Note: Descriptions are shown in the official language in which they were submitted.
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APPLICATION FOR PATENT
Inventors: DR. JOSEPH S. TANG and BRADFORD C. HARKER
Title: USE OF TRACERS TO MONITOR IN SI~U
MISCIBILITY OF SOLVENT IN OIL RESERVOIRS
DURING EOR
SPECIFICATION
Field of the Invention
The present invention relates to a method of using
tracers to determine the in situ miscibility of oil
reservoirs. More specifically, the present invention
relates to a method for determining the degree of in situ
miscibility of a solvent with subterranean reservoir oil
in miscible enhanced oil recovery projects by observing
the chromatographic separation of two or more tracers
having different vapor pressures.
Background of the Invention
Enhanced oil recovery operations are becoming
increasingly more popular as reservoirs age and oil
production declines. Waterflooding is by far the most
~` widely used method, but it is sometimes economical to
inject other fluids, such as hydrocarbon solvents, into a
partially depleted oil field in an effort to recover oil
which was not produced with waterflooding. When a solvent
is used in an enhanced oil reco~ery operation, it is
injected into the reservoir as a fluid which is miscible
with the reservoir oil. This class of enhanced oil
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recovery is commonly known as a miscible flood because a
miscible solvent is injected into the reservoir to
mobilize and push the oil out of the reser~oir.
Two different miscibility conditions can develop,
depending on the solvent used and the reservoir
conditions. The simplest and most direct method for
achieving miscible displacement is to inject a solvent
which completely mixes with the oil in all proportions
when it first contacts the oil. This type of method
produces mixtures of the solvent and oil in a single
phase, and it is commonly called first contact miscible
flooding. Hydrocarbons of intermediate molecular weight,
such as propane, butane, or mixtures of LPG, are solvents
that have been used most often for first contact miscible
flooding.
If an operation uses a solvent which is not
completely dissolved in the oil upon first contact, it is
known as multiple contact miscible flooding. Since first
contact miscible displacement is more effective than
multiple contact miscible displacement in recovering oil,
it is important to select a solvent composition to ensure
the existence of first contact miscible conditions
throughout the displacement process. The solvent
composition and pressure necessary for miscibility can be
determined from calculations, but constructing the
necessary pseudoternary diagrams is time consuming and
difficult to obtain experimentally.
In principle, the first contact miscible conditions
can be determined by calculating vapor/liquid equilibria
with appropriate equations of state or K-value
correlations while concurrently mathematically simulating
the multiple contacting and in situ mass transfer of
components. However, this approach has several
disadvantages. First, equations of state and K-value
correlations are usually not sufficiently accurate in the
region of interest. Therefore, the calibration of the
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correlations or equations of s-tate must be made with the
aid of experimental phase behavior da-ta.
Another approach is to use available correlations of
experimental miscibility data. However, some of these
correlations are seriously in error, perhaps by 1000 psi
or more. Correlations may be useful for purposes of
screening reservoirs for suitability of miscible
processes, but unless there is a large margin in operating
pressure to allow for potential errors in the correlation
estimates, miscibility pressure should be determined
experimen-tally.
Flow experiments are preferred over calculations as a
method for determining miscibility conditions. Selection
of solvent composition and miscibility pressure is usually
done in the laboratory using any one of a number of
displacement techniques, e.g., slim-tube tests. Criteria
for interpreting the displacements have included
breakthrough and ultimate recoveries at a given volume of
solvent injection, visual observations of core effluent,
composition of produced gases, shape of the breakthrough
and ultimate recovery curves vs. pressure, or combinations
of these criteria. The different experimental techniques
and interpretation criteria have led to vastly different
conclusions.
; 25 Steps have been taken to increase the accuracy-and
precision of the experimental determinations. However,
regardless of how accurate the laboratory work is, there
is always a question as to whether the solvent determined
to be first contact miscible in the laboratory will be
firs-t contact miscible with the oil in the reservoir.
Since an accurate solvent design is essential to the
success of an enhanced oil recovery project, it is highly
desirable to use a technique which can monitor the
miscibility in situ, i.e., in the reservoir, rather than
in the laboratory.
Present in situ techniques for monitoring miscibility
include the sampling and analysis of the produced
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hydrocarbon and gas. The process is determined to be
first contact miscible if the gas/oil ra-tio and the
compositions of the produced gas and hydrocarbon can be
represented as a linear combination of reservoir oil and
solvent. However, measurement errors in the gas/oil ratio
and oil and gas compositions, combined with the inadequacy
of the equations of state, make the results of this
"recombination" -technique inaccurate and ambiguous. In
addition, this technique is not sufficiently sensitive to
small changes in produced fluid proper-ties, such as those
which arise when the solvent and reservoir oil are
slightly immiscible.
Other factors contribute to the inaccuracy of the
"recombination" technique. The recombined reservoir oil
may not truly represent the actual reservoir oil because
of either improper sampling or the great variability of
the oil and gas properties throughout -the reservoir. This
variability is particularly pronounced in reservoirs which
had previously produced for an extended period of time by
solution gas drive at below bubble point pressure followed
by waterflooding in order to increase the reservoir
pressure in preparation for the enhanced oil recovery
project. When producing at below bubble point pressure
for extended periods of time, the gas saturation, gas/oil
ratio, and oil properties will be heterogeneous through
the reservoir because of the vast differences in
mobilities of gas and oil. Even where the reservoir is
later pressurized above the bubble point, pockets of free
gas will remain because the gas is slow to dissolve into
the oil.
Consequently, there is still a need in the industry
for an accurate method to monitor miscibility o~ a solvent
in a miscible flood operation. The present invention,
which is a direct, in situ method of monitoring
miscibility fulfills this need because it is accurate and
highly sensitive, and it requires simple and dependable
data interpretation.
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Summary of the Invention
The present invention relates to a process in which
the miscibility of a solvent in reservoir oil in a
hydrocarbon-containing formation is determined by
injecting a fluid containing at least two properly
selected tracers into the formation. The tracers are
selected from the group consisting of halocarbons,
halo-hydrocarbons, tritiated or carbon 14 tagged
hydrocarbons, sulfur hexafluoride, tritium gas and
radioactive isotopes of inert gases. The tracers must
have different vapor pressures and preferentially
partition into the oil phase and gas phase, if both phases
exist. Preferred tracers useful in this invention are
sulfur hexafluoride, tritiated methane and tritiated
heptane. The presence and amounts of the tracers are
detected at a production well in another location. The
boiling points of the tracers should be at least 50F
apart, preferably 200F or above, to provide detectable
separation of the tracers.
Brief Description of the Drawin~s
Fig. l is a ternary diagram showing the miscibility
of the Cl/C3/ClO system used in Examples l, 2 and 3-.
Fig. 2 is the tracer production curves for Example 1.
~5 Fig. 3 is the tracer production curves for Example 2.
Fig. 4 is the tracer production curves for Example 3.
~igs. 5a - 5d are the tracer production curves for the
four production wells used in Example 4.
Description of the Invention
In this invention, a mixture of tracers is inje~ted
with the solvent as a slug or continuously at any stage in
a miscible ~lood project. The tracers are later produced
from production wells in the vicinity of the injection
well. The tracers consis-t of at least two compounds
selected from the following groups: halo-hydrocarbons,
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halocarbons, sulfur hexafluoride, tr.itiated or carbon 14
tagged hydrocarbons, tritium gas and radioactive isotopes
of inert gases.
The tracers mus-t be miscible with the solvent so tha-t
the solvent and tracers can be injected into the reservoir
in a single phase. The tracers selec-ted must also have
different boiling points and should have a difference of
at least 50F in their boiling points, and preferably at
least 200F. In the preferred embodiment of this
invention, tracers can be selected so that the tracer
separation test can be completely eliminated by selectiny
one low boiling point and one high boiling point tracer,
e.g., -100F and 200F, respectively. The low boiling
point tracer will be produced predominantly with the gas
phase, while the high boiling point tracer will be
produced predominantly with the oil phase. The tracers
selected should also be detectable at -the parts per
million level by appropriate analytical means in order to
be economically feasible.
The produced tracers may be found in -the produced
gas, in the produced oil, or in both, depending on the
vapor pressure of the tracers and the separator condition.
The produced tracers in the gas and oil phases are
separated and analyzed. Analysis of the halo-hydrocarbons,
halocarbons and sulfur hexafluoride is usually by gas
` chromatography equipped with an electron capture detector.
Tritiated or carbon 14 tagged hydrocarbons ~including
tritium gas) can be measured using li~uid scintillation
counter and gas proportional counter.
The various tracers which can be used in practicing
this invention are partitioning tracers. Wherever the gas
phase and oil phase coexist in a reservoir, the tracers
will distribute themselves between these phases, according
to the tracer's K-values or Henry's law constants. When
passing through a reservoir having both an oil and a gas
phase, the tracer with a low boiling point (high vapor
pressure~ will be produced in the gas stream ahead of the
high boiling point (low vapor pressure) tracer. When this
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occurs, a distinct separation of the tracers will be
observed.
If the displacement solvent is first contac-t
miscible, there should be only one phase throughout the
reservoir. In this case, separation of the tracers would
be impossible, and the tracers would have the same scaled
production functions. By contrast, if the solvent is not
irst contact miscible wi-th the reservoir oil, a two-phase
zone will develop. The size of the two-phase zone will be
directly related -to the misci;bility o~ the solvent with
the oil. The tracers will separate and partitlon in the
two-phase zone.
The tracer separation in the two-phase zone is
extremely sensitive to the miscibility condition of the
solvent with the reservoir oil. Even slight deviations
from the first contact miscible condition results in a
significant tracer separation. The tracer separation is
thus a highly useful miscibility indicator, with the
degree of separation being directly rela-ted to the size of
the immiscible zone which is, in turn, directly related to
the deviation from first con-tact miscibili-ty.
The following examples are for illustrative purposes
only and are not intended to limit the scope of this
; invention:
Examples
Four examples were performed. One was conducted in
situ. The others were conducted in the laboratory using a
slim-tube with various mixtures o~ methane (Cl) and
propane (C3) as the solvent and decane (C10) as the oil.
The ternary diagram ~or the Cl/C3/C10 system at 610 psig
and 50C is shown in Figure 1. Solvent leaner than 18% C1
was below bubble point at the above pressure and
temperature. Hence, the solvent could not be richer in C1
than 18%. The solvent composition corresponding to the
first contact miscible condition was approximately 10% C1
and 90% C3. The sIim--tube was first saturated wi-th C10
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(simulated oil). Solvent made up o~ a mixture of C1 and
C3 was then injected to displace C10. The solvent was
spiked with three tracers -- sulfur hexafluoride (SF6),
bromo-trifluoro-methane (Fl3B1), and
dichloro-difluoro-methane (}~12). The tracer
concentrations in the produced gas were measured by a gas
chromatograph with an electron capture detector.
Example 1
The solvent used had a composition of 15% C1 and 85%
C3. This corresponds to a condition just slightly more
immiscible than first contact miscible. As shown in
Figure 2, the tracers broke through at nearly -the same
time. SF6, which has the highest vapor pressure of the
three tracers, was produced first, followed by F13B1 and
F12. There was little separation of F13B1 and F12,
probably due to the very small immiscible zone which
developed during the test.
Example 2
This test was run at more immiscible conditions with
a leaner solvent. The solvent used had a concentration of
17% Cl and 83% C3. Because a bigger immiscible zone
; formed, the tracers were widely separated, as shown in
Figure 3.
Example 3
This test was run at ~irs-t contact miscible condition
using the same solvent composition as in Example 2 but at
an elevated pressure of 800 psig. In sharp contrast with
Example 2 (Figure 3), there was no -tracer separation, as
shown in Figure 4.
Example ~
The invention was applied to a solvent miscible pilot
project in a carbonate oil reservoir. The test was
performed by injecting into the center injector of a
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five-spot pattern a slug of tracers containing sulfur
hexafluoride, tritiated me-thane and tritiated heptane.
Production from the four producers was analyzed for the
three tracers. The tracer production da-ta for the four
producing wells are shown in Figure 5, which shows that
sulfur hexafluoride and tritiated heptane tracked each
other closely. The absence of a separation between -the
sulfur hexafluoride (sublimation tempera-ture -82.~F) and
the tritiated heptane (boiling point 209F) indicates -that
the solvent was first contacl miscible in the reservoir
oil. The tritiated methane response was spurious and was
not included in Figure 5.
It should be no-ted that the recoveries for Examples
1, 2, and 3 were all above 98% at one pore volume of
solvent injection. From the recoveries, it would appear
that all three tests are essentially first contact
miscible. However, the vast differences in the production
curves in the three tests clearly reveal -the presence of
three distinct miscibili-ty conditions. Therefore, the
tracer separation obtained in practicing this invention is
a more sensitive and reliable indicator than the commonly
used misci~ility criteria based on fractional recovery at
one pore volume injection.
Flow tests are often performed on misci~le enhanced
oil recovery projects. These tests consist of injecting
and producing solvent tagged with a single tracer in an
effort to ~ain information on channelling, communication,
and solvent distribution. Implementation of this
invention would simply require the inclusion of a second
tracer in one of the flow tests and would provide highly
useful miscibility information for very slight incremental
cost.
The principle of the invention, a detailed
description of one specific application of the principle,
and the best mode in which it is contemplated to apply
;` that principle have been described. It is to he
understood that the foregoing is illustra-tive only and
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that other means and techni~ues can be employed without
departing from the true scope of the invention defined in
the following claims.
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