We studied plugging of crossflow behind oil-well casing with nitrogen foam under reservoir conditions. We have experimentally demonstrated the feasibility of the gas-liquid injection mode and suggest injecting the foam into the water-producing layer. If the nitrogen is injected first, the pressure in the oil-producing layer exceeds the pressure in the water-producing layer. The nitrogen can displace some of the water from the pores, reducing dilution of the foaming agent. As the experimental results show, the plugging strength of the foam injected into the water-producing layer is reduced as the plugging time increases. When the plugging time is longer than 200 hours, the pressure gradient can exceed 1.22 MPa/m and as a result, the nitrogen foam continues to plug the water. However, the lifetime of the nitrogen foam in the oil-producing layer is limited. The experimental study of plugging crossflow behind the casing with nitrogen foam and the field tests conducted confirmed the feasibility of plugging crossflow behind the casing with nitrogen foam.