An Innovative Characterization of Deep Coalbed Methane Reservoirs Through the Integration of 2D Nuclear Magnetic Resonance and Gamma-Ray Spectroscopy Measurements: Case Studies from China

Abstract Following a significant production breakthrough in the deep coalbed methane (CBM) reservoir located in the Ordos Basin, developing deep CBM reservoirs has become a strategic focus for various operators across different basins in China. These deep CBM reservoirs exhibit far more intricate intrinsic mechanisms and heterogeneous properties than other unconventional reservoirs. The mineral composition and content in deep CBM are highly diverse. The storage capacity of deep CBM reservoirs is a complex interplay of cleats and various types of pores. The way gas is stored in deep CBM reservoirs is also markedly different from others. These complex and heterogeneous characteristics pose significant challenges in evaluating the mineralogy, porosity, and saturation levels of deep CBM reservoirs. To gain a thorough understanding of these intricate systems, an integration of two-dimensional nuclear magnetic resonance (2D NMR) and gamma-ray spectroscopy measurements was employed in several basins. These techniques were used to characterize the mineral composition, pore structure, porosity, saturation, and gas in place (GIP) for the deep CBM reservoirs. The 2D NMR measurements were employed to characterize pore structures, porosity, saturation, and fluid components using a customized T1-T2 interpretation model. Gamma-ray spectroscopy was used to analyze the mineralogy and perform proximate analysis. The T1-T2 map provided a schematic illustration of fluid components, combined with NMR core analysis, enabling the development of a customized T1-T2 interpretation model. This model facilitated the accurate partitioning of porosity and identification of fluid components, enabling precise estimation of total porosity, effective porosity, gas volume, water volume, saturation, etc. Furthermore, X-ray diffraction analysis on core samples contributed to the establishment of a customized mineral evaluation model. This model enabled the accurate estimation of mineral components, especially for coal, their contents, and proximate analysis metrics, including fixed carbon, ash, volatile matter, and moisture content. By integrating the porosity and saturation data obtained from 2D NMR measurements with the mineral components and proximate analysis results derived from gamma-ray spectroscopy, the studies successfully quantified the GIP for the deep CBM reservoirs. Meanwhile, this integration provides a comprehensive understanding of the deep CBM reservoir's characteristics. The studies provide a comprehensive characterization of deep CBM reservoirs, detailing their porosity, fluid components, saturation levels, mineral composition, proximate analysis, and GIP. This was achieved through the integrated application of 2D NMR and gamma-ray spectroscopy logging techniques.