The fractured-vuggy carbonate reservoirs formed under different karst geological backgrounds in Tahe oilfield are being developed by nitrogen injection, with varying effects and unknown controlling factors, which will affect overall planning and deployment of subsequent nitrogen injection. On the basis of revealing main mechanism of nitrogen injection to enhance oil recovery in fractured-vuggy reservoirs, by using the “two baselines and three zones” economic evaluation method for nitrogen injection and field statistics method, the effects of nitrogen injection in these fractured-vuggy reservoirs were clarified, and the key controlling factors were analyzed on the basis of the dynamic and static parameters of the reservoirs. The results indicate some differences in various reservoirs: for weathered crust reservoirs, the proportion of ineffective wells is 40% for individual wells, and 31% for well groups, demonstrating the problems such as long gas injection time and difficulty in continuing conventional gas injection; for composite reservoirs, the proportion of ineffective wells is 24% for individual wells, and 27% for well groups, remaining in the stage of low-cycle gas injection and promising for nitrogen injection in the future; and for fault-controlled reservoirs, the proportion of ineffective wells is 57% for individual wells, and 66% for well groups, recording the poorest adaptability to gas injection. Key factors controlling the single-well gas flooding effect are determined as the reservoir type, attic size, reservoir compartmentalization, structural amplitude, remaining oil reserves at the vug top, energy of the bottom water, and injection/production parameters. Key factors controlling the nitrogen injection effect of well-groups are clarified as the injection-production site, dominant channel between wells, aquifer volume multiple, injection/production parameter, and injector pattern.
