Abstract: The Ordovician oil reservoirs in the Tahe oil field are typical carbonate fractured-vuggy oil reservoirs. The reservoir spaces and immigration channels mainly consist of pores, caves and fissures, which were formed by superposition reformations including tectonic action, karstification and diagenesis. In view of the rapid decline of formation energy, water injection replacing oil has been implied in a great scale in the Northwest oil field since the success of water injection in well TK741 in 2005. However, the failure wells in water injection replacing oil are gradually increasing, and the failure well proportion is up to 22% in No. 3 Oil Production Plant region. The failure reasons of water injection replacing oil were analyzed and summarized based on injection-production ratio, water injection displacement, energy indicative curve and water injection indicative curve. Material balance mechanism and oil reservoir engineering technique were applied to analyze remaining oil in view of different fai-lure reasons. Some countermeasures were proposed and had achieved good effects in field practices.