Morphological and petro physical estimation of Eocene tight carbonate formation cracking by cryogenic liquid nitrogen; a case study of Lower Indus basin, Pakistan

Abstract

Significant amounts of hydrocarbons are contained in tight carbonate rocks like those within the Indus basin of Pakistan, but due to their unconventional nature, their extraction is difficult. This can be completed/conducted by properly characterizing its mineralogical, geochemical and microstructural properties. In this context, Hydraulic fracturing and acidizing are the most common fracture stimulation techniques. However, they suffer significant technical and environmental flaws. Liquid Nitrogen (LN2) fracking is considered as one of the best alternatives compare to hydraulic fracturing or acidizing due to its eco-friendly nature. In fact, after exposer to LN2, super cryogenic feature of the rock causes porosity, permeability and fracture conductivity enhancement due to thermal shock without any environmental consequences. The effect of Liquid Nitrogen fracking on tight carbonate outcrops collected from the Indus basin is investigated in the present study. Scanning Electron Microscopy (SEM) reveals the clear appearance of wide pore fractures with sizes from 4 μm to 50 μm, when subjected to liquid Nitrogen (LN2) treatment ranging from 30 to 90 min’ duration. Furthermore, SEM and Atomic Force Microscopy (AFM) images also indicate clear pore fractures in tight carbonate rocks along with increased pore connectivity. Moreover, following liquid nitrogen treatment of up to 90 min the permeability of tight carbonate increases by 53% and increases in porosity by 73%. Results from SEM studies suggest that visible fractures occur in the tight carbonate samples with liquid nitrogen treatment because of the freezing temperature (−196 ⸰C) of Liquid Nitrogen. In addition to this, Nano indentation moduli depict significant decreases in tight carbonate rocks, both before and after exposure to LN2, as a result of increases in liquid rock compressibility. In this regard, a 50 mN force decreased from 50.57 GPa to 24.37 GPa and for a 200 mN force this decreased from 45.77 GPa to 26.67 GPa. This research depicts the significant effect of Liquid Nitrogen freezing on the fracturing of carbonate rocks.