8/10/2019 Determination of Residual Oil Saturation in a Carbonate Reservoir http://slidepdf.com/reader/full/determination-of-residual-oil-saturation-in-a-carbonate-reservoir 1/13 Copyright 2001, Society of Petroleum Engineers Inc. This paper was prepared for presentation at the SPE Asia Pacific Improved Oil Recovery Conference held in Kuala Lumpur, Malaysia, 8–9 October 2001. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract Single-well tracer testing has been widely accepted as a standard method for measuring residual oil saturation to waterflood. Residual oil saturation is an important parameter in the evaluation of tertiary oil recovery potential for depleted reservoirs. At an advanced stage of depletion, Leduc, a Canadian carbonate reservoir, has been considered as a candidate for enhanced oil recovery. As part of the evaluation process, single-well tracer tests were conducted at two watered-out producers to determine residual oil saturation to waterflood. The tracer production profiles were found to be highly skewed with long tails and early arrival times, which are typical for carbonate reservoirs. Two different models, namely a double-porosity model where tracer could distribute between the flowing and non-flowing pores through mass transfer and a single-porosity model where a fictitious water drift rate was assumed in the test zone, were used to interpret the data. It was found that either model could match the data to the same degree of accuracy regardless of the flow mechanisms assumed and the residual oil saturation derived from these two models were 35% and 38% respectively. This demonstrates the robust nature of the test that the non- uniqueness of the match does not affect residual oil saturation determination. The residual oil saturation determined by simple analytical models including mass balance method, peak method and mean retention volume method were all in the range of 34% to 38%, in excellent agreement with the simulation results. As well, the S orw obtained by the SWTT method compared favorably with those determined by interwell tracing (35%) and sponge coring (33%). Introduction The amount of oil left in a reservoir after secondary operations is needed to evaluate the potential of enhanced oil recovery processes. Various conventiona methods for S orw determination, such as production history, laboratory waterflood tests, core analyses logging, log-injection-log, interwell and single-wel tracing tests (SWTT) have been extensively reported and compared 1-3 in the literature; each technique offers certain advantages, limitations and different depths of investigation. Of all the methods available to date 1,2 SWTT is unique in its large and variable depth of investigation, and relatively free of the near-well bore effect. SWTT is still the most widely accepted method in the industry for measuring residual oil saturation though an increasing number of interwell tracer testing has been reported recently 4,5 . Since its invention in 1971, more than 200 SWTT have been run both in sandstone and carbonate reservoirs to determine residua oil saturation to waterflood S orw . Deans 6,7 and Tomich 8 have described the single- well tracer testing methods for measuring S orw . It is based on the chromatographic separation of partitioning and non-partitioning tracers in the test zone, a well- known phenomenon in chemistry for hundred years Single-well tracer test, which constitutes the most common application of partitioning tracers in the oi industry 4 , has been well demonstrated for homogeneous sandstone reservoirs. It has, however, been recognized to have some shortcomings when applied to heterogeneous carbonate reservoirs owing to the complicated pore structure and microscopic conformance, which could render long production tails SPE 72111 Determination of Residual Oil Saturation in A Carbonate Reservoir Joseph Tang, Pei-Xin Zhang, China Istitute of Atomic Energy, JT Petroleum
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8/10/2019 Determination of Residual Oil Saturation in a Carbonate Reservoir
Copyright 2001, Society of Petroleum Engineers Inc.
This paper was prepared for presentation at the SPE Asia Pacific Improved Oil RecoveryConference held in Kuala Lumpur, Malaysia, 8–9 October 2001.
This paper was selected for presentation by an SPE Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paperfor commercial purposes without the written consent of the Society of Petroleum Engineers isprohibited. Permission to reproduce in print is restricted to an abstract of not more than 300
words; illustrations may not be copied. The abstract must contain conspicuousacknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.
Abstract
Single-well tracer testing has been widely accepted as a
standard method for measuring residual oil saturation to
waterflood. Residual oil saturation is an important
parameter in the evaluation of tertiary oil recovery
potential for depleted reservoirs. At an advanced stage of
depletion, Leduc, a Canadian carbonate reservoir, has
been considered as a candidate for enhanced oil
recovery. As part of the evaluation process, single-welltracer tests were conducted at two watered-out producers
to determine residual oil saturation to waterflood. The
tracer production profiles were found to be highly
skewed with long tails and early arrival times, which are
typical for carbonate reservoirs. Two different models,
namely a double-porosity model where tracer could
distribute between the flowing and non-flowing pores
through mass transfer and a single-porosity model where
a fictitious water drift rate was assumed in the test zone,
were used to interpret the data. It was found that either
model could match the data to the same degree of
accuracy regardless of the flow mechanisms assumedand the residual oil saturation derived from these two
models were 35% and 38% respectively. This
demonstrates the robust nature of the test that the non-
uniqueness of the match does not affect residual oil
saturation determination. The residual oil saturation
determined by simple analytical models including mass
balance method, peak method and mean retention
volume method were all in the range of 34% to 38%, in
excellent agreement with the simulation results. As
well, the Sorw obtained by the SWTT method compared
favorably with those determined by interwell tracing
(35%) and sponge coring (33%).
Introduction
The amount of oil left in a reservoir after secondary
operations is needed to evaluate the potential of
enhanced oil recovery processes. Various conventiona
methods for Sorw determination, such as productionhistory, laboratory waterflood tests, core analyses
logging, log-injection-log, interwell and single-wel
tracing tests (SWTT) have been extensively reported and
compared1-3 in the literature; each technique offers
certain advantages, limitations and different depths of
investigation. Of all the methods available to date1,2
SWTT is unique in its large and variable depth of
investigation, and relatively free of the near-well bore
effect. SWTT is still the most widely accepted methodin the industry for measuring residual oil saturation
though an increasing number of interwell tracer testing
has been reported recently4,5. Since its invention in
1971, more than 200 SWTT have been run both in
sandstone and carbonate reservoirs to determine residua
oil saturation to waterflood Sorw.
Deans6,7 and Tomich8 have described the single-
well tracer testing methods for measuring Sorw. It is
based on the chromatographic separation of partitioning
and non-partitioning tracers in the test zone, a well-
known phenomenon in chemistry for hundred years
Single-well tracer test, which constitutes the mostcommon application of partitioning tracers in the oi
industry4, has been well demonstrated for homogeneous
sandstone reservoirs. It has, however, been recognized
to have some shortcomings when applied to
heterogeneous carbonate reservoirs owing to the
complicated pore structure and microscopicconformance, which could render long production tails
SPE 72111
Determination of Residual Oil Saturation in A Carbonate ReservoirJoseph Tang, Pei-Xin Zhang, China Istitute of Atomic Energy, JT Petroleum
8/10/2019 Determination of Residual Oil Saturation in a Carbonate Reservoir
The double-porosity model assumed no drift and the
skewed profiles were simulated by adjusting the
effective pore diffusion coefficient L. Dispersion was
controlled by the grid size. The match was found to be
insensitive to the oil saturation in the dead-end pores
!orw, the simulated tracer profiles were hardly affected by varying !orw from 30 to 40%. Since it is immaterial
as to what value !orw was used in the simulation, same
oil saturation was assigned to both flowing and dead-end
pores. The best match was at Sorw= !orw=35%. Since it
is believed that the oil saturation in the dead-end pores
should be higher than that in the flowing pores, the Sorw
of 35% thus underestimates the total pore-average
residual oil saturation. Only a single layer was required
for matching the tracer data.
The quality of the match was excellent except
for methanol and i-butanol. In general, there was a good
match of the peak height and peak position for all tracers but the match for the long tail was only marginal.
Methyl acetate (Figure 2) peaks at about 24 m3,
significantly less than the injection volume. The
methanol profile has the same level of dispersion as the
methyl acetate profile. In comparison with the methyl
acetate fit, the match for the methanol profile, especially
in the tail, was only satisfactory (Figure 3). The fit for
the methanol profile, together with the sensitivity runs at
three Sorw levels, i.e., 40%, 35% (best) and 30%, is
shown in Figure 3. It is clear from the sensitivity study
that the residual oil saturation was 35±3%. The methyl
acetate and methanol profiles are plotted in Figure 4 forcomparison. The methanol peak is ahead of the methyl
acetate peak as predicted by the theory.
The response of the cover tracer i-propanol is
normal (Figure 5). The concentration is high at early
production and continuously declining with production
volume. The match of the mass balance tracers n-
propanol and i-butanol are displayed in Figures 6 and 7.
While the match for n-propanol was excellent, the match
for i-butanol was inferior due to sample analysis
problems.
Single-porosity with Drift
Though it is recognized that the skewed tracer response
is due to the double porosity structure in carbonates, the
profiles can be matched to the same degree of accuracy
by a single-porosity model with a fictitious high driftrate. In the single-porosity simulation, the drift rate was
set to be 0."5 m/day and the dispersion length was at
2."m. To better match the tails, a thin layer was
incorporated into the model. This layer, constituting
only 5% of the total porosity-thickness, conducted "0%
of the flow. With these parameters input into the model
the methyl acetate profile was successfully matched as
shown in Figure 8. The match of the methanol profile
together with the sensitivity study cases, is given in
Figure 9, from which the residual oil saturation was
estimated to be 38±3%. Similar to the double-porosity
model, the match for i-butanol was poor relative to other
tracers. The close agreement between the single and
double porosity models demonstrates the robust nature
of SWTT that though the fit was not unique, the flow
mechanisms assumed did not affect the determination of
Sorw to any significant extent. This is because Sorw
depends mainly on the relative separation of the primary
and secondary tracers but not the absolute flow paths of
individual tracers.
Mass Balance Method
The mass balance tracer n-propanol and cover tracer i-
propanol were used to calibrate the primary tracer lossaccording to the procedures detailed in reference ""The n-propanol curve is deemed to be more credible for
loss calibration purposes because it was injected together
with the primary tracer. Nonetheless, there is only a
subtle 2% difference between the Sorw calculated by
these two tracers. To estimate the uncertainty in Sorw
caused by the hydrolysis rate measurement error, the
calculated Sorw is plotted against hydrolysis rate in
Figure "0. It is evident that for a "0% error in K H, the
corresponding error in Sorw is only 3%. Sorw was
estimated to be 34±3% based on the n-propanol loss-
calibration curve.
Other Analytical Methods
The mean retention volume method relates Sorw with the
mean retention volume ratio for the primary and
secondary tracers through Eq.9. The mean retention
volumes for various tracers were calculated and
summarized in Table 3, from which Sorw was determined
to be 36%. In comparison, Sorw was calculated to be
38% from the primary/secondary peak volume ratio aftercorrecting for the wellbore holdup volume.
The Sorw determined by various interpretation
methods fall into the range of 34-38%. The consistency
of the results validates the interpretation techniques as
well as the credibility of the SWTT method.
8/10/2019 Determination of Residual Oil Saturation in a Carbonate Reservoir