Note: Descriptions are shown in the official language in which they were submitted.
123~SO
F-2923
A METHOD FOR RECOVERING HYDROCARBONS
FROM FRACTURED OR HIGHLY STRATIFIED
LOW VISCOSITY SUBSURFACE RESERVOIRS
The present invention relates to a method for
recovering low viscosity hydrocarbons from subterranean
formations and, more particularly, to a method for
alternately injecting water and gas into such formations
to enhance recovery.
It is well known in the oil industry that
so-called secondary recovery processes are employed to
produce additional volumes of gas and oil from a
subterranean reservoir after production by primary means
has declined to an uneconomical level. The more
commonly known secondary recovery procedures involve
injecting either gas or water into a partially depleted
reservoir through an injection system to drive oil or
the like toward a production system from which the oil
is produced along with portions of the driving fluid.
When the ratio of driving fluid to oil reaches an
uneconomical level, the reservoir is normally abandoned,
even though a substantial amount of residual oil still
remains in the reservoir.
It has been recognized that greater recovery
efficiencies can be obtained by flooding with both gas
and water in a single recovery operation. It is
theoriæed that by injecting gas before or after a
waterflood, a gas saturation can be established within
the reservoir wherein slugs of "trapped gas" will occupy
space within the reservoir which otherwise would contain
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trapped oil. This inherently reduces the amount of
trapped or residual oil present in the reservoir and,
accordingly, provides greater recovery.
While the foregoing described secondary
recovery methods of alternating gas and water flooding
have been applied to oil reservoirs, they have only met
with limited success. The theory behind these
alternating gas-water injection methods is that the gas
would cause relative fluid mobility barriers in the
permeable streaks such that the following water could
flood out more of the oil zone. The problem with this
theory had to do with the injected gas channeling so
badly due to its low viscosity that the benefits ~o oil
recovery were limited.
In accordance with the present invention, there
is provided a method for recovering low viscosity
hydrocarbons from fractured or stratified subsurface
formations. A high viscosity blocking fluid is injected
into the fractured or stratified formation through an
injection well to fill the fractures or high
permeability strata (thief zones) and create a fluid
mobility barrier. A low viscosity flooding fiuid is
also injected into the formation through the injection
well. The fluid mobility barrier created by the
blocking fluid prevents channeling of the lower
viscosity flooding fluid into the formation fractures or
thief zones, thereby enhancing the sweep efficiency of
the lower viscosity flooding fluid and increasing
recovery of low viscosity hydrocarbons from the
formation through a production well. The injections of
blocking fluid and flooding fluid may be carried out
simultaneously or in successive injection cycles or
phases.
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The viscosity difference between the blocking
fluid and the flooding fluid may be increased to furthef
enhance hydrocarbon production. The viscosity of the
blocking fluid may be increased by the addition of a
thickening additive, such as a polymer, oil, or an
emulsion. The additive may be a foam for introducing a
minimal gravity effect. The additive may be a
thixotropic additive for increasing viscosity with
time. The additive may be added during the latter
portion of the injection phase of the blocking fluid.
In a further aspect, the viscosity difference
between the blocking fluid and the flooding fluid is
increased to an extent such that the ratio of fracture
fluid mobility or thief zone fluid mobility to matrix
fluid mobility is reduced to a predetermined minimum
level.
In a still further aspect, the location of high
fluid mobility fractures or thief zones in the formation
may be identified. The high viscosity blocking fluid is
injected directly into such fractures or thief zones to
create the fluid mobility barrier, while the low
viscosity flooding fluid is injected directly into the
formation matrix.
In a yet further aspect, the blocking fluid is
water and the flooding fluid is a gas. Firstly, the gas
is injected into the formation through the injection
well and hydrocarbons are recovered at the production
well until the production ratio of gas to hydrocarbons
reaches an unacceptable level due to channeling of the
gas through the formation fractures or thief zones.
Secondly, water is injected into the formation and
migrates into the fractures or thief zones to create the
desired fluid mobility barrier. Thirdly, gas is again
injected into the formation, the sweep efficiency of the
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gas being increased due to the fluid mobility barrier
created by the water filled fractures or thief zones,
thereby resulting in increased low viscosity hydrocarbon
production through the production well.
In the drawings appended to this specification:
Figure 1 of the present invention illustrates a
subterranean reservoir penetrated by an injection and a
production well;
Figures 2 and 3 are graphical representations
of production rates from a hydrocarbon reservoir using
the alternating water-gas injection method of the
present invention and
Figures 4 and 5 illustrate hydrocarbon sweep
efficiency patterns along a highly stratified or
fractured subsurface reservoir.
The alternating water and gas injection method
of the present invention has been shown to greatly
increase sweep efficiency in a highly stratified or
fractured low viscosity hydrocarbon reservoir, such as a
gas condensate or volatile oil reservoir. This increase
occurs because the injection of high viscosity water
tends to block the reservoir fractures or thief zones,
thus forcing the trailing injection gas to invade the
reservoir matrix with little channeling loss.
Referring to Fig. 1, an injection well 10 and a
production well 12 extend from the earth's surface 14
through a highly stratified or fractured low viscosity
hydrocarbon reservoir 16. Both the injection well 10
and the production well 12 are provided with
perforations 18 to establish fluid communication with
the reservoir 16. Firstly, a low viscosity flooding
flood, such as a gas, may be injected into well 10 for
flooding the reservoir 16. Production of hydrocarbons
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through the production well under the drive of the
flooding gas will continue until the production ratio of
flooding gas to hydroearbons reeovered reaches an
unacceptable level, primarily due to channeling of the
low viscosity flooding gas through naturally occurring
fractures or other permeable streaks in the reservoir
16. At this point, a high viscosity bloeking fluid,
sueh as water, is injeeted through injeetion well 10
into the reservoir where it migrates to and fills the
reservoir fractures or thief zones. Beeause of its 10
to 20-fold higher viscosity, the water acts as an
effective block to channeling of a trailing gas drive
and forces the trailing gas into a superior flood
pattern through the reservoir matrix.
To further overcome high stratification or
fracture induced problems, the viscosity difference
between the blocking gas and the flooding water may be
increased by the addition of a thiekening additive to
the water. Any number of thiekeners may be utilized,
sueh as polymers, for example. Other possibilities are
foams, oils and emulsions. Foams eould be quite
signifieant, espeeially in fraetured reservoirs since
foams would only be minimally affected by gravity
forces. Whenever gravity causes detrimental results, a
foam ean be designed to eounteraet gravity. Such
additives can be injected along with the initial water
injection or can be injected during the latter portion
of the water injection or blocking phase. Also, the
additive may be a thixotropic additive that increases
the viscosity of the injeeted water with time. In
addition, two or more fluid injeetion eycles may be used
during the bloeking phase wherein a trailing fluid
interacts with one injected earlier to form a more
viscous blocking of the fractures or permeable streaks.
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soth the water and gas injection phases can be
carried out continuously and simultaneously. secause of
relative permeabilities, the water would tend to migrate
to and fill the more permeable reservoir fractures or
thief zones, thereby blocking gas flow and forcing such
gas into the less permeable reservoir matrix or strata.
In a further aspect, the location of the fractures or
thief zones could be identified with the water injection
being directly into the fractures and the gas injection
being directly into the reservoir matrix. Conventional
well packers could be used to isolate the water and gas
injection.
An additional feature of the invention is to
increase the viscosity difference to an extent such that
the ratio of fracture or thief zones fluid mobility kt
to matrix fluid mobility km is reduced to a
predetermined level. Fig. 2 illustrates the effect of
reducing the ratio to 10 from the base case of lO0.
Curve I shows that the high fluid mobility or fracture
zone is no longer a thief zone and that good sweep is
achieved by nitrogen injection only. The maximum
possible production from this reservoir is about
1.9 x 105 barrels, so that almost all the wet gas is
produced by nitrogen injection in about 500 days. The
alternating water-gas injection method of the invention
reduces the production by about 3% due to some residual
gas to waterflood. The thief zone has no effect.
However, for a ratio of 500, the effect of the thief
zone on production is significant. Curve IV attains a
78% increase in cumulative wet gas production as
compared with nitrogen injection only of curve V. In
this case, it would be necessary to thicken the water
with an additive in order to achieve recoveries as high
as those experienced for the kt/km of lO0.
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Fig. 3 illustrates the results of a model study
for base case fluid-rock properties in which wet gas
production in reservoir barrels is plotted against time
in days. The reservoir fluid and fluid-rock properties
approximately represent those of the Anschutz Ranch
field in ~tah. For the nitrogen injection only of Curve
I, nitrogen breakthrough at the production well begins
in 20 to 30 days, which represents about a one pore
volume slug in the thief zone. wet gas production rate
drops as nitrogen production rises because nitrogen is
channeling through the thief zone. As with any miscible
flood where crossflow occurs, hydrocarbon recovery goes
on forever, but at continually declining rates. For
water injection only, there is an initial high wet gas
recovery due to the advantageous mobility ratio of water
to wet gas. However, eventually this hydrocarbon
recovery curve drops below the one for nitrogen only
because of high residual gas saturation after
waterflood. The alternating water-gas injection process
of the invention produces a much greater recovery (Curve
III), in this case, sweep efficiency is increased about
40% in 600 days over that of the gas only injection.
Such increase in sweep efficiency occurs because the
high viscosity water tends to block the thief zone so as
to prevent gas channeling, thus causing the trailing gas
to invade the reservoir matrix. The principal factors
governin~ the process are the ratio of the thief zone to
matrix permeabilities, the viscosities of injected water
and gas, and the residual saturation to water.
It can be seen from the foregoing description
that by utilizing the alternating water-gas injection
method of the invention in a highly stratified or
fractured low viscosity hydrocarbon reservoir, the
viscosity difference in the blocking water and the
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trailing gas flood will be working against any gas
channeling through the fractures or permeable streaks.
This is in contrast with the alternating gas-water
injection methods previously used in which gas
channeling was so severe that any blocking effects of
the gas to the trailing water flood is limited. This is
~uite graphically depicted in Figs. 4 and 5, where the
water blocking and gas flooding sweep pattern of the gas
condensate reservoir of Fig. 5 is greatly increased over
the gas blocking and water flood sweep pattern of oil
reservoir of Fig. 4.
