页岩气储层含水饱和度影响因素及计算方法——以焦石坝区块五峰组—龙马溪组为例

Influential factors and calculation methods for water saturation in shale gas:Examples from Wufeng Formation-Longmaxi Formation,Jiaoshiba block

与砂岩、泥岩和碳酸盐岩等常规地层相比,页岩地层黏土含量高,有机孔和无机孔并存,纳米孔非常发育,黄铁矿广泛发育,地层导电性较为复杂,以上因素增大了页岩地层含水饱和度的评价难度。为了提高页岩气储层含水饱和度计算的准确度,以四川盆地焦石坝地区上奥陶统五峰组—下志留统龙马溪组海相页岩地层为研究对象,对含水饱和度的影响因素进行了探讨。研究结果表明:①黏土矿物中存在吸附水、层间水和结构水等多种形式的束缚水,是页岩气储层束缚水的主要储集空间,容易形成较高的含水饱和度;②有机孔表面具有亲油气性,连通性好,是烃类的主要赋存空间,容易形成较低的含水饱和度;③黄铁矿使地层电阻率降低,导致计算的含水饱和度增大;④对比分析孔隙度—饱和度交会法、Archie公式、Simandoux公式以及Total Shale公式的应用效果和存在的问题,总结出适用于研究区页岩气储层含水饱和度的评价方法。该项研究对四川盆地五峰组—龙马溪组地层水的主要组成形式、含水饱和度影响因素以及不同计算方法之间的异同进行了分析,可以为其他地区页岩地层含水饱和度的评价和计算提供参考。

Different from conventional sandstone,mudstone,and carbonate strata,shale ones are characterized by high clay content,coexistence of organic and inorganic pore,quite developed nanopore,widely developed pyrite,and complex electric conductivity,which may increase a little difficulty to evaluate water saturation in shale gas reservoirs.So,some ma⁃rine shale of Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation,Jiaoshiba block,Sichuan Basin,was taken as an objective to make clear the influential factors of water saturation in shale strata.Results show that(1)there are many forms of irreducible water in clay minerals,such as adsorbed water,interlayer water,and structural water,and they are the main reservoir space of irreducible water in shale gas reservoirs,so the water saturation in clay minerals is likely to be higher;(2)for organic pores,the surface is hydrophobic,and they are well connected as the main occur⁃rence space of hydrocarbon,which tends to form lower water saturation;(3)pyrite leads to a decrease of formation resistivi⁃ty to further cause an increase of calculated water saturation;and(4)after comparing porosity-saturation relationship meth⁃od with Archie formula,Simandoux formula,and Total Shale formula from aspects of their application effect and problems,one method to evaluate shale gas saturation,which is suitable for the study area,is provided.In conclusion,the main com⁃position forms of formation water in Wufeng-Longmaxi formations in Sichuan Basin,the influential factors of water satura⁃tion,and the similarities and differences between different calculation methods can provide reference for evaluating and cal⁃culating the water saturation of shale strata in other areas.