Asynchronous and synchronous huff-n-puff water injection for enhanced oil recovery in water-wet tight oil reservoirs

The exploitation of tight oil faces significant challenges due to their low permeability, complex pore structures, and rapid production decline rates. Water-based enhanced oil recovery (EOR) methods hold great promise due to their low cost and wellbore safety. However, their effectiveness is significantly constrained by low injectivity and clay swelling. The water huff-n-puff strategy has demonstrated success in the Tuha Oilfield achieving economic production. Nevertheless, deeper mechanistic understanding is needed to elucidate the underlying recovery mechanisms. In this study, the effectiveness of two water-based EOR methods—asynchronous huff-n-puff (ASHNP) and synchronous huff-n-puff (SHNP)—was investigated using core flooding experiments on tight cores from the Santanghu Basin, China. Nuclear magnetic resonance was utilized to analyze oil recovery, characterize water front movement, and evaluate displacement dynamics across multiple huff-n-puff cycles. Results indicate that ASHNP exhibits a water front advancement pattern like power function with a power exponent of 0.67, while all the swept cores exhibited a recovery as high as 40%. In contrast, SHNP demonstrated rapid initial advancement in water front, which is twice that of ASHNP. However, the oil recovery of the rock not adjacent to the inlet is less than 30% and it decreases sharply with increasing depth. The reservoir fluid between two wells would be locked due to the symmetrical injection. Comparative analysis suggests that ASHNP is a more effective water injection strategy for water-wet tight oil reservoirs with deeper water penetration depth and higher sweeping efficiency. These findings provide insights into optimizing water-based EOR methods for tight oil reservoirs.