An Investigation of the Influence of Liquid Nitrogen on Tight Sands Through Micro-CT Scan Imaging

Hydraulic fracturing is essential for extracting hydrocarbons from tight sands but often results in suboptimal performance due to formation damage caused by water. Optimizing hydraulic fracturing fluid (HFF) with CO₂ or N₂ has led to only marginal improvements. This study proposes using liquid nitrogen to induce microfractures via thermal shock, reducing fracture initiation pressure through rock stress redistribution. A Scioto sandstone sample (5 mm in diameter) was heated to 70°C for 24 hours and then exposed to liquid nitrogen for 45 minutes. Changes in pore structure were captured using a high-resolution micro-CT scanner at a resolution of 2.4 microns before and after exposure. The image stacks were processed, and fifty-two cubic sub-samples (each 100x100x100 voxels) were extracted from four spots on the x-y plane and along the z-axis. These sub-samples were segmented into pore and solid grain regions using a machine learning algorithm, and the rock's porosity and permeability were calculated. Results showed an increase in average porosity and permeability from 15.4% and 6.3µD in the initial scan to 17.2% and 7.2µD in the final scan. Visual analysis revealed new pore spaces in the form of long channels, primarily near the outer boundary of the sample. These findings suggest that thermal shock induced rock stress failure, creating fractures that might reduce fracturing pressure. Using liquid nitrogen offers significant advantages, including preventing formation damage by converting to gas and flowing out easily. It also reduces the applied fracking pressure due to induced stress failure and serves as an environmentally friendly alternative to HFF, protecting water resources and avoiding disposal costs and contamination risks.