Page 64 - Fundamentals of Enhanced Oil and Gas Recovery

52 Ehsan Mahdavi and Fatemeh Sadat Zebarjad

2.4 THERMAL METHODS

Thermal EOR methods consist of injecting steam or hot water or creating com-
bustion in the reservoir which all of them increase the temperature of the production
zone that reduces the oil viscosity leading to greater oil recovery factor. The mecha-
nism of thermal EOR is to enhance the microscopic displacement efficiency by
decreasing interfacial tension and also the macroscopic displacement efficiency by
decreasing viscous forces. Other mechanism may include gas drive, emulsification
formed by oil/water and thermal swelling. A fundamental parameter in thermal EOR
is oil steam ratio (OSR) that is described as barrels of oil produced by injecting one
barrel of steam. The minimum amount of OSR is 0.15, but normally the successful
treatments that are considered economical have a higher OSR value [61].

2.4.1 Steam Flooding
One of the most famous methods in thermal EOR is steam flooding (SF) that steam
is generated on the surface then injected to the well. SF application started in the early
1960s; it was used to extract viscous oils such as bitumen (20,000 cp). For oils with
viscosities lower than 20 cp, waterflooding is a better option compared to SF. Steam is
injected through the target zone and pushes the oil to the production well. Screening
criteria for this type of thermal EOR is summarized by Green [62] and Taber [8].
Table 2.1 presents range and average of some reservoir parameters which have been
determined based on field data of SF projects.
Permeability of the reservoir requires to be higher than 200 mD, since the steam
should move fast enough through the porous media to avoid heat lost as much as pos-
sible. Reservoir thickness is another parameter that should be noticed due to the heat
loss; a minimum of 20 ft is required to avoid heat lost. The average thickness is
reported to be 70 ft. Moreover, the pressure over the well is an important aspect in
heat lost; therefore, the depth and spacing between injection and production well
should also be considered. As the target zone is deeper, the well spacing should
increase [61]. SF treatments are most often reported with a recovery of 50% OOIP
and OSR of 0.195 [63]. Some SF projects have been implemented in sandstones in
the past four decades such as Yorba Linda and Kern River fields in the United States
[64] and Mene Grande field in Venezuela [65]. In addition, steam injection projects
have been reported in carbonates such as Garland field in the United States [66].
Sandstone is the mostly reported formation in which SF is implemented; except
few cases, that formation was carbonate [67,68]. Type of the clays existing in the sand-
stone should be determined prior to the treatment, while some types of the clays swell
as they get in contact with formation water. There are different injection pattern and
well spacing used for SF. The most famous one is the inverted five-spot model, but