Abstract: After large-scale fracturing of shale oil horizontal wells,unreasonable shut-in and flowback-production regimes may cause sand production,wellbore blockage and other problems,thereby seriously affecting development performance.To solve this problem,a study was conducted on optimizing shut-in time and flowback-production regimes after large-scale fracturing of shale oil horizontal wells with the aim of determining the shut-in time and appropriate production regime under the current fracturing-fluid system.The results show that reservoir wettability experiments,spontaneous imbibition experiments,pressurized imbibition experiments,matrix damage experiments and fracturing-fluid flowback experiments were carried out to determine the imbibition time of reservoir cores,imbibition recovery factor,degree of reservoir damage and flowback rate of fracturing fluid,based on which the shut-in time was preliminarily determined to be 20-28 d.A shale oil horizontal well model considering imbibition was then established to simulate the velocity field,pressure field,pressure-gradient field and production performance at different times during imbibition,based on which the shut-in time was preliminarily determined to be 25-30 d.By integrating laboratory experiments,numerical simulation and production practice,the reasonable shut-in time was ultimately determined to be 20-40 d or until the daily pressure decline remained below 0.1 MPa for 7 consecutive days.By establishing a proppant flowback model and a fracturing-fluid velocity calculation model,the relationships among nozzle size,maximum flowback rate and different pressures were investigated,the nozzle size and flowback rate under different pressures were clarified,thereby establishing a reasonable flowback-production regime after fracturing.The research results were applied to the N4-2H Well area,and the initial daily oil production increased by more than 9%.These findings are of great significance for improving the initial productivity and development performance of shale oil horizontal wells in the Ordos Basin,and can also provide guidance for establishing production regimes in similar reservoirs.

Key words: shale oil, shut-in time, flowback-production regime, imbibition experiment, Ordos Basin