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.