Phase Behavior of Nitrogen Transport in Fractures During Liquid Nitrogen Fracturing in Deep Geothermal Reservoirs

Abstract

Liquid nitrogen (LN2) fracturing is a relatively new technology which can induce complex fracture networks in the much-warmer rocks under reservoir conditions. Numerous studies have investigated the fracturing mechanisms and performances of LN2 fracturing in geothermal wells based on laboratories experiments. However, nitrogen transport in fractures does not keep liquid and cryogenic state all the time during fracturing. The phase behavior of nitrogen during fracturing is still unclear. Hence, an unsteady-state fluid flow and heat transfer 3D model for LN2 fracturing is developed. The phase transition of nitrogen and the heat transfer between nitrogen and formation during LN2 fracturing are taken into account. Phase distribution of nitrogen in fracture and the temperature distribution of rock around the fracture are analyzed. The results indicated that the thermal damage range around fracture is in a conical shape which is the largest around the injection point and decreased with increasing distance from the wellbore. With the increase of the flow distance in the fracture, nitrogen at the fracture tip gradually changes into supercritical state. The formation of deep fractures during LN2 fracturing is not affected by thermal stress, but by fluid expansion and viscosity reduction. In addition, the thermal stress of rock in LN2 fracturing is greater than that in water fracturing near the injection point. But with the increase of flow distance, the thermal stress in LN2 fracturing is gradually lower than that in water fracturing. The main findings of this study provide a new insight into LN2 fracturing mechanisms in high-temperature geothermal reservoirs.