Page 66 - Fundamentals of Enhanced Oil and Gas Recovery

54 Ehsan Mahdavi and Fatemeh Sadat Zebarjad

quality but low amount of steam should be injected into the well for the first cycle
since the damage and plugging near the wellbore will be pushed far down in the res-
ervoir and the flow back process to eliminate the plugging will get harder to perform.
The production rate in the second and third injections are known be higher than the
first cycle.
The number of the steam stimulation cycles is required to be economical (6 7
times) and not more than 10 times [61], while the maximum production rate is
observed in the second and third cycles. It is also recommended that when oil pro-
duction rate reaches one-third of initial value at start of the cycle, the cycle should be
ended and the next cycle should be initiated; actually this is highly proposed to main-
tain the performance of the cycles high enough.
Cold Lake oil field in Canada, Midway-Sunset oil field in the United States, and
Gudao field in China are some CCS projects worldwide [15].

2.4.3 Steam-Assisted Gravity Drainage

Steam-assisted gravity drainage (SAGD) is one of the in situ thermal recovery methods
which was recently discovered to extract extraheavy oils and bitumen from Alberta’s
reservoirs [71]. 1.7 trillion barrels of bitumen is laying in Alberta making it second
large hydrocarbon resource on earth [72]. High recovery rates have been reported up
to 70% of OOIP, and most of the cases are in Canada. Most of the commercial
SAGD projects have been carried out in Canada, where there are many heavy and
extraheavy oil reservoirs; almost all successful commercial SAGD projects have been
reported in the reservoirs of Athabasca region (McMurray formation), for instance,
Hanginstone, Foster Creek, Christina Lake, and Firebag reservoirs [73,74]. In the case
of fractured carbonate reservoirs, no promising economically efficient result is
expected in a fractured reservoir, due to the existence of fracture networks leading to
early breakthrough and low oil recovery factor.
Commonly in SAGD method, two horizontal wells are drilled in the target zone
with the distance of 4 6 m from each other. The steam is injected through the upper
well which is called “steam chamber,” and the heat diffusing in the formation mobi-
lizes the heavy crude oil and increases its viscosity; thus, oil flows to the lower well
due to gravity and then is produced from the lower well.
The energy produced from the steam is roughly divided into three portions, one-
third of which is lost in the formation rock. The second part remains in the chamber
and last part is produced [75]. Therefore, large amount of natural gas is required as
fuel to produce the steam for SAGD treatment that results in considerable amount of
greenhouse emission, also high treatment cost. To overcome the aforementioned
issues, a method recognized as solvent SAGD treatment adds a chemical to reduce the
energy consumption [76,77]. It is worthwhile to note that an average cumulative