Determination of shut-in time of shale gas well: Implication from differences in pore structure and imbibition characteristics of marine shale reservoir, SW China

Abstract Shut-in after fracturing is one of the important technical means to improve the recovery of shale gas well. At present, there is no strong scientific basis to support the formulation of shut-in time after fracturing. Our work has established a method to determine the reasonable shut-in time of shale gas well after fracturing by combining laboratory experiment data and field production data. For shale reservoirs with well-developed pores in the study area, the maximum water imbibition per unit volume and equilibrium time are 0.018542 g/cm 3 and 18 days, which are much larger than those shale reservoirs with micro fractures (0.00939 g/cm 3 and 9.6 days). Shale core will not generate new MF after water imbibition, but will form dissolution pores and expand existed MF. The time for the width of MF to reach its maximum value after water imbibition for shale cores with developed pores is 14 days, which is greater than that in shale cores with developed MF (7 days). The permeability of shale reservoirs with developed pores decreases by 8.1% after water imbibition, while the permeability of shale reservoirs with developed micro fractures (MF) loses by 92.5%, indicating that the gas phase flow channel of the former is better, while the latter has water blocking the gas flow channel and needs to be discharged as soon as possible. So, the former requires longer shut-in time than the latter. The purpose of shut-in is to maintain the opening of artificial fractures and the stability of formation energy. The time when the formation closure pressure is equal to the bottom hole flow pressure is selected as the shortest shut-in time, the time when the volume of core water imbibition is the highest is selected as the maximum time, and the best shut-in time is when hydration promotes the MF width to expand to the maximum value. Therefore, shale gas wells with good MF development, the shut-in time is 3 ~ 9.6 days, and the optimum shut-in time is 5 days; shale gas well with good pore development, the optimum shut-in time is 12.5 days.