170
The Chemistry and Technology of Petroleum
dilation wave moves through the porous medium at a velocity of about 50–100 ft/s (40–80 m/s), the
small expansion and contraction of the pores with the passage of each packet of wave energy helps
unblock pore throats, increase the velocity of liquid flow, overcome part of the effects of capillary
blockage, and reduce some of the negative effects of instability due to viscous fingering, coning, and
permeability streak channeling.
Although a very new concept dating only since 1999 in small-scale field experiments, PPT promises to be a major adjunct to a number of oil production processes, particularly all pressure-driven
processes, where it will both accelerate flow rates as well as increase oil recovery factors. It is also
now used in environmental applications to help purge shallow aquifers of nonmiscible phases such
as oil, with about six successful case histories to date.
VAPEX is a new process in which the physics of the process is essentially the same as for SAGD
and the configuration of wells is generally similar. The process involves the injection of vaporized
solvents such as ethane or propane to create a vapor chamber through which the oil flows due to
gravity drainage (Butler and Mokrys, 1991, 1995, 1997; Butler and Jiang, 2000). The process can be
applied in paired horizontal wells, single horizontal wells, or a combination of vertical and horizontal wells. The key benefits are significantly lower energy costs, potential for in situ upgrading, and
application to thin reservoirs, with bottom water or reactive mineralogy.
In the process, a pair of horizontal wells is drilled as production and injection similar to SAGD
process. The injection well is located above production well and a mixture of solvents will be
injected through that. The solvents start to move toward cap rock, and when they reach the cap rock
(or a barrier bed), they spread along that barrier till solvents start to move downward to production
well by gravity drainage force. Well configurations are very important and deasphalting and the
effect of that on the diluted heavy oil or bitumen can relate to well configuration.
The selection of well spacing in the VAPEX process is as a function of oil mobility at reservoir
conditions. For reservoirs where the heavy oil or bitumen mobility is low, the well spacing should
be small. The lateral spacing between injector and producer has a negligible effect on the ultimate
oil recovery and just causes a long delay in start of well communication. Moreover, the length of
horizontal well is important. However, long horizontal wells lead to an economical production rate
because more solvent can spread into porous media and affect a bigger area of untouched heavy oil.
Because of the slow diffusion of gases and liquids into viscous oils, this approach, used alone,
perhaps will be suited only for less viscous oils although preliminary tests indicate that there are
micro-mechanisms that act so that the VAPEX dilution process is not diffusion rate limited and the
process may be suitable for the highly viscous tar sand bitumen and there is the potential for asphaltene separation if the solvent is insufficient to maintain the stability of asphaltene constituents in the
crude oil (Das and Butler, 1994; Yang and Gu, 2005a,b).
Nevertheless, VAPEX can undoubtedly be used in conjunction with SAGD methods. As with
SAGD and IGI, a key factor is the generation of a three-phase system with a continuous gas phase
so that as much of the oil as possible can be contacted by the gaseous phases, generating the thin
oil film drainage mechanism. As with IGI, vertical permeability barriers are a problem, and must
be overcome through hydraulic fracturing to create vertical permeable channels, or undercut by the
lateral growth of the chamber beyond the lateral extent of the limited barrier, or baffle.
Hybrid approaches that involve the simultaneous use of several technologies are evolving and
will see greater applications in the future. Some of the evolving options that will be tried at the field
scale in the next decade include the following:
1. A mixture of steam and miscible and noncondensable hydrocarbons is being field tested as
a hybrid SAGD-VAPEX approach, with apparent good success and reduction of steam–oil
ratios.
2. Single horizontal laterally offset wells can be operated as moderate pressure cyclic steam
stimulation wells in combination with SAGD pairs to widen the steam chamber and reduce
steam–oil ratios by about 20%.
The Chemistry and Technology of Petroleum
dilation wave moves through the porous medium at a velocity of about 50–100 ft/s (40–80 m/s), the
small expansion and contraction of the pores with the passage of each packet of wave energy helps
unblock pore throats, increase the velocity of liquid flow, overcome part of the effects of capillary
blockage, and reduce some of the negative effects of instability due to viscous fingering, coning, and
permeability streak channeling.
Although a very new concept dating only since 1999 in small-scale field experiments, PPT promises to be a major adjunct to a number of oil production processes, particularly all pressure-driven
processes, where it will both accelerate flow rates as well as increase oil recovery factors. It is also
now used in environmental applications to help purge shallow aquifers of nonmiscible phases such
as oil, with about six successful case histories to date.
VAPEX is a new process in which the physics of the process is essentially the same as for SAGD
and the configuration of wells is generally similar. The process involves the injection of vaporized
solvents such as ethane or propane to create a vapor chamber through which the oil flows due to
gravity drainage (Butler and Mokrys, 1991, 1995, 1997; Butler and Jiang, 2000). The process can be
applied in paired horizontal wells, single horizontal wells, or a combination of vertical and horizontal wells. The key benefits are significantly lower energy costs, potential for in situ upgrading, and
application to thin reservoirs, with bottom water or reactive mineralogy.
In the process, a pair of horizontal wells is drilled as production and injection similar to SAGD
process. The injection well is located above production well and a mixture of solvents will be
injected through that. The solvents start to move toward cap rock, and when they reach the cap rock
(or a barrier bed), they spread along that barrier till solvents start to move downward to production
well by gravity drainage force. Well configurations are very important and deasphalting and the
effect of that on the diluted heavy oil or bitumen can relate to well configuration.
The selection of well spacing in the VAPEX process is as a function of oil mobility at reservoir
conditions. For reservoirs where the heavy oil or bitumen mobility is low, the well spacing should
be small. The lateral spacing between injector and producer has a negligible effect on the ultimate
oil recovery and just causes a long delay in start of well communication. Moreover, the length of
horizontal well is important. However, long horizontal wells lead to an economical production rate
because more solvent can spread into porous media and affect a bigger area of untouched heavy oil.
Because of the slow diffusion of gases and liquids into viscous oils, this approach, used alone,
perhaps will be suited only for less viscous oils although preliminary tests indicate that there are
micro-mechanisms that act so that the VAPEX dilution process is not diffusion rate limited and the
process may be suitable for the highly viscous tar sand bitumen and there is the potential for asphaltene separation if the solvent is insufficient to maintain the stability of asphaltene constituents in the
crude oil (Das and Butler, 1994; Yang and Gu, 2005a,b).
Nevertheless, VAPEX can undoubtedly be used in conjunction with SAGD methods. As with
SAGD and IGI, a key factor is the generation of a three-phase system with a continuous gas phase
so that as much of the oil as possible can be contacted by the gaseous phases, generating the thin
oil film drainage mechanism. As with IGI, vertical permeability barriers are a problem, and must
be overcome through hydraulic fracturing to create vertical permeable channels, or undercut by the
lateral growth of the chamber beyond the lateral extent of the limited barrier, or baffle.
Hybrid approaches that involve the simultaneous use of several technologies are evolving and
will see greater applications in the future. Some of the evolving options that will be tried at the field
scale in the next decade include the following:
1. A mixture of steam and miscible and noncondensable hydrocarbons is being field tested as
a hybrid SAGD-VAPEX approach, with apparent good success and reduction of steam–oil
ratios.
2. Single horizontal laterally offset wells can be operated as moderate pressure cyclic steam
stimulation wells in combination with SAGD pairs to widen the steam chamber and reduce
steam–oil ratios by about 20%.