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Measurements of gas tracer retention under simulated reservoir conditions
Journal of Petroleum Science and Engineering, 10 ( 1993 ) 17-25
17
Elsevier Science Publishers B.V., A m s t e r d a m
Measurements of gas tracer retention under simulated reservoir conditions 13. Dugstad, T. Bjornstad and I.A. Hundere lnstitutt for Energiteknikk, Postbox 40, N-2007 Kjeller, Norway (Received September 17, 1992; revised version accepted May 26, 1993)
ABSTRACT Measurements are carried out to investigate the behaviour of gas tracers at reservoir conditions. The interactions between the tracer and the rock matrix, and between the tracer and the residual oil are studied. Two chemical compounds, perfluoromethylcyclopentane and perfluorometyl-cyclohexane, are studied to evaluate their potential as gas tracers in oilfield studies. The chemical tracers are compared with the behaviour of radioactive methane and ethane tracers.
I. Introduction
Tracing gas and water which are injected into reservoirs is a widely used m e t h o d to increase the knowledge of fluid movements. The tracers applied are mainly radioactively tagged molecules. These molecules can, using special techniques, be measured down to very small concentrations. This is important, because of the very high dilution of the injected fluid in the reservoir. The radioactive nuclides used are isotopes of elements in naturally occurring reservoir gases, or are tagged on chemicals which are not c o m m o n in the reservoir. The tracers most frequently reported in the literature are tritiated hydrogen or methane and 85Kr. 14Clabelled light hydrocarbon gases are also applicable as tracers, but are less commonly used, mainly because of higher costs. Armstrong (1960) reports on the application of gas tracers to define fracturing directions in reservoirs. Directional permeability can be identified early in reservoir processes so that steps can be taken to maximise recovery efficiency. In order to be able to trace the injected gas
from various injection wells, each well has to be labelled with different tracers. At present, the number of available and applicable radioactive tracers are restricted, and it is, therefore, of interest to find additional tracer candidates which can be applied in reservoir measurements. Before implementation of new tracers in field applications, the behaviour of the compounds should be extensively studied in laboratory experiments, in order to ensure the most efficient injection strategy. An ideal tracer should follow the traced phase closely. With injection of methane gas, radioactively labelled methane will be an ideal tracer. Methane can be radioactively labelled either by substituting one of the hydrogen atoms by tritium, or, likewise, to exchange ~2C by 14C. Though these tracers may be considered as ideal relative to methane, there will be a partitioning between the oil phase and the gas phase, as is the case with stable methane itself. The partitioning coefficient of methane is known in reservoir engineering literature. All gas molecules have partitioning between phases. The tracer molecules will travel with the velocity of the liquid phase when dissolved
0 9 2 0 - 4 1 0 5 / 9 3 / $ 0 6 . 0 0 © 1993 Elsevier Science Publishers B.V. All rights reserved.
I8
in the liquid and with the velocity of gas when in the gas phase. A tracer molecule with a high partitioning in other phases is, therefore, considerably delayed in the reservoir with respect to the average gas flow. Tracers may also be delayed through interaction with the rock surface. These delay mechanisms can in some cases cause difficulties in the interpretation of the results. In the search for new applicable tracers, it is, therefore, important to quantify these delays to improve the interpretation of the data collected from field tests. 2. Gas tracers in reservoir studies
Radioactive tracers
Several authors report on the use of radioactive tracers in oilfield applications. The most c o m m o n tracers are tritiated hydrogen gas (Fearon, 1957; Calhoun and Title, 1968; Calhoun and Hurford, 1970; Tinker, 1973; Rupp et al., 1984; Mayne and Pendleton, 1986), tritiated methane ( G o n d u o i n et al., 1967; Calhoun and Tittle, 1968; Calhoun and Hurford, 1970; Davis et al., 1976; Rupp et al., 1984; Welge, 1985; Mayne and Pendleton, 1986 ) and 85Kr (Welge, 1955; Armstrong, 1960; Howell et al., 1961; Calhoun and Title, 1968; Calhoun and Hurford, 1970; Tinker, 1973; Davis et al., 1976; Rupp et al., 1984; Mayne and Pendleton, 1986). Some papers have described the application of tritiated ethane, propane and even butane as tracers ( G o n d u o i n et al., 1967; Mayne and Pendleton, 1986). All these tracer molecules, except 85Kr, are tritium-labelled compounds. Tritium is a weak beta emitter. Small concentrations of tritium-labelled tracers are detected directly in the gas phase by a proportional counter technique, or the gas is oxidised to produce water which is counted by liquid scintillation technique. Chemical tracers
Only a few papers on the application of chemical gas tracers in reservoir studies are
O. DUGSTAD ET AL.
published. Craig (1985) has reported on the use of halocarbons and sulphur hexafluoride as tracers for carbon dioxide. These tracers were applied successfully in a miscible flood. As early as 1946, Frost ( 1946 ) reported on the use of helium as a tracer for gas injection. When searching for new "near-ideal" tracers, the following main principles are taken into consideration. A tracer must satisfy the following criteria: - - the tracer must have a very low detection limit; - - it must have stability under reservoir conditions; - - it should follow the tagged phase and must have a minimal partitioning into other phases; k it must have no absorption to rock material; and - - the use of the tracer must have m i n i m u m environmental consequences. Dietz (1987) reported the application of per-fluorinated hydrocarbons as tracers for air in environmental studies. These tracers have a very low detection limit which make them compatible with radioactive tracers with respect to detectability. The fluorine compounds are reported to be analysed in concentrations down to 10-15_ 10-161/1 in air samples. Such a high sensitivity makes these molecules interesting as oil field tracers. Analyses of perfluorocarbons (PFC) are performed using a gas-chromatograph (GC) connected to an electron capture (EC) detector. The EC detector is extremely sensitive to perfluorinated hydrocarbons, especially the cyclic compounds. Senum et al. ( 1989 ) have reported a method to analyse the PFC content in a hydrocarbon gas from a production stream. The gas contained in pressure bombs is flushed through active carbon to absorb the PFCs. The PFC containing pellets are thermally desorbed and the gas is directed through a system of a precolumn, catalysts and traps to remove the hydrocarbons before the PFCs enter the main
GAS TRACER RETENTION UNDER SIMULATEDRESERVOIRCONDITIONS
separation column to determine the amount of each tracer. Such tracers are applied in pilot projects on the Norwegian continental shelf.
3. Experimental studies In the experiments reported here two perfluorocompounds, perfluoromethylcyclopentane ( P M C P ) and perfluoromethylcyclohexane ( P M C H ) , and three radioactive tracers, tritiated methane, 14C-labelled ethane and 85Kr, were studied. The purpose of the experimental work is to study the behaviour of the tracers under elevated temperature and pressure, i.e., at simulated reservoir conditions. Some physical constants of the tracers are listed in Table 1, where it is evident that the PFCs are liquids under standard conditions. Methane, ethane and krypton are real gases with a boiling point far below 0 ° C. The tracers were studied in dynamic flooding experiments. Retention was measured while the gas containing the tracer was f o o d e d through a porous medium. The experimental equipment and procedures applied in this study are described by Dugstad et al. (1992). The tracer mixture confined in a small-volume tubing section just ahead of the entrance of the slim tube is flushed by the injection gas through the column. The effluent is analysed by automatically sampling the gas into scintillation vials for counting the radioactivity and by automatic injection into a gas chromatograph for the detection of the chemical tracer. Several series of experiments were carried TABLE 1 Physical constants for the studied gas tracers PMCP PMCH Methane Ethane
Formula C6FI2 CvFl4 CH3T MW BP(°C)
300 48.0
350 76.3
18 -164
Krypton
14CH3-CH3 S~Kr 32 85 -88.6 -152
19
out to study the retention caused by the interaction between the tracer and the rock matrix. The packing material applied in the tubes was dry sand (Ottawa sand--nearly pure quartz ) or limestone. Different amounts of kaolinite clay were added to the pure Ottawa sand to measure the effect of the clay. The addition of kaolinite was carried out, making a water slurry of the clay and the Ottawa sand. The mixture was dried in a climate chamber, providing a thin layer of clay distributed onto the quartz grain surface. Two different clay concentrations were prepared, 0.5% and 2.5% by weight, respectively. The applied limestone was composed of crushed material from the Ekofisk formation sampled onshore in Denmark. The prepared tubes were 6 m long, with an inner diameter of 0.5 cm. One series of experiments was performed to investigate the interaction between the tracer molecules and the oil in the porous medium. In a porous m e d i u m partly filled with different fluids, the main retention mechanism is the tracer partitioning between the different phases. To study this effect, a 12 m long slim tube with a inner diameter of 0.5 cm was used. The tube filling material was pure Ottawa sand. All gases will, to some extent, dissolve into liquid phases until equilibrium is achieved. The transport velocity of the tracer in the reservoir will, therefore, be composed of the sum of the time fraction spent in the liquid phase, multiplied by the liquid velocity and the time spent in the gas phase multiplied by the gas velocity. Tracers which partition between the gas and liquid phases will, therefore, be retained when the liquid flows with a lower velocity than the gas. In the retention studies here reported, the column was prepared to residual oil saturation with model oil (decane) and the oil phase is, therefore, considered as stationary. No water was present. The injection gas was methane. The experiments were carried out at four different temperatures and four different pressures. Because a model oil is used, the conditions are
20
not identical to those in a reservoir. If the oil in the tube is characterised by standard oil characterisation, it has a mole fraction of methane in the range of 0.3-0.7 and a hept a n e + fraction in the range of 0.7-0.3, depending on temperature and pressure. The heptane + fraction (pure decane) has a mole weight of 142. According to the Handbook of Natural Gas Engineering (Katz et al., 1959), the partitioning coefficients for methane in this system are listed in NGAA (Brown et al., 1948) data charts drawn for a convergence pressure of 5000 psia. To enable the calculation of effective pore volumes, an estimation ofoil and gas densities are carried out. In the present paper, data from Sage et al. (1940) are used to calculate the density of decane. The NGAA compressibility factor charts for pure methane are used to calculate the gas density. 4. Results and discussion
The results from the experiments carried out to study retention caused by the interaction oftracers with the surface of reservoir material are listed in Table 2. Because the injected gas is methane, tritium-labelled methane molecules behave as an ideal tracer. The results presented in the table are given as the ratio between the retention (measured at the mass middle point of the production profile ) of the methane and the actual tracer. The experiments are carried out at 100 bar. When pure Ottawa sand is applied no differences in retention time are measured between methane, ethane, PMCP and PMCH. When kaolinite is added to the porous matrix a delay is measured. The delay increases with increasing clay content. Limestone causes the most serious delay, in the order of 10% at 100°C for the PFCs. Ethane, however, seems to follow the methane closely. Other types of clay material, different from kaolinite, which frequently appear in reservoir rocks, also contribute to the retention. The ef-
O. DUGSTADET AL. TABLE 2 Tracer retention relative to tritiated methane in fraction of pore volumes Substrate
PMCP
PMCH
t4CIq3.CI-I3
1.00 1.03 1.06 1.13
1.00 1.00 1.00 1.01
1.00 1.03 1.06 1.14
1.00 1.00 1.00 1.00
1.00 1.02 1.06 1.13
1+00 1.00 1.00 1.02
Pressure 100 bar, temperature 800C Ottawa Ottawa + 0.5% kaol Ottawa + 2.5% kaol CaCO3
1.00 1.02 1.03 1.08
Pressure 100 bar, temperature 100*C Ottawa Ottawa +0.5% kaol Ottawa + 2.5% kaol CaCO3
1.00 1.02 1.04 1.09
Pressure 100 bar, temperature 1200C Ottawa Ottawa +0.5% kaol Ottawa + 2.5% kaol CaCO 3
1.00 1.02 1.04 1.09
fect of clay on retention is, however, reduced, because the grains are covered with water or oil. The gas tracers under study are expected to have a negligible partitioning into water. It is, therefore, expected that water will not contribute significantly to retention. Figures 1, 2 and 3 illustrate the results from three independent tracer experiments. The tracer response curves are plotted as a function of injected pore volumes. In Fig. 1 the experiment is carried out on a dry slim tube. The temperature was 50°C and the pressure was 250 bar. A small retention is measured for the PFCs. At identical pressure and temperature (50°C, 250 bar), but with a decane saturation of approximately 30%, a time lag is measured for all tracers, even for the methane. The PMCH, the perfluorocarbon with the highest molecular weight and the highest boiling point, is the most heavily retained tracer. At these conditions, more than 1.7 pore volumes are needed to flush the P M C H through the porous medium. When decreasing the pressure to 150 bar, 2.2 pore volumes are needed before the P M C H moved through the slim tube. Comparing the PMCP and ethane, it is noteworthy
21
GAS TRACER RETENTION UNDER SIMULATED RESERVOIR CONDITIONS 26
Z O
24
FE-, Z
22
•
CH3T
20
+
14CH3- CH3
o
PMCP
a
PMCH
18 16
L) Z 0
14 12
~q N
10 8
<
6 4
0 Z
2 0 0.8
12
I
1A
1.6
18
2
212
214
2.6
RELATIVE TIME (inj.vol/PV)
Fig. 1. Tracer retention as a function of injected pore volume with the following parameters: Temperature 50°C, pressure 250 bar, flow rate 0.45 ml/min, So, 0%.
15
Z 14 O 13
CH~T
•
e¢ tl EZ to m L) Z ©
+
14CH3- CH3
o
PMCP
s M © Z
RELATIVE TIME (inj.vol/PV)
Fig. 2. Tracer retention as a function of injected pore volume with the following parameters: Temperature 50 ° C, pressure 250 bar, flow rate 0.24 ml/min, So, 31%.
Z
14
2 <
~3 12 I1
Z C)
lo 9
~
~ JN <
a5678
. . L
°! 2
© Z
•
Ct43T
+
l'l CH3- CH3
o
PMCP
a
PMCH
2 1
i -_ ~ a .
0
0.8
,
1
-~
- - ~ - - - - F
1,2
.m
. . . .• 1.4
. . . .
,
,
1.6
,
i
1.8
r
2
"'r
. . . .
r . . . .
2.2
r
. . . .
r
24
r'"
2.6
RELATIVE TIME (inj.vol/PV)
Fig. 3. Tracer retention as a function of injected pore volume with the following parameters: Temperature 50 ° C, pressure 150 bar, flow rate 0.24 ml/min, Sor 31%.
22
O. DUGSTAD ET AL.
that at 250 bar the P M C P is in front of the ethane, but at 150 bar the situation is reversed and the ethane flows with a higher average velocity. Table 3 gives the relative retention of ethane, PMCP and P M C H with respect to methane at four pressures ( 100 bar, 150 bar, 200 bar and 250 bar) and at four temperatures (50°C, 80°C, 100°C and 120°C). The calculation of the relative time (the xaxis in the figures) will be influenced by the accuracy of several process parameters. A small shift may, therefore, occur and the most valuable method to compare the results will, therefore, be to give the relative retention with respect to the ideal methane tracer. This is shown in Table 3. For each experiment the liquid saturation is given. The saturation must be measured individually in each experiment and is a TABLE3 Retention of PMCP, PMCH and 14C labelled ethane relative to tritiated methane as a function of temperature, pressure and oil saturation Temperature 50°C Pressure (bar) Oil sat. (%) 14CH3- CH3 PMCP PMCH
100 32.7 1.47 1.95 2.84
150 30.7 1.40 1.47 1.91
200 29.9 1.27 1.31 1.51
250 31.3 1.21 1.18 1.29
Temperature 80°C Pressure (bar) Oil sat. (%) 14CH3-CH3 PMCP PMCH
100 19.6 1.27 1.49 1.95
150 22.6 1.24 1.33 1.59
200 25.6 1.19 1.23 1.37
250 30.1 1.17 1.18 1.28
consequence of the applied temperature and pressure. The relative retention cannot, therefore, be compared directly. An increase in saturation gives an increase in the relative retention value. The retention measured can, however, be recalculated to give the retention at 20% saturation. This is done by calculating the ratio between the concentration of the tracer in the gas phase (Cm) and in the liquid phase (Cs). This ratio is a constant independent of saturation and can, therefore, be used to derive the retention at a new saturation. From the measurements, the fraction of the time the tracer is in the gas phase can be calculated by:
FR=
vm vr
Vm is volume of mobile phase; and Vr=trv, where t~ is retention time and v is flow rate. This parameter may also be expressed as the ratio between the number of tracer molecules in the gas phase divided by the total number of tracer molecules. This is expressed by:
FR=
VmCm (VmCm-~-VsCs)
150 22.5 1.24 1.35 1.62
200 23.9 1.16 1.21 1.34
250 26.2 1.11 1.11 1.18
Temperature 120*C Pressure (bar) 100 Oil sat. (%) 12.0 14CH3.CH3 1.11 PMCP 1.19 PMCH 1.33
150 13.9 1.09 1.13 1.22
200 17.2 1.98 1.12 1.19
250 24.2 1.09 1.10 1.15
(2)
where Vs is the volume of the stationary phase. By combining Eqs. 1 and 2, the constant relation Cm/C, which relates the retention volume and the saturation, is found. By expressing the relative retention time as the retention volume divided by the mobile pore volume, the following equality is found:
(Rtl - Temperature IOOOC Pressure (bar) 100 Oil sat. (%) 20.8 14CH3. CH3 1.27 PMCP 1.49 PMCH 1.94
(1)
1 ) Vml
Vm2
- (R,2- 1)--
Vs2
(3)
where: R , = Vff Vm. Prefix 1 denotes the condition in the experiment and prefix 2 the conditions at a new saturation. This equation is applied to recalculate our experimental results to yield the retention at 20% decane saturation. The relative retention times as a function of pressure are plotted in Fig. 4 for PMCP, P M C H and ethane. The relative retention is influenced by the pres-
23
GAS TRACER RETENTION U N D E R SIMULATED RESERVOIR CONDITIONS
PMCP
ETHANE •
Z
2.1
2.1
2 1.9 1.8 1.7 1.6, 1.5'
1.9 OZ ~ 1.8 _~ 1.7 1.6 1.5 ~ 1.4 <~ 1.3 ,--1 1.2 ~ 1.1 1
~
k
> 1,4
5 100
150
200
2~
1
100
150
200'
250
PRESSURE (bar)
PRESSURE (bar) PMCH 2.1
2 k
E-
1.9
Z 1.8 ~ ~.~ ~¢ t~ >
1.7 1.6 1.5
•
50°C
+
80~C
V
10{YC
A
12{YC
1.4
~--~ 1.3
< 1.2 1.1 1
10O
150
200
250
PRESSURE (bar) Fig. 4. T r a c e r r e t e n t i o n r e l a t i v e to t r i t i a t e d m e t h a n e as a f u n c t i o n o f p r e s s u r e w i t h t h e t e m p e r a t u r e as p a r a m e t e r a n d n o r m a l i z e d to 2 0 % Sot.
sure. An increase in pressure gave for the three components PMCP, P M C H and ethane, a reduction in relative retention. At elevated pressures, the behaviour of these tracers is more similar to that of methane. The four isotherms for each tracer indicate an increase in relative retention by a decrease in temperature. Only small variations are recorded at the temperatures 50°C, 80°C and 100°C, but at 120°C the experiments show a significantly lower relative retention, especially at 100 bar. The ratio between the retention of ethane and methane shows approximately a linear behaviour with respect to pressure. The P M C P and P M C H curves are steeper and are expressed by an exponentially declining equation. If the pressure is further increased, miscibility is achieved and the whole system will be one phase flowing with the same velocity. P M C H is the tracer with the most dramatical reduction in retention when the pressure is in-
creased. At 100 bar and 50 °C the average flow rate of P M C H is more than 100% higher than that of methane but at 250 bar this difference is reduced to approximately 20%. PMCP shows always a smaller delay than PMCH. A small delay of less than 20% is acceptable in studies of directional flow and fracturing in reservoirs. An increase in delay increases the time spent in the field test before a tracer response is detected in the production well. For testing the communication in the reservoir it is preferable that the tracer follows the injected gas closely. When the pressure in the system increases, more methane is dissolved in the liquid phase. By applying the concept of convergence pressure, the mole fraction of methane in the liquid phase at 150°C and 100 bar is estimated to be 0.3; at 50°C and 250 bar the mole fraction is estimated to be 0.7. The chemical properties of the liquid phase are, therefore, con-
24
O. DUGSTAD
24 22
•
cwr
+
85Kr
20 lg 16 14 12
8 6 4
o 1
12 RELATIVE
14
1.6
1 f4
TIME (inj.vol/PV)
Fig. 5. Tracer retention as a function of injected pore volume with the following parameters: Temperature 100°C, pressure 250 bar, flow rate 0.22 ml/min, Sot 12%.
ET AL.
At the highest pressure investigated, 250 bar, none of the tracers were retained more than 20% with respect to methane. The dynamic experiments carded out at high pressures show that the PFC tracers follow methane to such a standard that they can become valuable tracers in reservoir characterisation studies. When parameters such as gas velocity and sweep efficiency are going to be estimated, however, retention of the compounds should be taken into consideration. The chromatographic effect of the gas tracers demonstrated in these experiments can be used for the determination of residual oil saturation (i.e., gas-contactable oil ) in a reservoir.
Acknowledgement siderably changed from experiment to experiment. When the a m o u n t of methane in the liquid phase increases, the dissolution of ethane and the PFCs are reduced. By increasing the temperature, more tracer molecules enter the gas phase and retention decreases. A comparison of methane and 85Kr results in a small retention of krypton with respect to methane (Fig. 5 ). This retention is, however, minimal. Among the four tracers compared with methane, 85Kr is the one which follows methane most closely.
5. Conclusions Perfluorocarbons have properties which make them suitable as tracers in reservoir evaluation programs. They fulfil the criteria of high detectibility and stability under reservoir conditions. No retention is measured for the tracers when they are flooded through pure Ottawa sand. With the addition of clay, a small retention is found. Limestone shows a retention of the PFCs, ethane is following methane closely. Experiments carried out on a porous medium containing a certain oil (decane) saturation show considerable variations in retention times, when the total pressure is changed.
We are indebted to the British Petroleum Company Norway for financial support of these experiments and for the permission to publish this paper.
References Armstrong, F.E., 1960. Field use of radioactive gas tracers. Pet. Eng., 32( 13): B34-B36. Brown, G.G., Katz, D.L., Oberfell, G.G. and Allen, R.C., 1948. Natural gasoline and the volatile hydrocarbons. Natl. Gasoline Assoc. Am., Tulsa, Okla. Calhoun, T.G. and Hurford, G.T., 1970. Case history of radioactive tracer and techniques in fairway field. J. Pet. Technol., 22( 10): 1217-1224. Calhoun, T.G. and Tittle, R.M., 1968. Use of radioactive isotopes in gas injection. SPE, 2277. Craig, F.F,, 1985. Field use of halogen compounds to trace injected CO> SPE, 14309. Davis, J.A., Blair, R.K. and Wagner, O.R., 1976. Monitoring and control program for a large scale miscible flood. SPE, 6097. Dietz, R.N., 1987. Perfluorocarbon tracer technology. BNL, 38847. Dugstad, O., Bjornstad, T. and Hundere, I.A., 1992. Construction of a slim-tube equipment for gas tracer evaluation at simulated reservoir conditions. Appl. Radiat. Isot., 43(24): 527-535. Fearon, R.E., 1957. Tritium--Newest tool for tracing reservoir flow. World Oil, 145:144-148. Frost, E.M., 1946. Helium tracer studies in the Elk Hills, California. Field, BM-R 1-3897. Gonduoin, M., Comier, J.J. and Thibierge, P., 1967. Etude
GAS TRACER RETENTION UNDER SIMULATED RESERVOIR CONDITIONS
du champ petrolifbre d'Hassi Messaoud par injection de methane, ethane, propane, et butane trities. Proc. Symp. Radioisotopic Tracers in Industry and Geophysics. Int. Atom. En. Agency, Vienna, pp. 161-175. Howell, W.D., Armstrong, F.E. and Wade Watkins, J., 1961. Radioactive gas tracer survey aids waterflood planning. World Oil, 152 (2): 41-43. Katz, D.L., Cornell, D., Kovayashi, R., Poettman, F.H., Vary, J.A., Elenbaas, J.R. and Weinaug, C.F., 1959. Handbook of Natural Gas Engineering. McGraw-Hill, New York, N.Y. Mayne, C.J. and Pendleton, R.W., 1986. Fordoche: An enhanced oil recovery project utilizing high pressure methane and nitrogen injection. SPE, 14058. Rupp, K.A., Nelson, W.C., Christian,L.D., Zimmerman,
25
K.A., Metz, B.E. and Styler, J.W., 1984. Design and implementation of a miscible water-alternating-gas flood at Prudhoe Bay. SPE, 13272. Sage, B.H., Lavender, H.M. and Lacey, W.E., 1940. Phase equilibria in hydrocarbon systems. Ind. Eng. Chem., pp. 743-750. Senum, G.I., Cote, E.A., D'Ottavio, T.W. and Dietz, R.N., 1989. Hydrocarbon precombusting catalyst survey and optimization for perfluorocarbon tracer analysis in subsurface tracer applications. BNL Inf. Rep. Tinker, G.E., 1973. Gas injection with radioactive tracer to determine reservoir continuity East Coaling Field, California. J. Pet. Technol., 25( 11 ): 1251-1254. Welge, H.J., 1955. Super sleuths tracer flow of injected gas. Oil Gas J., 54: 77-79.
17
Elsevier Science Publishers B.V., A m s t e r d a m
Measurements of gas tracer retention under simulated reservoir conditions 13. Dugstad, T. Bjornstad and I.A. Hundere lnstitutt for Energiteknikk, Postbox 40, N-2007 Kjeller, Norway (Received September 17, 1992; revised version accepted May 26, 1993)
ABSTRACT Measurements are carried out to investigate the behaviour of gas tracers at reservoir conditions. The interactions between the tracer and the rock matrix, and between the tracer and the residual oil are studied. Two chemical compounds, perfluoromethylcyclopentane and perfluorometyl-cyclohexane, are studied to evaluate their potential as gas tracers in oilfield studies. The chemical tracers are compared with the behaviour of radioactive methane and ethane tracers.
I. Introduction
Tracing gas and water which are injected into reservoirs is a widely used m e t h o d to increase the knowledge of fluid movements. The tracers applied are mainly radioactively tagged molecules. These molecules can, using special techniques, be measured down to very small concentrations. This is important, because of the very high dilution of the injected fluid in the reservoir. The radioactive nuclides used are isotopes of elements in naturally occurring reservoir gases, or are tagged on chemicals which are not c o m m o n in the reservoir. The tracers most frequently reported in the literature are tritiated hydrogen or methane and 85Kr. 14Clabelled light hydrocarbon gases are also applicable as tracers, but are less commonly used, mainly because of higher costs. Armstrong (1960) reports on the application of gas tracers to define fracturing directions in reservoirs. Directional permeability can be identified early in reservoir processes so that steps can be taken to maximise recovery efficiency. In order to be able to trace the injected gas
from various injection wells, each well has to be labelled with different tracers. At present, the number of available and applicable radioactive tracers are restricted, and it is, therefore, of interest to find additional tracer candidates which can be applied in reservoir measurements. Before implementation of new tracers in field applications, the behaviour of the compounds should be extensively studied in laboratory experiments, in order to ensure the most efficient injection strategy. An ideal tracer should follow the traced phase closely. With injection of methane gas, radioactively labelled methane will be an ideal tracer. Methane can be radioactively labelled either by substituting one of the hydrogen atoms by tritium, or, likewise, to exchange ~2C by 14C. Though these tracers may be considered as ideal relative to methane, there will be a partitioning between the oil phase and the gas phase, as is the case with stable methane itself. The partitioning coefficient of methane is known in reservoir engineering literature. All gas molecules have partitioning between phases. The tracer molecules will travel with the velocity of the liquid phase when dissolved
0 9 2 0 - 4 1 0 5 / 9 3 / $ 0 6 . 0 0 © 1993 Elsevier Science Publishers B.V. All rights reserved.
I8
in the liquid and with the velocity of gas when in the gas phase. A tracer molecule with a high partitioning in other phases is, therefore, considerably delayed in the reservoir with respect to the average gas flow. Tracers may also be delayed through interaction with the rock surface. These delay mechanisms can in some cases cause difficulties in the interpretation of the results. In the search for new applicable tracers, it is, therefore, important to quantify these delays to improve the interpretation of the data collected from field tests. 2. Gas tracers in reservoir studies
Radioactive tracers
Several authors report on the use of radioactive tracers in oilfield applications. The most c o m m o n tracers are tritiated hydrogen gas (Fearon, 1957; Calhoun and Title, 1968; Calhoun and Hurford, 1970; Tinker, 1973; Rupp et al., 1984; Mayne and Pendleton, 1986), tritiated methane ( G o n d u o i n et al., 1967; Calhoun and Tittle, 1968; Calhoun and Hurford, 1970; Davis et al., 1976; Rupp et al., 1984; Welge, 1985; Mayne and Pendleton, 1986 ) and 85Kr (Welge, 1955; Armstrong, 1960; Howell et al., 1961; Calhoun and Title, 1968; Calhoun and Hurford, 1970; Tinker, 1973; Davis et al., 1976; Rupp et al., 1984; Mayne and Pendleton, 1986). Some papers have described the application of tritiated ethane, propane and even butane as tracers ( G o n d u o i n et al., 1967; Mayne and Pendleton, 1986). All these tracer molecules, except 85Kr, are tritium-labelled compounds. Tritium is a weak beta emitter. Small concentrations of tritium-labelled tracers are detected directly in the gas phase by a proportional counter technique, or the gas is oxidised to produce water which is counted by liquid scintillation technique. Chemical tracers
Only a few papers on the application of chemical gas tracers in reservoir studies are
O. DUGSTAD ET AL.
published. Craig (1985) has reported on the use of halocarbons and sulphur hexafluoride as tracers for carbon dioxide. These tracers were applied successfully in a miscible flood. As early as 1946, Frost ( 1946 ) reported on the use of helium as a tracer for gas injection. When searching for new "near-ideal" tracers, the following main principles are taken into consideration. A tracer must satisfy the following criteria: - - the tracer must have a very low detection limit; - - it must have stability under reservoir conditions; - - it should follow the tagged phase and must have a minimal partitioning into other phases; k it must have no absorption to rock material; and - - the use of the tracer must have m i n i m u m environmental consequences. Dietz (1987) reported the application of per-fluorinated hydrocarbons as tracers for air in environmental studies. These tracers have a very low detection limit which make them compatible with radioactive tracers with respect to detectability. The fluorine compounds are reported to be analysed in concentrations down to 10-15_ 10-161/1 in air samples. Such a high sensitivity makes these molecules interesting as oil field tracers. Analyses of perfluorocarbons (PFC) are performed using a gas-chromatograph (GC) connected to an electron capture (EC) detector. The EC detector is extremely sensitive to perfluorinated hydrocarbons, especially the cyclic compounds. Senum et al. ( 1989 ) have reported a method to analyse the PFC content in a hydrocarbon gas from a production stream. The gas contained in pressure bombs is flushed through active carbon to absorb the PFCs. The PFC containing pellets are thermally desorbed and the gas is directed through a system of a precolumn, catalysts and traps to remove the hydrocarbons before the PFCs enter the main
GAS TRACER RETENTION UNDER SIMULATEDRESERVOIRCONDITIONS
separation column to determine the amount of each tracer. Such tracers are applied in pilot projects on the Norwegian continental shelf.
3. Experimental studies In the experiments reported here two perfluorocompounds, perfluoromethylcyclopentane ( P M C P ) and perfluoromethylcyclohexane ( P M C H ) , and three radioactive tracers, tritiated methane, 14C-labelled ethane and 85Kr, were studied. The purpose of the experimental work is to study the behaviour of the tracers under elevated temperature and pressure, i.e., at simulated reservoir conditions. Some physical constants of the tracers are listed in Table 1, where it is evident that the PFCs are liquids under standard conditions. Methane, ethane and krypton are real gases with a boiling point far below 0 ° C. The tracers were studied in dynamic flooding experiments. Retention was measured while the gas containing the tracer was f o o d e d through a porous medium. The experimental equipment and procedures applied in this study are described by Dugstad et al. (1992). The tracer mixture confined in a small-volume tubing section just ahead of the entrance of the slim tube is flushed by the injection gas through the column. The effluent is analysed by automatically sampling the gas into scintillation vials for counting the radioactivity and by automatic injection into a gas chromatograph for the detection of the chemical tracer. Several series of experiments were carried TABLE 1 Physical constants for the studied gas tracers PMCP PMCH Methane Ethane
Formula C6FI2 CvFl4 CH3T MW BP(°C)
300 48.0
350 76.3
18 -164
Krypton
14CH3-CH3 S~Kr 32 85 -88.6 -152
19
out to study the retention caused by the interaction between the tracer and the rock matrix. The packing material applied in the tubes was dry sand (Ottawa sand--nearly pure quartz ) or limestone. Different amounts of kaolinite clay were added to the pure Ottawa sand to measure the effect of the clay. The addition of kaolinite was carried out, making a water slurry of the clay and the Ottawa sand. The mixture was dried in a climate chamber, providing a thin layer of clay distributed onto the quartz grain surface. Two different clay concentrations were prepared, 0.5% and 2.5% by weight, respectively. The applied limestone was composed of crushed material from the Ekofisk formation sampled onshore in Denmark. The prepared tubes were 6 m long, with an inner diameter of 0.5 cm. One series of experiments was performed to investigate the interaction between the tracer molecules and the oil in the porous medium. In a porous m e d i u m partly filled with different fluids, the main retention mechanism is the tracer partitioning between the different phases. To study this effect, a 12 m long slim tube with a inner diameter of 0.5 cm was used. The tube filling material was pure Ottawa sand. All gases will, to some extent, dissolve into liquid phases until equilibrium is achieved. The transport velocity of the tracer in the reservoir will, therefore, be composed of the sum of the time fraction spent in the liquid phase, multiplied by the liquid velocity and the time spent in the gas phase multiplied by the gas velocity. Tracers which partition between the gas and liquid phases will, therefore, be retained when the liquid flows with a lower velocity than the gas. In the retention studies here reported, the column was prepared to residual oil saturation with model oil (decane) and the oil phase is, therefore, considered as stationary. No water was present. The injection gas was methane. The experiments were carried out at four different temperatures and four different pressures. Because a model oil is used, the conditions are
20
not identical to those in a reservoir. If the oil in the tube is characterised by standard oil characterisation, it has a mole fraction of methane in the range of 0.3-0.7 and a hept a n e + fraction in the range of 0.7-0.3, depending on temperature and pressure. The heptane + fraction (pure decane) has a mole weight of 142. According to the Handbook of Natural Gas Engineering (Katz et al., 1959), the partitioning coefficients for methane in this system are listed in NGAA (Brown et al., 1948) data charts drawn for a convergence pressure of 5000 psia. To enable the calculation of effective pore volumes, an estimation ofoil and gas densities are carried out. In the present paper, data from Sage et al. (1940) are used to calculate the density of decane. The NGAA compressibility factor charts for pure methane are used to calculate the gas density. 4. Results and discussion
The results from the experiments carried out to study retention caused by the interaction oftracers with the surface of reservoir material are listed in Table 2. Because the injected gas is methane, tritium-labelled methane molecules behave as an ideal tracer. The results presented in the table are given as the ratio between the retention (measured at the mass middle point of the production profile ) of the methane and the actual tracer. The experiments are carried out at 100 bar. When pure Ottawa sand is applied no differences in retention time are measured between methane, ethane, PMCP and PMCH. When kaolinite is added to the porous matrix a delay is measured. The delay increases with increasing clay content. Limestone causes the most serious delay, in the order of 10% at 100°C for the PFCs. Ethane, however, seems to follow the methane closely. Other types of clay material, different from kaolinite, which frequently appear in reservoir rocks, also contribute to the retention. The ef-
O. DUGSTADET AL. TABLE 2 Tracer retention relative to tritiated methane in fraction of pore volumes Substrate
PMCP
PMCH
t4CIq3.CI-I3
1.00 1.03 1.06 1.13
1.00 1.00 1.00 1.01
1.00 1.03 1.06 1.14
1.00 1.00 1.00 1.00
1.00 1.02 1.06 1.13
1+00 1.00 1.00 1.02
Pressure 100 bar, temperature 800C Ottawa Ottawa + 0.5% kaol Ottawa + 2.5% kaol CaCO3
1.00 1.02 1.03 1.08
Pressure 100 bar, temperature 100*C Ottawa Ottawa +0.5% kaol Ottawa + 2.5% kaol CaCO3
1.00 1.02 1.04 1.09
Pressure 100 bar, temperature 1200C Ottawa Ottawa +0.5% kaol Ottawa + 2.5% kaol CaCO 3
1.00 1.02 1.04 1.09
fect of clay on retention is, however, reduced, because the grains are covered with water or oil. The gas tracers under study are expected to have a negligible partitioning into water. It is, therefore, expected that water will not contribute significantly to retention. Figures 1, 2 and 3 illustrate the results from three independent tracer experiments. The tracer response curves are plotted as a function of injected pore volumes. In Fig. 1 the experiment is carried out on a dry slim tube. The temperature was 50°C and the pressure was 250 bar. A small retention is measured for the PFCs. At identical pressure and temperature (50°C, 250 bar), but with a decane saturation of approximately 30%, a time lag is measured for all tracers, even for the methane. The PMCH, the perfluorocarbon with the highest molecular weight and the highest boiling point, is the most heavily retained tracer. At these conditions, more than 1.7 pore volumes are needed to flush the P M C H through the porous medium. When decreasing the pressure to 150 bar, 2.2 pore volumes are needed before the P M C H moved through the slim tube. Comparing the PMCP and ethane, it is noteworthy
21
GAS TRACER RETENTION UNDER SIMULATED RESERVOIR CONDITIONS 26
Z O
24
FE-, Z
22
•
CH3T
20
+
14CH3- CH3
o
PMCP
a
PMCH
18 16
L) Z 0
14 12
~q N
10 8
<
6 4
0 Z
2 0 0.8
12
I
1A
1.6
18
2
212
214
2.6
RELATIVE TIME (inj.vol/PV)
Fig. 1. Tracer retention as a function of injected pore volume with the following parameters: Temperature 50°C, pressure 250 bar, flow rate 0.45 ml/min, So, 0%.
15
Z 14 O 13
CH~T
•
e¢ tl EZ to m L) Z ©
+
14CH3- CH3
o
PMCP
s M © Z
RELATIVE TIME (inj.vol/PV)
Fig. 2. Tracer retention as a function of injected pore volume with the following parameters: Temperature 50 ° C, pressure 250 bar, flow rate 0.24 ml/min, So, 31%.
Z
14
2 <
~3 12 I1
Z C)
lo 9
~
~ JN <
a5678
. . L
°! 2
© Z
•
Ct43T
+
l'l CH3- CH3
o
PMCP
a
PMCH
2 1
i -_ ~ a .
0
0.8
,
1
-~
- - ~ - - - - F
1,2
.m
. . . .• 1.4
. . . .
,
,
1.6
,
i
1.8
r
2
"'r
. . . .
r . . . .
2.2
r
. . . .
r
24
r'"
2.6
RELATIVE TIME (inj.vol/PV)
Fig. 3. Tracer retention as a function of injected pore volume with the following parameters: Temperature 50 ° C, pressure 150 bar, flow rate 0.24 ml/min, Sor 31%.
22
O. DUGSTAD ET AL.
that at 250 bar the P M C P is in front of the ethane, but at 150 bar the situation is reversed and the ethane flows with a higher average velocity. Table 3 gives the relative retention of ethane, PMCP and P M C H with respect to methane at four pressures ( 100 bar, 150 bar, 200 bar and 250 bar) and at four temperatures (50°C, 80°C, 100°C and 120°C). The calculation of the relative time (the xaxis in the figures) will be influenced by the accuracy of several process parameters. A small shift may, therefore, occur and the most valuable method to compare the results will, therefore, be to give the relative retention with respect to the ideal methane tracer. This is shown in Table 3. For each experiment the liquid saturation is given. The saturation must be measured individually in each experiment and is a TABLE3 Retention of PMCP, PMCH and 14C labelled ethane relative to tritiated methane as a function of temperature, pressure and oil saturation Temperature 50°C Pressure (bar) Oil sat. (%) 14CH3- CH3 PMCP PMCH
100 32.7 1.47 1.95 2.84
150 30.7 1.40 1.47 1.91
200 29.9 1.27 1.31 1.51
250 31.3 1.21 1.18 1.29
Temperature 80°C Pressure (bar) Oil sat. (%) 14CH3-CH3 PMCP PMCH
100 19.6 1.27 1.49 1.95
150 22.6 1.24 1.33 1.59
200 25.6 1.19 1.23 1.37
250 30.1 1.17 1.18 1.28
consequence of the applied temperature and pressure. The relative retention cannot, therefore, be compared directly. An increase in saturation gives an increase in the relative retention value. The retention measured can, however, be recalculated to give the retention at 20% saturation. This is done by calculating the ratio between the concentration of the tracer in the gas phase (Cm) and in the liquid phase (Cs). This ratio is a constant independent of saturation and can, therefore, be used to derive the retention at a new saturation. From the measurements, the fraction of the time the tracer is in the gas phase can be calculated by:
FR=
vm vr
Vm is volume of mobile phase; and Vr=trv, where t~ is retention time and v is flow rate. This parameter may also be expressed as the ratio between the number of tracer molecules in the gas phase divided by the total number of tracer molecules. This is expressed by:
FR=
VmCm (VmCm-~-VsCs)
150 22.5 1.24 1.35 1.62
200 23.9 1.16 1.21 1.34
250 26.2 1.11 1.11 1.18
Temperature 120*C Pressure (bar) 100 Oil sat. (%) 12.0 14CH3.CH3 1.11 PMCP 1.19 PMCH 1.33
150 13.9 1.09 1.13 1.22
200 17.2 1.98 1.12 1.19
250 24.2 1.09 1.10 1.15
(2)
where Vs is the volume of the stationary phase. By combining Eqs. 1 and 2, the constant relation Cm/C, which relates the retention volume and the saturation, is found. By expressing the relative retention time as the retention volume divided by the mobile pore volume, the following equality is found:
(Rtl - Temperature IOOOC Pressure (bar) 100 Oil sat. (%) 20.8 14CH3. CH3 1.27 PMCP 1.49 PMCH 1.94
(1)
1 ) Vml
Vm2
- (R,2- 1)--
Vs2
(3)
where: R , = Vff Vm. Prefix 1 denotes the condition in the experiment and prefix 2 the conditions at a new saturation. This equation is applied to recalculate our experimental results to yield the retention at 20% decane saturation. The relative retention times as a function of pressure are plotted in Fig. 4 for PMCP, P M C H and ethane. The relative retention is influenced by the pres-
23
GAS TRACER RETENTION U N D E R SIMULATED RESERVOIR CONDITIONS
PMCP
ETHANE •
Z
2.1
2.1
2 1.9 1.8 1.7 1.6, 1.5'
1.9 OZ ~ 1.8 _~ 1.7 1.6 1.5 ~ 1.4 <~ 1.3 ,--1 1.2 ~ 1.1 1
~
k
> 1,4
5 100
150
200
2~
1
100
150
200'
250
PRESSURE (bar)
PRESSURE (bar) PMCH 2.1
2 k
E-
1.9
Z 1.8 ~ ~.~ ~¢ t~ >
1.7 1.6 1.5
•
50°C
+
80~C
V
10{YC
A
12{YC
1.4
~--~ 1.3
< 1.2 1.1 1
10O
150
200
250
PRESSURE (bar) Fig. 4. T r a c e r r e t e n t i o n r e l a t i v e to t r i t i a t e d m e t h a n e as a f u n c t i o n o f p r e s s u r e w i t h t h e t e m p e r a t u r e as p a r a m e t e r a n d n o r m a l i z e d to 2 0 % Sot.
sure. An increase in pressure gave for the three components PMCP, P M C H and ethane, a reduction in relative retention. At elevated pressures, the behaviour of these tracers is more similar to that of methane. The four isotherms for each tracer indicate an increase in relative retention by a decrease in temperature. Only small variations are recorded at the temperatures 50°C, 80°C and 100°C, but at 120°C the experiments show a significantly lower relative retention, especially at 100 bar. The ratio between the retention of ethane and methane shows approximately a linear behaviour with respect to pressure. The P M C P and P M C H curves are steeper and are expressed by an exponentially declining equation. If the pressure is further increased, miscibility is achieved and the whole system will be one phase flowing with the same velocity. P M C H is the tracer with the most dramatical reduction in retention when the pressure is in-
creased. At 100 bar and 50 °C the average flow rate of P M C H is more than 100% higher than that of methane but at 250 bar this difference is reduced to approximately 20%. PMCP shows always a smaller delay than PMCH. A small delay of less than 20% is acceptable in studies of directional flow and fracturing in reservoirs. An increase in delay increases the time spent in the field test before a tracer response is detected in the production well. For testing the communication in the reservoir it is preferable that the tracer follows the injected gas closely. When the pressure in the system increases, more methane is dissolved in the liquid phase. By applying the concept of convergence pressure, the mole fraction of methane in the liquid phase at 150°C and 100 bar is estimated to be 0.3; at 50°C and 250 bar the mole fraction is estimated to be 0.7. The chemical properties of the liquid phase are, therefore, con-
24
O. DUGSTAD
24 22
•
cwr
+
85Kr
20 lg 16 14 12
8 6 4
o 1
12 RELATIVE
14
1.6
1 f4
TIME (inj.vol/PV)
Fig. 5. Tracer retention as a function of injected pore volume with the following parameters: Temperature 100°C, pressure 250 bar, flow rate 0.22 ml/min, Sot 12%.
ET AL.
At the highest pressure investigated, 250 bar, none of the tracers were retained more than 20% with respect to methane. The dynamic experiments carded out at high pressures show that the PFC tracers follow methane to such a standard that they can become valuable tracers in reservoir characterisation studies. When parameters such as gas velocity and sweep efficiency are going to be estimated, however, retention of the compounds should be taken into consideration. The chromatographic effect of the gas tracers demonstrated in these experiments can be used for the determination of residual oil saturation (i.e., gas-contactable oil ) in a reservoir.
Acknowledgement siderably changed from experiment to experiment. When the a m o u n t of methane in the liquid phase increases, the dissolution of ethane and the PFCs are reduced. By increasing the temperature, more tracer molecules enter the gas phase and retention decreases. A comparison of methane and 85Kr results in a small retention of krypton with respect to methane (Fig. 5 ). This retention is, however, minimal. Among the four tracers compared with methane, 85Kr is the one which follows methane most closely.
5. Conclusions Perfluorocarbons have properties which make them suitable as tracers in reservoir evaluation programs. They fulfil the criteria of high detectibility and stability under reservoir conditions. No retention is measured for the tracers when they are flooded through pure Ottawa sand. With the addition of clay, a small retention is found. Limestone shows a retention of the PFCs, ethane is following methane closely. Experiments carried out on a porous medium containing a certain oil (decane) saturation show considerable variations in retention times, when the total pressure is changed.
We are indebted to the British Petroleum Company Norway for financial support of these experiments and for the permission to publish this paper.
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GAS TRACER RETENTION UNDER SIMULATED RESERVOIR CONDITIONS
du champ petrolifbre d'Hassi Messaoud par injection de methane, ethane, propane, et butane trities. Proc. Symp. Radioisotopic Tracers in Industry and Geophysics. Int. Atom. En. Agency, Vienna, pp. 161-175. Howell, W.D., Armstrong, F.E. and Wade Watkins, J., 1961. Radioactive gas tracer survey aids waterflood planning. World Oil, 152 (2): 41-43. Katz, D.L., Cornell, D., Kovayashi, R., Poettman, F.H., Vary, J.A., Elenbaas, J.R. and Weinaug, C.F., 1959. Handbook of Natural Gas Engineering. McGraw-Hill, New York, N.Y. Mayne, C.J. and Pendleton, R.W., 1986. Fordoche: An enhanced oil recovery project utilizing high pressure methane and nitrogen injection. SPE, 14058. Rupp, K.A., Nelson, W.C., Christian,L.D., Zimmerman,
25
K.A., Metz, B.E. and Styler, J.W., 1984. Design and implementation of a miscible water-alternating-gas flood at Prudhoe Bay. SPE, 13272. Sage, B.H., Lavender, H.M. and Lacey, W.E., 1940. Phase equilibria in hydrocarbon systems. Ind. Eng. Chem., pp. 743-750. Senum, G.I., Cote, E.A., D'Ottavio, T.W. and Dietz, R.N., 1989. Hydrocarbon precombusting catalyst survey and optimization for perfluorocarbon tracer analysis in subsurface tracer applications. BNL Inf. Rep. Tinker, G.E., 1973. Gas injection with radioactive tracer to determine reservoir continuity East Coaling Field, California. J. Pet. Technol., 25( 11 ): 1251-1254. Welge, H.J., 1955. Super sleuths tracer flow of injected gas. Oil Gas J., 54: 77-79.