Characterization and interpretation of organic matter, clay minerals, and gas shale rocks with low-field NMR

Highlights

• To investigate the T₁-T₂ correlation spectrum of isolated components of the shale.

• Accurately quantifying the relaxation mechanism of the specific constituents in clay mineral.

• To complement the position of bitumen and clay mineral in T₁-T₂ mapping of gas-shale.

• To imply temperature affects on the relaxation times on kerogen, bitumen, and clay mineral.

Abstract

Nuclear magnetic resonance (NMR) has emerged as an essential method for appraising gas-shales, quantifying their petrophysical properties, classifying their fluid types, and discerning their productivity. Comprehensive measurement campaigns, as well as reliable thermal-heating experiments, are required to fully understand the NMR mechanism of shale and to further develop future gas-shale exploitation strategies. NMR signatures from individual matrix components, which comprise shale, are investigated to gather data to better estimate its petrophysical parameters. Subsequently, we perform T₁ and T₂ measurements of kerogen, solid bitumen, and clay minerals. Next, we analyze the temperature effect on the NMR signal interpretation of different compositions of gas-shale samples in detail, coupled with the position of clay minerals and solid bitumen from the T₁-T₂ mapping. A shale plug sample was quantified by the NMR T₁-T₂ relaxation signals under initial conditions and different temperature conditions of 110 °C, 250 °C, 450 °C, and 650 °C. The fluid volume significantly decreased as the temperature increased. The organic matter maturity changed at 450 °C and 650 °C. According to a thermal-heating experiment with powdered clays, we obtained the hydroxyl group and crystal water signals and determined the critical temperature for different dried types of water in clay mineral. It is reasonable to infer that deconvoluting the entire shale sample into different components can further determine the NMR relaxation mechanisms. The ultimate goal of this project was to develop a holistic methodology to understand the NMR mechanisms of isolated components and to assess the positions of kerogen, solid bitumen, illite, kaolinite, smectite, and chlorite from T₁-T₂ mapping. This work contributes to the fundamental understanding of shale formations by quantifying the NMR relaxation mechanisms of various constituents in shale and provides implications for shale rock evaluations.