Abstract: An apparatus for estimating a gas-oil ratio for a well in an unconventional reservoir comprises a processor associated with a control system. The processor is configured to determine a bottomhole pressure for the well. The processor is further configured to determine one or more pressure, volume, and temperature (PVT) properties associated with fluid flow and to determine transient well performance. The processor is further configured to determine a Productivity Index (PI) for the well for a plurality of time steps and to determine an inflection point. The processor is further configured to estimate the gas-oil ratio of the well after the inflection point based on the determined transient well performance and to perform the fluid production rate forecast based on the determined PI. The processor is further configured to apply the fluid production rate forecast to the estimated gas-oil ratio to determine an instantaneous gas-oil ratio.