Modeling of Calcite Scaling and Estimation of Gas Breakout Depth in a Geothermal Well by Using PHREEQC

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1 PROCEEDINGS, Fortieth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 26-28, 2015 SGP-TR-204 Modeling of Calcite Scaling and Estimation of Gas Breakout Depth in a Geothermal Well by Using PHREEQC Taylan AKIN 1, Aygün GÜNEY 2, Hulusi KARGI 1 1 Pamukkale University, Geological Engineering Department, Denizli, TURKEY takin@pau.edu.tr, hkargi@pau.edu.tr 2 Zorlu Energy Group, Levent 199, Büyükdere Caddesi No:199, Şişli, İstanbul, 34394, TURKEY aygun.guney@zorlu.com Keywords: Kizildere, geothermal, phreeqc, calcite scaling, gas breakout depth ABSTRACT Calcite scaling is widely encountered in geothermal wells and it has to be inhibited since it prevents production. As a result of a pressure drop, thermal fluids start to boil and degas of CO 2 while fluids rise in a wellbore. Thermal fluid becomes saturated to calcite as a result of both CO 2 exsolution and concentration increase of calcium and carbonates as a consequence of boiling. When the first gas bubble is formed, CO 2 exsolution affects the ph and also carbonate species. In order to prevent calcite scaling effectively, inhibitor must be injected into the wellbore at a depth below gas breakout point where thermal fluid is still in liquid phase. Thermal fluid transforms from 100% liquid to both liquid and gas phases at the gas breakout depth when the sum of partial pressures of water vapor and noncondensable gases exceed the wellbore pressure under flowing conditions. There are a lot of ways to predict the gas breakout depth in geothermal wellbores. One of them, the easiest but primitive, is dynamic PT survey evaluation. This method is common in industry but limited with applicable flow rate. Therefore determining gas breakout depth from dynamic pressure profile is not sensitive. In this study, major chemical analysis of brines, in situ measurements (ph, CO 2 /H 2 O ratios), steam fractions at sampling conditions, dynamic wellbore temperature and pressure data were used to compute gas breakout depth in a well at Kizildere geothermal field. All calculations were performed with thermodynamic approach by using PHREEQC software. 1. INTRODUCTION Mineral precipitation and consequently reduction in production capacity is widely encountered at geothermal fields all around the world. Physical and chemical properties of thermal fluids which at or close to equilibrium with host rocks in a reservoir, change as ascending throughout the well and some minerals may precipitate in wellbore. Calcite and quartz are common scale minerals that are observed in both well and surface equipment. Calcite dissolution gradually decreases with increasing temperature along the flow path from recharge inflow of fresh water toward the reservoir. Geothermal fluids are generally at equilibrium with calcite in reservoir but it becomes saturated within extraction well while the geothermal fluids are being produced. Calcite scaling is mainly related to both evaporation of brine which increases calcium and carbonate concentrations and simultaneously degassing of CO 2 which causes to increase in the saturation index of calcite as a result of ph rising. Various chemical inhibitors are used to restrain precipitation, but chemicals must be injected into liquid phase to provide effective inhibition. For this purpose, gas breakout depth where liquid phase starts to form gas bubble and transforms to both liquid and gas phase should be determined. The point that begins to differentiate from linearity on wellbore dynamic pressure profile is generally used for determining gas breakout depth. However, significant amount of vaporization must be occurred to change slope of line and lead to curve on pressure profile. Moreover, the depth of that point is most probably shallower than the depth of first gas bubble. Brine chemistry especially ph simultaneously change when first gas bubble formed. Therefore, both detection of gas breakout depth and evaluation of calcite scaling based on curve of dynamic pressure profile are not sensitive. Haizlip et. al. (2012) discussed the effect of high non-condensable gas on reservoir and wellfield production in the deep Kizildere reservoir. In their study, partial pressures of dissolved CO 2 and water in reservoir were calculated based on Henry s law and water steam tables. The gas bubble depth was detected according to matching of calculated pressure with dynamic PT measurements. Montegrossi and Lopez (2012) investigated reactive transport model that takes into account the kinetics of dissolution and precipitation of carbonates based on data from the geothermal field at Ahuachapan, El Salvador. They used an upward linear model with two infinite volume grid blocks simulating the extraction pressure and the reservoir pressure condition measured in the dynamic regime. The well scaling model was computed by means of the TOUGHREACT software package and then calibrated by varying the CO 2 partial pressure in the reservoir in order to match the ph measured at the well head. In this study, data of a geothermal well that is located in Kizildere geothermal field (SW Turkey) was used for modeling of calcite scaling and estimation of gas breakout depth. For this purpose, chemical analysis of sampled thermal fluids, in-situ measurements (ph, Gas/Steam etc.), dynamic pressure and temperature data of wellbore were used. Modeling studies were carried out in PHREEQC software by using thermodynamic equilibrium approach. Furthermore, probable wellbore dynamic pressure and temperature profile at several flow rate was modeled by WELLBORE simulator. Thus, maximum flow rate that well can be operated without lowering the gas breakout depth to reservoir level was determined by arranging PHREEQC input file with WELLBORE simulator results. Accurate 1

2 estimation of gas breakout depth in a geothermal well is possible with this approach that, in fact, is a significant contribution to inhibitor operation in practice. 2. GEOTHERMAL FIELD SETTINGS Kizildere geothermal field is located at 40 km west of Denizli, in Northeastern part of Büyük Menderes graben system. Anatolia is tectonically active region with regard to Alpine-Himalayan system and consequently many horst-graben structures such as Büyük Menderes graben was formed. The geothermal system is controlled by active graben faults. The reservoir rocks in the geothermal field are the limestone and conglomerate units within Neogene sediments and the marble-quartzite units within Paleozoic metamorphic formations (Şimşek, 2003). Heat source of the Büyük Menderes graben is magmatic intrusion in deeper part of the region as a result of tectonic extension. The δ 3 He surplus in thermal waters of Kızıldere reveal interactions of thermal fluids with basic to intermediate igneous rocks of mantle origin that are still cooling and the existence of a subvolcanic intrusion (Ozgur 2013). According to 18 O- 2 H analysis, origin of the thermal water are meteoric and there is intensive water-rock interaction along with long residence time (Haklidir, 2011). The development of the Kizildere Geothermal Field started more than 50 years ago by the General Directorate of Mineral Research and Exploration of Turkey (MTA). In mid 1960s. tens of wells were drilled to discover the reservoir area. In 1980s very first single flash geothermal power plant was installed for trials to generate electricity for the village located in the vicinity. In 1984 the first geothermal power plant (GPP) with a capacity of 20 MWe production was constructed. The GGP was operated by the government up to the year Next to production of electricity the produced geothermal fluids was as well utilized for district heating. After 2008, the GGP was privatized and acquired by Zorlu Energy Group with opportunity to operate field for 30 years. Zorlu started further development of the Kizildere geothermal resource with improving the productivity of the existing geothermal wellfield by acidization of the wells and optimizing the re-injection plan. Secondly Zorly continued developing the Kizildere geothermal field by conducting geophysical and geological studies. After analysis of the geophysical and geological data, 20 additional wells have been drilled including injectors and observation wells. On the basis of new data obtained great engineering process has been started and a 80 MWe capacity power plant has been installed. The power plant has triple flash and additional two binary turbines. Further engineering works are still in progress with new technological applications and studies. 3. MATERIALS AND METHODS Figure 1: Geological map of Kizildere geothermal field (Şimşek et al., 2009). Water chemistry and partial pressure of dissolved CO 2 at reservoir condition are essential with dynamic pressure and temperature data for determining first gas bubble and calcite scaling depth in a geothermal well. Wellbore dynamic pressure and temperature can be obtained by numerically modeling or in situ measuring. In this study, measured data were used to compare end member of modeled chemical results with sampled brine analysis and used numerically modeled pressure-temperature data to evaluate gas breakout depth at different flow rates. 3.1 Geochemical Sampling and In Situ Measurements Brine, steam condensate and gas samples were collected for geochemical analysis. All chemical data related to geothermal well were obtained via both condenser and portable separator which were mounted to the surface equipment (Figure 2a). Non condensable gas and steam ratio of gas phase was also measured with gas flow meter (Figure 2b). Filtered brine was sampled in two distinct bottles for cation and anion analysis. The ph of the cation sample was reduced to 2 with ultra pure nitric acid. Brine was diluted ten times for silica analysis. Major chemical constituents of brine samples were analyzed by both ICP-MS and ion chromatography at MTA laboratories. 2

3 Non condensable gas samples and steam condensates were taken from gas outlet of separator (Figure 2c). Gas bottles were filled with 6M NaOH solution and subsequently evacuated before sampling. Gas and steam condensate samples were analyzed at CNR Italy. Carbonates amount were determined at site by titration with hydrochloride acid. ph, Eh and electrical conductivities were measured in situ with WTW ph-cond While geochemical sampling was carrying out, dynamic wellbore pressure and temperature data were also recorded by Kuster brand PTS tool (Figure 2d). The measurement was taken every 40 to 50 cm. along the well. (a) (b) (c) (d) Figure 2: In situ measurements. a) Portable separator mounted to surface equipment, b) measurements of in situ gas/steam ratio, c) taking gas and steam condensate samples, d) dynamic PT survey. 3.2 Geochemical Calculation Scheme According to reservoir pressure and temperature, thermal fluid is in liquid phase at reservoir condition. However, all obtained chemical data from surface just represent separator (multi phase) condition. Therefore thermal fluid properties at reservoir condition were calculated from surface parameters. The enthalpies of the thermal fluid at casing inlet and separator condition were computed based on dynamic temperature data and used for steam fraction calculation. Steam condensate and brine were mixed with respect to steam fraction and mixture temperature was increased from separator to reservoir condition. An assumption was made that gas phase consists of only CO 2 based on gas analysis which reveals %99 of non condensable gases is carbon dioxide. Mole of dissolved CO 2 per kg of thermal water at reservoir state was calculated by using gas/steam ratio and steam fraction (Table 1). Afterward, the calculated value was irreversibly reacted with reservoir water. Because inhibitor was not being used while sampling, the amount of calcium in the brine sample was lower than initial quantity of reservoir. Hence, calculated reservoir fluid was reacted with calcite until equilibrium achieved. Consequently, fluid chemistry and partial pressure of dissolved CO 2 at reservoir condition (Casing inlet) was determined as initial condition. This calculated initial fluid chemistry was incrementally equilibrated with calcite, partial pressure of both water and CO 2 at each measured dynamic pressure and temperature state throughout the well. Approximately 2910 records were taken by PTS tool during dynamic survey of a well. A code was written in C# language for transformation of huge PT data to PHREEQC input file. Since dynamic survey is limited with regard to flow rate, PT profile data at higher flow rates were computed with Wellbore sim code. These data then were put into PHREEQC to calculate gas bubble depth by taking initial brine chemistry constant. Wellbore sim code assumes that reservoir pressure, temperature, gas content, salinity and friction loses are constant during computation to provide continues steady flow. The logic behind the code is stated in Garg et al., 2004 in detailed. 3

4 Table 1: Calculation of dissolved CO 2 at reservoir condition. Well head pressure (Barg) 12.5 Separator pressure (Barg) 11.5 Separator temperature ( 0 C) 182 Casing inlet temperature ( 0 C) 215 Water enthalpy at separator (kj/kg) Steam enthalpy at separator (kj/kg) Steam enthalpy at casing inlet (@1812m (kj/kg)) Steam Fraction (%) 7.41 nco 2 /nh 2 O at separator nco 2 /nh 2 O at reservoir nco 2 /mh 2 O at reservoir (mole/kg) MODEL RESULTS AND DISCUSSIONS According to the computation results, ph of the thermal water, initial calcium concentration and total pressure of both water vapour and dissolved CO 2 at reservoir condition is calculated as 5.86, 7.84 and atm respectively. ph is increasing from 5.86 to 6.84 as a result of degassing of CO 2 whereas calcium concentration is decreasing from 7.84 to 1.26 ppm in consequence of calcite scaling in wellbore. Model results also reveal that first gas bubble is formed at meter below the surface and ph starts to increase simultaneously (Figure 3a). On the other hand, calcite is not saturated as soon as gas bubble arises, it starts to precipitate after 80 meters (Figure 3b). (a) (b) Figure 3: a) Measured dynamic PT variation along with computed ph and dissolved gas pressure. First gas bubble arise at meters. b) ph profile versus total calcium. ph changes as soon as gas bubble form but calcite becomes saturated after 80 meters. Carbon dioxide and water vapour generates first gas bubble to compensate for the pressure difference at the depth where total partial pressure of both dissolved CO 2 and H 2 O exceed dynamic pressure. The gas exsolution shifts equation 3 to left by combining bicarbonate with hydrogen. As a result, ph increases and some consecutive reactions that involve carbonate species occur. Decreases of hydrogen and bicarbonate concentration lead to rearrangement of ion activities based on equation 2 and 4 (Figure 4). 4

5 CaCO 3(s) Ca +2 2 (aq) + CO 3(aq) (1) + CaCO 3(s) + H (aq) Ca +2 (aq) + HCO 3(aq) + CO 2(g) + H 2 O (l) H (aq) + HCO 3(aq) (2) (3) HCO 3(aq) 2 + CO 3(aq) + H (aq) (4) Brine is equilibrated with calcite in the reservoir but it tends to dissolve calcite up to first gas bubble arise while ascending in the casing. When first gas bubble forms, saturation index of calcite starts to increase and reach zero after 80 meters (Figure 5a). Effect of vaporization on calcium ions is negligible because steam fraction is % between first gas bubble formation and calcite saturation depth. While molality of total calcium remains constant until scaling start, activity of Ca +2 ion increases from 8.27E-06 mol/kg in the reservoir to 8.78E-06 mol/kg and 8.84E-06 mol/kg at first formation of gas bubble and calcite scaling depth respectively (Figure 5b). Increase in Ca +2 activity as a result of speciation from other calcium complex is more effective than evaporation to achieve calcite saturation. Figure 4: Showing that at a depth where total gas pressure exceeds dynamic pressure, non condensable gases (%99 CO 2 ) and steam generate first gas bubble and consequently ph and CO 3 increase, HCO 3 and dissolved CO 2 (H 2 CO 3 ) decrease and finally calcite becomes saturated. 5

6 (a) (b) Figure 5: a) Logarithm of ion activity product and equilibrium constant of calcite, b) total calcium versus Ca +2 ion Calcite dissolution reaction can be expressed in various equation based on database. For instance, llnl database calculates equilibrium constant and saturation index based on equation 2 whereas wateq4f, minteq and pitzer use equation 1. While variations in pressure have little effect on the equilibrium constant, the temperature variations are important. The value of equilibrium constant at various temperature is usually calculated with the Van t Hoff equation: dlnk dt = ΔH r RT 2 (5) Where ΔH r is the reaction enthalpy, K is the equilibrium constant, R is the ideal gas constant and T is the temperature in Kelvin. However, if the database includes essential coefficients, PHREEQC can calculate the equilibrium constant from an analytical expression of the type: log k = A 1 + A 2. T + A 3 /T + A 4. logt + A 5 /T 2 (6) Where T is temperature in Kelvin and the numbers are fit parameters. In this study llnl database was used which calculates log k based on the above polynomial expression that validity range is between degrees. Local equilibrium assumption was used and modeled end-member fluid chemistry then compared with sampled brine chemistry for checking the assumption. According to the correlation, there is quite similarity among modeled and sampled fluid chemistry (Table 2). Therefore it can be assumed that, calcite precipitation kinetic is rather fast, so local equilibrium assumption can be used to predict first gas bubble depth and calcite scaling profile in wellbore. Although quartz has tendency to precipitation due to both adiabatic and conductive cooling during production, it was not modeled since its slow reaction kinetic. Table 2: Comparison of modeled and analyzed brine chemistry. Modeled brine chemistry at reservoir Modeled brine chemistry at separator Sampled brine chemistry at separator ph Na Ca K SiO2 Cl S(6) F Li B Br Modeling of wellbore pressure distribution in various flow rates was revealed that gas bubble depth changes between meters (Figure 6). Beginning depth of slotted liner of the well is 1715 meters so, flashing does not occur at reservoir level if reservoir condition does not change. Computation of gas bubble depth based on wellbore simulator demonstrates slight difference from calculation based on measured dynamic pressure. Reason of the differences is most probably friction parameters of the well. Because of steady production schedule, flow rate of the well was not changed during preparation of the study. Hence, verification and calibration of dynamic PT model at various flow rates was not carried out. 6

7 5. CONCLUSIONS Figure 6: Computed gas bubble depth versus flow rates based on constant reservoir conditions This study demonstrates that ph is dependent on dissolved CO 2 and it changes immediately as soon as gas bubble occurs. However, calcite scaling needs some activity increase of calcium and carbonate due to both vaporization and speciation. Quartz also gains tendency to precipitation while ascending through well since thermal water cools both adiabatically and conductively. But, quartz has slow reaction kinetic and its precipitation must be modeled based on kinetic approach. Calcite scaling is one of the main problems that prevent sustainable production from geothermal wells. Effective inhibition to scaling can be supplied by injecting chemical inhibitors into liquid phase in casing. The depth of gas bubble formation in a geothermal well can be estimated more precisely using modeled or measured PT along with geochemical calculation. Although study with measured PT restricts calculation to one flow rate, it gives the possibility to validate geochemical modeling. On the other hand, fluctuation of gas bubble depth along with several flow rate can be evaluated integrating modeled PT data into geochemical computation. Calibration of the PT model was not carried out because of constant flow rate as a result of strategy of electricity production. There was no available data about calcite scaling in wellbore because, the well was not being allowed to produce without inhibitor usage during the study. The only available data were sampled brine chemistry at separator and PT survey of wellbore. In order to validate the calcite scaling model, only results of modeled and sampled brine chemistry were matched. Comparison of modeled and sampled brine chemistry at separator condition imply that local equilibrium assumption is applicable for modeling of gas bubble depth and calcite scaling since they have fast reaction response. Additionally, absence of reliable data and uncertain parameters like mineral surface area makes kinetic approach more questionable than thermodynamic equilibrium model. ACKNOWLEDGEMENTS This study was supported by the Scientific Research Program (BAP) of the Pamukkale University (No.2013KRM024). The authors wish to thank Mr. Ali Kindap from Zorlu Energy group for permission to discuss data collected in the Kizildere geothermal field. Thanks also to Ms. Fusun Tut Haklidir for both sharing of organization of chemical analysis and field works. REFERENCES Appelo, C.A.J., and Postma, D.: Geochemistry, groundwater and pollution: 2nd Edition, A.A. Balkema Publishers, Great Britain, (2005), p. Garg, S.K., Pritchett, J.W., Alexander, J.H.: A new liquid hold-up correlation for geothermal wells. Geothermics, 33, (2004), Haizlip, J.R., Guney, A., Tut Haklidir, F.S., Garg, S.K.: The impact of high noncondensible gas concentrations on well performance Kizildere geothermal reservoir, Turkey, Proceedings, 37th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA (2012). Montegrossi, G., Lopez, F.E.M.: Modeling of calcite scaling in a geothermal well, Proceedings, TOUGH Symposium, Lawrence Berkeley National Laboratory, Berkeley, CA (2012) Ozgur, N.: Hydrogeological, hydrogeochemical and isotope geochemical modeling of the thermal waters of the Menderes massif, western Anatolia, Turkey, Proceedings, 38th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA (2013). Parkhurst, D.L., Appelo, C.A.J.: User s guide to PHREEQC (Version 2) A computer program for speciation, batch-reaction, one dimensional transport, and inverse geochemical calculations, U.S. Geological Survey Water-Resources Investigations Report, , (1999), 312 pp. 7

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