Page 65 - Fundamentals of Enhanced Oil and Gas Recovery

Screening Criteria of Enhanced Oil Recovery Methods 53

Chinese favorite pattern is inverted nine-spot. The maximum pressure that we can
inject into the steam is formation fracture pressure while low injection pressures are
not desirable, since low injection rate will end up in more heat lost. Zhang [69] pro-
posed an injection strength around 1.3 bbl/(d ac ft) for a successful treatment.
Soaking process is recommended for more viscous oil the duration of which is
around 4 years. Large amount of water is required in SF projects; the ratio of water
required for oil production is 4 5. Therefore, water treatment plant is required to
maintain the water quality for the boilers. Actually, water treatment plant removes the
oxygen level (,0.05 ppm) and lowered the water hardness to 0.1 ppm. Moreover,
sodium particles and any suspended particles are also removed in water treatment
plants.

2.4.2 Cyclic Steam Stimulation
Cyclic steam stimulation (CSS) also known as huff-n-puff is another thermal EOR
method which only requires one well, and it consists of three levels. First, high-
pressure steam is injected through the target zone for several weeks to reduce the oil
viscosity; then, in the next step, a soaking period is given to the steam to diffuse
through the reservoir. Finally, oil is produced from the same well. In multilayer reser-
voirs, the treatment starts from the bottom layer and moves up to the top layer.
Screening criteria for steam treatments was reported by Taber [8] and Green [62]
without considering the soaking time. It is noted that Taber and Green classified SF
and CSS into one category. Sheng [15] presented modified screening criteria includ-
ing the soaking effect and pointed out that CSS treatment can be performed in wider
ranges of screening criteria, Accordingly, a soaking time of 1 4 days was proposed in
each cycle of CCS process. Summarized general screening criteria are illustrated in
Table 2.1.
The CSS process is generally carried out with a combination of SF after the CSS
process when the oil viscosity range is 10,000 50,000 cp. In lower viscosities (average
viscosity of 100 cp), waterflooding is more favorable before the SF. On the other
hand, gas cap is not desirable in the CSS treatments since it increases the gravity over-
ride of the steam. Also, a bottom aquifer acts as a competitor for crude oil in receiving
the heating energy of steam; therefore, CSS method is not recommended for this type
of reservoir.
The soaking period is another crucial screening parameter. Optimum amount of
time is required to allow the heat distribution through the reservoir to produce the
maximum level of oil; also, fining optimum soaking time is mandatory to avoid heat
loss with long soaking periods or heat accumulation in short period of times. For the
production period of half a year, an average field data reported for soaking period is
6.25 days [70]. Liu [61] also noticed that 2 3 days of soaking is enough. High steam