渝东南地区五峰—龙马溪组页岩气储层孔缝发育特征及其地质意义

Characteristics and geological significance of pore and fracture of shale gas reservoirs in Wufeng-Longmaxi Formation,southeastern Chongqing

选择渝东南地区五峰—龙马溪组页岩储层为研究对象,采用场发射扫描电镜、低温液氮吸附、覆压渗透率孔隙度测试等实验,分析页岩中微观孔缝类型、孔隙结构及其分形学特征等。研究发现,页岩宏观裂缝主要为页理缝与构造缝,盆缘高压页岩气井样品中裂缝多数呈雁列状分布,盆外常压页岩气井样品中裂缝形态较为平直。宏观裂缝极为发育的位置微裂缝也较为发育,含气量与渗透率高。深部与浅部页岩孔缝特征存在明显差异。深部样品裂缝发育较浅部样品少,微裂缝以收缩缝与溶蚀缝为主。浅部样品中孔隙类型、形态、结构在纵向上具有相似的分布特征,平均孔径与覆压孔隙度明显大于深部样品。石英与黏土矿物对微孔和中孔的发育有一定的影响,显微类型是有机孔发育的重要影响因素。埋深大于3500 m时,孔隙形态与孔径分布的主要影响因素为深度,深部页岩经历更为复杂的成岩作用,孔隙遭受强烈的后期改造,孔缝结构不再是被有机质主控。具有这类孔缝赋存特征的深部页岩压裂时,缝网形成的难度较大,需要对深部页岩缝网形成机制开展进一步的研究。通过对不同尺度裂缝的观测,可快速判断易压裂层位的位置,即使这类孔缝占比原位孔隙体积的比例极小,对储层的渗透率没有作用,但是在压裂改造下,这类孔缝群体会被激发,作为页岩储层的主要渗流通道。

The types of micro-pores,pore structures and fractal characteristics of shale have been analyzed for the shale reservoir of Wufeng-Longmaxi Formation in Southeastern Chongqing by field emission scanning electron microscope(FESEM),low temperature liquid nitrogen adsorption and overpressure permeability porosity test.It is found that the macroscopic fracture types are mainly foliated fractures and structural fractures,and most of the fractures in the samples along the basin are arranged in an echelon series,while the fractures in the atmospheric pressure shale gas well samples outside the basin are relatively straight.The positions where the macroscopic fractures develop most are also the places where the microfractures develop well.Usually,it has high gas content and permeability.There are obvious differences between the occurrence characteristics of pore fractures in deep shale and relatively shallow shale.The fractures in the deep samples are less developed compared with those in the shallow samples,and the microfractures are mainly shrinkage and dissolution fractures.The pore type,shape and structure of the shallow samples have similar longitudinal distribution characteristics.The average pore diameter and overburden porosity of shallow samples are obviously larger than that of deep samples.Quartz and clay have a certain influence on the development of micropores and mesopores.The microscopic type is an important influence factor on the development of organic pores.When the burial depth is greater than 3500 m,depth is the main influence factor on pore morphology and pore size distribution.The deep shale undergoes more complex diagenesis,the pores undergo intense late transformation,and the pore structure is no longer dominated by organic matter.It is difficult to form fracture network when fracturing the deep shale with such pore fracture occurrence characteristics,so further research on the formation mechanism of deep shale fracture network is needed.Through the observation of fractures with different scales,the layer easy to fracture can be quickly determined.Even though such fractures account for a very small proportion of in-situ pore volume and have no effect on the permeability of the reservoir,such pore groups can be stimulated because of fracturing and serve as the main seepage channels of shale reservoir.