NMR petrophysical interpretation method of gas shale based on core NMR experiment

•Different NMR distribution characterizes different types of shale.•Kerogen or pyrite lead to underestimation of NMR porosity in organic shale.•New NMR-based petrophysical models for gas shale evaluation are proposed.•Kerogen and adsorbed gas overlap together with clay bound water in T2 distribution.

Nuclear magnetic resonance (NMR) characteristics provide critical information to study the storage of organic shale. Based on NMR petrophysical experiment of Haynesville Shale, the response characteristics of transverse relaxation time (T2) are analyzed. According to the shape and amplitude of T2 distribution of shale, Haynesville Shale can be categorized into two types: continuous and discontinuous T2 spectrums. Compared with routine density porosity, NMR porosity is generally underestimated for organic shale. Therefore, the controlling factors are investigated in detail. Through constructing the correlations of NMR porosity and clay volume, total organic carbon (TOC) content, kerogen content and pyrite volumetric concentration, the pyrite and kerogen are finally regarded as dominant factors. Subsequently, a new NMR porosity correction model is proposed. Moreover, the permeability model based on NMR bin porosity is built, and water, gas and oil saturation models are also established through building some correlations between gas saturation and NMR bin porosity. NMR experiment indicates that the kerogen and adsorbed gas are located in bound fluid part of NMR T2 distribution, and the gas storage is dominant as absorbed gas rather than free gas. The study provides a novel petrophysical interpretation method for gas-bearing shale.