Page 174 - Fundamentals of Enhanced Oil and Gas Recovery

162 Forough Ameli et al.

the produced oil, 0.5 kg of solvent is needed, whereas 3 kg of steam is required. Such
estimations have also been reported by some other researchers [65]. For the processes
in which solvent and steam are used simultaneously, the cost would be higher in com-
parison to VAPEX and less energy is required in comparison to SAGD. Some operat-
ing costs include solvent and water purchase and their handling, separation cost of
water or solvent from oil. The required amount of steam for SAGD process with high
steam-to-oil ratio is reduced but there would be difficulty in separating oil and water
[60,63]. A flash vaporizer is applied to separate light hydrocarbons in low temperature.
This process leads to solvent recovery of about 90%.
Application of VAPEX and warm VAPEX increases the chance of asphaltene
elimination. Asphaltene is removed as the equilibrium condition for its solution
wipes out. This leads to reduction of heavy oil viscosity. If asphaltene content of the
crude oil is reduced from 16% to zero, the viscosity would reduce 20-order of mag-
nitude [66]. If asphaltene is present in the solution, it would increase the operating
costs regarding to oil upgrading. This would be from thermal and catalytic operation
points of view. On the other hand, as permeability of the system is reduced, the
sweeping efficiency would also be affected. In reservoirs, where aquifer in the bot-
tom or top water layer exists, thermal operations could not be run [67].
Hydrocarbon solvents are slightly soluble in water. A bottom aquifer leads to better
connection of injection and production wells. The production rate is increased due
to changing the flow mechanism from gravity drainage to countercurrent flow
regime [59,68,69]. It could be concluded that VAPEX is the only appropriate pro-
cess for reservoirs with overlying layers and bottom aquifers [58,69]. The heat loss is
also the case for tight and shallow reservoirs. SAGD process is also inappropriate for
reservoirs with high clay content which is due to clay swelling as a result of water
condensations. This actually happens for reservoirs with clay contents more than
10% [63]. The recovery of such reservoirs could be enhanced using VAPEX process.

5.2.7 Formation Heating by Hot Fluid Injection
By injection of a hot fluid into porous media, heat transfer occurs in the rock matrix
and the contained fluid in it. The same thing happens at overburden and underbur-
den. The mechanism of heat transfer includes convection and conduction. If phase
change occurs in this media, equations would become more complicated [70].For
hot fluid injection process, the mechanism of heat transfer to fluid and matrix include
convection and conduction. This fluid causes the movement of water, gas, and oil in
place and heating them by conduction and convection mechanisms. The porous
media is also warm up by conduction. Of course, the equilibrium rate is a function of
injected fluid properties, namely, viscosity and density. For modeling heat transfer pro-
cesses, it is assumed that the rock and fluid temperatures are the same. As heat transfer