Pore Structure Heterogeneity and Probability Measurement Areas in Fractal Analysis of a Shale Reservoir

Understanding the pore structure in shale reservoirs is key to estimating the resource potential and recovery efficiency of shale gas. In this study, the probability measurement areas of the T2 spectrum in multifractal analysis were determined and quantified, and the pore structure of shale samples was evaluated by the fractal theory using low-field nuclear magnetic resonance (LF-NMR) experiments. The results show that the shale reservoirs exhibit a complex and strongly heterogeneous pore structure, with typical single- and multifractal characteristics. The single-fractal analysis has limitations in quantifying bound fluid pores and overall pores. Subsequently, the multifractal analysis provides a robust approach for evaluating local pore heterogeneity. The multifractal parameters D–10s – D0s exceeding D0s – D10s suggest that low-probability measurement areas control the heterogeneity of the pore structure. Consequently, the high- and low-probability measurement areas are identified and quantified by the calculation of local singularity strength. The two high-probability measurement areas are directly associated with the two peaks of the T2 spectrum and mainly consist of bound fluid pores with few movable fluid pores. The range of T2 intervals for high-probability measurement areas is greatly correlated with both the heterogeneity of the pore structure and physical properties, such as porosity. The results can provide an in-depth understanding for the characteristics of the pore structure and an effective method to correlate the multifractal analysis with specific pore size.