Investigating effects of cryogenic treatment on physical and mechanical properties of geothermal-formation samples – an experimental study

The fracture network in geothermal reservoirs influences heat extraction performance. Geothermal reservoirs, in general, have poor permeability, making extraction of the injected fracturing fluid difficult. The traditional fracturing technique used in oil and gas wells is water-based hydraulic fracturing treatment. Although this approach has been employed to improve permeability in geothermal formations, it has environmental and technical drawbacks. In this study, a waterless stimulation method, using liquid nitrogen (LN2), was used to stimulate geothermal rock samples. The main objective of this study is to investigate effects of cryogenic treatment and the treatment-time on the physical and mechanical properties of granite core samples using LN2. Eleven core samples were obtained from a granite block. The core samples were heated to different temperatures (up to 482 oF) and treated with LN2 for different times of 10, 20, and 30 minutes. CT-scan, porosity, gas permeability, and compressional and shear velocities tests were conducted before and after LN2 treatment. Besides, unconfined compressive strength (UCS) and X-Ray Diffraction (XRD) tests were conducted on them. The results of the experiments indicate that the physical and mechanical properties of the core sample were altered after implementing the LN2 treatment. CT scan results shows that there is formation of the fractures in the samples after the cryogenic treatment. The permeability increase in samples was more significant than the increase in the porosities. The ultrasonic velocities of the samples were decreased after the cryogenic treatment owing to the creations of the new fractures inside the samples. The results reveal that increasing the temperature of the samples and the treatment time with LN2 result in significant increase in the permeability of samples.

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