LN2 cryo-fracturing stimulation for future geothermal energy production from a depleted oil field: A case study of LN2 immersion in heated granite subsurface core specimens from Southwestern Kazakhstan

This work examines the contingency of an active Kazakhstani oil field to be transformed, once depleted, to a geothermal source following stimulation by a liquid nitrogen (LN2) fracturing method through thermal shock, which will create fractures and increase the contact area. Unlike the bulk of literature on granite stimulation, this work uses subsurface granite core gathered from an oilfield coring operation rather than outcrop samples. Heat transfer analysis establishes mild, controllable stress upon sample heating and thermal stresses exceeding the peak stress load of untreated granite upon sudden cooling by LN2. Brazilian and compression tests showed positive results after LN2 treatment, with their lowest values occurring at 600 °C with LN2 treatment at 14.4 kN and 19.77 MPa, respectively. The permeability of the elevated heated granites with LN2 treatment augments remarkably after LN2 treatment, while density, P-wave velocity, and maximum load are reduced. Nitrogen adsorption and mercury intrusion show a positive correlation, indicating an increase in the number or size of pores as temperature increases with LN2 treatment. With more significant thermal shocks, imagery showed compound fractures through a preferred mineral and the generation of through fractures, contributing to the enlargement of contactable surfaces by geothermal working fluids.