Developing deep coalbed methane (CBM) reservoirs is becoming a key direction for different operators after the production breakthrough in the Ordos Basin in China. The deep CBM reservoirs have more complex and heterogeneous properties and intrinsic mechanisms than other unconventional reservoirs such as shale gas, shale oil, etc. The storage space of deep CBM reservoirs is a coupling of cleats and various pores such as dissolution pores, wedge-shaped intergranular pores, deformed residual plant tissue pores, gas pores, moldic pores, etc. These complex and heterogeneous pores and cleats make it challenging to characterize the pore structures, porosity, and saturation in deep CBM reservoirs by traditional triple-combo logs. To have a better understanding of deep CBM reservoirs, 2D nuclear magnetic resonance (NMR) logs were performed to characterize the pore structure, porosity, saturation, and gas in place (GIP) for the Longtan Formation in Sichuan Basin.
In this paper, we performed 2D NMR loggings to characterize the porosity, saturation, and fluid components with a customized T1-T2 interpretation model for the deep CBM reservoir. The 2D NMR tool provides continuous T1-T2 measurements, which not only enables accurate porosity estimates but also enables the separation and quantification of different fluids in the pores by the 2D data analytics technique. Based on the schematic illustration of fluid components on the T1-T2 map, NMR core analysis, and experience with the T1-T2 map in shale gas reservoir, the customized T1-T2 interpretation model was established to make the porosity partitions and fluid components identification. Then, the total porosity, effective porosity, gas volume, water volume, and saturation were estimated accurately in this deep CBM reservoir in the Longtan Formation. Combining the accurate proximate analysis from gamma-ray spectroscopy measurements, porosity, and saturation from 2D NMR measurements, the GIP including the adsorbed gas and free gas was achieved sequentially.
The case study is presented from a deep CBM reservoir of the Permian Longtan Formation in Sichuan Basin. It showed that the accurate porosity, fluid components, and saturation were estimated quantitively from 2D NMR logs through the 2D data analytics techniques. The porosity and saturation match well with tight rock analysis (TRA) core measurements. The GIP also matches well with the core measurement in this CBM reservoir. The study showed the value of 2D NMR logs for direct porosity, fluid components, and saturation estimation in deep CBM reservoirs.
2D NMR measurements help characterize the pore structure, porosity, fluid components, and saturation accurately and robustly in deep CBM reservoirs. which enhanced the understanding of deep CBM reservoir characteristics.
