Development of microcracks in granitic rock by liquid CO2 fracturing

Abstract

Hydraulic fracturing by liquid carbon dioxide (LCO2) generates fracture and cracking patterns that vary from those generated by water injection. The use of LCO2 as a fracturing fluid can minimize water usage and potentially sequester CO2. In this study, hydraulic fracturing by LCO2 and water in a granitic specimen was performed, and the micro-scale characteristics of generated microcracks were investigated using an X-ray imaging technique and thin-section analysis. The results revealed that LCO2 and water injection produced different fracture characteristics. The injection of LCO2 with less viscosity and high compressibility required a greater fluid volume for fracture initiation to generate a lower breakdown pressure, and also generated higher crack-density zones located near the borehole hole possibly because of facilitated permeation amount into the rock matrix as compared to the water-injection case. In both the LCO2- and water-injection cases, the fractures developed along the rift cleavage plane, and an increase in microcrack density was observed in regions within 6 mm from the borehole. It was confirmed that the statistical and spatial distributions of developed microcracks were affected by the fracturing fluid and anisotropic properties of granitic rocks. The results of this study could be applied to fracturing that employs less water, CO2 sequestration, and recovery of geothermal energy.