Page 152 - Fundamentals of Enhanced Oil and Gas Recovery

140 Forough Ameli et al.

due to miscibility of the solvent in oil. Howbeit, some heavy fractions of the crude
oil, namely, asphaltenes are immiscible in such solvents [8]. Another alternative to
steam injection processes is in situ combustion (ISC), which is applied for highly vis-
cous oils. Oil displacement in this technique leads to 95% recovery. The generated
heat front is so difficult to control. This leads to just few successful field experiences [9].
The nature of combustion reactions including cracking and oxidation, which happen
in heterogeneous reservoirs, increases the complexity of this process [10]. In tight
reservoirs, using electromagnetic methods is incumbent for increasing the recovery,
although using various thermal EOR techniques would not lead to full recovery of
the system as a result of fluid channeling in heterogonous media or heat losses occur-
ring in underburden or overburden of thin reservoirs [11]. Another leading technique
is application of this method in a specified part of the reservoir. In this method, con-
torting the wave’s penetration and their absorption is of paramount importance.
To deeply understand the mechanism of thermal EOR processes, it is necessary to
review various mechanisms of heat transfer including conduction, convection, and
radiation. The momentum equation reveals the dependency of flow to viscosity.
Variations of interfacial tension and the phase change lead to application of mass trans-
fer equations and complicated reactions which occur in thermal EOR processes. This
complexity and difficulty in prediction of the reservoir behavior may lead to review-
ing of thermal EOR processes. In this section, various methods of thermal recovery
are represented and compared to each other.

5.2.1 Steam Flood and Steam-Assisted Gravity Drainage
To increase the recovery of heavy oil and bitumen, the SAGD technique was devel-
oped. In this technique, two uneven horizontal wells are drilled. The upper wellbore
is occupied with continuous stream of high pressure gas. This heat leads to oil viscos-
ity reduction and moves the heated oil from upper well to the lower one and pumps
it out. As heat transfer has occurred in this process, the steam which is injected creates
a “steam chamber.” Steam and other gases are accumulated within the upper well due
to their lower density in comparison to oil and fill its empty space left by the oil. The
associated gas forms an insulating space over the steam. No vapor is produced in the
lower well [12]. A countercurrent flow of oil and water is produced by gravity drain-
age in the lower well. This fluid is pumped to the surface using cavity pump which is
appropriate for viscous fluids containing suspended solids.
ISC process was first introduced as forward dry combustion which starts with igni-
tion of crude oil downhole using an air stream to initiate the combustion. By propa-
gating the front of generated flame, much energy is lost. To reduce this phenomenon,
the process is reversed in which the injection of air stream occurs in another well and
the stated well is ignited. In other words, the air stream and flame move in opposite