Summary: Water huff ‘n’ puff is an effective enhanced oil recovery (EOR) technology for tight oil reservoirs. However, the oil production of horizontal wells declines seriously after several huff ‘n’ puff cycles, and a large amount of oil is still trapped in the reservoir due to the heterogeneity of fracturing sections. The temporary plugging agent had been used for plugging high-permeability areas and thus diverting the following fluid into small permeability areas. It would improve the sweep efficiency of flooding fluid, enhancing oil recovery. However, the use of the oil-soluble particulate temporary plugging agent in the water huff ‘n’ puff application is barely reported. Therefore, the feasibility and influencing factors of oil-soluble particulate temporary plugging agent-assisted water huff ‘n’ puff (TAWHP) in enhancing oil recovery was investigated in this study. First, based on the evaluation of the performance of the oil-soluble particulate temporary plugging agent, the oil recovery of fractured core samples with different apertures for water huff ‘n’ puff and TAWHP was compared via the parallel-core experiment to verify the feasibility of TAWHP in enhancing oil recovery. The temporary plugging agent had good oil solubility, a low residual rate in the formation, and little damage to the formation. The oil recovery yielded by TAWHP was 5.17% higher than the traditional water huff ‘n’ puff process. More oil (i.e., about 1.71%) could be expelled from the fractured core samples with a small aperture. It indicated that the EOR performance yielded by water huff ‘n’ puff after several cycles could be enhanced by adding the oil-soluble particulate temporary plugging agent. After that, a mathematical model of TAWHP was established to investigate the effect of TAWHP parameters on EOR performance. The simulation results showed that the cumulative oil production increased with the increase in injection time of the temporary plugging agent solution, but the trend would level-off after 10 minutes. Moreover, as the diversion index increased, the effect of the injection rate on cumulative oil production gradually enhanced while the effect of the soaking time gradually weakened. Furthermore, the difference in cumulative oil production at different diversion indexes gradually increased as the huff ‘n’ puff cycle increased. This work could provide theoretical guidance for water huff ‘n’ puff enhancing oil recovery after several cycles.

In the waterflooding development of fractured ultra-low permeability reservoirs, the heterogeneity is becoming increasingly serious. The development of large fracture channels leads to serious water channelling and low recovery, and the effect of conventional profile control is not ideal. This paper proposed gel foam composite profile control and flooding technology to solve the above problems. Herein, the new intelligent gel and foaming agent systems were optimized through laboratory experiments, and their performance was evaluated. The new intelligent gel system has the characteristics of low viscosity, easy preparation, good injection, slow cross-linking, high strength, and long-term effectiveness. The injection parameters were optimized, and the indoor injection scheme was formulated, that is, the optimal injection volumes of gel and foam slugs were 0.3 and 0.6 PV, respectively. The injection sequence of composite slugs was to inject gel slugs first, then foam slugs. The injection mode of air foam slugs was multiple rounds of small slug injection. The final recovery rate in the indoor dual tube oil displacement experiment reached 35.01%, increasing by 23.69%. Furthermore, an oil output increase of 899 t and an average water cut decrease of 5% were acquired in the oil field test. It shows that the injection scheme can effectively improve oil recovery. The gel foam compound profile control and flooding technology herein has good adaptability in similar reservoirs and has good promotion prospects.

High-angle fractures are widely developed in ultra-low permeability reservoirs in the Ordos Basin, and their propagation has a significant influence on waterflooding performance. However, few studies have focused on the identification of high-angle fractures, let alone the impact of their propagation on waterflooding performance. Therefore, a waterflooding model considering dynamic fracture propagation was developed to evaluate the impact of fracture propagation on waterflooding performance in this study. Firstly, based on the logging response characteristics of high-angle fracture intervals, the high-angle fracture identification model was established by the fuzzy comprehensive evaluation method, and a comprehensive index was proposed to identify high-angle fracture intervals. A threshold of approximately 70 effectively distinguishes high-angle fracture intervals from non-high-angle fracture intervals. After that, the waterflooding model considering dynamic fracture propagation was established to investigate the effect of its parameters on waterflooding performance. A smaller initial leak-off coefficient and a faster decline rate reduce fluid loss, promote fracture propagation, and ultimately improve oil recovery. Higher injection rates promote dynamic fracture propagation, accelerate water cut rise, mitigate production decline, and enhance cumulative oil production, while improving the waterflooding response of wells along the fracture direction. Optimal well group performance is achieved at a fracture orientation of 45°, beyond which production declines. Increasing well spacing reduces the water cut rise rate and the production decline rate, thereby improving cumulative oil production. However, it leads to a faster production decline in the early development stage. This work could provide theoretical guidance for waterflooding in ultra-low permeability reservoirs.