In response to the issues of poor water flooding efficiency, low oil production rates, and low recovery rates during the high-water-cut period in the low-permeability reservoirs of the Mutou Oilfield, the non-steady-state (NSS) CO₂ huff-n-puff oil recovery technology was explored. The NSS CO₂ huff-n-puff can improve the development effect of low-permeability reservoirs by replenishing the reservoir energy and significantly increasing the crude oil mobility. Experimental investigations were carried out, including a crude oil and CO₂–crude oil swelling experiment, minimum miscibility pressure testing experiment, high-temperature and high-pressure microfluidic experiment, and NSS CO₂ huff-n-puff oil recovery on-site pilot test. The experimental results showed that the main mechanisms of NSS CO₂ huff-n-puff include dissolution, expansion, viscosity reduction, and swept volume enlargement, which can effectively mobilize the remaining oil from the various pore throats within the reservoir. The high-temperature and high-pressure microfluidic experiment achieved an ultimate recovery rate of 83.1% for NSS CO₂ huff-n-puff, which was 7.9% higher than the rate of 75.2% obtained for steady injection. This method can effectively utilize the remaining oil in the corners and edges, enlarge the swept volume, and increase the recovery rate. Field trials of NSS CO₂ huff-n-puff in a low-permeability reservoir in the Mutou Oilfield indicated that it cumulatively increased the oil production by 1134.5 tons. The achieved results and insights were systematically analyzed and could provide key technical support for the application of NSS CO₂ huff-n-puff technology in low-permeability reservoirs, promoting the innovative development of this technology.

This study presents a tailored nitrogen foam flooding system developed for the Mu146 block’s medium-high permeability reservoir conditions. Through systematic optimization, we establish an optimal formulation comprising 0.40% FP2398 foaming agent and 0.13% WP2366 stabilizer. The formulated foam demonstrates superior performance characteristics with a generated volume of 850 mL and extended stability duration of 1390 s, exhibiting exceptional structural integrity under oil-bearing conditions. Core flooding experiments conducted on berea cores reveal a 33.20% incremental oil recovery factor following waterflooding that achieves 53.60% primary recovery. The non-steady-state nitrogen foam huff-and-puff (NSSNFHF) field test at Well Mu146-61 shows significant reservoir response, with post-treatment analyses indicating an average chloride ion concentration increase of 540.20 mg/L and total salinity elevation of 1194.20 mg/L across five monitoring wells. These chemical signatures confirm effective volumetric sweep enhancement through the NSSNFHF field test, demonstrating a flooding-like mechanism that mobilizes bypassed oil in previously unswept zones. The field test encompassing Well Mu146-61 and four offset producers yield substantial production improvements, including a 74.55% increase in fluid production rates and a sustained oil yield of 1.80 tons per day. The validity period of the NSSNFHF field test is more than 12 months. The technology demonstrates dual functionality in conformance control and enhanced recovery, effectively improving both oil productivity and ultimate recovery factors.