Page 67 - Fundamentals of Enhanced Oil and Gas Recovery

Screening Criteria of Enhanced Oil Recovery Methods 55

3
steam-to-oil ratio of 3 t/m is suggested for a SAGD process in order to be efficient
from an economical point of view.
Resource quality is an essential parameter in SAGD treatment performance that
includes
1. High thickness of the pay zone in order to be able to drill two vertically aligned
horizontal wells; also a thick pay zone means less energy loss. A reasonable range
for thickness is reported to be 10 15 m.
2. High vertical permeability (an average of 2700 mD) is necessary since low perme-
ability limits the steam rise and from the chamber well and considerably reduces
the drainage head. Therefore, geological studies before drilling are highly recom-
mended to avoid any shale layers in sandstone formations located between the
injection and production well.
3. High oil concentration (mostly high content of bitumen) while it is directly
related to thermal efficiency; higher the oil content, more oil is produced from
the formation with the same thermal energy and lower steam-to-oil ratio. The
operations are economically viable if 10% bitumen content is present in the
formation.

2.4.4 In Situ Combustion
In situ combustion (ISC) method was first presented in 1923, a recovery treatment by
burning of oil originally existed in the reservoir [78], which oil ignition acquires
spontaneously or artificially. ISC front produced from the ignition moves through the
reservoir and pushes the heavy unburned oil out of the reservoir and to the produc-
tion well. Continuous injection of air keeps the ISC front moving. The ISC treatment
is an exothermic process that assists the improvement of oil recovery by reducing the
viscosity of the oil by the generated heat from the burned oil.
ISC treatment generates less greenhouse gas emission since the compressed air is
injected to the reservoir instead of steam. Moreover, less energy is consumed in ISC
compared to other thermal recoveries. Although ISC is known as the second thermal
EOR recovery [79], there are several drawbacks such as very low process control
resulting in poor sweep efficiency and completions adversely getting effect by the
ignition. Also, greater number of experienced and knowledgeable personnel is
required compared to other thermal methods due to the complexity of the process.
ISC method is generally used for very light and very heavy crude oil, since low-
pressure profile (due to shallower reservoirs) and low corrosion rate in heavy crude oil
formations assist the ISC procedure. Also, in very light crude oils, integral oxygen
consumption and eliminating the ignition process (due to deep reservoirs) are great
impetuses for ISC treatment. ISC is not commercialized in viscosity range between
2 and 60 cp [80]. Formations with high permeability, shallow and homogenous