169
Recovery of Heavy Oil and Tar Sand Bitumen
After initial cyclic steam stimulation cycles, the SAGD wells are started and operate similarly to
conventional SAGD. When the steam chambers of cyclic steam stimulation wells and SAGD wells
make contact with each other, steam injection into the cyclic steam stimulation wells is ceased but
not steam injection through the SAGD wells. During this time, heavy oil or bitumen is produced
from cyclic steam stimulation wells. CHOPS is now widely used as a production approach in unconsolidated sandstones.
The process results in the development of high-permeability channels (wormholes) in the adjacent low-cohesive strength sands, facilitating the flow of oil foam that is caused by solution gas
drive. The key benefits of the process are improved reservoir access, order-of-magnitude higher oil
production rates (as compared to primary recovery), and lower production costs. The outstanding
technical issues involve sand handling problems, field development strategies, wormhole plugging
for water shut-off, low ultimate recovery, and sand disposal. Originally, cold production mechanisms were thought to apply only to vertical wells with high-capacity pumps. It is now believed
that addition of resins, similar to addition of dispersants, increased foam stability, presumably
by decreasing the size of asphaltene aggregates (McLean and Kilpatrick, 1997; Zaki et al., 2002)
(Chapters 12 and 13).
Thus, instead of blocking sand ingress by screens or gravel packs, sand is encouraged to enter the
wellbore by aggressive perforation and swabbing strategies. Vertical or slightly inclined wells (vertical to 45°) are operated with rotary progressive cavity pumps (rather than reciprocating pumps) and
old fields are converting to higher-capacity progressive cavity pumps, giving production boosts to
old wells. Productivity increases over conventional production and a CHOPS process can produce
as much as 12% to perhaps as much as 25% of the OOIP can be recovered, rather than the 0%–5%
typical of primary production without sand in such cases. Finally, because massive sand production
creates a large disturbed zone, the reservoir may be positively affected for later implementation of
thermal processes.
The CHOPS process increases productivity for the following reasons: (1) if the sand can move
or is unconsolidated, the basic permeability to fluids is enhanced; (2) as more sand is produced,
a growing zone of greater permeability is generated, similar to a large-radius well that gives better
production; (3) gas coming out of solution in heavy oil does not generate a continuous gas phase;
rather, bubbles flow with the fluid and do not coalesce, but expand down-gradient, generating an
internal gas drive, referred to as foamy flow. This also helps to locally destabilize the sand, sustaining the process; (4) continuous sand production means that asphaltene or fines plugging of the
near-wellbore environment potentially do not occur, so there is no possibility of an effect to impair
productivity; and (5) as sand is removed, the overburden weight acts to shear and destabilize the
sand, helping to drive sand and oil toward the wellbore.
Typically, a well placed on CHOPS production will initially produce a high percentage of sand,
greater than 20% by volume of liquids. However, this generally drops after some weeks or months.
The huge volumes of sand are disposed of by slurry fracture injection or salt cavern placement or by
sand placement in a landfill in an environmentally acceptable manner.
Pressure pulsing technology (PPT), which involves a radically new aspect of porous media
mechanics, was discovered and developed into a production enhancement method during the period
1997–2003. Pressure pulse flow enhancement technology (PPT) is based on the discovery that large
amplitude pressure pulses that are dominated by low-frequency wave energy generate enhanced
flow rates in porous media. For example, in preliminary experiments in heavy oil reservoirs in
Alberta, PPT has reduced the rate of depletion, increased the oil recovery ratio, and prolonged the
life of wells. Also, it has been found that very large amplitude pressure pulses applied for 5–30 h to
a blocked producing well can reestablish economic production in a CHOPS well for many months,
even years.
The mechanism by which PPT works is to generate a porosity dilation wave (a fluid displacement
wave similar to a tsunami); this generates pore-scale dilation and contraction so that oil and water
flow into and out of pores, leading to periodic fluid accelerations in the pore throats. As the porosity
Recovery of Heavy Oil and Tar Sand Bitumen
After initial cyclic steam stimulation cycles, the SAGD wells are started and operate similarly to
conventional SAGD. When the steam chambers of cyclic steam stimulation wells and SAGD wells
make contact with each other, steam injection into the cyclic steam stimulation wells is ceased but
not steam injection through the SAGD wells. During this time, heavy oil or bitumen is produced
from cyclic steam stimulation wells. CHOPS is now widely used as a production approach in unconsolidated sandstones.
The process results in the development of high-permeability channels (wormholes) in the adjacent low-cohesive strength sands, facilitating the flow of oil foam that is caused by solution gas
drive. The key benefits of the process are improved reservoir access, order-of-magnitude higher oil
production rates (as compared to primary recovery), and lower production costs. The outstanding
technical issues involve sand handling problems, field development strategies, wormhole plugging
for water shut-off, low ultimate recovery, and sand disposal. Originally, cold production mechanisms were thought to apply only to vertical wells with high-capacity pumps. It is now believed
that addition of resins, similar to addition of dispersants, increased foam stability, presumably
by decreasing the size of asphaltene aggregates (McLean and Kilpatrick, 1997; Zaki et al., 2002)
(Chapters 12 and 13).
Thus, instead of blocking sand ingress by screens or gravel packs, sand is encouraged to enter the
wellbore by aggressive perforation and swabbing strategies. Vertical or slightly inclined wells (vertical to 45°) are operated with rotary progressive cavity pumps (rather than reciprocating pumps) and
old fields are converting to higher-capacity progressive cavity pumps, giving production boosts to
old wells. Productivity increases over conventional production and a CHOPS process can produce
as much as 12% to perhaps as much as 25% of the OOIP can be recovered, rather than the 0%–5%
typical of primary production without sand in such cases. Finally, because massive sand production
creates a large disturbed zone, the reservoir may be positively affected for later implementation of
thermal processes.
The CHOPS process increases productivity for the following reasons: (1) if the sand can move
or is unconsolidated, the basic permeability to fluids is enhanced; (2) as more sand is produced,
a growing zone of greater permeability is generated, similar to a large-radius well that gives better
production; (3) gas coming out of solution in heavy oil does not generate a continuous gas phase;
rather, bubbles flow with the fluid and do not coalesce, but expand down-gradient, generating an
internal gas drive, referred to as foamy flow. This also helps to locally destabilize the sand, sustaining the process; (4) continuous sand production means that asphaltene or fines plugging of the
near-wellbore environment potentially do not occur, so there is no possibility of an effect to impair
productivity; and (5) as sand is removed, the overburden weight acts to shear and destabilize the
sand, helping to drive sand and oil toward the wellbore.
Typically, a well placed on CHOPS production will initially produce a high percentage of sand,
greater than 20% by volume of liquids. However, this generally drops after some weeks or months.
The huge volumes of sand are disposed of by slurry fracture injection or salt cavern placement or by
sand placement in a landfill in an environmentally acceptable manner.
Pressure pulsing technology (PPT), which involves a radically new aspect of porous media
mechanics, was discovered and developed into a production enhancement method during the period
1997–2003. Pressure pulse flow enhancement technology (PPT) is based on the discovery that large
amplitude pressure pulses that are dominated by low-frequency wave energy generate enhanced
flow rates in porous media. For example, in preliminary experiments in heavy oil reservoirs in
Alberta, PPT has reduced the rate of depletion, increased the oil recovery ratio, and prolonged the
life of wells. Also, it has been found that very large amplitude pressure pulses applied for 5–30 h to
a blocked producing well can reestablish economic production in a CHOPS well for many months,
even years.
The mechanism by which PPT works is to generate a porosity dilation wave (a fluid displacement
wave similar to a tsunami); this generates pore-scale dilation and contraction so that oil and water
flow into and out of pores, leading to periodic fluid accelerations in the pore throats. As the porosity