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Abstract
Vapor extraction (VAPEX) process is solvent analogue of SAGD process in which
low molecular weight solvent are used to
reduce the viscosity of heavy oil and bitumen. This process may be a good
candidate to be applied in problematic reservoirs in
which thermal methods are not efficient due to severe heat losses.
Asphaltene precipitation as a result of compositional alteration of
solvent/bitumen has substantial effects on performance of
VAPEX. In this process if operating pressure is very close to dew point
pressure, asphaltene precipitation occurs. In-situ
deasphalting of heavy oil and bitumen reduces the viscosity of the produced oil
and creates the oil that is more easily refined,
but the main concern is whether this advantage can outweigh reduced
permeability and plugging of formation caused by
adsorption and precipitation of asphaltenes.
In current work experiments have been conducted in low permeability
sandpacks in both dry and non-dry systems to assess the
performance of Vapex process when asphaltene precipitation occurs. Experiments
are carried out in a 2D visual cell using
highly asphaltenic heavy oil from one of Iranian reservoirs (Kuh-e-Mond) and
propane as a solvent.
The results show that performance of Vapex process increases when asphaltene
precipitation occurs provided that optimal
pressure has been designed to prevent extensive asphaltene deposition and
subsequent plugging of porous media. It has been
observed that adsorption of asphaltenes has considerably decreased in non-dry
systems. Movement of precipitated asphaltene
and less adsorption in non-dry experiments results in a more stable and
efficient Vapex process while in dry experiments
higher adsorption of asphaltenes resulted in plugging of low permeability
porous medium.
Introduction
Concurrent with increase in oil demand, there is marked decline in
conventional oil production. Huge heavy oil and bitumen
resources are increasingly recognized as a strategic resource and potential
contributor to worldwide energy security.Thermal
processes like SAGD and CSS have increased tremendously worldwide reserves
especially in Canada and probably SAGD
could be considered as one of the most effective EOR techniques applied in
petroleum industry. But, thermal processes cannot
be applied in cases with severe heat losses like thin formations and heavy oil
in deep reservoirs and offshore. Solvent-based
methods like VAPEX process may be a suitable option for the recovery of heavy
oil in problematic cases.
The Vapour extraction is basically an analogue of the steam assisted gravity
drainage (SAGD) process. In VAPEX process,
vaporized solvents are used instead of high temperature steam and the oil
viscosity is lowered by in-situ dilution instead of
heating. A vaporized light hydrocarbon or a mixture of solvents, such as
propane or butane, is injected through a horizontal
well into the formation containing the viscous oil or bitumen (Butler &
Mokrys 1991, Das 1997).
It has been experimentally shown that the optimum solvent injection
condition is near solvent dew point where the vapour
phase has the maximum solubility and diffusivity in the heavy liquid phase
(Butler & Mokrys, 1993).Carrying out experiments
with pure solvents in these conditions leads to asphaltene precipitation as a
result of high solvent concentration in boundary
layer (Das & Butler 1998).
Asphaltene flocculation and deposition during natural depletion and/or
miscible gas injection in enhanced oil recovery (EOR)
processes is a common problem in oilfields throughout the world. Changes in
some environmental parameters, such as
pressure and composition (addition of solvent or dispersant injection), can
change the stable condition of an oil mixture to
another condition where the mixture will be unstable and heavy organics, such
as asphaltenes or waxes, flocculate and deposit
(Mofidi & Edalat, 2006).
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