Uniaxial compressive strength (UCS) and SEM study of liquid nitrogen for waterless hydraulic fracturing in coalbed methane reservoirs of Karaganda Basin in Kazakhstan

Four different cryo-fracturing liquid nitrogen (LN2) treatment processes are compared for stimulation of coal. Namely, peak compression strength, acoustic emission, and energy absorption tests, plus scanning electron microscope (SEM) observations for fracture evolution were conducted for freezing time (FT), freezing-thawing cycle (FTC), negative freezing-thawing cycle (NFTC) and positive freezing-thawing cycle (PFTC) LN2 exposure processes for both dried and water-saturated specimens. The positive and negative freezing-thawing cycle experimental processes are novel variations on multi-contact cryogenic stimulation reported for the first time. In all freezing-thawing processes, water-saturated experiments exhibited lower Uniaxial Compression Strength (UCS) peaks compared to dry coal samples, presumably due to the additional frost forces degrading the overall sample integrity. For all LN2 exposure processes using dry coal specimens, the compaction line decreased and the elastic line decreased in acoustic emission (AE) tests, concurrent with a reduction in compressive strength, as LN2 treatment time increased. This trend reversed in water-saturated samples, highlighting the importance of sample water content on fracturing potential. Energy absorption analyses clearly showed that the strength of coal samples decreased with increasing the freezing time and freezing-thawing cycles, as the LN2 treatment produced more cracks and damage to coal. Furthermore, SEM analysis and image processing confirmed that all LN2 treatment processes resulted in the generation of fractures evident in fracture trace statistic changes on the surface of coal samples. FTC and PFTC processes showed the greatest promise as stimulation methods due to the extent of the network of fractures created. Engineering results were augmented with the inclusion of the geological setting, important for the transportability of these outcomes to coals of similar origin.

Automated summary

This study compared four different cryo-fracturing liquid nitrogen (LN2) treatment processes for coal stimulation. The experiments included peak compression strength, acoustic emission, and energy absorption tests, as well as scanning electron microscope observations. The results showed that water-saturated samples exhibited lower compression strength peaks compared to dry coal samples. The experiments also indicated the importance of sample water content on fracturing potential. The freezing-thawing cycle and positive freezing-thawing cycle LN2 exposure processes showed the most promising results due to the extent of fracture creation.