Research and application of low resistivity shale gas reservoir saturation equation:A case study of Wufeng:Longmaxi Formation in Changning area,southern Sichuan
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 Rt＜5 Ωm)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.
Low resistivity shale gasSaturation equationSouthern SichuanWufeng—Longmaxi FormationGraphitization of organic matter
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