Cryogenic liquid nitrogen fracking effects on petro-physical and morphological characteristics of the sembar shale gas formation in the Lower Indus Basin, Pakistan
Energy and Fuels
School of Engineering / Centre for Sustainable Energy and Resources
Shale gas resources have emerged as critical resources in global energy. Pakistan has a substantial unconventional shale gas potential. However, shale gas in Pakistan, like in the United States and China, is still poorly developed compared to other unconventional resources. Shale reservoirs exhibit extremely poor permeability for production by conventional means. Numerous approaches have been applied to extract unconventional shale gas energy resources, including acid treatment and hydraulic fracturing. However, due to copious concerns, many countries have banned such practices. Hence, waterless fracturing techniques have the potential to alleviate such issues. In such situations, cryogenic fracturing by liquid nitrogen (LN2) is an imperative stimulation option for tight formations. Hence, we obtained samples from the Sembar shale formation of the lower Indus basin in Pakistan and experimentally investigated the effects of pre- and post-treatment using LN2 under various time cycles (30, 60, and 90 min). The study provides information about mineralogical, microstructural, and petro-physical properties. Sembar shale samples were exposed to LN2 for aging at various time cycles to generate fractures. The cryogenic LN2 pre- and post-treatment effects were quantitatively characterized using atomic force microscopy (for topography), scanning electron microscopy (for microstructural properties), energy dispersive spectroscopy (for elemental composition), petro-physical quantification (for porosity and permeability), and nanoindentation (for rock-mechanical properties). The results reveal that, before LN2 treatment, Sembar exhibited nominal pore-throat sizes. However, the post-treatments of cryogenic LN2 were very effective in creating microcracks of up to 17 μm. Similarly, the petro-physical properties exhibited a significant increase after LN2 treatment. The mean indentation moduli decreased from 24.34 to 12.25 GPa for 50 mN force and 20.15 to 10.42 GPa for 200 mN force. The topographic study found increased surface roughness from 356 to 778 nm due to the LN2 treatment of 90 min. Cryogenic LN2 treatment can improve the pore connectivity of shale samples and significantly enhance the flowing capacity, which may greatly increase the productivity of unconventional shale gas reservoirs.