This paper is a continuation of earlier papers on the development of the SAGP process presented at Annual Technical Meetings of the Petroleum Society.
SAGP improves the thermal efficiency of SAGD by adding non-condensible gas to the steam. Significant steam savings are achieved by lowering the average temperature in the reservoir and by reducing heat loss to the overburden. Rising gas fingers increase the pressure towards the top of the reservoir and displace oildownwards even though the temperature is below that of saturated steam.
The gas hold-up in the reservoir equals the volume of the produced oil, with allowance for the effects of pressure, temperature and partial pressure of the steam. The gas in the chamber comes from the combination ofadded gas, solution gas and gas generated by chemical reactions, minus gas produced with the oil and the net gas driven to or coming from outside the depleted region by pressure difference. The gas hold-up and gas dissolved are estimated and it is found that, in general, more gas is required for higher pressures.
Effects of layered sands on SAGD and SAGP performance are studied experimentally using a physical model. In SAGD, steam spreads below low-permeability layers and oil can not drain from the above until the steam vapour can penetrate to replace it. In SAGP, gas fingers rise into low-permeability layers and displace the oil downwards below steam temperature. Mechanisms for the enhancement of oil drainage from the lowpermeability layers by gas fingers are discussed together with the experimental results.
SAGP continues to show promising results and it is thought that results in the field will be better than in our experiments.
Several papers [1,2,3,4] have been presented on the theoretical and laboratory studies of the Steam and Gas Push (SAGP) process since it was described first in 1997 at the 48th Annual Technical Meeting of the Petroleum Society of CIM. The process improves the thermal efficiency for SAGD by adding a small amount of noncondensible gas to the injected steam. Steam condenses and leaves concentrated gas in the upper portion of the vapour chamber. As a result, only the region near the injector and producer, where the gas concentration is low, is heated to the temperature of saturated steam. The upper part of the reservoir remains at relatively low temperature and the heat loss is low; significant steam is saved.
The gas required to achieve the SAGP effect is obtained from gas dissolved in the oil, gas generated by chemical reaction, net reservoir gas flowing into the recovery region and gas supplied from injection. In heavy oil and bitumen reservoirs such as those in Athabasca and Cold Lake the operating pressure for SAGD is usually higher than the initial reservoir pressure and gas injection is required.
The required gas hold-up in the vapor chamber is related to the volume of oil produced, with allowance for operating pressures and temperature. To make the injected gas more effective in the chamber, the production of gas should be minimum.