Effect of Oil-Viscosity-Gradient Presence on SAGD

The presence of an oil-viscosity gradient appears to be a real phenomenon in the field, and its influence on the steam-assisted-gravity-drainage (SAGD) process has been studied by various researchers. However, there is potential to refine previous results to exploit the impact of viscosity gradients on SAGD-oil-recovery-process performance. This paper covers our simulation study of the impact of an oil-viscosity-gradient presence on SAGD operation. Various SAGD operating scenarios in the reservoir with the presence of a vertical oil-viscosity gradient were investigated and simulated. The conclusions are summarized as follows: 1. The presence of a vertical oil-viscosity gradient in the reservoir had a mild effect on the SAGD-oil-recovery rate, while the effect on the oil-recovery factor was insignificant. 2. J-well SAGD or J-well and gravity-assisted steam stimulation (JAGASS), in which a J-well producer, the production well that is inclined along its length, is placed below the injector well, did not perform as well as SAGD with a standard well configuration (two parallel horizontal wells). These two conclusions are not in agreement with previous findings by other research groups. 3. Observations from some SAGD field tests indicate that the steam chamber could fail to reach the top of reservoirs. For those cases, gas injection could potentially help to recover the oil from the layer above the steam chamber. If that is true, the presence of an oil-viscosity gradient might have some impact on SAGD performance with gas injection. Because local steam-trap control is required for J-well SAGD (JAGASS) operation to optimize performance, the challenge and feasibility of local steam-trap control are also discussed in this paper. Some of the SAGD field evidence will be provided to support our simulation results.