Integrated NMR and FE-SEM methods for pore structure characterization of Shahejie shale from the Dongying Depression, Bohai Bay Basin

Abstract

Of particular importance for shale reservoirs is the evaluation of pore structure characteristics that significantly influence a reservoir's storage capacity and flow mechanisms. Due to the presence of multiscale pore sizes in shale reservoirs, a series of experiments, including total organic carbon (TOC), Rock-Eval analysis, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and nuclear magnetic resonance (NMR), were conducted on shale samples from the lower submember of the third member (Es3L) and upper submember of the fourth member of the Eocene Shahejie Formation (Es4U) in the Dongying Depression, Bohai Bay Basin. The results indicate that calcite (averaging 30.5%) and clay (averaging 29.1%) are the dominant minerals. Based on analysis of geochemistry data, the organic matter type of LY1 is I and II. Four main types of reservoir spaces are observed in LY1: intercrystalline pores, dissolved pores, intergranular pores, and microfractures. Shale samples in the study area have a wide range of pore size distributions and complex pore structures because of the wide range of transversal relaxation time (T2) distributions and messy T2 curves of the NMR experiment. Per the NMR results, the percentage of micropores, mesopores and macropores (MS1, MS2, and MS3) were calculated, and the relationships between these proportions and pore structure parameters (porosity, permeability, T2 cutoff value, T2 geometric mean value) and structural characteristics were investigated. MS1 increased with a decrease of T2 cutoff values and geometric mean values, suggesting that shale samples with higher MS2 and MS3 have better pore structures and strong percolation potential. Under similar porosity conditions, samples with high permeability have a larger MS3 value. Laminated and layered shale samples have larger values of MS2 and MS3 than massive shale. Therefore, MS1, MS2 and MS3 are useful to indicate the quality of pore structures; the smaller the MS1, the better the pore structure. Finally, taking three representative shale samples as an example, the relationships between MS1, MS2, and MS3 and the effectiveness of the reservoir were discussed using the logging profile of LY1. The research concludes that MS1, MS2, and MS3 are effective NMR parameters for evaluating the effectiveness of the reservoir.