- Email: [email protected]
Steam flow measurement using alcohol tracers
Geothermics 30 (2001) 641–654 www.elsevier.com/locate/geothermics
Steam flow measurement using alcohol tracers Brian G. Lovelock* Sinclair Knight Merz, PO Box 9806, Newmarket, Auckland, New Zealand Received 6 January 2001; accepted 8 June 2001
Abstract Tracer flow testing procedures are increasingly being used for routine measurement of well output in operating geothermal fields. A new procedure developed in New Zealand uses low boiling-point, liquid alcohol (isopropanol or butan-2-ol) as the steam-phase tracer. The use of alcohol tracers allows major simplifications to field procedures, including injection with conventional liquid-dosing pumps, sampling into open sample bottles and preparation of composite steam/water tracers. These practical benefits far outweigh the only drawback of alcohol tracers, which is the need to correct for alcohol gas dissolved in the liquid phase. This paper reviews development work with alcohol tracers over the past five years and discusses the properties of alcohols and the practical aspects of their use in tracer-dilution testing. # 2001 Published by Elsevier Science Ltd on behalf of CNR. Keywords: Tracers; Alcohol; Isopropanol; Mass flow; Chemistry
1. Introduction Measurement of mass flow and enthalpy in two-phase pipelines is an important monitoring task at operating geothermal fields. Traditionally this has been accomplished by using dedicated wellhead separators or by periodically taking wells out of service and bypassing the discharge to a total-flow atmospheric silencer. With modern steamfield design, wellhead separators are less common. In many cases, twophase flow from production wells is combined and piped to centralised production separators. At the same time, there have been increasing environmental constraints on atmospheric well testing. These factors have provided the impetus for the development of non-invasive, on-line methods for measuring steam and water flow in * Fax: +64-9-913-8901. E-mail address: [email protected] 0375-6505/01/$20.00 # 2001 Published by Elsevier Science Ltd on behalf of CNR. PII: S0375-6505(01)00020-7
642
B.G. Lovelock / Geothermics 30 (2001) 641–654
two-phase pipelines. To date, the only on-line method in routine use is tracer flow testing—sometimes called the tracer-dilution method. Because of the potential cost savings and environmental benefits, on-line testing is likely to become the standard operating procedure in the future. Tracer flow testing procedures for well output testing were developed in the early 1990s by workers in the United States (e.g. Hirtz et al., 1993; Hirtz and Lovekin, 1995). This technology is now available as a commercial service and is in regular use at operating fields in the United States, the Philippines, Indonesia, Iceland and Japan. This paper reviews development work on a new approach to tracer flow testing using low boiling-point alcohols.
2. Tracer flow testing methodology To measure total mass flow and enthalpy in two-phase pipelines, it is necessary to measure the flow of the individual phases, steam and liquid-water (brine), separately. Tracer flow testing involves the quantitative injection and dilution of small quantities of chemical tracers (one for steam and one for liquid) in a pipeline. The tracers mix completely inside the pipeline, and at downstream sampling points separated water and steam are collected at the pipeline pressure. Using a steamphase tracer that travels entirely in the steam (e.g. SF6 or propane), the steam flow is calculated as: Steam flow ðt=hÞ ¼
steam tracer injection rate ðg=hÞ Steam tracer concentration in steam ðg=tÞ
ð1Þ
The calculation is analogous for water flow, where the liquid-phase tracer (e.g. benzoate or bromide) is totally dissolved in the brine. Total discharge enthalpy (Ht) can then be calculated from the measured steam and water flows (SF, WF): Ht ¼
SF hg þ WF hf SF þ WF
ð2Þ
where hg and hf are the enthalpy of steam and water at the pipeline temperature (obtained from steam tables). The above expressions do not include noncondensible gases (CO2, H2S, etc.), which may constitute several percent of the total mass flow from a well. These must be determined separately. Tracer flow testing is currently the only viable on-line method for measurement of two-phase flow in pipelines. The failure of other methods (e.g. Doppler-based systems) is due to the fact that the two phases are present in varying proportions, and may not be uniformly distributed across the pipeline. This is not an issue with tracer flow testing, where the two phases are measured independently. On-line, non-invasive testing has a number of major advantages over traditional methods (atmospheric testing or total flow separators):
B.G. Lovelock / Geothermics 30 (2001) 641–654
643
power station operations are not interrupted; generation load losses associated with taking wells out of service are avoided; stable flow conditions are more likely (since production is not disturbed); there is no need for permanent wellhead test equipment, aside from sampling ports; . atmospheric discharge of wastewater is avoided; . there is greater flexibility in scheduling testing. . . . .
Tracer flow testing can provide the greatest benefits when implemented at the steamfield design stage, rather than by retro-fitting. With new geothermal developments, it is common to do away with dedicated wellhead test equipment in favour of tracer flow testing methods. In addition to well testing, tracer flow testing procedures can be used for power plant steam audits (e.g. as a check on annubar1 measurements) and for measurement of combined flow from several wells.
3. Practical development of tracer flow testing methods Tracer flow testing procedures developed in the United States currently use SF6 gas as the steam-phase tracer and sodium benzoate as the liquid-phase tracer (Macambac et al., 1998). The benzoate solution is injected into the pipeline using a liquid-metering pump. The SF6 is injected from a compressed-gas cylinder using a specialised gas-metering system. Sealed gas flasks are used to collect steam samples for analysis of SF6. In the mid-l990s, Lovelock (1997) began investigating various low boiling-point, organic liquids as steam-phase tracers, including chloroform, hexane and alcohols. With this approach, the organic liquid is injected into the pipeline where it boils and travels down the pipeline as a gas. The tracer is then collected as a liquid in condensed steam samples. Low boiling-point, organic tracers are an attractive alternative to compressed-gas tracers because they can be injected using conventional liquid-dosing pumps and offer the potential for simpler sampling procedures. Much of Lovelock’s early work focused on the practical aspects of sampling and analysing organic liquids. The main goal was to identify tracers that could be sampled quantitatively without gas flasks and which could be analysed with low-cost methods (e.g. ultraviolet-visible photometric or titrimetric). While some success was had with chloroform, alcohols were found to offer the greatest practical advantages. 3.1. Alcohol tracers Early work with alcohol tracers focused on their potential for reservoir tracer tests. Adams (1995) showed that some alcohols have the thermal stability required
1
A tradename of a differential pressure flow measurement sensor.
644
B.G. Lovelock / Geothermics 30 (2001) 641–654
of reservoir tracers but recognised the limitations of analytical methods, which do not have the low detection limits of tracers such as iodine-131 and SF6. Alcohols were first used as quantitative mass flow tracers in New Zealand in the mid-1990s (Lovelock, 1997). Isopropanol was used as the steam-phase tracer and bromide as the liquid-phase tracer. This work helped to characterise the vapourliquid distribution character of alcohols and showed that with simple corrections for liquid-phase alcohol concentrations, accurate steam flow measurements were possible. The work also showed that alcohol in condensate could be sampled quantitatively into open sample bottles, even when gas contents were high. Isopropanol was analysed by a standard head-space, gas chromatographic (GC) method developed for blood-alcohol analysis. Mass flow tracer measurements were found to be in good agreement with measurements obtained by the empirical lip-pressure method of James (1970). Simple injection and sampling procedures were developed. More rigorous field testing of alcohols was carried out in 1999 at the Rotokawa field in New Zealand (Lovelock and Stowell, 2000). This is a high-temperature field with two production wells supplying a 25 MWe combined-cycle plant. The two-phase pipelines operate at 20–25 bar gauge (215–225 C). Mean steam flow measurements from seven separate alcohol-injection runs were within 1–4% of flows measured independently by annubar. Representative results are given in Table 1. These trials showed good reproducibility for multiple samples (< 2% variability for most tests). Both isopropanol and butan-2-ol were shown to be viable tracers. Subsequently, alcohol tracers have been used for routine well testing at the Mokai geothermal field in New Zealand, which was commissioned in 1999. This is a high-temperature field with five production wells supplying a 50 MWe power plant. For the five tracer flow testing surveys conducted at Mokai since commissioning, steam flow rates, when summed, were within 5% of total steam flow rate measured at the power plant (using an orifice plate). 3.2. Field equipment and procedures In New Zealand, tracer flow testing with alcohols is carried out using a small portable injection rig which includes a high pressure, high stroke-rate, positive-displacement dosing pump with variable injection rate up to 420 ml/min (Fig. 1). A composite steam-water tracer is used, comprising 50 wt.% isopropanol and 10 wt.% benzoate (as sodium benzoate). The injection rate is metered by weight loss over the period of the test, typically 5–15 min. Samples are collected from downstream sampling points using miniature sampling separators operating at line pressure (Fig. 2). Separated steam and water are passed through condensing coils and collected into screw-cap containers. Dissolved hydrogen sulphide is precipitated with zinc acetate and an antibacterial agent is added.
4. Discussion In assessing the attributes of alcohols for tracer flow testing, it is useful to compare their properties with those of other organic liquids. Table 2 lists several common
Table 1 Selected tracer flow measurements at Rotokawa field, New Zealand, using alcohol, and composite isopropanol (IPA) and benzoate tracers. Sampling pressure: 20–25 bar gauge (from Lovelock and Stowell, 2000) Tracer
Time
16–17 June 1999 RK5
Butan-2-ol
RK5
Isopropanol
26 August 1999 RK5
RK9
a
Composite: 10 wt.% Benzoate 40 wt.% Isopropanol
Composite: 10 wt.% Benzoate 40 wt.% Isopropanol
Tracer injection rate (g/min)
IPA in liquid (mg/kg)
Steam flow (t/h)
13:56 98.5 44.0 13:58 44.0 14:00 44.2 12:02 130 57.5 12:06 57.5 12:10 57.2 Mean steam flow by tracera Mean steam flow by orifice plate
2.7 2.5 2.7 4.5 4.6 4.5
122 123 121 120 120 121 121 119
13:06
7.6 8.0 8.3 8.7 7.8 8.1 7.6 8.3 8.0
74.1 73.7 72.3 68.8 68.9 69.5 69.9 68.8 68.6 141 140–143
296
IPA in steam (mg/kg)
78.2 78.1 # 78.9 81.8 13:23 83.9 10:10 297 82.6 83.3 # 82.9 10:26 83.6 Mean flow by tracers (RK5+RK9) Total flow by orifice plate (RK4+RK9)
Benzoate in brine (mg/kg)
Brine flow (t/h)
10.3 10.5 10.5 10.5 10.4 10.5 10.5 10.5 10.3
172 169 169 169 171 169 169 169 173 340 331–336
B.G. Lovelock / Geothermics 30 (2001) 641–654
Well
Based on water flow of 200 t/h measured by orifice plate.
645
646
B.G. Lovelock / Geothermics 30 (2001) 641–654
Fig. 1. Schematic layout of portable tracer injection rig used in New Zealand.
Fig. 2. Schematic layout of tracer sampling equipment, showing sampling separators for steam and brine condensing/cooling coils.
647
B.G. Lovelock / Geothermics 30 (2001) 641–654
Table 2 Properties of some common organic liquids, arranged in order of increasing solubility in water (Liley and Gambill, 1973) Compound
Formula
Formula weight (g/mol)
Density (g/ml)
Boiling point ( C)
Solubility in liquid (mg/l)
Hexane Benzene Chloroform Butan-2-ol n-Propanol iso-Propanol Ethanol
CH3(CH2)4CH3 C6H6 CHCl3 CH3CH(OH)CH2CH3 CH3(CH2)2OH (CH3)2CHOH CH3CH2OH
86.2 78.1 119.4 74.1 60.1 60.1 46.1
0.66 0.88 1.49 0.81 0.80 0.79 0.79
69.0 80.1 61.2 99.5 97.4 82.5 78.4
140 700 8200 125,000 1 1 1
organic compounds that have properties potentially suitable for use as steam-phase tracers. Selecting the best tracer is always an exercise in compromise, balancing benefits and drawbacks for the different candidate tracers. Criteria for selection fall into two broad categories: (1) physical properties and (2) practicality of use. 4.1. Physical properties of steam-phase tracers There are a number of physical properties that are required of steam-phase tracers. These are: . the tracer exists as a gas over the full range of pipeline conditions; . the tracer is chemically and thermally stable over the full range of pipeline temperatures and in the presence of other geothermal constituents; . the tracer has negligible background concentration in the geothermal fluid; . the tracer in gaseous form has a low solubility in water.
The first three properties are shared by all of the compounds in Table 2. In the case of thermal stability, it is only necessary that the tracer be stable at pipeline temperatures for the duration of the injection period, which in most cases will be less than one minute. Indeed, it is preferable that any tracer that is injected will eventually degrade, thus ensuring a long-term, low background concentration in the reservoir. In the case of chemical stability, it is necessary that the tracer remain unreacted until analyses are completed. In two-phase pipelines it is preferable that the steam-phase tracer travels entirely in the steam phase, with a negligible amount dissolved in the brine. This is not the case with alcohol gas, which has a significant solubility (due to its polar character). Isopropanol has a steam/water concentration ratio (Cv/CL) of 10–25 in the temperature range 150–220 C (Fig. 3), with an inverse temperature dependence. The data in Fig. 4 show that the steam-water distribution ratio falls within a well-defined range, which in turn suggests that equilibrium is achieved quickly. This is supported by the fact that some of the data in Fig. 3 (from Japan) were obtained for injectionto-sampling distances of only 10 m. Because some of the alcohol tracer is dissolved
648 B.G. Lovelock / Geothermics 30 (2001) 641–654
Fig. 3. Distribution of isopropanol between steam and liquid-phase in two-phase pipelines. CV/CL is the steam-water concentration ratio.
B.G. Lovelock / Geothermics 30 (2001) 641–654 Fig. 4. Changes in dissolved isopropanol (IPA) concentration as a result of heating from 20 to 80 C and with air-purging (for 1 min at each temperature step). 649
650
B.G. Lovelock / Geothermics 30 (2001) 641–654
in the liquid-phase, and, therefore, not performing as a steam tracer, a correction must be applied to Eq. (1): Steam flow ðt=hÞ ¼
Tracer injection rate ðg=hÞ ½WFðt=hÞ TW ðg=tÞ TS ðg=tÞ
ð3Þ
where TW and TS are the alcohol-tracer concentrations in water and steam, and WF is the water flow. Both the water flow and the alcohol concentration in the water need to be measured for this correction to be applied. The water flow is obtained from a liquid-phase tracer (e.g. benzoate). The size of the correction in Eq. (3) increases with decreasing discharge enthalpy (higher water fraction). Above 2000 kJ/kg the correction can almost be ignored. For a 1500 kJ/kg discharge, a 10% error in the brine alcohol analysis will add 1% to the total error in the steam flow measurement (for a 180 C flow). At 1000 kJ/kg, a 10% error in the brine analysis will add 10% to the steam flow error. In other words, accurate liquid-phase alcohol analyses are more important for low enthalpy discharges. At low discharge enthalpies, steam flow contributes relatively less to the total mass flow and total enthalpy. Therefore, the accuracy of the total mass flow and total enthalpy measurements is unchanged at lower enthalpies, as a result of the correction. For high enthalpy discharges, where the correction is small, it may be reasonable to assume a liquid-phase alcohol concentration based on distribution ratios from earlier testing. Isopropanol tracer testing has been carried out successfully at discharge enthalpies as low as 900 kJ/kg (Lovelock, 1997). The liquid-phase correction is smaller if a less soluble alcohol such as butan-2-ol is used. However, butan-2-ol is more expensive than isopropanol and is not miscible, so it cannot easily be mixed with liquid-phase tracers. Another potential tracer is npropanol, which is miscible in water but has a higher boiling point than isopropanol (97 C, Table 2). The more volatile compounds in Table 2 (chloroform, benzene and hexane) can be expected to have minimal solubility as gases, and the correction in Eq. (3) would not be required. However, these compounds would require special procedures for them to be sampled quantitatively. 4.2. Practical considerations Once it has been shown that a mass flow tracer is thermally and chemically stable under the conditions in which it is being used, it should then be judged on how practical it is to use. This should consider the following: . . . . . .
ease of injection; ease of sampling; ease of analysis; health and safety; tracer availability and cost; potential for minimising errors.
B.G. Lovelock / Geothermics 30 (2001) 641–654
651
4.2.1. Injection All the organic compounds in Table 2 can be injected with conventional highpressure metering pumps. The alcohols with lower molecular weights have the advantage of being miscible in water, which means that they can be mixed with a liquid-phase tracer (e.g. benzoate) to form a composite steam/water tracer. Where the tracer injection rate is metered by weight loss, having only one tracer solution halves the amount of metering information that is collected, thereby reducing the errors. Injecting the steam and water tracers simultaneously is also important if the well is cycling over a short period, i.e. it is possible to measure steam and water flowrate at the same time. Alcohol will flash immediately upon injection into a pipeline and the admixture of alcohol to a liquid-phase tracer (e.g. benzoate) may help in dispersing the liquid tracer across the diameter of the pipeline. The more rapid the dispersal of tracer, the shorter the required distance between injection and sampling points. This is important on multiple well pads where the two-phase branch lines of individual wells are often very short. A requirement of composite tracer solutions is that the mixture be chemically stable. Repeat analyses of a solution of 50% isopropanol and 10% benzoate have shown it to be stable for at least 4 months. 4.2.2. Sampling Sampling of tracers for mass flow measurement must be quantitative, i.e. all the tracer present in the steam sample must be collected and preserved in the sample container. If the steam tracer is gaseous (e.g. SF6) or volatile (e.g. hexane, benzene), then the pipeline steam must be collected into specially sealed containers (e.g. Rotoflo-type gas flasks) to prevent loss of the tracer. It may also be necessary to use an alkali solution to absorb CO2 and H2S so that sufficient steam is condensed for determination of tracer-in-steam concentrations. In this case, sample concentrations must be corrected for alkali addition and for the weight of absorbed gas. In the case of the volatile organic liquids (e.g. hexane, benzene), it may be necessary to extract the tracers into an organic medium (e.g. diethyl ether). None of the above procedures are necessary when alcohol tracers are used. Lovelock and Stowell (2000) showed that both isopropanol and butan-2-ol can be collected quantitatively by simple condensation of steam using a condensing coil and storage in screw-cap glass bottles (10 ml is sufficient). The ability to collect steam samples into screw-cap bottles affords a number of benefits: . minimal skills are required for sample collection; . the alcohol concentration in the sample is the concentration in the steam, i.e. no corrections necessary for gas or caustic pre-treatment; . samples can be air-mailed to central laboratories without the restrictions imposed on caustic-containing gas flasks.
In New Zealand, tracer flow testing with alcohol has been used successfully in wells with gas contents over 10% by weight. Under these conditions, condensate flowing from a condensing coil (see Fig. 2) will be highly charged with gas (CO2, H2S), which is exsolved from samples. Bench-top experiments have shown that isopropanol
652
B.G. Lovelock / Geothermics 30 (2001) 641–654
is stable in 40 C water even when purged with air (Fig. 4). Therefore, one-step exsolution of gas from cold condensate would be expected to cause negligible loss of dissolved alcohol. Significant loss of alcohol can occur at temperatures above 60 C (the boiling point of isopropanol is 82 C), so it is important that testing procedures stipulate the need for collecting cold samples. 4.2.3. Alcohol analyses Like other steam-phase tracers (e.g. SF6), alcohols must be analysed by GC techniques. For tracer flow testing carried out in New Zealand, samples are analysed by head-space GC at a large blood-alcohol laboratory. Details of this method are given in Lovelock and Stowell (2000). A blood-alcohol laboratory was chosen primarily for expediency. No method development was required because standard ethanol-inblood methods are directly applicable to geothermal samples. Also, because the laboratory was highly automated and dedicated to a single analysis, it could provide rapid turn-around at modest cost. Excellent precision could be expected because blood-alcohol results are used for evidential purposes. If a blood-alcohol laboratory were not available, it is unlikely that head-space GC techniques would be implemented specifically for alcohol tracer analyses because direct-injection methods of analysis are simpler, less expensive and provide greater sensitivity. JMC Geothermal Engineering Co., LTD (Geo-E) in Japan has used direct-injection procedures for the analysis of isopropanol in geothermal samples (Kasai, pers. com.). During tracer flow testing trials carried out in Japan in 2000, split samples analysed by direct injection (Geo-E) and head-space GC (in New Zealand) gave results that agreed within 2% (for condensate samples with 85 mg/kg isopropanol). Direct injection has greater sensitivity than head-space GC methods and column injection is simpler. 4.2.4. Health and safety Isopropanol presents minimal health risks. It is used widely as a rubbing alcohol and disinfectant. The pure compound is flammable, but this risk is reduced when the alcohol is diluted with an aqueous liquid-phase tracer (e.g. sodium benzoate solution). 4.2.5. Cost The cost of alcohol tracer chemicals is small compared with other tracer flow testing costs (e.g. labour and equipment). In New Zealand the cost of isopropanol consumed in each well test is about US$5. Butan-2-ol is approximately four times the price of isopropanol but is still quite affordable. 4.2.6. Minimising errors The following are the main sources of error in tracer flow measurements: . poor sampling: incomplete separation of phases at line pressure, inadequate sample preservation; . poor analysis: standard preparation, calibration, contamination, calculation errors, etc.;
B.G. Lovelock / Geothermics 30 (2001) 641–654
653
. incomplete tracer mixing in pipeline: this may be unavoidable if the injectionto-sampling distance is short (e.g. on multi-well pads). Incomplete mixing may be apparent from the variability of data for multiple samples; . injection rate errors: these should be small and can be checked against rate curves for the pump being used.
Strict procedures must be in place to ensure that the sampling separators are operating properly. To avoid systematic errors related to incorrect preparation of bulk tracer solutions, standards should be prepared from the same tracer that was used when the samples were collected. In this way, any errors in preparation of the stock solutions will be cancelled out, because the numerator and denominator in Eq. (1) will change by the same amount. Cumulative errors can be reduced by preparing a composite steam/water tracer (e.g. using alcohol and benzoate), which requires only one injection rate measurement to be made. 4.3. Further work Selection of tracers for a particular task is always an exercise in balancing the positive and negative features of each tracer. The development work with alcohol tracers has demonstrated the practical benefits of low boiling-point organic liquids, and further work should focus on these compounds. The ideal steam-phase tracer would be a compound that has properties similar to isopropanol but with a steam/ water distribution ratio greater than 100 (so that a liquid-phase correction is not needed), and which can be analysed precisely by simple procedures. Such a compound may not exist, but there may be compounds with properties that come close. As the steam-water distribution ratio of a tracer increases, the practical advantages of tracers such as isopropanol are lost. Firstly, above a certain distribution ratio (around 30), the liquid tracer will no longer be miscible in water, so that composite steam/water tracer solutions can no longer be prepared (i.e. the steam tracer must be injected separately). At still higher distribution ratios (possibly around 1000), the volatility of the tracer will probably reach the point where it can no longer be sampled quantitatively into open bottles. Such tracers include hexane and benzene. A compromise solution is to select a tracer that is not miscible and has a high distribution ratio (e.g. around 100), but which can still be sampled quantitatively into open bottles. Lovelock (1997) measured a distribution ratio of 500 for chloroform at 150 C and was able to sample this tracer quantitatively, albeit in a very low-gas well. On this basis, one would not expect to have trouble sampling a tracer with a ratio of 100–200. There may exist a few affordable organic compounds that fulfill these criteria, and these should be identified and studied to see whether they have significant advantages over isopropanol.
5. Summary A tracer flow testing method using alcohol as the steam-phase tracer has been successfully developed in New Zealand. With this procedure, isopropanol is injected
654
B.G. Lovelock / Geothermics 30 (2001) 641–654
into the pipeline where the alcohol then boils and travels down the pipeline as a gas. At the downstream sampling point, alcohol is condensed into the steam condensate sample. Alcohol offers significant practical advantages over compressed gas tracers including: . alcohol can be injected with conventional liquid-dosing pumps; . isopropanol can be mixed with the brine tracer (e.g. benzoate) to produce a composite steam/brine tracer, thus halving the number of injection rate measurements; . alcohol can be sampled quantitatively into simple screw-cap sample bottles; . corrections for gas content are not required.
It is believed that these benefits far out-weigh the only drawback of alcohol tracers, which is the need to correct for alcohol gas dissolved in the liquid phase.
Acknowledgements Development work with alcohol tracers in 1999 was funded by Tuaropaki Power Company, owners of the Mokai geothermal power station in New Zealand. The field work was carried out at the Rotokawa power station, owned and operated by Mighty River Power Ltd. JMC Geothermal Engineering Co., LTD (Geo-E) has contributed to the development of GC methods for analysing alcohol in geothermal samples. The assistance of Mr. Kaichiro Kasai and his staff is gratefully acknowledged.
References Adams, M.C., 1995. Vapor, liquid and two-phase tracers for geothermal systems. Proceedings of the 1995 World Geothermal Congress, Florence, pp. 1875–1880. Hirtz, P.N., Lovekin, J., 1995. Tracer dilution measurements for two-phase geothermal production: comparative testing and operating experience. Proceedings of the 1995 World Geothermal Congress, Florence, pp. 1881–1886. Hirtz, P.N., Lovekin, J., Copp, J., Buck, C.L., Adams, M.C., 1993. Enthalpy and flowrate measurements for two-phase production by tracer dilution techniques. Proceedings of the 18th Workshop on Geothermal Reservoir Engineering, Stanford University, pp. 17–27. James, R., 1970. Factors controlling borehole performance. Proceedings U.N. Symposium on the Development and Use of Geothermal Resources (special issue). Geothermics 2, 1502–1515. Liley, P.E., Gambill, W.R., 1973. Physical properties of pure substances. In: Perry, R.H., Chilton, C.H. (Eds.), Chemical Engineers’ Handbook, 5th Edition. McGraw-Hill, pp. 3-1–3-250. Lovelock, B.G., 1997. Steam flow measurement in two-phase pipelines using volatile organic liquid tracers. Proceedings of the 19th New Zealand Geothermal Workshop, Auckland University, New Zealand, pp. 75–80. Lovelock, B.G., Stowell A., 2000. Mass flow measurement by alcohol tracer dilution. Proceedings of the 2000 World Geothermal Congress, Japan, pp. 2701–2706. Macambac, R.V., Salazar, A.T.N., Villa, R.R., Alcober, E.H., Magdadaro, M.C., Hirtz, P.N., Kunzman, R., 1998. Field-wide application of chemical tracers for mass flow measurement in Philippine geothermal fields. Proceedings of the 19th Annual PNOC-EDC Geothermal Conference, Manila, pp. 153–159.
Steam flow measurement using alcohol tracers Brian G. Lovelock* Sinclair Knight Merz, PO Box 9806, Newmarket, Auckland, New Zealand Received 6 January 2001; accepted 8 June 2001
Abstract Tracer flow testing procedures are increasingly being used for routine measurement of well output in operating geothermal fields. A new procedure developed in New Zealand uses low boiling-point, liquid alcohol (isopropanol or butan-2-ol) as the steam-phase tracer. The use of alcohol tracers allows major simplifications to field procedures, including injection with conventional liquid-dosing pumps, sampling into open sample bottles and preparation of composite steam/water tracers. These practical benefits far outweigh the only drawback of alcohol tracers, which is the need to correct for alcohol gas dissolved in the liquid phase. This paper reviews development work with alcohol tracers over the past five years and discusses the properties of alcohols and the practical aspects of their use in tracer-dilution testing. # 2001 Published by Elsevier Science Ltd on behalf of CNR. Keywords: Tracers; Alcohol; Isopropanol; Mass flow; Chemistry
1. Introduction Measurement of mass flow and enthalpy in two-phase pipelines is an important monitoring task at operating geothermal fields. Traditionally this has been accomplished by using dedicated wellhead separators or by periodically taking wells out of service and bypassing the discharge to a total-flow atmospheric silencer. With modern steamfield design, wellhead separators are less common. In many cases, twophase flow from production wells is combined and piped to centralised production separators. At the same time, there have been increasing environmental constraints on atmospheric well testing. These factors have provided the impetus for the development of non-invasive, on-line methods for measuring steam and water flow in * Fax: +64-9-913-8901. E-mail address: [email protected] 0375-6505/01/$20.00 # 2001 Published by Elsevier Science Ltd on behalf of CNR. PII: S0375-6505(01)00020-7
642
B.G. Lovelock / Geothermics 30 (2001) 641–654
two-phase pipelines. To date, the only on-line method in routine use is tracer flow testing—sometimes called the tracer-dilution method. Because of the potential cost savings and environmental benefits, on-line testing is likely to become the standard operating procedure in the future. Tracer flow testing procedures for well output testing were developed in the early 1990s by workers in the United States (e.g. Hirtz et al., 1993; Hirtz and Lovekin, 1995). This technology is now available as a commercial service and is in regular use at operating fields in the United States, the Philippines, Indonesia, Iceland and Japan. This paper reviews development work on a new approach to tracer flow testing using low boiling-point alcohols.
2. Tracer flow testing methodology To measure total mass flow and enthalpy in two-phase pipelines, it is necessary to measure the flow of the individual phases, steam and liquid-water (brine), separately. Tracer flow testing involves the quantitative injection and dilution of small quantities of chemical tracers (one for steam and one for liquid) in a pipeline. The tracers mix completely inside the pipeline, and at downstream sampling points separated water and steam are collected at the pipeline pressure. Using a steamphase tracer that travels entirely in the steam (e.g. SF6 or propane), the steam flow is calculated as: Steam flow ðt=hÞ ¼
steam tracer injection rate ðg=hÞ Steam tracer concentration in steam ðg=tÞ
ð1Þ
The calculation is analogous for water flow, where the liquid-phase tracer (e.g. benzoate or bromide) is totally dissolved in the brine. Total discharge enthalpy (Ht) can then be calculated from the measured steam and water flows (SF, WF): Ht ¼
SF hg þ WF hf SF þ WF
ð2Þ
where hg and hf are the enthalpy of steam and water at the pipeline temperature (obtained from steam tables). The above expressions do not include noncondensible gases (CO2, H2S, etc.), which may constitute several percent of the total mass flow from a well. These must be determined separately. Tracer flow testing is currently the only viable on-line method for measurement of two-phase flow in pipelines. The failure of other methods (e.g. Doppler-based systems) is due to the fact that the two phases are present in varying proportions, and may not be uniformly distributed across the pipeline. This is not an issue with tracer flow testing, where the two phases are measured independently. On-line, non-invasive testing has a number of major advantages over traditional methods (atmospheric testing or total flow separators):
B.G. Lovelock / Geothermics 30 (2001) 641–654
643
power station operations are not interrupted; generation load losses associated with taking wells out of service are avoided; stable flow conditions are more likely (since production is not disturbed); there is no need for permanent wellhead test equipment, aside from sampling ports; . atmospheric discharge of wastewater is avoided; . there is greater flexibility in scheduling testing. . . . .
Tracer flow testing can provide the greatest benefits when implemented at the steamfield design stage, rather than by retro-fitting. With new geothermal developments, it is common to do away with dedicated wellhead test equipment in favour of tracer flow testing methods. In addition to well testing, tracer flow testing procedures can be used for power plant steam audits (e.g. as a check on annubar1 measurements) and for measurement of combined flow from several wells.
3. Practical development of tracer flow testing methods Tracer flow testing procedures developed in the United States currently use SF6 gas as the steam-phase tracer and sodium benzoate as the liquid-phase tracer (Macambac et al., 1998). The benzoate solution is injected into the pipeline using a liquid-metering pump. The SF6 is injected from a compressed-gas cylinder using a specialised gas-metering system. Sealed gas flasks are used to collect steam samples for analysis of SF6. In the mid-l990s, Lovelock (1997) began investigating various low boiling-point, organic liquids as steam-phase tracers, including chloroform, hexane and alcohols. With this approach, the organic liquid is injected into the pipeline where it boils and travels down the pipeline as a gas. The tracer is then collected as a liquid in condensed steam samples. Low boiling-point, organic tracers are an attractive alternative to compressed-gas tracers because they can be injected using conventional liquid-dosing pumps and offer the potential for simpler sampling procedures. Much of Lovelock’s early work focused on the practical aspects of sampling and analysing organic liquids. The main goal was to identify tracers that could be sampled quantitatively without gas flasks and which could be analysed with low-cost methods (e.g. ultraviolet-visible photometric or titrimetric). While some success was had with chloroform, alcohols were found to offer the greatest practical advantages. 3.1. Alcohol tracers Early work with alcohol tracers focused on their potential for reservoir tracer tests. Adams (1995) showed that some alcohols have the thermal stability required
1
A tradename of a differential pressure flow measurement sensor.
644
B.G. Lovelock / Geothermics 30 (2001) 641–654
of reservoir tracers but recognised the limitations of analytical methods, which do not have the low detection limits of tracers such as iodine-131 and SF6. Alcohols were first used as quantitative mass flow tracers in New Zealand in the mid-1990s (Lovelock, 1997). Isopropanol was used as the steam-phase tracer and bromide as the liquid-phase tracer. This work helped to characterise the vapourliquid distribution character of alcohols and showed that with simple corrections for liquid-phase alcohol concentrations, accurate steam flow measurements were possible. The work also showed that alcohol in condensate could be sampled quantitatively into open sample bottles, even when gas contents were high. Isopropanol was analysed by a standard head-space, gas chromatographic (GC) method developed for blood-alcohol analysis. Mass flow tracer measurements were found to be in good agreement with measurements obtained by the empirical lip-pressure method of James (1970). Simple injection and sampling procedures were developed. More rigorous field testing of alcohols was carried out in 1999 at the Rotokawa field in New Zealand (Lovelock and Stowell, 2000). This is a high-temperature field with two production wells supplying a 25 MWe combined-cycle plant. The two-phase pipelines operate at 20–25 bar gauge (215–225 C). Mean steam flow measurements from seven separate alcohol-injection runs were within 1–4% of flows measured independently by annubar. Representative results are given in Table 1. These trials showed good reproducibility for multiple samples (< 2% variability for most tests). Both isopropanol and butan-2-ol were shown to be viable tracers. Subsequently, alcohol tracers have been used for routine well testing at the Mokai geothermal field in New Zealand, which was commissioned in 1999. This is a high-temperature field with five production wells supplying a 50 MWe power plant. For the five tracer flow testing surveys conducted at Mokai since commissioning, steam flow rates, when summed, were within 5% of total steam flow rate measured at the power plant (using an orifice plate). 3.2. Field equipment and procedures In New Zealand, tracer flow testing with alcohols is carried out using a small portable injection rig which includes a high pressure, high stroke-rate, positive-displacement dosing pump with variable injection rate up to 420 ml/min (Fig. 1). A composite steam-water tracer is used, comprising 50 wt.% isopropanol and 10 wt.% benzoate (as sodium benzoate). The injection rate is metered by weight loss over the period of the test, typically 5–15 min. Samples are collected from downstream sampling points using miniature sampling separators operating at line pressure (Fig. 2). Separated steam and water are passed through condensing coils and collected into screw-cap containers. Dissolved hydrogen sulphide is precipitated with zinc acetate and an antibacterial agent is added.
4. Discussion In assessing the attributes of alcohols for tracer flow testing, it is useful to compare their properties with those of other organic liquids. Table 2 lists several common
Table 1 Selected tracer flow measurements at Rotokawa field, New Zealand, using alcohol, and composite isopropanol (IPA) and benzoate tracers. Sampling pressure: 20–25 bar gauge (from Lovelock and Stowell, 2000) Tracer
Time
16–17 June 1999 RK5
Butan-2-ol
RK5
Isopropanol
26 August 1999 RK5
RK9
a
Composite: 10 wt.% Benzoate 40 wt.% Isopropanol
Composite: 10 wt.% Benzoate 40 wt.% Isopropanol
Tracer injection rate (g/min)
IPA in liquid (mg/kg)
Steam flow (t/h)
13:56 98.5 44.0 13:58 44.0 14:00 44.2 12:02 130 57.5 12:06 57.5 12:10 57.2 Mean steam flow by tracera Mean steam flow by orifice plate
2.7 2.5 2.7 4.5 4.6 4.5
122 123 121 120 120 121 121 119
13:06
7.6 8.0 8.3 8.7 7.8 8.1 7.6 8.3 8.0
74.1 73.7 72.3 68.8 68.9 69.5 69.9 68.8 68.6 141 140–143
296
IPA in steam (mg/kg)
78.2 78.1 # 78.9 81.8 13:23 83.9 10:10 297 82.6 83.3 # 82.9 10:26 83.6 Mean flow by tracers (RK5+RK9) Total flow by orifice plate (RK4+RK9)
Benzoate in brine (mg/kg)
Brine flow (t/h)
10.3 10.5 10.5 10.5 10.4 10.5 10.5 10.5 10.3
172 169 169 169 171 169 169 169 173 340 331–336
B.G. Lovelock / Geothermics 30 (2001) 641–654
Well
Based on water flow of 200 t/h measured by orifice plate.
645
646
B.G. Lovelock / Geothermics 30 (2001) 641–654
Fig. 1. Schematic layout of portable tracer injection rig used in New Zealand.
Fig. 2. Schematic layout of tracer sampling equipment, showing sampling separators for steam and brine condensing/cooling coils.
647
B.G. Lovelock / Geothermics 30 (2001) 641–654
Table 2 Properties of some common organic liquids, arranged in order of increasing solubility in water (Liley and Gambill, 1973) Compound
Formula
Formula weight (g/mol)
Density (g/ml)
Boiling point ( C)
Solubility in liquid (mg/l)
Hexane Benzene Chloroform Butan-2-ol n-Propanol iso-Propanol Ethanol
CH3(CH2)4CH3 C6H6 CHCl3 CH3CH(OH)CH2CH3 CH3(CH2)2OH (CH3)2CHOH CH3CH2OH
86.2 78.1 119.4 74.1 60.1 60.1 46.1
0.66 0.88 1.49 0.81 0.80 0.79 0.79
69.0 80.1 61.2 99.5 97.4 82.5 78.4
140 700 8200 125,000 1 1 1
organic compounds that have properties potentially suitable for use as steam-phase tracers. Selecting the best tracer is always an exercise in compromise, balancing benefits and drawbacks for the different candidate tracers. Criteria for selection fall into two broad categories: (1) physical properties and (2) practicality of use. 4.1. Physical properties of steam-phase tracers There are a number of physical properties that are required of steam-phase tracers. These are: . the tracer exists as a gas over the full range of pipeline conditions; . the tracer is chemically and thermally stable over the full range of pipeline temperatures and in the presence of other geothermal constituents; . the tracer has negligible background concentration in the geothermal fluid; . the tracer in gaseous form has a low solubility in water.
The first three properties are shared by all of the compounds in Table 2. In the case of thermal stability, it is only necessary that the tracer be stable at pipeline temperatures for the duration of the injection period, which in most cases will be less than one minute. Indeed, it is preferable that any tracer that is injected will eventually degrade, thus ensuring a long-term, low background concentration in the reservoir. In the case of chemical stability, it is necessary that the tracer remain unreacted until analyses are completed. In two-phase pipelines it is preferable that the steam-phase tracer travels entirely in the steam phase, with a negligible amount dissolved in the brine. This is not the case with alcohol gas, which has a significant solubility (due to its polar character). Isopropanol has a steam/water concentration ratio (Cv/CL) of 10–25 in the temperature range 150–220 C (Fig. 3), with an inverse temperature dependence. The data in Fig. 4 show that the steam-water distribution ratio falls within a well-defined range, which in turn suggests that equilibrium is achieved quickly. This is supported by the fact that some of the data in Fig. 3 (from Japan) were obtained for injectionto-sampling distances of only 10 m. Because some of the alcohol tracer is dissolved
648 B.G. Lovelock / Geothermics 30 (2001) 641–654
Fig. 3. Distribution of isopropanol between steam and liquid-phase in two-phase pipelines. CV/CL is the steam-water concentration ratio.
B.G. Lovelock / Geothermics 30 (2001) 641–654 Fig. 4. Changes in dissolved isopropanol (IPA) concentration as a result of heating from 20 to 80 C and with air-purging (for 1 min at each temperature step). 649
650
B.G. Lovelock / Geothermics 30 (2001) 641–654
in the liquid-phase, and, therefore, not performing as a steam tracer, a correction must be applied to Eq. (1): Steam flow ðt=hÞ ¼
Tracer injection rate ðg=hÞ ½WFðt=hÞ TW ðg=tÞ TS ðg=tÞ
ð3Þ
where TW and TS are the alcohol-tracer concentrations in water and steam, and WF is the water flow. Both the water flow and the alcohol concentration in the water need to be measured for this correction to be applied. The water flow is obtained from a liquid-phase tracer (e.g. benzoate). The size of the correction in Eq. (3) increases with decreasing discharge enthalpy (higher water fraction). Above 2000 kJ/kg the correction can almost be ignored. For a 1500 kJ/kg discharge, a 10% error in the brine alcohol analysis will add 1% to the total error in the steam flow measurement (for a 180 C flow). At 1000 kJ/kg, a 10% error in the brine analysis will add 10% to the steam flow error. In other words, accurate liquid-phase alcohol analyses are more important for low enthalpy discharges. At low discharge enthalpies, steam flow contributes relatively less to the total mass flow and total enthalpy. Therefore, the accuracy of the total mass flow and total enthalpy measurements is unchanged at lower enthalpies, as a result of the correction. For high enthalpy discharges, where the correction is small, it may be reasonable to assume a liquid-phase alcohol concentration based on distribution ratios from earlier testing. Isopropanol tracer testing has been carried out successfully at discharge enthalpies as low as 900 kJ/kg (Lovelock, 1997). The liquid-phase correction is smaller if a less soluble alcohol such as butan-2-ol is used. However, butan-2-ol is more expensive than isopropanol and is not miscible, so it cannot easily be mixed with liquid-phase tracers. Another potential tracer is npropanol, which is miscible in water but has a higher boiling point than isopropanol (97 C, Table 2). The more volatile compounds in Table 2 (chloroform, benzene and hexane) can be expected to have minimal solubility as gases, and the correction in Eq. (3) would not be required. However, these compounds would require special procedures for them to be sampled quantitatively. 4.2. Practical considerations Once it has been shown that a mass flow tracer is thermally and chemically stable under the conditions in which it is being used, it should then be judged on how practical it is to use. This should consider the following: . . . . . .
ease of injection; ease of sampling; ease of analysis; health and safety; tracer availability and cost; potential for minimising errors.
B.G. Lovelock / Geothermics 30 (2001) 641–654
651
4.2.1. Injection All the organic compounds in Table 2 can be injected with conventional highpressure metering pumps. The alcohols with lower molecular weights have the advantage of being miscible in water, which means that they can be mixed with a liquid-phase tracer (e.g. benzoate) to form a composite steam/water tracer. Where the tracer injection rate is metered by weight loss, having only one tracer solution halves the amount of metering information that is collected, thereby reducing the errors. Injecting the steam and water tracers simultaneously is also important if the well is cycling over a short period, i.e. it is possible to measure steam and water flowrate at the same time. Alcohol will flash immediately upon injection into a pipeline and the admixture of alcohol to a liquid-phase tracer (e.g. benzoate) may help in dispersing the liquid tracer across the diameter of the pipeline. The more rapid the dispersal of tracer, the shorter the required distance between injection and sampling points. This is important on multiple well pads where the two-phase branch lines of individual wells are often very short. A requirement of composite tracer solutions is that the mixture be chemically stable. Repeat analyses of a solution of 50% isopropanol and 10% benzoate have shown it to be stable for at least 4 months. 4.2.2. Sampling Sampling of tracers for mass flow measurement must be quantitative, i.e. all the tracer present in the steam sample must be collected and preserved in the sample container. If the steam tracer is gaseous (e.g. SF6) or volatile (e.g. hexane, benzene), then the pipeline steam must be collected into specially sealed containers (e.g. Rotoflo-type gas flasks) to prevent loss of the tracer. It may also be necessary to use an alkali solution to absorb CO2 and H2S so that sufficient steam is condensed for determination of tracer-in-steam concentrations. In this case, sample concentrations must be corrected for alkali addition and for the weight of absorbed gas. In the case of the volatile organic liquids (e.g. hexane, benzene), it may be necessary to extract the tracers into an organic medium (e.g. diethyl ether). None of the above procedures are necessary when alcohol tracers are used. Lovelock and Stowell (2000) showed that both isopropanol and butan-2-ol can be collected quantitatively by simple condensation of steam using a condensing coil and storage in screw-cap glass bottles (10 ml is sufficient). The ability to collect steam samples into screw-cap bottles affords a number of benefits: . minimal skills are required for sample collection; . the alcohol concentration in the sample is the concentration in the steam, i.e. no corrections necessary for gas or caustic pre-treatment; . samples can be air-mailed to central laboratories without the restrictions imposed on caustic-containing gas flasks.
In New Zealand, tracer flow testing with alcohol has been used successfully in wells with gas contents over 10% by weight. Under these conditions, condensate flowing from a condensing coil (see Fig. 2) will be highly charged with gas (CO2, H2S), which is exsolved from samples. Bench-top experiments have shown that isopropanol
652
B.G. Lovelock / Geothermics 30 (2001) 641–654
is stable in 40 C water even when purged with air (Fig. 4). Therefore, one-step exsolution of gas from cold condensate would be expected to cause negligible loss of dissolved alcohol. Significant loss of alcohol can occur at temperatures above 60 C (the boiling point of isopropanol is 82 C), so it is important that testing procedures stipulate the need for collecting cold samples. 4.2.3. Alcohol analyses Like other steam-phase tracers (e.g. SF6), alcohols must be analysed by GC techniques. For tracer flow testing carried out in New Zealand, samples are analysed by head-space GC at a large blood-alcohol laboratory. Details of this method are given in Lovelock and Stowell (2000). A blood-alcohol laboratory was chosen primarily for expediency. No method development was required because standard ethanol-inblood methods are directly applicable to geothermal samples. Also, because the laboratory was highly automated and dedicated to a single analysis, it could provide rapid turn-around at modest cost. Excellent precision could be expected because blood-alcohol results are used for evidential purposes. If a blood-alcohol laboratory were not available, it is unlikely that head-space GC techniques would be implemented specifically for alcohol tracer analyses because direct-injection methods of analysis are simpler, less expensive and provide greater sensitivity. JMC Geothermal Engineering Co., LTD (Geo-E) in Japan has used direct-injection procedures for the analysis of isopropanol in geothermal samples (Kasai, pers. com.). During tracer flow testing trials carried out in Japan in 2000, split samples analysed by direct injection (Geo-E) and head-space GC (in New Zealand) gave results that agreed within 2% (for condensate samples with 85 mg/kg isopropanol). Direct injection has greater sensitivity than head-space GC methods and column injection is simpler. 4.2.4. Health and safety Isopropanol presents minimal health risks. It is used widely as a rubbing alcohol and disinfectant. The pure compound is flammable, but this risk is reduced when the alcohol is diluted with an aqueous liquid-phase tracer (e.g. sodium benzoate solution). 4.2.5. Cost The cost of alcohol tracer chemicals is small compared with other tracer flow testing costs (e.g. labour and equipment). In New Zealand the cost of isopropanol consumed in each well test is about US$5. Butan-2-ol is approximately four times the price of isopropanol but is still quite affordable. 4.2.6. Minimising errors The following are the main sources of error in tracer flow measurements: . poor sampling: incomplete separation of phases at line pressure, inadequate sample preservation; . poor analysis: standard preparation, calibration, contamination, calculation errors, etc.;
B.G. Lovelock / Geothermics 30 (2001) 641–654
653
. incomplete tracer mixing in pipeline: this may be unavoidable if the injectionto-sampling distance is short (e.g. on multi-well pads). Incomplete mixing may be apparent from the variability of data for multiple samples; . injection rate errors: these should be small and can be checked against rate curves for the pump being used.
Strict procedures must be in place to ensure that the sampling separators are operating properly. To avoid systematic errors related to incorrect preparation of bulk tracer solutions, standards should be prepared from the same tracer that was used when the samples were collected. In this way, any errors in preparation of the stock solutions will be cancelled out, because the numerator and denominator in Eq. (1) will change by the same amount. Cumulative errors can be reduced by preparing a composite steam/water tracer (e.g. using alcohol and benzoate), which requires only one injection rate measurement to be made. 4.3. Further work Selection of tracers for a particular task is always an exercise in balancing the positive and negative features of each tracer. The development work with alcohol tracers has demonstrated the practical benefits of low boiling-point organic liquids, and further work should focus on these compounds. The ideal steam-phase tracer would be a compound that has properties similar to isopropanol but with a steam/ water distribution ratio greater than 100 (so that a liquid-phase correction is not needed), and which can be analysed precisely by simple procedures. Such a compound may not exist, but there may be compounds with properties that come close. As the steam-water distribution ratio of a tracer increases, the practical advantages of tracers such as isopropanol are lost. Firstly, above a certain distribution ratio (around 30), the liquid tracer will no longer be miscible in water, so that composite steam/water tracer solutions can no longer be prepared (i.e. the steam tracer must be injected separately). At still higher distribution ratios (possibly around 1000), the volatility of the tracer will probably reach the point where it can no longer be sampled quantitatively into open bottles. Such tracers include hexane and benzene. A compromise solution is to select a tracer that is not miscible and has a high distribution ratio (e.g. around 100), but which can still be sampled quantitatively into open bottles. Lovelock (1997) measured a distribution ratio of 500 for chloroform at 150 C and was able to sample this tracer quantitatively, albeit in a very low-gas well. On this basis, one would not expect to have trouble sampling a tracer with a ratio of 100–200. There may exist a few affordable organic compounds that fulfill these criteria, and these should be identified and studied to see whether they have significant advantages over isopropanol.
5. Summary A tracer flow testing method using alcohol as the steam-phase tracer has been successfully developed in New Zealand. With this procedure, isopropanol is injected
654
B.G. Lovelock / Geothermics 30 (2001) 641–654
into the pipeline where the alcohol then boils and travels down the pipeline as a gas. At the downstream sampling point, alcohol is condensed into the steam condensate sample. Alcohol offers significant practical advantages over compressed gas tracers including: . alcohol can be injected with conventional liquid-dosing pumps; . isopropanol can be mixed with the brine tracer (e.g. benzoate) to produce a composite steam/brine tracer, thus halving the number of injection rate measurements; . alcohol can be sampled quantitatively into simple screw-cap sample bottles; . corrections for gas content are not required.
It is believed that these benefits far out-weigh the only drawback of alcohol tracers, which is the need to correct for alcohol gas dissolved in the liquid phase.
Acknowledgements Development work with alcohol tracers in 1999 was funded by Tuaropaki Power Company, owners of the Mokai geothermal power station in New Zealand. The field work was carried out at the Rotokawa power station, owned and operated by Mighty River Power Ltd. JMC Geothermal Engineering Co., LTD (Geo-E) has contributed to the development of GC methods for analysing alcohol in geothermal samples. The assistance of Mr. Kaichiro Kasai and his staff is gratefully acknowledged.
References Adams, M.C., 1995. Vapor, liquid and two-phase tracers for geothermal systems. Proceedings of the 1995 World Geothermal Congress, Florence, pp. 1875–1880. Hirtz, P.N., Lovekin, J., 1995. Tracer dilution measurements for two-phase geothermal production: comparative testing and operating experience. Proceedings of the 1995 World Geothermal Congress, Florence, pp. 1881–1886. Hirtz, P.N., Lovekin, J., Copp, J., Buck, C.L., Adams, M.C., 1993. Enthalpy and flowrate measurements for two-phase production by tracer dilution techniques. Proceedings of the 18th Workshop on Geothermal Reservoir Engineering, Stanford University, pp. 17–27. James, R., 1970. Factors controlling borehole performance. Proceedings U.N. Symposium on the Development and Use of Geothermal Resources (special issue). Geothermics 2, 1502–1515. Liley, P.E., Gambill, W.R., 1973. Physical properties of pure substances. In: Perry, R.H., Chilton, C.H. (Eds.), Chemical Engineers’ Handbook, 5th Edition. McGraw-Hill, pp. 3-1–3-250. Lovelock, B.G., 1997. Steam flow measurement in two-phase pipelines using volatile organic liquid tracers. Proceedings of the 19th New Zealand Geothermal Workshop, Auckland University, New Zealand, pp. 75–80. Lovelock, B.G., Stowell A., 2000. Mass flow measurement by alcohol tracer dilution. Proceedings of the 2000 World Geothermal Congress, Japan, pp. 2701–2706. Macambac, R.V., Salazar, A.T.N., Villa, R.R., Alcober, E.H., Magdadaro, M.C., Hirtz, P.N., Kunzman, R., 1998. Field-wide application of chemical tracers for mass flow measurement in Philippine geothermal fields. Proceedings of the 19th Annual PNOC-EDC Geothermal Conference, Manila, pp. 153–159.