Changes in the petrophysical properties of coal subjected to liquid nitrogen freeze-thaw - A nuclear magnetic resonance investigation

- NMR was used to explore the petrophysical properties of frozen-thawed coal.
- Freeze-thaw cycles had the most significant effect on petrophysical properties.
- The modification on different coal ranks was affected by coal's initial porosity.

Liquid nitrogen (LN2), a non-aqueous medium, has attracted attention in recent years as a fluid for fracturing in the petroleum/energy industry. This study proposes a freeze-thaw method using LN2 to improve coal permeability for the production of coal bed methane. Experiments were conducted using nuclear magnetic resonance (NMR) to explore the physical properties of frozen-thawed coal. The coal samples were subjected to different LN2 freezing times and to freeze-thaw cycles and coals of different rank and with different moisture contents were tested. Changes in these four freeze-thaw variables changed the petrophysical properties of the frozen-thawed coal samples; the pore structure, porosity, and permeability of the coals were modified. Of these variables, the number of freeze-thaw cycles had the most substantial effect on modifying the coal's petrophysical properties. The degree of modification on the coals of different rank was affected by the coal's initial porosity. In general, lignites were modified the most, anthracite coal was modified less, and bituminous coal was modified the least. The study analyzed three of the classic NMR transforms for determining permeability and found that the Schlumberger-Doll Research (SDR) model matched the measured gas permeabilities most consistently. Based on this SDR permeability model, equations suitable for predicting the permeability of frozen-thawed low-rank coals were derived. In addition, results from scanning electron microscope studies showed that a fracture network with fracture widths of as much as 32.3 μm was formed in the coal after 30 freeze-thaw cycles. Additionally, micron-size particles falling from the coal surface gradually increased as the number of freeze-thaw cycles increased, indicating that freeze-thaw using LN2 materially modified the physical properties of the coal.