Experimental study on liquid production law, oil recovery mechanism, and influencing factors of water huff-n-puff in the tight sedimentary tuff oil reservoir

Water huff-n-puff (WHP) is an effective means of improving the recovery factor (RF) of the tight sedimentary tuff oil reservoir. In this study, through the tight oil WHP experimental platform monitored by online nuclear magnetic resonance, the liquid production law, oil recovery mechanism, and influencing factors of tight sedimentary tuff WHP were investigated. The results show that the optimal cut-off pressure and depletion rate of core natural energy depletion are 5 MPa and 0.3 ml/min, respectively. WHP can effectively improve the RF of the tight sedimentary tuff, and the core RF increased by 23.04% after five cycles of the WHP. In multiple cycles of the WHP, the volume of the injected water and produced liquid of each cycle is relatively stable, but the oil production gradually decreases. The first three cycles of the WHP are high-efficiency stage, the fourth and fifth cycles are low-efficiency stage, and there is no WHP value after the fifth cycle. The range of the WHP oil production gradually extends from the injection end to the far end with increasing huff-n-puff cycles, and the oil comes mainly from the medium pores of the core. Both elastic and imbibition displacements are important mechanisms for WHP oil recovery. In five cycles of the WHP, the proportion of elastic displacement oil production to the total oil production of each cycle was 89%, 88.22%, 85.92%, 22.78%, and 0%. When the water injection pressure of each cycle is fixed, the elastic displacement oil production of the WHP is mainly affected by the water shielding, multiphase flows resistance, carrying and scouring, and capillary resistance. Meanwhile, imbibition displacement oil production is mainly affected by the contact area of oil and water. The RF of the WHP has a parabolic relationship with the water injection and reinjection pressures, and a positive correction relationship with the well soaking time. • Water huff-n-puff technology can effectively improve the recovery factor of the tight sedimentary tuff oil reservoir. • Medium pores are the main force of multiple cycles of water huff-n-puff oil production of the tight sedimentary tuff. • With increasing huff-n-puff cycles, the proportion of elastic displacement oil production gradually decreases. • There are optimal values for injection and reinjection pressures, and the well soaking time can be extended appropriately.