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Steam assisted gravity drainage

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Steam assisted gravity drainage (SAGD) is an outstanding example of a steam injection process devised for a specific type of heavy oil reservoir utilizing horizontal wells. It is widely used in Alberta, Canada for recovery of heavy oil and tar sand resources.

Basic process

Several variations of the basic process have been developed, and are being tested. The original SAGD process, as developed by Butler, McNab, and Lo[1] in 1979, utilizes two parallel horizontal wells in a vertical plane: the injector being the upper well and the producer the lower well (Fig. 1, taken from Butler[2]). If the oil/bitumen mobility is initially very low, steam is circulated in both wells for conduction heating of the oil around the wells. Steam is then injected into the upper well, while producing the lower well. As a result, steam rises forming a steam chamber with oil flowing at the sides of the chamber and condensate flowing inside the chamber, as shown in Fig. 1. This is an idealized situation. Other flow regimes may occur depending on the oil and formation properties.

Estimating production rates

For the simplest case, the oil production rate qoh, in B/D per ft well length, is given by (multiplier "2" indicates flow from two sides of the steam chamber)

Vol5 page 1355 eq 001.png....................(1)

where the kinematic viscosity of oil (in centistokes) at the steam temperature, Ts, is given by νs, and that at any other temperature, T, is given by

Vol5 page 1355 eq 002.png....................(2)

where m is derived from the viscosity-temperature relationship of the oil.

Eq. 2 predicts rates of 0.1 to 0.7 B/D per ft for a horizontal well for an oil viscosity of 100,000 cp. For example, a 2,000-ft long well may be expected to produce about 800 B/D at a steam temperature of 400°F. The theory has been verified by laboratory experiments. Field results to-date have been encouraging. One commercial project (EnCana’s Foster Creek Project), consisting of 22 well pairs, has been in operation since October 2001. In 2002, steam/oil ratios were averaging 2.5 bbl oil/bbl of steam. Earlier field tests of SAGD in Athabasca tar sands were successful at a depth of about 600 ft, which is too deep for surface mining and not deep enough for high-pressure steam injection.

Considerations

SAGD is a complex process because gravity flow strongly relies on a high vertical permeability. The initial oil mobility determines the vertical spacing of the two wells. In a million cp tar sand, the spacing would be 5 to 6 m [16 to 20 ft]. It is also important that the steam chamber be sealed. There is no steam migration to offset vertical wells. In California, SAGD failed to achieve commercial success because of relatively high initial mobility of oil, as well as other reasons.

Variations

One variation of SAGD is known as single-well SAGD. Here, insulated tubing is used to inject steam into a single horizontal well, with production from the annulus. This process was successful in a few cases but generally failed. Another variation (Vapex) utilizes a suitable solvent (such as ethane, propane, etc.) instead of steam and is being field tested.

Nomenclature

hh = fluid level in stimulated reservoir, ft [m]
k = reservoir permeability, md [μm3]
kro = relative permeability to oil
m* = exponent in Eqs. 1, and 2
qoh = hot oil production rate, B/D [m3/d]
So = oil saturation
Sors = residual oil saturation to steam fraction
T = average temperature in heated reservoir, °F
TR = unaffected reservoir temperature, °F
Ts = steam temperature, °F
Ф = porosity

References

  1. Butler, R.M., McNab, G.S., and Lo, H.Y. 1981. Theoretical studies on the gravity drainage of heavy oil during in-situ steam heating. The Canadian Journal of Chemical Engineering 59 (4): 455-460. http://dx.doi.org/10.1002/cjce.5450590407
  2. 2.0 2.1 Butler, R.M. 1985. A new approach to the modeling of steam-assisted gravity drainage. J Can Pet Technol 24 (3): 42–51. http://dx.doi.org/10.2118/85-03-01

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See also

Thermal recovery by steam injection