Sampling and analysis of geothermal fluids

Transcription

1 Geofluids (2006) 6, doi: /j x Sampling and analysis of geothermal fluids S. ARNÓRSSON, J. Ö. BJARNASON, N. GIROUD, I. GUNNARSSON AND A. STEFÁNSSON Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland ABSTRACT Sampling of geothermal fluids presents some problems not encountered when sampling surface and nonthermal ground waters. Specific collection techniques are required to obtain representative samples because of the elevated temperature and boiling of these fluids, the effect of exposing them to the atmosphere and cooling of the samples. Sample treatment during collection depends on the analytical method to be used. When sampling wetsteam wells, both the liquid and the vapour fractions should be collected at the same fluid separation pressure. When sampling fumarole steam, maximum information is obtained if the total discharge is collected into a single container without separating the gas and the steam condensate fractions. Silica polymerization affects the solution ph. The only way to obtain reliable ph measurement of a water sample supersaturated with respect to amorphous silica is to measure it on site, before the onset of polymerization. This paper provides an outline of the geothermal sampling techniques and analytical methods currently in use in Iceland. Sampling of hot-water and wet-steam wells is described, as is sampling of hot springs, fumaroles and gas bubbling through hot-spring waters. Detailed procedures are given for the analysis of total carbonate carbon and total sulphide sulphur in geothermal water and steam condensate samples. Key words: analysis, fumaroles, geothermal fluid, hot springs, sampling, wet-steam wells Received 24 August 2005; accepted 28 March 2006 Corresponding author: Stefan Arnorsson, Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland. stefanar@raunvis.hi.is. Tel: Fax: Geofluids (2006) 6, INTRODUCTION Active geothermal systems consist of hot fluid and hot rock within the upper part of the Earth s crust. The fluid is, for the most part or solely, meteoric water or seawater by origin, or a mixture thereof. Generally, geothermal systems develop by deep density-driven convection of ground water. Yet, in some systems of relatively low temperature (< 100 C) the convection may be driven by hydraulic head only. The water convection transports heat from the deeper to the shallower parts of geothermal systems. The heat source may be a magma intrusion (in volcanic geothermal systems) or the hot rock at the roots of the convection cycle (in nonvolcanic geothermal systems). Geothermal fluids rising to the surface include both hot water and steam with dissolved solutes and gases. They may be discharged from hot springs, fumaroles, hot-water wells and wet and dry-steam wells. The source of the dissolved constituents in the geothermal fluids is the rock with which the deep ground water has reacted and constituents present in the parent water. In the case of volcanic geothermal systems, the magma heat source may also supply chemical constituents to geothermal fluids through its degassing. The chemical composition and isotope ratios of geothermal fluids provide important information about the geological, chemical and hydrological characteristics of geothermal systems. An understanding of these characteristics is essential for the exploration and development of geothermal resources and the environmental impact of their utilization (see e.g. Truesdell 1976; Ellis & Mahon 1977; Fournier 1977; D Amore 1991; Arnorsson et al. 2000; World Geothermal Congress 1995, 2000, 2005). The study of geothermal fluid chemistry has also provided valuable insight into the formation of various types of hydrothermal ore deposits and hydrothermal alteration processes (e.g. Spycher & Reed 1989; Hedenquist et al. 1998; Simmons & Browne 2000). Ó 2006 Blackwell Publishing Ltd

2 204 S. ARNÓRSSON et al. Obtaining representative samples of geothermal fluids requires specific sampling techniques, which relate to the elevated temperature, boiling, exposure to the atmosphere, and cooling of the samples. Although some components in geothermal fluids remain stable upon storage, others will react and change their concentrations. This has to be taken into account during sampling, and the samples treated appropriately to preserve them. For a few components the best results may be obtained by analysis on site. Sample treatment during collection for sample preservation depends on the analytical method to be used. It is, therefore, essential to decide which components should be determined in geothermal fluids and by which methods before embarking on sample collection. Some physical data are required for the interpretation of the chemical and isotopic composition of geothermal fluids and these must be gathered during sampling. In particular, for wet-steam wells it is necessary to have information on sampling pressure, discharge enthalpy and the depth level of producing horizons (see e.g. Arnorsson et al. 2000; Arnorsson & Stefansson 2005a,b). For hot springs, geological information pertinent to the mixing of the geothermal water with surface or shallow ground water is important, and so is the contact area with the atmosphere and the flow rate. For fumaroles, it is important to have information on the steam discharge rate and the permeability of the soil, as the fumarole steam may have become contaminated with air close to the surface before sampling. The collection of samples of geothermal fluids presents several problems that do not arise when sampling surface or nonthermal ground waters. The elevated temperature of the water needs to be taken into consideration to avoid evaporation when collecting samples into glass bottles such as for the determination of the stable isotopes of water, ph and carbonate carbon. Conductive cooling in upflow zones below thermal springs and depressurization boiling are known to lead to chemical reactions that modify the composition of the geothermal fluids rising from depth. Depressurization boiling in wet-steam wells and their producing aquifers may cause extensive deposition of various components, again causing the discharge to differ in composition from the source fluid. The present contribution summarizes sampling procedures for fluids from hot springs and hot-water wells, fumaroles and dry-steam wells, and especially wet-steam wells. It is largely based on practices in Iceland over the last few decades. They are similar to sampling techniques developed in other countries, notably in New Zealand. Several reports are available, which focus on the sampling of geothermal fluids (Klyen 1982; Ólafsson 1988; Giggenbach & Goguel 1989; Fahlquist & Janik 1992), but to our best knowledge only two publications (D Amore et al. 1991; Arnorsson et al. 2000). Giggenbach (1975) developed a small gas sampling bottle, the so-called Giggenbach bottle, partially filled with strong OH-solution for collection of geothermal steam and volcanic gasses that is now being used globally. Downhole sampling of geothermal fluids is not covered in this contribution. The reader is referred to Klyen (1982), Brown & Simmons (200) and Arnorsson & Stefansson (2005a). SELECTION OF COMPONENTS FOR CHEMICAL ANALYSIS The method of sample collection and sample treatment on site to preserve the samples until analysed depends on which elements are to be determined and by which analytical methods. For analyses of all major components, gases and most trace elements in geothermal water and gas samples, relatively few analytical instruments are needed: ICP- AES, ICP-MS and ion (IC) and gas chromatographs (GC), in addition to a ph meter and mass spectrometers for isotope analysis. Total carbonate carbon (TCC) and sulphide sulphur are most often determined by titration methods, although TCC may also be analysed by Reagent-Free TM IC (RFIC TM, Dionex 2000 with IonPac Ò AG-11 column and KOH eluent; see Stefansson et al., 2006), and sulphide sulphur by absorption spectrophotometry. The advantages of using these analytical instruments and methods over many others are that they are precise, relatively rapid and have low inter-elemental interference, which is vital when analysing multicomponent samples. Chemical analysis of elements occurring in different oxidation states is of central importance to understanding many geochemical processes, for example Fe(II) and Fe(III), As(III) and As(V), and sulphur species with oxidation states between sulphide and sulphate. Such analysis for geothermal fluids is very complex. Concentrations and redox conditions may be altered during the ascent of the fluid from the deep reservoir to the surface, during sampling and sample storage and during analysis. Separation and storage on resins followed by various analytical techniques has been used, as have colorimetric and IC methods, often carried out on site (To et al. 1999; Druschel et al. 200; McCleskey et al. 2004). Determinations of isotope ratios rely on the use of various mass spectrometers. Sample preparation for isotope analysis may be complex, but it is relatively simple for chemical components. Some analytical methods provide information on element concentrations. Others measure compound concentrations (e.g. sulphate, ammonia, methane). Still others give individual species activities (e.g. H +,F ) ). Appendix A lists analytical methods together with their current detection limits for a series of components routinely analysed by our group. The sampling procedures described below assume the use of the analytical instruments listed in Appendix A.

3 Geothermal fluid sampling 205 SAMPLING OF HOT SPRINGS AND HOT-WATER WELLS When embarking on sampling of geothermal fluids from a particular field, it is very useful to have information on the distribution of the thermal manifestations in the area as well as on the temperatures and flow rates of the springs. The intensity of surface activity between and within geothermal fields is highly variable. It may not be practical or serve a purpose to sample all springs in a particular field. Specific sites need to be selected for sampling. The criteria used to select hot springs for sampling include temperature, flow rate, geographic distribution, whether the water issues from soil or directly from the bedrock and any hydro-geological observations that might indicate mixing of the geothermal water with surficial water. Generally speaking, the most favourable sites are springs with the highest temperature, the highest flow rate, the smallest aperture and minimum contact with any soil. Sampling and analysis of ground and surface waters is important for evaluation of possible mixing of the geothermal water with such shallow water in upflow zones. The layout of the apparatus needed to collect samples of hot-spring water is shown in Fig. 1. The layout is similar for hot-water wells except that the cooling coil is connected to a tap on the wellhead or a pipe from the well. If the water temperature is below 0 40 C, a cooling coil is not necessary as thermal contraction of water is not significant when cooled from this temperature to room temperature ( %). If the water in the well is under pressure, the peristaltic pump is not needed to push the sample through the filtering apparatus (see Fig. 1). Contact of the water to be sampled with the atmosphere should be minimal, certainly for those components which may undergo changes in their concentration upon exposure to air. This can be done by immersing tubing into the hot springs well below the surface, if the surface area of the hot spring is sufficiently large and deep, and withdrawing water through the tubing with a peristaltic pump and pushing it through the cooling coil and the filtering apparatus (Fig. 1A). If the aperture is small and on a slope, it is convenient to place a funnel just below the aperture and let the water flow by gravity into the tubing to the peristaltic pump. Samples for major cation and trace metal analysis by ICP-AES and ICP-MS need to be filtered and acidified on site, whereas samples for major anions to be determined by IC need only be filtered (Appendix A). Samples for ph and TCC should be collected into gas sampling bulbs or bottles with air-tight caps. If the concentrations of iron and trace elements present in steel are of interest, the cooling coil should be omitted, if possible, and the water samples for these analyses collected without cooling. Sulphide sulphur present in the sample may oxidize upon storage, ultimately adding to the sulphate sulphur initially present. The sulphide sulphur may be removed from the sample on site by various methods. The most favourable involves bubbling N 2 gas through the sample for a few minutes subsequent to its acidification. If sulphide sulphur is removed from the sample immediately after collection, determination of total sulphur by ICP- AES can be taken to represent SO 4, at least if sulphurbearing species in other oxidation states than +6 are not present in significant concentrations. Alternatively, the sulphide sulphur may be precipitated from an unacidified sample by adding 1 ml of 1% (CH COO) 2 Zn solution to a 100 ml sample and filtered from the solution. In samples treated in this way, SO 4 may be determined by ion chromatography (Arnorsson et al. 2000). In the presence of sulphur species other than sulphide and sulphate, the only way to obtain samples for reliable sulphate determination is to separate it from other S-species on site by collection into an anion resin (Druschel et al. 200). A Fig. 1. Equipment for sampling hot spring waters. (1) 1/4 diameter silicone tubing. (2) Peristaltic pump. () Bucket with cold water and cooling coil of stainless steel (N16). (4) Teflon filter holder with 20 cm diameter 0.2 lm filter membrane. (5) Sample bottle. (6) Funnel, about 10 cm in diameter to fit the silicone tubing. B

4 206 S. ARNÓRSSON et al. Samples for determination of stable isotopes of hydrogen (d 2 H), oxygen (d 18 O) and carbon (d 1 C), as well as 14 C, should be cooled, filtered and stored in glass bottles with air-tight caps to avoid evaporation and exchange with the atmosphere. Specifically, for carbon isotope determination, the sample should be dosed with a few drops of 1% HgCl 2 solution to impede changes in the carbon isotope content of the sample by biological activity. Determination of the isotope ratios of heavier elements does not, in general, require other treatment on site than filtering. SAMPLING OF FUMAROLES, DRY-STEAM WELLS AND GASES FROM THERMAL SPRINGS The selection of fumaroles for sampling is not as straightforward as that of hot springs. It is generally best to sample small outlets, which discharge steam at considerable velocity, in areas of the most intense acid surface alteration. Mud pools or pools of steam-heated surface water should be avoided, if possible. The gas phase discharged from such thermal manifestations tends to be low in H 2 S compared with that of adjacent fumaroles because of its oxidation by oxygen dissolved in the surface water through which the steam passes (Arnorsson 1987). Sampling fumarole steam provides information on individual gas concentrations in the steam and is for this reason also preferred to the sampling of gases bubbling through steam-heated or hot-spring water. The latter only provide information on the relative amounts of the gases in the gas phase and not their absolute concentrations in the steam. Warm, moist air emerging from hot ground may look like a fumarole. Temperature measurement will reveal that it is not. For this reason, it is useful to measure the temperature when collecting samples from fumaroles so as to eliminate any uncertainty about the nature of the discharge. Steaming ground may be very conspicuous from a distance. However, closer inspection may reveal that the discharging steam is diffuse and that it is difficult to find any well-defined vents. This is particularly the case when the steam issues in highly permeable formations like young lavas. Experience shows that such discharges may be difficult if not impossible to sample. Slight restriction to flow in the tubing and the valve of the sampling bulb may cause the steam in the fumarole to be diverted to another outlet. The types of fumarole steam samples to be collected depend on which chemical and isotopic constituents are to be analysed. Chemical analyses for applied studies of geothermal systems routinely include CO 2, H 2 S, NH, H 2, CH 4, O 2, N 2 and Ar (e.g. D Amore & Panichi 1980; Arnorsson & Gunnlaugsson 1985). Analysis may also include the noble gases (e.g. Mazor & Truesdell 1984; Nuti 1984; Mazor et al. 1990), CO, Rn and the heavier hydrocarbons (e.g. Semprini & Kruger 1984). Isotopes routinely determined in steam include d 2 H and d 18 O and occasionally d He and d 20 Ne. Sometimes isotope ratios are also determined in individual gas components. They include d 2 HinCH 4 and H 2, and d 1 CinCO 2 and CH 4 (e.g. Hulston 1977; Árnason 1977; Ono et al. 199). Three types of gas samples have been collected from fumaroles and hot springs (Fig. 2). One is a sample of the total discharge of a fumarole (Fig. 2A). The second type is the gas fraction in the fumarole discharge (Fig. 2B) and the third type consists of gas bubbles emerging from hot springs (Fig. 2C). No analysis or sample treatment is required on site for these steam and gas samples. Before collecting a sample, it is necessary to allow steam to pass through the tubing and the sample ports of the gas sampling bulb, and through the wash bottle when used, to displace any air in the system. The sampling methods depicted in Fig. 2A,B assume the sampling bulb to have been evacuated in the laboratory before going into the field. For sampling of gas from hot springs, the gas sampling bulb, which must have a stopcock at both ends, should not be evacuated. After immersing the funnel upside-down into the spring water over the rising gas bubbles, the peristaltic pump is used to suck up water to displace any air in the whole system, including the sampling bulb. Gas is then allowed to accumulate under the funnel, and is subsequently pumped into the sampling bulb. Sampling by the methods shown in Fig. 2B,C provides information on relative gas concentrations only. Furthermore, these relative concentrations are skewed because some of the CO 2 and H 2 S in the steam will dissolve in the condensate. Sampling by the method shown in Fig. 2A provides information on absolute gas concentrations in the steam on the other hand. To maximize information, it is advisable to collect the total discharge of fumaroles rather than the gas phase only, i.e. to use the method shown in Fig. 2A. For gases which are highly water soluble, like NH, it is necessary to collect separately a small sample of condensate by passing the steam from the fumarole through the cooling coil. The condensate is invariably acid, and will thus dissolve the NH quantitatively. Samples for the determination of the stable isotopes of water in the steam (d 2 H and d 18 O) should be collected as just described for NH. The condensate samples should be collected into air-tight containers. For analysis of the noble gases and determination of isotopes in gases, it is considered best to apply sampling method 2A. Some studies focus on analysing dissolved gases in water, such as the noble gases, including Rn. Because of its short half-life (.8 days), Rn must be analysed soon after collection. Sampling methods for such studies are those used to sample hot springs and hot-water wells. Samples for He analysis must be collected into vessels which are not per-

5 Geothermal fluid sampling A 2 4 Fig. 2. Equipment for sampling fumarole steam (A and B) and gases bubbling through hot spring waters (C). (1) Steam outlet, (2) Funnel, () 1/ 4 diameter thick-walled silicone tubing. Although quite permeable to gases, silicone tubing is preferred to other types of tubing because it is flexible and therefore easy to handle over a wide range of temperature (0 100 C). If desired, one may use tygon tubing which has very low permeability to most gases. (4) Bucket with cold water. (5) Gas sampling bulb, 00 ml, with teflon stopcock and rubber o-ring seal. A piece of tubing, about 5 cm in length, is connected to the distal sample port and fitted with a clamp. After flushing the sample ports with steam for 1 2 min to displace any air, the short tubing is closed with the clamp and the evacuated bottle opened to collect a sample. (6) Bucket with cold water and cooling coil of stainless steel (N16). (7) Washing bottle (approximately 00 ml) for collection of condensate. (8) Peristaltic pump. (9) Sampling bulb with stopcocks at both ends, 00 ml. (10) Small bucket with water to prevent air from entering the sampling bulb through the lower stopcock. B C meable to this gas, such as copper tubing or special Al-silicate glass. The gas phase in geothermal steam is most often largely CO 2 and H 2 S (> 90% by volume). In order to obtain better analytical results for the less abundant major gases (H 2, CH 4,N 2,O 2, Ar), the most common practice is to collect samples into evacuated bulbs containing a solution of either NaOH (50 ml, 4 M) or KOH (10 ml, 50% w/v). The CO 2 and H 2 S present in the steam dissolve quantitatively in the alkaline solution, thus concentrating the remaining gases in the head space above the solution and allowing their more precise determination by gas chromatography. A freshly prepared hydroxide solution should be pipetted into the bulb, which is subsequently evacuated with a vacuum pump. When the alkaline solution boils, the vapour so formed will aid sweeping out any air remaining in the bulb, thus reducing contamination of the sample by air to practically nihil. The CO 2 and H 2 S in the steam condensate are usually determined by titration (see Appendix B) and, of course, corrected for the effect of dilution by the caustic solution. Analytical reagent grade NaOH and KOH always contain some carbonate. To correct for this, the concentration of carbonate in a blank sample must be determined. If it is of interest to determine CO, collection of samples using a hydroxide solution should be avoided because the CO tends to react with H 2 O in alkaline medium leading to low analytical values for CO. If it is considered desirable to collect samples for CO measurement into a bulb with an alkaline solution, the gas phase which contains the CO should be separated from the alkaline solution immediately after sampling by transferring it to another evacuated sampling bulb. When sampling fumarole steam the main concern is usually to avoid air contamination of the sample. Generally, the presence of oxygen in geothermal steam is taken as evidence for atmospheric contamination during sampling. However, oxygen in fumarole steam may have originated from degassing of near-surface steam-heated water and could truly be present in the steam. In the discharges of dry and wet-steam wells, on the other hand, the presence of oxygen in gas samples can safely be considered to be a measure of atmospheric contamination during sampling. Atmospheric contamination reduces the value of data on N 2, Ar and other atmospheric gases. It also tends to yield

6 208 S. ARNÓRSSON et al. low values for H 2 S, which is easily oxidized by atmospheric oxygen in alkaline solution. The best samples are generally obtained from fumaroles discharging from ground highly altered by acid surface leaching. The funnel should be placed upside down over the steam vent and covered with compacted clay using a shovel. The procedure for sampling dry-steam wells is the same as that for fumaroles according to method 2A, except that the tubing is connected to the wellhead or a pipeline. For safety reasons, it is advisable to have a one-way atmospheric valve between the pipe and the sampling bulb, as described in the section below on wet-steam wells. The discharge of dry-steam wells may contain some water droplets and solid material. To obtain representative samples from such discharges, an isokinetic sampling nozzle may be required. SAMPLING OF WET-STEAM WELLS Wet-steam wells withdraw fluid from > 100 C liquid-dominated geothermal reservoirs. They discharge a mixture of water and steam. The steam forms essentially by depressurization boiling, but a minor fraction of it may be initially present in the reservoir (two-phase reservoir) or form by conductive heat transfer from the aquifer rock to the fluid flowing into the well in the depressurization zone around it (see e.g. Horne et al. 2000; Pruess 2002; Li & Horne 2004). To obtain representative samples of the well discharge, it is necessary to collect the liquid water and steam phases separately. This may be done with a wellhead steam separator (Fig. ), which separates the total well discharge, or by a small Webre separator which is connected to the two-phase pipe that conveys the discharged fluid from the wellhead (Fig. 4). It is not possible to collect a representative sample of the total discharge without phase separation by connecting a cooling coil directly to a two-phase flow pipeline. The ratio of water to steam entering the cooling coil cannot be assumed to be the same as that in the total discharge. To calculate the composition of the total discharge from analysis of water and steam samples, it is necessary to have data on the separation pressure and the discharge enthalpy (see e.g. Arnorsson et al. 2000). Figures and 4 provide information on sampling using the two types of separators. Details of the design of the Webre separator are shown in Fig. 5. The most common practice is to collect steam samples at elevated pressure and water samples from the weirbox at atmospheric pressure (see Fig. and Arnorsson & Stefansson 2005a,b). By this method, some of the steam in the discharge is not collected or analysed, i.e. the fraction which forms by depressurization boiling from the steam sampling pressure to atmospheric pressure. This steam is secondary and can therefore be expected to contain little gas. Indeed, the approximation invariably made for calculation of total well discharge composition or aquifer fluid composition, when water and steam samples are collected at different pressures, is to take the gas content of the steam fraction not sampled to be zero. This approximation is unnecessary when water and steam samples are collected at the same pressure. Therefore, such sampling procedure is to be preferred. If a steam separator has been installed at each wellhead, the Webre separator is not necessary, and a cooling coil can be fitted directly on the respective ball valves (Fig. ). The location of the socket on the steam pipe of wellhead separators for collection of steam samples is not critical. A B Fig.. Wellhead steam separator. (1) Wellhead. (2) Pressure gauge. () Wellhead steam separator. (4) Teflon filter holder with 20 cm diameter 0.2 lm filter membrane. (5) Sample bottle. (6) Bucket with cold water and cooling coil of stainless steel (N16). (7) 1/4 diameter thick-walled silicone tubing. (8) Sampling bulb with stopcocks at both ends, 00 ml. (9) Steel-clad teflon tubing. (10) One-way atmospheric valve. (11) Bucket with cold water. (12) Gas sampling bulb, 00 ml. (1) Copper tubing (approximately 0 cm long), with special clamps at both ends, for the sampling of noble gases and individual gas components for isotopic measurements. (14) Small bucket with water to prevent air from entering the copper tubing. (15) Sample bottle for condensate for the determination of NH, d 2 H, d 18 O, etc.

7 Geothermal fluid sampling 209 A Fig. 4. Equipment for the sampling of water and steam (gases) from a wet-steam well discharge using a Webre steam separator. (1) Wellhead. (2) Pressure gauge. () Webre steam separator. (4) Steam outlet valve (see Fig. 5 for details). (5) Water outlet valve. (6) Steel-clad teflon tubing. (7) Bucket with cold water and cooling coil (approximately 6 m long) of stainless steel (N16). (8) 1/4 diameter thick-walled silicone tubing which may be either connected to the filter holder or to a gas sampling bulb with stopcocks at both ends. (9) Teflon filter holder with 20 cm diameter 0.2 lm filter membrane. (10) Sample bottle. (11) Gas sampling bulb with stopcocks at both ends, 00 ml, for determination of ph and TCC. (12) One-way atmospheric valve. (1) Bucket with cold water. (14) Gas sampling bulb, 00 ml. (15) Copper tubing (approximately 0 cm long), with special clamps at both ends, for the sampling of noble gases and individual gas components for isotopic measurements. (16) Small bucket with water to prevent air from entering the copper tubing. (17) Sample bottle for condensate for the determination of NH, d 2 H, d 18 O, etc. B C The socket on which the ball valve is fitted for collection of water samples should be low on the separator body but not on its bottom. Debris may collect on the separator bottom and may clog the socket and the adjacent valve. If a coil is not used for steam samples (see Fig. ), the 1/4 diameter silicone tubing with the one-way atmospheric valve is connected directly to the 1/2 ball valve on the steam line. When collecting samples using the Webre separator, it is important that steam samples are not contaminated with liquid water and, in particular, that water samples are not contaminated with steam. It is easy to verify the presence of liquid water in a steam sample by analysing it for Na or K. Sometimes Cl is determined, but this may not be reliable as Cl may be present in the steam as HCl, particularly at high sampling pressures and when the aqueous phase is saline. If steam comes with the water, so does a gas phase, and this can easily be verified by placing the end of the tubing conveying the cooled water from the separator into water in a beaker to see if gas bubbles emerge. If they do, steam is coming with the water. If they do not, the water sample is as it should be. The surest way to collect a steam-free liquid water phase is by adjusting the Webre separator in such a way that the water valve is relatively closed and the steam valve relatively open. This adjustment keeps the separator almost full of water, and the steam discharge very wet (Fig. 4A). Proper adjustment of the separator valves is easily verified by making certain that the steam is wet. This can be done by passing a tool or any other handy object through the plume; it will get wet if the steam is wet and stay dry if the steam is dry. When collecting steam samples, the adjustment of the valves should be reversed; the steam valve should be slightly open and the water valve relatively well open. Now the separator is largely filled with steam, and a mixture of water and steam is discharged from the water valve. The

8 210 S. ARNÓRSSON et al. Ball valve for pressure gauge PN 400, DN 15 Thermometer well Needle valve PN 400, DN 15 Water Steam Water Needle valve PN 400, DN , Flow from well Ball valve PN 40, DN 15 Steam Flow from well Fig. 5. Design of the Webre separator. All material is stainless steel (N16). Pipes are 1/2 in diameter except for the inlet valve (flow from well) which is 1 in diameter and the pipe for the thermometer, which is 1/ 4 in diameter. The numbers given for the height and diameter of the separator are in mm. steam flowing from the steam valve, if dry, is not visible until some 1 2 cm from the outlet, at which point some of it has condensed. A plume of dry steam is conical in shape, in contrast to a plume of liquid, which is often tulip-shaped. If the steam is truly dry, one can pass a hand quickly through the plume without pain or harm. These observations will confirm that the steam is indeed dry. The pressure drop from the pipeline to the separator should be minimal so that secondary flashing may be avoided. This is best achieved by having the inlet pipe and valve between the separator and the two-phase pipeline relatively large (1 inch) compared with the outlet valves on the separator (1/2 inch). Steam condensation in the Webre separator should be minimized by insulating it well against heat loss with rock or glass wool. The flow patterns of water steam mixtures in pipes depend on the relative volumes of the phases and their velocities and whether the pipe is vertical or horizontal. When the steam dominates the volume, the liquid water tends to exist as droplets in the steam phase. This pattern of flow is the most favourable for phase separation and sample collection. When the volume fraction of liquid water increases, the 410 flow may become annular. If the flow is slow enough in a horizontal pipe, the water may flow at the bottom of the pipe and the steam at the top because of their gravitational separation. Alternatively, the water may come in slugs. These flow patterns, in particular slug flow, make separation more difficult as they lead to variable flow of water and steam into the Webre separator, thus upsetting the adjustment of the valves on the separator during sampling. It may not be possible to collect steam-free water samples with a Webre separator from wells with a high discharge enthalpy (> 2500 kj kg )1 ), i.e. enthalpy approaching that of dry steam ( kj kg )1 ). If the Webre separator cannot be adjusted to discharge water only through the water valve, it is best to collect the water sample from the weirbox, if available. Yet, there is always a disadvantage in collecting water samples from the weirbox of wells with low water flow rates, because the water may evaporate substantially as it flows through the weirbox. This will change the water composition, especially its hydrogen and oxygen isotope content, the ph and dissolved components which form gaseous species, such as CO 2 and H 2 S. A cooling coil is needed to bring the temperature of the separated water well below 100 C in order to prevent its boiling (Figs and 4). The flow of water from the separator should be adjusted so that the water cools to at least approximately 80 C for collection of samples for IC and ICP analysis. Samples for determination of ph, TCC, sulphide sulphur and the stable isotopes of water should be cooled down to 0 40 C in order to prevent evaporation and thermal contraction of the sample in the air-tight container. Steam for collection may also be condensed with the aid of a cooling coil (see Figs and 4C). Alternatively, a steam sample may be collected into an evacuated gas sampling bottle by immersing this bottle into cold water, as described above for sampling of fumarole steam (Figs 2A and 4A). The latter method of steam sampling requires an atmospheric nonreturn valve on the tubing between the cooling coil and the gas sampling bulb, or between the separator and the gas sampling bulb, to ensure that pressure does not build up in the bulb that may cause the tubing to come off or the bulb to explode. The common practice is to have the cooling coil and the Webre separator (see Fig. 5) made of stainless steel (N16). Yet, experience shows that water, and especially samples of steam condensate, may be contaminated with the trace elements in steel (Co, Cr, Mn, Ni, V) as well as Fe. If data on these elements are sought, it may be better to collect water samples from the weirbox, if possible, or to use a steam separator of another material. It is anticipated that leaching of the casing and the wellhead equipment material is generally not important because observations show that sulphide phases deposit easily from geothermal waters as they boil and cool. The sulphide scale forms

9 Geothermal fluid sampling 211 a protective coating that prevents contact between the fluid and the steel in the casing. In fact, one may expect that the concentrations of many sulphide-forming metals in geothermal waters are lower at the wellhead than in the source aquifer water because of their precipitation from solution as the water cools by depressurization boiling. Sample treatment, such as filtering and acidification and analysis on site, is the same as that already described previously for water and steam samples collected from springs and hot-water wells. CHEMICAL ANALYSIS Samples of geothermal waters can be preserved by appropriate treatment upon collection in such a way that later analysis in the laboratory provide reliable information on all component concentrations in the sample when collected. The necessary sample handling for sample preservation is summarized in Appendix A. Several components can be conveniently analysed on site, including TCC, sulphide sulphur (Appendix B) and ph. At times, analysis on site may be necessary or preferred because of difficulties in preserving the samples with respect to these components. For example, water containing dissolved oxygen and some dissolved ferrous iron cannot be preserved for adequate ph measurement. Oxidation of the divalent iron and its subsequent precipitation as ferric hydroxide will lead to a decrease in the water ph. If samples are collected into airtight containers, however, ph and TCC are best determined in the laboratory where the temperature is stable. H 2 S is most conveniently determined on site. Some parameters must always be measured on site, including temperature, flow rate and redox potential. The measurement of redox potential with a platinum electrode is, however, meaningless for dilute waters, as the Pt-PtO half-cell potential of the electrode itself contributes very significantly to the potential reading (Stefansson & Arnorsson 2005). For interpretation of the fluid chemistry, it is important to describe the sampling site with respect to features such as the nature of the emergence point and whether the water issues from the bedrock or through an organic soil cover. It is also useful to relate the sampling site to tectonic or other geological structures, rock type and topography. Contact of geothermal water with organic soil tends to increase TCC, lower ph and, as a consequence, to increase the aqueous concentrations of Ca, Fe and Mg by enhancing primary mineral dissolution. Discharge temperature measurement is essential for the interpretation of chemical data from hot water wells. As already mentioned, information on sampling pressure and discharge enthalpy is necessary for the interpretation of chemical data of discharges from wet-steam wells. Below, we provide some detailed information on on-site analysis of ph and TCC. Appendix B provides a detailed procedure for analysis of TCC and sulphide sulphur. Measurement of ph Many minerals have a ph-dependent solubility, and the relative abundance of many aqueous species is sensitive to ph, such as that of hydrolysed cations and many other weak acids and bases. For this reason, accurate measurement of sample ph is critical for reliable data interpretation. The ph measurement gives the activity of H + in solution. This activity varies with the temperature of the solution because the dissociation constants for acids and bases present in the sample vary with temperature. It is therefore important to report the temperature at which the ph is measured and to calibrate the ph-meter with standard buffer solutions at the temperature at which the ph of the samples is measured. When samples of geothermal water are exposed to the atmosphere, they may lose or gain CO 2 depending on the partial pressure of this gas in the sample relative to that of the atmosphere. H 2 S may be lost, either through degassing or by its oxidation. These gas reactions change the sample ph as both dissolved CO 2 and H 2 S are weak acids. To avoid such changes, contact of the water with the atmosphere during sampling should be avoided as much as possible. This may be achieved by the sampling methods described above. Collection of samples into air-tight containers necessitates that they be cooled to 0 40 C before collection into the sample container because thermal contraction by cooling leads to their degassing. It is convenient to use gas sampling bulbs with stopcocks at both ends for this purpose, or else a glass bottle with an air-tight cap. Waters that contain dissolved oxygen may precipitate ferric hydroxide upon storage because of oxidation of Fe 2+ to Fe +. In this case, the solution ph will decrease because of removal of OH ) from solution. The only way to obtain a satisfactory value for the ph of such waters is to measure it on site. For data interpretation, the ph measured in the laboratory, or on site at a particular temperature, needs to be corrected to the in situ temperature of the thermal water or to another selected temperature, at which individual aqueous species activities are calculated. The change in ph with temperature of a particular water depends on the ph-buffering action of all weak acids and bases in solution. Sometimes this change may be limited, but at other times it is very large. For example, if a water has a ph of 9.70 at 25 C and its ph is only buffered by aqueous silica, its value at 90 C is We have: H 4 SiO 0 4 ¼ Hþ þ H SiO 4 ð1þ and

10 212 S. ARNÓRSSON et al. K d ¼ ½Hþ Š½H SiO 4 Š ½H 4 SiO 0 4 Š ð2þ The concentration of H SiO 4 is constant, except for a small (insignificant) change corresponding to the change in the H + concentration, so K d /[H + ] constant. Thus, the change in ph with temperature is about the same as the change in K d with temperature. The dissociation constant for silicic acid (H 4 SiO 0 4 ) at 25 and 90 C, respectively, is 10 )9.91 and 10 )9.04 (Arnorsson et al. 1982) so it increases by between these temperatures. Accordingly, the ph of this aqueous silica-buffered system must decrease by the same amount as K d, or from 9.70 at 25 C to 8.8 at 90 C. Another water with a ph of 6.50 at 25 C, and buffered by carbonic acid (dissolved CO 2 ) only, would have a ph of 6.52 at 90 C. In this latter example, the change in ph with temperature is insignificant because the dissociation constant for carbonic acid is almost the same at 25 and 90 C. To calculate the ph of a geothermal water of a particular composition at a specified temperature requires the use of a chemical speciation program and must take into account all bases in solution that can consume protons. The calculation thus assumes conservation of total alkalinity, i.e. X i A i ¼ k ðþ i where k is a constant. A i denotes the concentration of an aqueous species i that can consume protons and m i the number of protons consumed by this species. The value for P im i A i can be obtained by calculating the species distribution at the temperature at which the ph is measured. Calculation of ph at another temperature, e.g. the measured in situ temperature of a geothermal water, can be obtained by an iterative process until a ph is obtained that satisfies equation () and all associational equilibria for proton consuming species reactions. Species contributing to alkalinity include the conjugate ions of weak acids and hydrolysed cations, such as H SiO 4, HCO,CO 2,HS), H 2 BO and Al(OH) 4 to mention a few. Accurate calculation of ph at a temperature other than that at which it is measured, requires an accurate ph measurement, accurate determination of all aqueous species which may contribute to the alkalinity and their association constants as a function of temperature. Computer codes are available for calculating ph of solutions at elevated temperature from measured ph at around room temperature (e.g. Arnorsson et al. 1982; Reed & Spycher 1984). Samples of high-temperature geothermal water often become over-saturated with respect to amorphous silica upon cooling. Silica in excess of amorphous silica solubility tends to polymerize, the rate of polymerization depending on various parameters, i.e. the degree of oversaturation, temperature, ph and salinity (Gunnarsson & Arnorsson 2005). Polymerization causes the water-ph to change because it causes removal of monomeric silica from solution, which acts as a weak acid and the formation of polymers which are stronger acids than monomeric silica (Iler 1979; Tossell & Sahai 2000). The acid dissociation constants of silica polymers are not accurately known and their concentrations cannot be determined. Therefore, the only way to obtain a reliable measurement of ph of samples which contain monomeric silica in excess of amorphous silica solubility is to measure it on site before onset of the polymerization reaction. Determination of total carbonate carbon Primary and secondary alkalinity is often taken to represent bicarbonate and carbonate concentrations in natural waters. This is indeed a good approximation for many surface and ground waters as the bases reacting with the acid used for the alkalinity measurement are for the most part bicarbonate and carbonate ions. This is, however, not the case for geothermal waters and some other geofluids, such as oil field brines. Bases other than bicarbonate and carbonate may dominate the alkalinity. For geothermal fluids, such bases include bisulphide, ionized silica, borate, and even some hydrolysed cations, but organic acid anions in the case of oil field brines (e.g. Arnorsson et al. 2000; Palandri & Reed 2000). The TCC in such waters may be determined directly, either by using a new version of ion chromatographs that use KOH solution as eluent, or by alkalinity titration. In the latter case, titration by standard HCl solution is followed by removal of the CO 2 from the sample by bubbling N 2 gas through the acidified sample and subsequent back titration with standard NaOH solution. The difference of the two titres gives the sum of TCC and sulphide sulphur. The sulphide may be determined separately and the TCC obtained by difference. 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