Constant GOR as an infinite acting effect in reservoirs producing below (dew/bubble) saturation pressure

In recent years, constant producing gas-oil-ratio (GOR) trends have been repeatedly observed in many unconventional oil and gas reservoirs. This effect can last many years, even after the depleting system is believed to have crossed its saturation pressure. In this study, we corroborate that constant GOR could indeed be expected in depleting reservoir systems exhibiting early-transient infinite-acting flow under constant bottomhole pressure (BHP) production constraint for both linear and radial flow geometries. This effect had been shown to be present in infinite-acting linear systems by previous authors; but this study shows how this GOR trend and value can be fully predicted prior to availability of production data. Focusing on liquid-rich gas systems, we start with the applicable multiphase governing flow equations and provide semi analytical and rigorous solutions. Similarity theory is applied to transform the resulting system of equations and achieve the multiphase solution. The results show that a constant GOR effect could be maintained as long as the flow regime remains infinite-acting, which is a typical condition to be found in unconventional systems. Also, it is found that, in both linear and radial multiphase systems, this constant or stabilized GOR is shown to be present regardless of condensate phase mobility at sandface conditions.