Simulation of Cryogenic Fracturing/Cooling Around Boreholes with Liquid Nitrogen

Abstract

Mining operations for the Lihir gold mine in Papua, New Guinea, include packing explosives into drilled boreholes and detonating them in order to facilitate the removal of gold-containing rock. However, the mine is in a geothermally active volcano crater whose surface is at an elevated temperature, and this heat can degrade the explosives. A proposed remedy for this degradation is to inject liquid nitrogen into the boreholes in order to cool them. The process is similar to the situation of cold-fluid injection in oil and gas reservoirs. We are performing experimental and numerical studies of this process in order to determine the relationship between injected liquid nitrogen volume and ground cooling, the duration of ground/borehole cooling after injection, and the pressure buildup resulting from liquid nitrogen vaporization. The experimental studies use rock samples to investigate behavior during cryogenic application at the appropriate temperature and under a tri-axial confining stress field. For our numerical studies, we modified our existing simulator of fluid and heat flow and geomechanics in porous and fractured media to additionally simulate liquid nitrogen injection into boreholes, as wells as cryogenic fracturing of the surrounding formation. We validated our modified simulator by matching experimental data, namely fluid conductivity enhancement of rock samples and the temperature drop during and after the cryogenic application. We then applied the simulator to the field, and modeled various scenarios for cryogenic cooling in the vicinity of the boreholes, obtaining data relating injected liquid nitrogen volume and ground cooling and the duration of ground/borehole cooling after injection. This data was used to make recommendations for ground cooling operations. We concluded that our modified simulator is a useful tool for designing cryogenic cooling processes and it can be applied to other applications as well.