The imbibition has an important influence on the water injection development of ultra-low-permeability reservoirs. In this paper, the nuclear magnetic resonance (NMR) high-temperature and high-pressure displacement system was used to simulate the formation temperature and pressure and the spontaneous imbibition of single porosity medium and dual porosity medium (containing fractures), and imbibition displacement experiments at different injection rates were carried out. The NMR T2 spectrum curves of simulated oil signals in the pores, throats, and fractures were obtained. The characteristics of oil content change and oil displacement efficiency in dual porosity medium under different experimental conditions were quantitatively evaluated, and the contribution of spontaneous imbibition to oil displacement efficiency was clarified. The experimental results show that the oil displacement efficiency of single porosity medium is lower than that of dual porosity medium. The smaller the pore is, the greater the displacement speed is, and the greater the contribution rate of imbibition is. The porosity, permeability, and maximum pore throat radius are positively correlated with the oil displacement efficiency of spontaneous imbibition but are poorly correlated with the oil displacement efficiency of under the dual action of spontaneous imbibition and displacement. The poor correlation also includes the median pressure, maximum mercury saturation, median radius, and displacement rate. Fractures play a positive role in improving oil displacement efficiency.

The Yanchang Eastern Oilfield of Ordos Basin is a typical ultra-low permeability shallow reservoir. Because of the relatively low vertical pressure, horizontal artificial fractures are prone to take place in the case of oil well fracturing. Given the bigger contact surface between the horizontal fracture and the waterflood front of the water injection well, the oil well may be flooded fast in a short time, leading to a low mobilization degree of the reservoir. According to the characteristics of waterflooding of horizontal fractures, the development mode of waterflooding at the bottom of oil reservoirs was proposed. Through core sample displacement experiments and nuclear magnetic resonance online tests, combined with numerical simulation of reservoirs, field tracer test, and other comprehensive methods, the heterogeneity of reservoirs and the limit of waterflooding parameters were optimized. The research results show that the waterflooding effect is the best when the variation coefficient of permeability is less than 0.5, the permeability rush coefficient is less than 1.5, the permeability contrast is less than 5, the waterflooding rate is 0.06 mL/min, and the waterflooding pressure is 7 MPa. Through field practice, the daily oil production of a single well in the test well group increased from 0.054 t/d before the test to 0.179 t/d, the water cut decreased from 15% before the test to 10%, the formation pressure increased by 0.18 MPa, and it is predicted that the final recovery would increase by 2%. This study provides an experimental and theoretical basis for water injection development of ultra-low permeability shallow reservoirs with horizontal fracture, and also plays a good demonstration role for high-efficiency water injection development of shallow reservoirs.