A computer-implemented method comprising the steps: during normal operation of an injection well formed in a subterranean zone, determining a plurality of bottomhole pressures at a bottom of the injection well based on a respective plurality of surface injection pressures at a surface of the injection well, each surface injection pressure being a pressure in the injection well resulting from a respective injection flow rate at which injection fluid is flowed through the injection well from the surface toward the bottom; determining an injection well performance model for the injection well based on the plurality of bottomhole pressures and a plurality of injection flow rates, wherein each injection flow rate is caused by each surface injection pressure of the plurality of surface injection pressures; and after an injection well shut-in responsive to a subsurface leak in the injection well, wherein the leak causes a crossflow from a high pressure region in the subterranean zone to a comparatively low pressure region in another subterranean zone through the injection well i) modeling the shut-in injection well as an injection well having the injection well performance model determined during normal operation of the injection well, ii) modeling the shut-in injection well as a producing well having the injection well performance model determined during normal operation of the injection well, and iii) determining a crossflow rate in the injection well at a location of the subsurface leak in the injection well based on the injection well performance model of the modeled injection well and the injection well performance model of the modeled producing well. during normal operation of an injection well formed in a subterranean zone, determining a plurality of bottomhole pressures at a bottom of the injection well based on a respective plurality of surface injection pressures at a surface of the injection well, each surface injection pressure being a pressure in the injection well resulting from a respective injection flow rate at which injection fluid is flowed through the injection well from the surface toward the bottom; determining an injection well performance model for the injection well based on the plurality of bottomhole pressures and a plurality of injection flow rates, wherein each injection flow rate is caused by each surface injection pressure of the plurality of surface injection pressures; and after an injection well shut-in responsive to a subsurface leak in the injection well, wherein the leak causes a crossflow from a high pressure region in the subterranean zone to a comparatively low pressure region in another subterranean zone through the injection well i) modeling the shut-in injection well as an injection well having the injection well performance model determined during normal operation of the injection well, ii) modeling the shut-in injection well as a producing well having the injection well performance model determined during normal operation of the injection well, and iii) determining a crossflow rate in the injection well at a location of the subsurface leak in the injection well based on the injection well performance model of the modeled injection well and the injection well performance model of the modeled producing well. i) modeling the shut-in injection well as an injection well having the injection well performance model determined during normal operation of the injection well, ii) modeling the shut-in injection well as a producing well having the injection well performance model determined during normal operation of the injection well, and iii) determining a crossflow rate in the injection well at a location of the subsurface leak in the injection well based on the injection well performance model of the modeled injection well and the injection well performance model of the modeled producing well.