Gas-oil ratio forecasting in unconventional reservoirs

A method for modeling gas-oil ratio (GOR) for a well in an unconventional reservoir, comprising:

• receiving a tubing head pressure and flowrates of oil, gas, and water for the well during well production;
• determining a bottomhole pressure (BHP) for the well based, at least in part, on the tubing head pressure and the flowrates of oil, gas, and water;
• determining one or more pressure, volume, and temperature (PVT) properties associated with fluid flow from the well based at least on the flowrates of oil, gas, and water;
• determining a dataset of cumulative oil production values and cumulative gas production values for a plurality of time steps based on the flowrates of oil and gas for the well;
• analyzing the dataset to determine an inflection point in a log-log relationship between cumulative oil production and cumulative gas production, the inflection point representing a point at which gas produced from the well is solution gas;
• for one or more groups of continuous time steps following the inflection point, estimating average reservoir pressure by performing history matching of an estimated cumulative GOR to an observed cumulative GOR of the well, the estimated cumulative GOR being dependent at least on the average reservoir pressure, and the observed cumulative GOR calculated using the cumulative oil production and the cumulative gas production, wherein the history matching is performed based at least on the BHP and the PVT properties;
• determining a productivity index (PI) for the well for the plurality of time steps based at least on the estimated average reservoir pressure;
• performing a liquid production rate forecast for the well based at least on the determined PI;
• and
• forecasting a cumulative GOR for the well based at least on the liquid production rate forecast.