Optimization of huff-n-puff in shale oil horizontal wells based on EDFM

Continental shale oil horizontal wells have fast decreasing natural production and low recovery, for which single well water injection huff-n-puff can effectively replenish formation energy and improve recovery. Taking Ordos Chang7 shale oil as an example, numerical simulation method is used to carry out the optimisation study of water injection huff-n-puff in horizontal wells of continental shale oil. To enhance the accuracy of the numerical simulation model for shale reservoirs after volume fracturing, the Embedded Discrete Fracture Model（EDFM） is introduced. This model characterizes both natural and hydraulic fractures resulting from volume fracturing. Additionally, a conceptual model that considers imbibition and reservoir stress sensitivity is established. The timing, volume, and speed of injection, as well as the soaking period and huff-n-puff cycle, are optimized based on simulation results. These results indicate that a too-rapid injection rate causes water to flow along the fractures, decreasing the utilization rate of the injected water. As the huff-n-puff cycle increases, the oil increment per cycle tends to decrease. For the specific case of a shale reservoir in Ordos, the optimization of huff-n-puff parameters is as follows: Water injection should commence when the pressure coefficient drops to 0.706, with an optimal injection volume of 4 000 m³ at a rate of 300 m³/d. The recommended soak period is 15 days, with a total of six huff-n-puff rounds. This approach can increase the recovery rate by 4.95% and achieve a total oil-water replacement rate of 6.65%. This study provides valuable insights for water injection huff-n-puff strategies in shale reservoirs.