When non condensable gases (NCGs) are co-injected with steam in SAGD, the steam oil ratio is reduced without negative effects in the oil production rates. But most simulation studies show the opposite effect. In this work, a critical literature review is conducted to understand the mechanisms affecting the response of NCG in numerical simulations of SAGD, and a simulation study is performed to investigate the parameters affecting the NCG co-injection process.
The simulation tests were run in a two-dimensional homogeneous model based on available data of Surmont reservoir using CMG STARS. According to previous simulation studies, the time of co-injection, solubility data, ex-solution rate, injection pressure and amount of NCG co-injected are important mechanisms affecting the simulation of NCG in SAGD. These parameters were evaluated independently in the simulation study. In addition, the diffusion of methane in the gas phase and gas-liquid relative permeability curves were evaluated to study the flow of the gas in the steam chamber.
As observed in previous studies, we also found that the time of co-injection, the amount of NCG co-injected and the injection pressure are critical for the design and optimization of an NCG co-injection process in SAGD. However, we found that the gas-liquid relative permeabilities are the parameter that most affects the simulation results. There are not congruent gas-liquid relative permeability curves in the literature for the SAGD reservoir simulation, and if the proper gas-liquid relative permeability curves are not considered, the design of the process using numerical simulators may not represent the real field behaviour. The liquid production rates and ex-solution rate of the gas are also important mechanisms affecting the simulation of NCG in SAGD. Contrary to what was expected, there was not a significant effect in the simulation when the diffusion and solubility of gases in the liquid were evaluated.