Page 153 - Fundamentals of Enhanced Oil and Gas Recovery

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Thermal Recovery Processes

directions. Of course, in the absence of oxygen supply, the flame might shut off. This
is an economical process while the oil price is within 30 35 $/bbl.
The difference between boiling point of water at the lower well pressure and the
temperature of the producer is called subcool. The producer temperature is lowered
by increasing the liquid level in upper well which yields to higher subcool. As the
actual systems are highly heterogeneous, there would not be a uniform subcool
through the whole length of the well. In practice, a portion of steam is kept in the
producer to keep the bitumen warm and lower its viscosity which leads it to flow
toward colder regions of the reservoir. As process has a long shut-in time or start-up,
the steam is circulated in the lower well. This process is called partial SAGD. From
thermal stand point, low value of subcool would not be beneficial. This includes
reduction in the rate of steam injection. Low temperature would actually result in vis-
cosity increase and decrease in bitumen mobility [13]. Another point is that as the
subcool is very high, the steam pressure would be low, and maintaining the chamber
would not be possible leading the steam chamber to collapse. The condensed steam
would prevent the chamber development. If the processes of injection and production
are continuously operated at reservoir pressure, this would remove the instability issue
in this process. The output of SAGD process would be 70% 80% recovery of the
OIP for suitable reservoirs. This process is not affected by vertical barriers for fluid
flow and steam. By heating the rock and considering the conduction mechanism of
heat transfer, fluid and steam flow into the production well. This mechanism would
lead to 60% 70% oil recovery even with many shale barriers [13].
This technique was first proposed by Roger Butler, the engineer at Imperial Oil at
1970s. Then, he worked as the director of technical programs of Alberta Oil Sands
Technology and Research Authority (AOSTRA) which developed new technique for
increasing the recovery of heavy oil and oil sand. They welcomed SAGD technique
[14]. This process as the improvement to steam injection technique was applied for
Kern River Oil Field in California [15]. As this field was produced using CSS tech-
nique, the oil was recovered from some specific portions of oil sands, for instance,
Cold Lake oil sands. This approach was not efficient for bitumen production from
deeper layers in oil sand, namely, Athabasca and Peace River oil sands. However, most
of the reserves lie in that area. This led to developing the SAGD process in order to
increase the oil recovery with cooperation of AOSTRA and industry partners with
Bulter [16].
A number of geological formations that apply SAGD include Clearwater
Formation, Lloydminster Sand of the Mannville Group, General Petroleum Sand,
McMurray Formation, Grand Rapids Formation, a Stratigraphic range in the Western
Canadian Sedimentary Basin Canada which is now the largest supplier of the United
States oil with over 35% supply capacity. This value is more than the contributed share
of Venezuela or Saudi Arabia and also the OPEC countries [17]. SAGD technology