Note: Descriptions are shown in the official language in which they were submitted.
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SOLVENT PUSH-PULL PROCESS FOR IMPROVING
VERTICAL CONFORMANCE OF STEAM DRIVE PROCESS
The present invention pertains to a steam drive process for
recovering viscous oil from a subterranean, viscous oil-containing
formation. More particularly, the present invention involves an
improved steam drive and recovery method utilizing a solvent push-pull
process at the production well which increases the vertical con
formance of the steam proc2ss.
Many oil reservoirs have been discovered which contain vast
quantities of oil, but little or no oil has been recovered from many
of them because the oil present in ths reservoir is so viscous that it
is essentially immQbile at reservoir conditions, and little or no
petroleum flow will occur into a well drillPd into the ~ormaticn even
if a natural or artificially induced pressure differential exists
between the formation and the well. Some form of supplemental oil
recovery must be applied to these formations which decreases the
viscosity of the oil sufficiently that it will flow or can be
dispersed throu~h the formation to production well and therethrough to
the surface of the earth. Thermal recovery techniques are quite
suitable for visccus oil ~ormations, and steam flooding is khe most
successful thermal oil recovery technique yet employed commercially.
Steam may be utilized ~or thermal stimulation ~or viscous oil
production by means of a steam drive or steam throughput process, in
which steam is injected into the formation on a more or less
continuous basis ~y means of an injection well and oil i5 recovered
from the formation from a spaced-apart production well. While this
process is very effective with respect to the portion of the recovery
zone between the injection well and production well through which the
steam travels~ poor vertical and horizontal con~ormance is oFten
experlenced in steam drive oil recovery processes. By vertical
conformance, it is meant the portion of the vertical thickness of a
formation through which the injected steam passes. A ma~or cause of
poor vertical conformance is caused by steam~ being of lower density
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than other fluids present in the perrneable formation, migrating to the
upper portion of the oil formation to the remotely located production
well. Once steam channeling has occurred in the upper portion of the
formation, the permeability of the steam-swept ~one is increased due
to the desaturation or removal of oil ~rom the portion of the
formation through which steam has channeled. Thus subsequently-
injected steam will migrate almost exclusively through the steam-swept
channel and very little of the injected steam will move into the lower
portions of the formation, and thus very little additîonal oil ~rom
the lower portion of the formation will be produced. While steam
drive processes eff`ectively reduce the oil saturation in the portions
of the formation through which they travel by a significant amount, a
large portion of the recovery zone between the injection and
production systems i-~ not contacted by steam and so a significant
amount of oil remains in the ~ormation after ~ompletion of the steam
drive oil recovery process. The severity of the poor vertical
conformance problem increases with the thickness o~ the oil ~ormation
and with the viscosity of the oil contained in the ~ormation~
In view of the foregoing discussionS it can be appreciated
that there is a substantial, unfulfilled need for a method of
conducting a well-to-well throughout steam in~ection oil recovery
method in a manner which results in improved vertical conf`ormance.
The process of the present invention involves an improved
steam drive oil recoYery process with at least one in~ection well and
at least one spaced-apart production well ~or in~ecting steam into the
formation and recovering oil from the formatioll wherein a solvent
push-pull treatment is commenced in the lower portion of the formation
ad~acent to the production well once there is a breakthrough of steam
at the producing well. The injection well is in fluid communication
throughout the full or a substantial amount of the vertical thlckness
of the oil f`ormation or with the lower portion of the formation. The
production well is completed with two separate ~low means, one between
the surface and the lo~er portion of the formation, and the other
being in communication with the upper portion of` the formation. Steam
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is injected into the injection well and oil is recovered ~rom the
production well until steam breakthrough at the production well
occurs. At this time, production is terminated, and a solvent
injection-production process is applied by the flow path of the
production well in communication with the lower portion of the
formation. This process is applied simultaneously with the steam
drive process in a series of repetitive cycles throughout the entire
time that the steam drive sequence is being applied. The solvent
push-pull process comprises injecting a predetermined volume of
solvent into the lo~er portion of the formation or until the injection
pressure rises to a predetermined level which should be less than the
pressure which will cause fracture of the formation and/or overburden
formation. The volume of solvent injected i5 from 10 to 50 and
preferably from 10 to 20 barrels of solvent per foot of formation with
which the lower portion of the production well is in fluid communi-
cation. Once the predetermined volume of solvent has been injected,
or when the predetermined pressure has been reached, solvent injection
is stopped and fluid produ~tion is taken from the bottom of the
formation. Oil and solvent flow from the bottom of the formation back
into the flow path in ~luid communication with the lower portion of
the produoing well and production is continued until the amount of
solvent in the produced fluids has decreased to a value less than 12
percent by volume. Solvent injection is again applied followed by
another period of production o~ solvent and oil. The solvent push-
pull cycles are repeated until there is a breakthrough of steam or
water in the lower portion of the formation. The solvent push-pull
process promotes the sweeping action of the ste~m into the lower
portion of the formation thereby stimulatin0 the recovery,of oil in
that zone which would not be recovered in a conventional steam drive
process.
Fig. 1 illustrates a subterraneous formation penetrated by an
lnjection well and a production well being employed in ~ state-of-
the-art steam drive oil recovery method, illustrating how the injected
steam migrates to the upper portion of the formation as it
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travels thro~gh the recovery zone within the formation and between the
injection well and production well, thus bypassing a significant
amount of oil in the recovery zone.
Fig. 2 illustrates the initial phase of my process wherein
steam is injected into the injection well and simultaneously the lower
portion of the formation adjacent the production well is subjected to
a solvent push-pull treatment to draw steam into portions of the
formation belo~ the zone originally swept by steam.
Fig. 3 illustrates how after a plurality of solvent push-pull
treatments, the solvent-swept zone expands and additional portions of
the formation are swept by steam~
The problem o~ steam override for which the process o~ our
invention represents a solution may best be understood by re~erring to
the attached drawings, in which Fig. 1 illustrates how a relatively
thick, viscous oil formation 1 is penetrated by an injection well 2
and a production well 3 is used for a conventional steam drive oil
recovery process. Steam is injected into well 2, passes through thP
perforations in well 2 into the viscous oil formation. Conventional
practice is to perforate or establish fluid ~low communications
between the well and the formation throughout the ~ull vertical
thickness of the ~ormation, both ~ith respect to injection well 2 and
production well 3. Not withstanding the fact that steam is injected
into the full vertical thickness of the formation, it can be seen that
steam migrates both horizontally and in an upward direction as it
moves through the ~ormation between injection well 2 and production
well 3. The result is the creation of a steam-swept zone 4 in the
upper portion of the formation and zone 5 in the lower portion of the
formation through which little or no steam has passed. Once steam
break-through at production well 3 occurs, continued injection o~
steam will not cause any steam to ~low through zone 59 because (1) the
speci~ic gravity of the substantially all vapor phase steam is
significantly less than the specific gravity of the petroleum and
other liquids present in the pore spaces of the ~ormation, and so
gravltational ef~ects will cause the steam vapors to be confined
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exclusively in the upper portion of the formation, and (~) steam
passage through the upper portion of the formation displaces and
removes petroleum from that portion of the formation through which it
travels, and desaturation of the zone increases the relative perme-
ability of the formation significantly as a consequence o~ removing
the viscous petroleum therefrom~ Thus any injected fluid will travel
more readily through the desaturated portion of the formation 4 than
it will through the portion of the formation 5 which is near original
conditions with respect to viscous petroleum saturation.
The process of my invention is better understood by referring
now to Fig. 2 wherein formation 1 is penetrated by spaced-apart
injection and production wells 2 and 3~ respectively. Injection well
2 is in fluid communication with the full vertical thickness of the
formation. Production well 3 has two separate ~low paths with one
flow path in fluid communication with th~ upper portion of the
formation and a second flow path in fluid co~munication with the lower
portion of the formation~ In the particular embodiment illustrated in
Fig. 2, the annular Cpace between casing 8 and tubing 7 of well 3 is
used as the first communication path which is in ~luid communication
with the upper portion of the ~ormation, while tubing 7 is used for
the second communication path which is in fluid communication with the
lower portion of the formation.
In the first step of the process of the present invention, a
thermal reCQvery fluid comprising steam is injected into the formation
by means of injection well 2. Steam enters the portion of the
formation immediately adjacent to well 2 through all of the
perforations in well 2, and initially travels through substantially
all of the full vertical thicknesses of formation 1. As can be seen
ln Fig. 1, the steam vapors begin migrating in an upward direction
toward the top of the reservoir because of the difference in specific
yravity between steam vapor and formation fluids. This causes the
characteristic slanting interface 9 between the steam-swept zone 4 and
the unswept portion 5 of the formation 1. Thus by the time steam
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arrives at the upper perforations of production well 3, steam is
passing through only a small fraction of the full vertical thickness
of the formation. Oil i5 initially produced to the surface via the
two communication paths of well 3 in fluid communication with the
upper and lower portions of the formation.
The first step comprising injecting steam into in~ection well
2 and recovering fluids including oil from the upper and lower portion
of the formation by means of production well 3 continues until the
fluid recovered from the upper portion of the oil formation via the
first flow path of the production well comprises an unfavorable amount
of steam or water. Once there is sufficient steam or water
breakthrough at well 3, production is terminated.
After production has been terminated, the flow path in the
production well in fluid communication ~ith the upper portion of the
formation is shut in and a solvent injection-production sequence or
push-pull process is applied to the lower portion of the ~ormation
adjacent the producing well by means of the flow path which com~uni-
cates from the sur~are to the lower portion of the producing well
while continuing in~ection of steam into the injection well. This
sequence as illustrated in Fig. 2. comprises injecting a predetermined
amount of solvent into the lower portion of the formation via the flow
path which communicates from the surface to the lower portion of the
producing well. Tubing 7 of well 3 is used for this purpose in the
embodiment depicted in Fig. 2. The type of solvent injected into the
lower portion of the formation is preferably a hydrocarbon which is
liquid at formation temperature and injection pressure. Suitable
solvents include light hydrocarbons such as a C4-C10 hydrocarbons?
or a commercial blend such as natural gasoline) naphtha, lIght crude
oil, partially refined tar generally known as syncrude~ etcO
As the solvent is injected into the lower portion of the
formation it fingers into zone 10 of the formation and dissolves the
viscous oil, thus lowering its viscosity and thereby increasing its
mobili~y. After a predetermined amount of solvent is injected into
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the forfnation which is from 10 to 50 and preferably from 10 to 20
barrels of solvent per foot of formation thickness with which the
lower portion of the production well is in fluid communication,
production is immediately resumed by recovering flulds including oil
and solvent from the lower portion of the formation via the flow path
in communication therewith.
During injection of the solvent it is necessary to monitor
the injection pressure so that this pressure does not increase to the
point which will cause fracture of the formation and~or the over-
burden, if the value of this pressure is known. Therefore, in some
instances it is desirable to inject solvent for a fixed period of time.
When solvent injection is terminated and production of
solvent and oil from the solvent-swept zone 10 is begun through well 3
via the flow path in fluid communication with the lower portion of the
formation, the flow rate is usually quite high at first but declines
rapidly as the drive pressure declines. Production is continued until
the percentage of solvent of the fluid bPing produced decreases to a
predetermined level, preferably to a va~ue less than 12 percent by
volume. The sequence o~ solvent injection followed by fluid
production is then continued for a plurality of cycles until the fluid
being recovered from the production well contains an unfavorable
amount of steam or w~ter. Generally, the amount of solvent in the
production fluid at the termination of each production step will
gradually decrease within the specified range as the cycles of solvent
lnjection-fluid production increase.
The above-described sequence of solvent injection followed by
fluid production while simultaneously injecting steam into well 2
draws the steam away from the original steam-swept zone 4 so that it
invades an additional portion 12 of the formation 5 located below the
original steam-swept zone, as shown in Fig. 3. As the solvent
injection-production cycles are repeated, the solvent-swept zone
expands toward the inJeotion well 2 and steam passes deeper into
portion 5 of the formation~ The amount of solvent injected for each
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cycle is from 10 to 50 and preferably 10 to 20 barrels of solv~nt per
foot of formation thickness being treated and each fluid production
step is terminated when the amount of solvent in the produced fluids
decreases to a value less than 12 percent by volume. The solvent
injection-production cycles are continued until steam or steam
condensate production at well 3 via tubing 7 occurs to a unfavorable
~xtent.
In another embodiment of the invention, the above-described
process may be employed in those formations where the production well
does not initially have two separate flow paths in fluid communication
with the upper and lower portions of the formationO For example, the
injection well and production well may be in fluid communication with
a substantial portion or the entire portion of the formation. In such
a case, steam is in~ected into the injection well and fluids including
oil are recovered from the production well until the presence of steam
and steam condensate in the production ~luid occurs to a unfavorable
extent. Production is then terminated and the upper portion of the
production well in fluid communication with the forniation is closed
off by any convenient means. One method is to spot sufficient cement
in the upper portions of the production well so as to ccmpletely fill
and block-off communications between that portion of the well and the
formation~ The solvent push-pull treatment is then initiated while
simultaneously injecting steam into the injection well in the manner
described above with injection of the solvent into the production well
which is in fluid communication with the lower portlon of the
forma-tion. Steam injection and the simultaneous sequence of solvent
in~ection followed by production are continued until there is a
substantial breakthrough o~ steam or steam condensate at the
production well.
In still another embodiment of the invention, the above-
described process may be employed in those formations wherein the
in;jection well is in fluid communication with the lower portion of the
formation.
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It will be apparent to those skilled in the art that the
present method results ln an appreciable increase in the vertical
sweep efficiency of a steam drive process~ thereby significantly
increasing the recovery of oll from oil-bearing formations.
