PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 24-26, 2014 SGP-TR-202 1 Barite Scale Control at the Soultz-sous-Forêts (France) EGS Site Julia Scheiber 1 , Andrea Seibt 2 , Johannes Birner 3 , Nicolas Cuenot 1 , Albert Genter 1 and Wilfried Moeckes 1 1 GEIE, “Exploitation Minière de la Chaleur”, Route de Soultz, 67250 Kutzenhausen, France [email protected], [email protected], [email protected], [email protected]2 BWG Geochemische Beratung GmbH, Seestraße 7A, 17033 Neubrandenburg, Germany [email protected]3 GTN Geothermie Neubrandenburg GmbH, Seestraße 7A, 17033 Neubrandenburg, Germany [email protected]Keywords: barite, scaling, NORM, radiation protection, scale inhibitor, EGS ABSTRACT Scale formation in the surface and subsurface installations of the Soultz geothermal power plant affect negatively operational performance of the geothermal cycle. The Soultz EGS site operates a naturally fractured granitic reservoir percolated by Na-Ca-Cl brine with Total Dissolved Solids (TDS) up to 100 g/l. Geothermal brine is produced at 160°C/20 bars and re-injected at 70°C/18 bars. For power production, an Organic Rankine Cycle (ORC) is installed which includes three heat exchanger units, one evaporator and two preheaters. The heat transfer in this binary cycle is limited continuously by formation of scales in the tubular heat exchangers. Due to the temperature decrease of 90 K in the ORC heat exchanger system, strontium rich barite (Ba 0.6 Sr 0.4 SO 4 ) becomes oversaturated and forms a homogenous scaling layer. Additionally, minor amounts of sulfide minerals like galena (PbS) are present. Those scales form an insulation layer inside of the heat exchanger tubes and decreases thereby efficiency of the heat transfer between geothermal and organic fluid. In consequence, cleaning operations for scale removal are required in regular time intervals to keep up efficiency of energy production. During cleaning and disposal operations, strict regulations for safety at work have to be followed due to radiation-protection regulations. The scales have to be classified as NORM (Naturally Occurring Radioactive Material) related to the presence of 226 Ra and 210 Pb, incorporated by chemical substitution in the scales. Moreover, the inner diameter of the reinjection wells decrease slowly but continuously by deposition of scales. Recently, well loggings proofed the existence of a progressing precipitation front inside of the injection wells versus depth which can reach the open-hole section as a function of injected brine volume and brine temperature. For reasons of safety at work and long-term power plant operation, the formation of barite needs to be inhibited continuously. Therefore, several inhibitors, based on phosphonic-acid, were tested in laboratory experiments. These studies included tests for calcium tolerance, effectiveness and dose rate adjustment by tube blocking tests. Out of these products, Diethylenetriaminepentakis (methylenephosphonic acid) (DTPMP), showed the best results for barite inhibition. Therefore it was selected for additional tests which investigated long-term efficiency, thermal stability and reservoir rock–inhibitor interactions. The inhibitor suppressed successfully barite formation for 24 h at 90°C in contact with reservoir rock. Geothermal fluids trapped within deep-seated fractured crystalline rocks from the Upper Rhine Valley must be carefully investigated in order to minimize scaling (sulfate, sulfide) in geothermal power plants and seriously limit the natural radioactivity associated with. At the Soultz site an inhibitor injection system was designed, installed and tested for continuously inhibitor injection and on-site efficiency tests showed very good results during short time tests. 1. INTRODUCTION The geothermal power plant of Soultz-sous-Forêts is located in the NE of France, 50 km NE of Strasbourg, at the western rim of the Upper Rhine Graben (URG). The Graben structure was formed by a Tertiary rift system accompanied with the formation of a thermal anomaly of increased heat flow. Within the geothermal anomaly of the URG are a few areas located where the temperature gradient is even higher in comparison to surrounding areas. Temperature measurements in petroleum wells at the Pechelbronn oil field indicated one of these very local temperature anomalies to be connected to the Soultz horst, first mentioned by Haas and Hoffmann, (1929). 1400 m of sediments, clays, sands, marls, lacrustine limestone and sandstone, cover the crystalline, granitic basement at the Soultz location, Schnaebele et al. (1948). The Paleozoic granitic basement includes two different types of granite. A grey porphyritic monzo-granite to about 4500 m depth and a fine grained two-mica granite deeper. At levels above 4500 m, the fine grained two mica granite intersects part-wise the monzo-granite as dike structures. Top of the crystalline basement is highly hydrothermally altered and fractured, Genter (1989). Fractures occur either as individual fractures or in clustered zones up to 10-20 m thickness, Genter et al. (2010). Primary minerals like biotite and plagioclase show intense alteration or replacement by secondary minerals like illite, chlorite, corrensite, tosudite, geodic quartz, calcite, dolomite and sulfides as well as locally kaolinite in the well GPK-1, Genter and Traineau (1992). The Soultz-sous-Forêts project started in 1987, with the aim to develop heat exploitation of deep reservoirs in hot dry rocks (HDR) by creating an artificial deep heat exchanger in a closed environment, Gérard and Kappelmeyer (1987). Therefore, one exploration well, EPS-1, and four deep wells, GPK-1 to GPK-4, were drilled between 1987 and 2005 down to the crystalline basement of the Rhine Graben. During drilling and well testing it became obvious that native brine was circulating through the fracture network of
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PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, February 24-26, 2014
SGP-TR-202
1
Barite Scale Control at the Soultz-sous-Forêts (France) EGS Site
Julia Scheiber1, Andrea Seibt
2, Johannes Birner
3, Nicolas Cuenot
1, Albert Genter
1 and Wilfried Moeckes
1
1GEIE, “Exploitation Minière de la Chaleur”, Route de Soultz, 67250 Kutzenhausen, France
Scheiber, Seibt, Birner, Cuenot, Genter and Moeckes
2
the naturally fractured granite, Genter and Traineau (1992), Genter et al. (2010). The original HDR design was found to be
obsolete. Nevertheless, permeability of the reservoir decreased with increasing depth and the initially low permeability was
improved by hydraulic and chemical stimulations, creating an Enhanced Geothermal System (EGS), Gérard et al. (2006). The
power plant, equipped with an Organic Rankine Cycle (ORC) unit, was designed and installed between 2007 and 2009 with an
estimated capacity of 2.2 MWe. During power plant operation scale formation in the surface installations was observed and was
recognized as serious issue for power plant operation and safety at work.
2. SCALE FORMATION AT THE SOULTZ EGS SITE
The formation of scales in the surface and subsurface installations of the Soultz geothermal power plant, is connected with
circulation and cooling of geothermal brine of high salinity, 97 g/l Total Dissolved Solids, Sanjuan et al. (2010), Seibt (2011). Scale
deposition is strongly connected to the cold part of the power plant due to the significant temperature decrease of 90 K from 160˚C
to 70˚C in the ORC tube heat exchanger. Temperature decrease affects the saturation state of dissolved minerals in the brine and
strontium rich barite (Ba0.6Sr0.4SO4) becomes oversaturated and forms thin and brittle deposits, Figure 1. Additionally, minor
fractions of galena (PbS) and trace amounts of mixed sulfides ((Fe,Sb,As)Sx) are formed, Sanjuan et al. (2011), Scheiber et al.
(2012) and Nitschke (2012). Fine sulfide crystals cover the dominant barite layer, Figure 2. Sulfate and sulfide scales are deposited
in the heat exchanger system, inside of equipment downstream to the injection well and even inside of the injection wells.
Figure 1: Scale deposits in the ORC main heat exchanger cap. Thin and brittle barite scales were flushed out of the heat
exchanger tubes into the cap during operation. (Picture: GEIE, 2011)
Figure 2: Electron microscope exposure of a scale cross section in the back scattered mode (BSED). A thin layer of sulfides,
approximately 5-10 μm, covers the surface of the dominating barite layer, which has an average thickness of
200 μm. The deposit was sampled from the main ORC heat exchanger. Picture from Scheiber et al. (2012).
Barite and galena scales at Soultz have to be classified as NORM (Naturally Occurring Radioactive Material) due to the presence of 226Ra and 210Pb, Cuenot (2013). Those radionuclides accumulate in the scales by chemical substitution of 226Ra in barite/celestine
and 210Pb in galena during mineral formation, e.g. Doerner and Hoskins (1925), Ceccarello et al. (2004), Curti et al. (2010),
Kudryavskii and Rakhimova (2007) and Zielinski et al. (2001). The geothermal brine takes up radionuclides by rock-fluid
interaction in the reservoir by natural fluid circulation in fractured granite which is a naturally radioactive rock, Degering and
Köhler (2009), Degering and Köhler (2011), Eggeling (2010) and Eggeling et al. (2013).
Three serious issues are connected with the precipitation of industrial scales, classified as NORM, in the surface installations of the
Soultz power plant: First, scales act as an insulation material in the tube heat exchanger which leads to a significant efficiency
decrease in the heat transfer. In consequence, extensive time and cost consuming cleaning procedures had to be applied. Second,
specific protection regulation for workers and for scale disposal needed to be followed due to the presence of radionuclides in the
scales. Those scales are considered as radioactive waste deposits and had to be evacuated by ANDRA (French National Agency for
Radioactive Waste Management). Third, the inner diameter of the injection wells decrease slowly but continuously due to
Scheiber, Seibt, Birner, Cuenot, Genter and Moeckes
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deposition of scales, Figure 3. Recently, well loggings proofed the existence of a progressing precipitation front inside of the
injection wells versus depth which can possibly reach the open-hole section as a function of injected brine volume and brine
temperature.
Figure 3: The inner casing of the injection well GPK-1 is completely covered by barite scale in 80 m depth. Picture size:
5 cm x 4 cm, with courtesy of the Leibniz Institute of Applied Geophysics (LIAG) from Scheiber et al. (2012).
To avoid scale formation and thereby radioactive contamination of deposits, several measures were developed in the past in the oil
and gas industry. Most common method is the addition of scale inhibitors based on phosphonates, polyphosphates and
polycarboxylates (He et al., 1994).
At Soultz, a scale inhibitor, based on phosphonic-acid, was selected and tested in laboratory experiments for its compatibility with
the Soultz brine as well as the reservoir rocks. At the power plant, an injection system was installed and tested for efficiency and
dose rate adjustment. Scale inhibitor injection is expected to decrease the total amount of scales at the cold part of the power plant
significantly. In consequence, time frequency of required cleaning operations will be elongated. Moreover, the progressing
precipitation towards the reservoir will be significantly retarded.
3. INHIBITOR SELECTION: LABORATORY STUDIES
Strontium rich barite is the dominating precipitate in the Soultz scales. For inhibition of this type of sulfate mineral, chemical
products based on phosphonic acid were chosen to be tested for their effectiveness of scale avoidance at the Soultz geothermal site.
The study was also presented in Scheiber et al. (2013).
3.1. Efficiency of phosphonate based inhibitors
Phosphonates are well known scale inhibitors which are widely used in oilfield applications (Black et al., 1991 and He et al., 1994).
They act in two different ways: On one hand they complex specific cations by forming water soluble complexes. On the other hand,
they inhibit the growth of crystals by adsorption on growth-active surfaces which leads thereby effectively to retardation or
blocking of the growth rate.
The degree of efficiency of an inhibitor is influenced by several physical-chemical parameters: pH, coordination number of the
complex, thermal stability, adsorption affinity, and the presence and concentration of co-ions like Ca2+, Mg2+, Sr2+ or Pb2+ in
solution, the degree of oversaturation of barite as well as of the degree of oversaturation of the inhibitor and the thermal stability of
the phosphonate.
With decreasing pH, the inhibiting efficiency of phosphonate on sulfate mineral growth, like barite or gypsum, also decreases (Van
Rosmalen et al., 1980, He et al., 1994 and Rosenberg et al., 2012). Phosphonate is very effective at very low concentrations. This
type of threshold inhibition is related to phosphonate adsorption or their dissociated compounds on growth-active surfaces (Weijnen
and Van Rosmalen, 1986, He et al., 1996 and Pina et al., 2004). As a consequence of these adsorption processes, the crystal
morphology changes from typically idiomorphic crystals to smoothed and rounded surfaces (Black et al., 1991 and Pina et al.,
2004). A high concentration of co-ions can trigger the precipitation of metal-phosphonate (Nowak, 2003) and therefore
inhibitor/brine compatibility tests are necessary. With increasing temperature a higher amount of inhibitor is required (He et al.,
1994) and also thermal induced decomposition of phosphonate needs to be considered. Thermal stability depends on the specific
properties of the product in use.
3.2 Scale inhibitor selection for the Soultz site
Four products were tested for their calcium tolerance and for their effectiveness of blocking barite formation in the Soultz brine.
The products are described in the following as Inhibitor Green, Inhibitor Orange, Inhibitor Blue and Inhibitor Red.
3.2.1 Calcium tolerance
The produced fluid in Soultz is a Na-Ca-Cl based brine with a calcium concentration of 6.5 g/l. All four products were tested for the
formation of Ca-phosphates after inhibitor addition. Therefore, the four inhibitors were added in different concentrations to a Ba2+
and Sr2+ free artificial brine (#1). Composition of the artificial brine (#1), the inhibitor concentration and the results are listed in
Table 1. All products showed a good calcium tolerance and all brine/inhibitor mixtures kept a transparent state.
Scheiber, Seibt, Birner, Cuenot, Genter and Moeckes
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Table 1: Test of the calcium tolerance with artificial brine (#1) Composition of artificial brine (#1) and results of the calcium
tolerance test with various inhibitor concentrations.
Artificial Brine (#1)
Na in mg/l Cl in mg/l Ca in mg/l
25900 51520 6530
Results
Product Concentration Ca-Tolerance
Inhibitor Green 0.5, 3 and 5% good
Inhibitor Orange 0.5, 3 and 5% good
Inhibitor Blue 0.5, 3 and 5% good
Inhibitor Red 0.5, 3 and 5% good
3.2.2 Blocking of barite formation in artificial brines
For efficiency tests, the four products were mixed with artificial brine (#2). Within this brine, the Ba2+ concentration was six times
higher than in the original Soultz brine. Composition of artificial brine (#2) is listed in Table 2.
Table 2: Composition of artificial brine (#2).
Artificial Brine (#2)
Electrolytes Concentration in mg/l
Na 25430
Ca 6340
Ba 60 (overdose)
Sr 400
Cl 50770
SO4 1010 (overdose)
Active content of 5 and 15 mg/l respectively, of Inhibitor Green, Orange, Blue and Red were tested for the inhibition of barite
formation. The mixtures were shook continuously for 48 h at 60˚C. Barite formation was detected using turbidity measurements
and additionally the Ba2+ concentration was measured after 6 and 48 h. The effectiveness of Inhibitor Green (green line), Inhibitor
Blue (blue line) and Inhibitor Red (red line) are compared to the barite formation in brine without the addition of any inhibitor
(black line), Figure 5.
Figure 5: Ba2+ concentration as a function of time after the addition of Inhibitor Green (green line), Inhibitor Blue (blue
line) and Inhibitor Red (red line). The black line represents the barite formation in inhibitor-free brine.
3.2.3 Blocking of barite formation in original Soultz brine
After determination of calcium tolerance and inhibitor efficiency in artificial brines, the remaining three products (Inhibitor Orange,
Blue and Red) were tested for their inhibition effectiveness in the Soultz brine. Therefore, closed bottle tests were performed. This
experiment was already described in detail in Scheiber et al. (2012). This paragraph summarizes the findings.
Original Soultz brine was sampled at the production well GPK-2 by brine cooling from160˚C to70˚C in a special cooler. Inhibitors
were added immediately with active content of 5 mg/l and bottles were stored at room temperature. After 8 days the inhibitor/brine
samples were filtrated and filters residues were investigated visually and by electron microscopy. Additionally, the filtrate was
analyzed for Ba2+, Sr2+, Ca2+ and SO42- concentration. In the non-treated brine sample, the Ba2+ concentration decreases to half of
the original concentration. In the presence of the three inhibitors, no decrease of the Ba2+ concentration was monitored in the fluid
Scheiber, Seibt, Birner, Cuenot, Genter and Moeckes
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and no barite deposits were detected on the filter surface. Figure 6 shows filter residues on the filter surfaces. Most deposits,
including barite crystals, were found after filtration of the non-treated fluid sample, Figure 6a. Inhibitor Orange suppressed the
formation of barite but allowed co-precipitates like iron oxides and hydroxides to be formed, Figure 6b. Inhibitor Red, Figure 6c,
and Inhibitor Blue, Figure 6d, inhibit both, the formation of barite and co-precipitation of other minerals.
Figure 6: Filter residues of the closed bottle tests a) reference sample (brine without inhibitor), b) Inhibitor Orange/brine
mixture, c) Inhibitor Red/brine mixture and d) Inhibitor Blue/brine mixture (Scheiber et al., 2012).
Finally, Inhibitor Red was selected for the inhibition of barite at the Soultz geothermal power plant. This product showed a slightly
better performance than Inhibitor Blue and will be added continuously to the production stream of the geothermal power plant in
Soultz for scale inhibition. It is based on DTPMP ((Diethylenetriaminepentakis (methylene-phosphonic acid)).