Effective porosity in lignite using kerosene with low-field nuclear magnetic resonance

Effective porosity is a key factor in subsurface fluid flow. However, lignite is soft, may swell in water (depending on saturation state), and is friable. Thus the conventional saturated water NMR method has challenges when applied to lignite (coal swelling, and fragmentation during the centrification process to remove the pore water. Here an improved experimental method for effective porosity is demonstrated for lignite. To prevent sample breakage, the cores were wrapped with a heat shrinkable plastic and kerosene replaced water as the saturated fluid. The kerosene low-field NMR signal and porosity relationship was established and demonstrated for lignites from the Lower Cretaceous Saihantala Formation in the Shengli coalbed reservoirs in the Erlian Basin of Inner Mongolia, China. The lignite had a relatively high total porosity of 37.2-47.0% and an effective porosity of 25.0-34.6%. Thus, from this and additional reservoir considerations, this area is likely conducive to the production of coalbed methane.