative correlation with the number of fractures.Multi-scale fracture formation is mainly controlled by the differential rock mechanical properties,thickness of mechanical stratigraphic unit and tectonic stress.The greater tectonic stress is favorable for generating large-scale,cross-layer fractures,the propagation of which can be constrained only by relatively large differential rock mechanical properties and thick mechanical stratigraphic units;while the smaller tectonic stress is conducive to developing small-scale fractures,the propagation of which can be sufficiently constrained by relatively small differential rock mechanical properties and thin mechanical stratigraphic units.Fractures of multiple scales are of different contributions to the formation of tight,low-permeability reservoirs.Micro-scale fractures are the major reservoir space,improving the storage and permeability of tight,low-permeability reservoirs;thus they are key to the reservoir stable production of this kind;small-scale fractures act as both storage space and seepage channels,key to controlling the initial productivity of tight,low-permeability reservoirs;large- and meso-scale fractures mainly function as seepage channels,affecting the distribution pattern of remaining oil at the middle and late stages of reservoir development of the kind.