Analysis of micro-scale heterogeneity characteristics in marine shale gas reservoir: Pore heterogeneity and its quantitative characterization

ZHAO Difei; GUO Yinghai; ZHU Yanming; Geoff WANG; CHONG Xuan; HU Xinmeng

Abstract：To deepen the understanding of microscopic heterogeneity characteristics and its control on the storage and transportation of gas in shale reservoirs,samples from Wufeng formation and Longmaxi formation in southeast Chongqing were used to further study the microscopic heterogeneity features experimentally and theoretically by methods of field emission scanning electron microscopy (FESEM),mercury intrusion and liquid nitrogen gas adsorption on the basis of previous studies.The heterogeneity characteristics on genesis type,pore structure,pore network and scaling effect were summarized based on image processing,selection of structural parameters and fractal dimension modeling.The single sample evaluation model and comprehensive evaluation model for the characterization of shale pore heterogeneity were established using parameters of plane porosity,coefficient of variation,tortuosity and fractal dimension value.It indicates that pores and pore networks in shale reservoirs have significant heterogeneity characteristics controlling the gas reservoir capacity and the direction of seepage flow.The results show that different micro-layers with different types of pore networks are interbedded developed with strong heterogeneity characteristics and thus controlled the reservoir quality,including micro-percolation pore network,matrix adsorption-seepage pore network and interlayer or sand-layer pore network.The directional heterogeneity controlled by seepage pores as well as the different development degrees of adsorption pores are key factors affecting the storage and release properties of shale.The comprehensive evaluation and analysis suggest that the reservoirs with characteristics of organic nano-pores developed (more than 50 ％ of the reservoir space);the mass fraction of brittle minerals is larger than 40 ％;the pore tortuosity is less than 40;the pore network of a combination of micro-percolation pore network and matrix adsorption-seepage pore network (pore-micro-fracture collocation) have better reservoir properties and potential for exploration.