Physical Simulation and Improved Numerical Simulation Method of Non-Condensable Gas Co-Injection in SAGD Process

Abstract

Abstract There are great differences about the distribution of non-condensable gas (NCG) in steam chamber between field observations and numerical simulations. Field observations show that the process could reduce steam consumption significantly but no evidence of impairment to oil production especially in mid to late SAGD life. However, this point isn't consistent with current numerical simulations. Therefore, this paper presents an improved numerical simulation method of non-condensable gas co-injection to validate the physical simulation results. Solubility experiments of methane, nitrogen and carbon dioxide in oil phase at different pressure and temperature were conducted to analyze the interface properties, and 1D core experiments to understand the displacement characteristics. On the basis, 3D physical simulations were conducted with each gas in SAGD process. These results were inconsistent with conventional numerical simulations. In order to narrow the gap, sensitivity analysis was conducted by using CMG STARS software considering different grid size, solubility or K-value in oil and water phase, rate-dependent dissolution and ex-solution of NCG in oil. Non-equilibrium dissolution and ex-solution behavior and reaction equations were incorporated to successfully improve the numerical simulation results. The experimental tests indicate that the solubility and compressibility of methane fall in between nitrogen and carbon dioxide. Then 3D visualized physical simulations with methane, nitrogen or flue gas co-injection and shale interlayer existence were conducted to analyze the law of NCG distribution, steam chamber growth and SAGD performance. NCG tends to accumulate at the top rather than the drainage edge of steam chamber, decrease the temperature of reservoir top by about 30°C, reduce heat loss to the cap rock and improve steam to oil ratio (SOR). Compared with other gases, carbon dioxide decrease oil viscosity most and achieve higher displacement efficiency. Also, it succeeds in improving the flow characteristics of steam chamber front, maintaining pressure, stabilizing or slightly lowering oil production and reducing steam injection significantly by co-injection with these NCG at increasing proportion, when steam chamber reaches the top of reservoir. Particularly, NCG can penetrate thin shale interlayer, allow steam passing through and help accelerate vertical steam chamber growth. Finally, the parameters were optimized using improved field-scale model such as solvent type, co-injection concentrations and timing, etc. The novelty of this paper is successfully revealing NCG distribution law in steam chamber by visualized physical simulations. Particularly, it also captures the phenomenon that the existence of NCG can help steam chamber penetrate the shale interlayer. The findings narrow the gap among field observations, physical and numerical simulation and can guide NCG co-injection optimization.