页岩气储层孔隙连通性及其对页岩气开发的启示——以四川盆地南部下志留统龙马溪组为例

Connectivity of pores in shale reservoirs and its implications for the development of shale gas:A case study of the Lower Silurian Longmaxi Formation in the southern Sichuan Basin

大型水力压裂后,页岩气储层中的不连通含气孔隙有可能转变成"潜在可采孔隙",而目前的主流页岩气储层孔隙分类方法没有考虑上述不连通孔隙,对储层孔隙有效性评价的准确性有影响。为此,以四川盆地南部下志留统龙马溪组页岩为研究对象,开展柱塞样和碎样岩心孔隙度、饱和盐水后离心+渐变干燥核磁共振和核磁冻融实验,分析页岩气储层不连通孔隙体积、主要发育位置、主要孔径分布范围,划分页岩气储层孔隙系统,确定页岩含气连通孔隙有效孔径的下限,开展页岩气储层全孔隙有效性评价,并探讨页岩中不连通孔隙对于页岩气开发的影响。研究结果表明:①该区页岩气储层存在着大量的不连通孔隙,占比高达30.23%,孔径分布介于5~30 nm,主要发育于有机质和少量的黏土矿物中;②该区页岩气储层黏土束缚水核磁T2截止值为0.26 ms,对应孔径为5.35 nm,此为该区页岩气储层有效孔径的下限;③大型水力压裂可改善页岩气储层中孔径超过5.35 nm的不连通孔隙,实现页岩气有效开发;④水力压裂改造后的不连通孔隙可增加压裂液在基质中的储存空间,吸收裂缝中的压裂液,置换孔隙中的页岩气,促使页岩气储层自动缓解水锁,提高页岩气单井产量。结论认为,采用"离心+渐变温度干燥"法,结合核磁共振实验可实现页岩孔隙中流体赋存状态和孔隙系统的定量划分,高速离心+核磁共振实验可以确定可动水和毛细管束缚水,渐变干燥+核磁共振实验可以确定毛细管束缚水和黏土束缚水。

The unconnected gas-bearing pores in shale gas reservoirs may be transformed into"potential recoverable pores"after largescale hydraulic fracturing.However,the mainstream pore classification methods of shale gas reservoirs,do not take the unconnected pores into account,which impacts the evaluation accuracy of reservoir pore effectiveness.To solve this problem,this paper took the shale of Lower Silurian Longmaxi Formation in the southern Sichuan Basin as the research object to carry out experiments on core porosity,centrifugal+gradual drying NMR after saturated with brine and NMR freeze-thaw using plunger samples and fragmented samples to analyze the volume,main development location and main pore size distribution range of unconnected pores.Then,the pore systems were classified,and the lower limit of effective pore size of connected gas-bearing pores was determined.Finally,the total pore effectiveness of shale gas reservoirs was evaluated,and the influence of unconnected pores on the development of shale gas was discussed.And the following research results were obtained.First,there are a large number of unconnected pores in the shale gas reservoirs of the study area,accounting for 30.23%.Most of them are mainly developed in organic matters and a few are developed in clay minerals.The distribution of pore size ranges from 5 nm to 30 nm.Second,T2 cut-off value of clay bound water in the shale gas reservoirs of the study area is 0.26 ms,whose corresponding pore size is 5.35 nm,which is the lower limit of the effective pore size.Third,large-scale hydraulic fracturing can improve the unconnected pores with the size more than 5.35 nm,so effective development of shale gas will be realized.Fourth,after hydraulic fracturing stimulation,the unconnected pores can increase the storage space of fracturing fluid in the matrix,absorb the fracturing fluid in the fractures,replace the shale gas in the pores and promote the automatic alleviation of water lock in shale gas reservoirs,so the single-well shale gas production will be increased.In conclusion,fluid occurrence and pore system in shale pores can be quantitatively divided by means of centrifugal+gradual temperature drying method combined with NMR experiments,movable water and capillary bound water can be determined by the high speed centrifugal+NMR experiments,and capillary bound water and clay bound water can be determined by the gradual drying+NMR experiments.