Determining wellbore leak crossflow rate between formations in an injection well

A computer-readable medium storing instructions executable by one or more processors to perform operations comprising:

• during normal operation of an injection well formed in a subterranean zone,
• receiving a plurality of injection flow rates at which an injection fluid is flowed into an injection well formed in a subterranean zone;
• receiving a plurality of surface injection pressures measured at a surface of the injection well for the respective plurality of injection flow rates;
• determining, by the one or more processors, a plurality of bottomhole pressures at a bottom of the injection well based on the respective plurality of surface injection pressures and the respective plurality of injection flow rates;
• modeling, by the one or more processors, a flow of the injection fluid through the injection well from the surface to a bottom of the injection well;
• 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 the subterranean zone through the injection well:
• modeling, by the one or more processors, the shut-in injection well as a producing well having a same model as the modeled injection well during normal operation;
• assigning the subsurface location of the leak as a top node of the modeled producing well;
• and
• determining a crossflow rate in the injection well at the location of the subsurface leak in the injection well based on the modeled producing well.