Impacts of Initial Gas-to-Oil Ratio (GOR) on SAGD Operations
Authors: Yuan J.Y. , Nasr T.N. , Law D.H.S. 
Published: March 2010

Abstract
This study addresses the important role of initial gas-to-oil ratio (GOR) in steam assisted gravity drainage (SAGD) operations. A numerical model using CMG's STARS was validated through history matching of laboratory experiments conducted at the Alberta Research Council. The impacts of initial GOR on process performance were then studied using field scale numerical simulations. The results indicate that high initial GOR may have beneficial effects, namely, reduction of oil viscosity, and improvement of the oil-to-steam ratio (OSR). A detrimental impact, however, is also shown as the gas impedes the rate of steam chamber growth, and hence reduces oil production rates. Further analysis indicated that this is because of a "dynamic vacuum" effect due to steam condensation at the front of the steam chamber. This dynamic vacuum effect dominates the diffusion process and creates a gas-rich zone at the front of the steam chamber, thereby resisting further growth of the steam chamber and slowing oil production. The same effect occurred when noncondensable gas was co-injected with steam in either live oil or dead oil reservoirs.
This study addresses the important role of initial gas-to-oil ratio (GOR) in steam assisted gravity drainage (SAGD) operations. A numerical model using CMG's STARS was validated through history matching of laboratory experiments conducted at the Alberta Research Council. The impacts of initial GOR on process performance were then studied using field scale numerical simulations. The results indicate that high initial GOR may have beneficial effects, namely, reduction of oil viscosity, and improvement of the oil-to-steam ratio (OSR). A detrimental impact, however, is also shown as the gas impedes the rate of steam chamber growth, and hence reduces oil production rates. Further analysis indicated that this is because of a "dynamic vacuum" effect due to steam condensation at the front of the steam chamber. This dynamic vacuum effect dominates the diffusion process and creates a gas-rich zone at the front of the steam chamber, thereby resisting further growth of the steam chamber and slowing oil production. The same effect occurred when noncondensable gas was co-injected with steam in either live oil or dead oil reservoirs.

Introduction
It is generally assumed that when the steam assisted gravity drainage (SAGD) process is applied to a heavy oil reservoir with a high initial gas-to-oil ratio (GOR), there is better production than for a reservoir with a low initial GOR. This is a fairly common assumption for good reasons; the mobility of live oil in porous media is usually higher(1) than that of dead oil. The present numerical study was initiated to investigate this assumption.
The approach involves the history match of 1D laboratory tests and field scale simulations. The history match was used to verify the numerical techniques and to provide reliable parameters for the use of field scale simulations. The results support the commonly believed assumption of viscosity reduction in 1D live/dead oil experiments. In field scale simulations, production predictions were compared for reservoirs with different initial GORs. There were additional effects beyond viscosity reduction.