A system and method is provided that determines at least one bottomhole condition during flowback operations in a well that traverses a hydraulically fractured reservoir. The system and method measure fluid properties of fluids produced at a surface-location of the well during the flowback operations. A transient fluid flow simulator determines composition and properties of fluids in the well between the surface-location of the well and at least one bottomhole-location of the well based on the measured fluid properties. At least one bottomhole condition in the well is determined based on the composition and properties of fluids in the well between the surface-location and at least one bottomhole-location of the well.