low

eservoir engineers consider an EOR project,
questions are raised concerning the detailed
knowledge of the reservoir in which EOR is being
considered. Such questions are often asked (and
partially answered) when water flooding has
occurred, but the far greater cost of any EOR
process compared with water flooding puts a
greater emphasis on the need for a detailed
description of the reservoir to be exploited [8] .
Figure 2: Characterize the heterogeneity
Tracers for the determination of
residual oil saturation (partitioning
tracers in single well or in well-to-well
experiments)
Measurement of remaining oil saturation
in a near well region, using a single-well
chemical tracer test (SWCTT) is commonly used
in the oil industry. This method exploits the time
lag of back-produced ester vs. hydrolyzed alcohol.
Partitioning inter-well tracer tests (PITTs), which
can be used to assess inter-well oil saturation,
are frequently used to investigate the presence
and remediation of non-aqueous phase liquids
(NAPLs) in aquifers. However PITTs are rare in the
oil industry, with a few notable exceptions dating
back to the 1990’s [9].
Single-well chemical tracer (SWCTT)
The single-well chemical tracer (SWCTT) test is an
in-situ method for measuring fluid saturations in
reservoirs. The most common use is the assessment
of residual oil saturation (Sor) after water flood
operation. When monitoring displacement of oil
from a reservoir, it is important to benchmark
the amount that remains following secondary
recovery. SWCTT are non-destructive and can be
run in either sandstone or carbonate reservoirs
over widely varying formation characteristics. The
test is based on injection of a partitioning tracer
(i.e. an ester) into the reservoir, where part of it
partitions in the remaining oil phase and the other
part undergoes a hydrolysis reaction to produce
a non partitioning tracer. This hydrolysis process
takes place over a period of few days while the
well is shut-in.
Reaction for ethyl acetate is:
CH 3
COOCH 2
CH 3
+ H 2
O CH 3
CH 2
OH + CH 3
COOH
i.e.
Ester + Water Alcohol + Acid
The well is then back-produced and wellbore
samples are analyzed for tracer returns. The
analysis of the flowed-back samples are plotted
as in Figure 3, where concentration vs. produced
volume is generated.
Figure 3: Typical tracer production curves used for interpretation
from a SWCTT
From the differences in arrival times (maximum
of the peaks) and the partitioning coefficient
value, one can obtain the remaining or residual
oil saturation (ROS or Sor). The partitioning
coefficient is a physical property that relates
tracer concentration in oil and water phases at
equilibrium as shown below:
K d
=C o
/C w
Where, C o
and C w
are tracer concentrations in oil
and water phase respectively.
From chromatographic theory, the retardation
factor (1+ β) which is equivalent to the peaks ratio
Q a
/Q b
(Figure 3) as defined as follow:
Q
Q
kS
= (1 + β ) =
(1 − S )
a d or
Rearranging this formula gives:
b
Commonly utilized esters in SWCTTs are propyl
format and ethyl acetate.
β
Sor
=
β + k
The SWCTT can also be used to evaluate the
effectiveness of EOR processes to mobilize
residual oil (S or
) or “trapped oil”. First, SWCTT
is used to determine Sor to water flood. Then
the EOR fluid is injected for a certain volume
followed by water in the test well/interval. Lastly,
the SWCTT is carried out again for the second
time to determine Sor to the EOR process. The
results of the test will give direct indications of
the effectiveness of the EOR process to mobilize
residual oil [10] .
d
or
Figure 4: The partitioning inter-well tracer test (PITT)
The important features of SWCTT are summarized
below [11] :
• The Sor measurement is made in situ in the
water flooded layers of the target formation.
The tracers can go only where the injected
water goes.
• Compared to coring or logging method results,
the S or
results are from a relatively large
reservoir volume.
• The S or
measurement is carried out on an
existing well and usually in an existing
completion, which can be perforated or open
hole.
• Because the Sor measured actually is the
volume fraction of oil in the pore space, the
measurement is independent of porosity.
Partitioning inter-well tracer test (PITT)
The partitioning inter-well tracer test (PITT) is
a non-intrusive low-cost test that can provide
measurement of oil saturation in the region
between injectors and producers in an oilfield.
The test is run during normal operation of both
injector and producer and thus neither cause loss
of production nor halt of injection.
Although the comparison with a SWCTT or
saturation obtained from core floods are indeed
useful and required as an independent verification
of the PITT methodology, it should be noted that
58 OIL INNOVATORS International Journal MAR. 2018 59