A comprehensive study on huff-n-puff oil recovery in tight sedimentary tuff oil reservoirs

Huff-n-puff (HnP) is currently an important means to enhance the oil recovery of tight oil reservoirs. In this study, taking tight sedimentary tuff oil reservoirs in Santanghu Basin, China as an example, the oil production law, pore utilization characteristics, and influencing factors of water, N2, CO2, N2–water alternating, and CO2–water alternating HnP (WHP, NHP, CHP, NWAHP, CWAHP) are comprehensively studied. The results show that middle pores (MPs) are the main pore utilization interval for various HnP mediums. The recovery rates (RRs) of five rounds of WHP, NHP, and CHP are 39.32%, 44.45%, and 61.24%, with contribution rates (CRs) of MPs being 68.60%, 47.87%, and 56.00%. The RRs and CRs of various pores are closely related to their pore distribution and the oil recovery mechanism of the medium. Under the experimental conditions of this study, the RR of CHP does not have an obvious rock sample size effect, with sweep depth exceeding 1.25 cm in each round. The RR of CHP is positively correlated with injection pressure and shut-in time, and the injection exceeding the rock fracture fracturing and 300 min shut-in time are recommended. The RRs of six rounds of NWAHP and CWAHP are 51.98% and 62.75%, with CRs of MPs being 60.39% and 58.32%. The oil recovery effect (ORE) of NWAHP is better than that of their individual, while the ORE of CWAHP is better than that of WHP but close to that of CHP. The alternating sequence affects the stage RR of NWAHP and CWAHP. The NWAHP and CWAHP have great advantages in improving RR, reducing injection cost, and stabilizing production, and are expected to achieve rapid development.