While heavy oils can be produced by horizontal wells at economic rates, the extent of recovery is small because of loss of drive and/or watering out of the production. In typical cases a recovery of only a few percent of the OOIP is achievable. SAGD overcomes these problems by allowing high production rates with only a small drawdown. The reservoir pressure is maintained by the injection of the steam and operation without steam coning is practicable; very high recoveries are normally obtained.
In conventional SAGD the depleted reservoir becomes filled with steam at the saturation temperature corresponding to the operating pressure - typically 200 to 250 °C. Large quantities of steam are required to heat the depleted reservoir, the reservoir surrounding the steam chamber and, particularly with thin reservoirs, the overburden. With high quality, thick reservoirs SOR's of about 2.5 to 3 are obtained but in thinner reservoirs, and in reservoirs with lower oil saturations, SOR's are much higher and the process is uneconomic. The problem is compounded by the steam chamber having long sloping boundaries that extend the heated area.
In this paper it is shown that it is not necessary for the whole steam chamber to be at a uniform high temperature. Only the lowest part, where the coning tendency is greatest, needs to be at the highest temperature. High temperatures in the upper part of the chamber are an " "overkill" that results in wasted heat. It is proposed that the SAGD process be modified by injecting non-condensible gas, such as natural gas, with the steam from a horizontal injection well somewhat above the producer. Gas accumulates in the chamber above the injector and lowers the temperature. Considerable savings can be made by this approach without a substantial decline in performance. In one example it is shown that the heat stored in the chamber per cubic meter of produced oil will be only 62 % of that for conventional SAGD and that the heat lost to the overburden will be a very much smaller fraction. This concept should greatly extend the range of reservoirs that can be produced economically.
For the efficient and economic recovery of heavy oils it is necessary to employ means for facilitating the flow of the viscous fluid through the reservoir. Not only is it necessary to offset the effect of their very high viscosities by providing extended contact and/or viscosity reduction by heating, but it is also necessary to maintain the drive needed to move the oil as the reservoir becomes depleted.
Some approaches such as the use of long horizontal wells or cyclic steaming are effective in promoting flow but only produce limited recovery. Steamflooding and, more recently, SAGD also maintain reservoir drive and allow high recoveries. However, because of their considerable heat requirements, these processes are limited in their economic use to higher quality reservoirs.
It is the purpose of this paper to describe a new concept for the production of heavy oils that is a modification of the SAGD process[1, 2, 3, 4,5].