低电阻率页岩气储层饱和度方程研究及应用——以川南长宁地区五峰组—龙马溪组为例

Research and application of low resistivity shale gas reservoir saturation equation:A case study of Wufeng:Longmaxi Formation in Changning area,southern Sichuan

页岩气含水(含气)饱和度是游离气量评价至关重要的参数,由于高成熟有机质石墨化、黄铁矿、黏土矿物及地层水等多种导电因素的影响,川南长宁、泸州、渝西等多个区块页岩气勘探开发进程中相继一些页岩气井出现测井超低电阻率现象(R_(t)<5Ωm),造成基于传统电法的饱和度计算方法(Archie公式、Simandoux方程等)不具适用性.因此,为了明确不同导电因素对低电阻率页岩气储层的影响规律,进而建立合理的饱和度评价模型实现提高低电阻率页岩气储层饱和度计算的精度,以川南长宁地区五峰组—龙马溪组低电阻率页岩气储层为例,首先,利用X衍射、地化、激光拉曼、镜下薄片、饱和度测试及测井等资料,在确立低电阻率页岩气储层岩石体积物理模型的基础上,依据并联导电模型,推导了考虑石墨化有机质、黄铁矿等多因素的改进饱和度电性响应方程;其次,通过电阻率响应数值模拟,结合实际井黏土矿物含量、黄铁矿含量、有机质石墨化程度、地层水饱和度及矿化度资料,明确了长宁区块五峰组—龙马溪组页岩高演化有机质石墨化、高含水饱和度是形成超低电阻率页岩气储层的最核心因素;最后,依据建立的饱和度响应方程,利用最优化迭代方法计算了低电阻率页岩气储层的含水饱和度,计算结果与保压取心饱和度测试数据的相关系数达到了0.90以上,有效解决了低电阻率页岩气储层饱和度精确计算难度大的问题,为降低低电阻率页岩气勘探开发风险提供了依据.

Water(gas)saturation of shale gas is a crucial parameter for free gas evaluation.Due to the influence of various conductive factors,such as graphitization of highly mature organic matter,pyrite,clay minerals and formation water,the phenomenon of ultra-low logging resistivity(logging resistivity R_(t)<5Ωm)appears successively in shale gas exploration and development in Changning,Luzhou,western Chongqing and other blocks in southern Sichuan.As a result,the traditional saturation calculation methods based on electrical methods(Archie formula,Simandoux equation,etc.)are not applicable.Therefore,in order to clarify the influence law of different conductive factors on low resistivity shale gas reservoirs,and then establish a reasonable saturation evaluation model to improve the calculation accuracy of low resistivity shale gas reservoir saturation,taking Wufeng—Longmaxi Formation low resistivity shale gas reservoirs in Changning area of southern Sichuan as an example,first,Based on the data of X-ray diffraction,geochemistry,Laser Raman,microscopic thin section,saturation test and logging,and on the basis of the rock volume physical model of low resistivity shale gas reservoir,the improved saturation electrical response equation considering graphitized organic matter,pyrite and other factors is derived based on the parallel conduction model.Secondly,through the numerical simulation of resistivity response,combined with the actual well clay mineral content,pyrite content,graphitization degree of organic matter,formation water saturation and salinity data,it is clear that the highly evolved organic matter graphitization and high water saturation are the core factors for forming ultra-low resistivity shale gas reservoirs in the Wufeng—Longmaxi Formation of Changning block.Finally,based on the established saturation response equation,the optimal iterative method is used to calculate the water saturation of low resistivity shale gas reservoir.The correlation coefficient between the calculated results and the test data of pressure retaining core saturation is more than 0.90,which effectively solves the difficult problem of accurate calculation of low resistivity shale gas reservoir saturation.It provides a basis for reducing the risk of shale gas exploration and development with low resistivity.