GRC Transactions, Vol. 36, 2012

863

KeywordsSulfide scales, sulfide, Cerro Prieto, geothermal wells, cotun-nite, chalcopyrite, cubanite, bornite

ABSTRACT

Chemical and mineralogical characterization of sulfide scales, formed in well 624 from the CP II sector of the Cerro Prieto geothermal field, was carried out by reflected light mi-croscopy, X-ray diffraction and scanning electron microscopy. Samples come from a depth of 551 m, 1765 m, 2155 m and 2355 m. Scale samples consist of chalcopyrite, sphalerite, galena, bornite, cubanite, chalcocite and covellite. Chalcopyrite is the dominant phase in all samples. Electron microscope analyses indicate sphalerite and galena intergrowths in chalcopyrite. The proportions of the minerals assemblages do not show relation to the well temperature. In order to determine the solubility of sulfide scales in an acidic media, samples were treated in 10% hot HCl. As soon as the acid solution contacts the samples, H2S is formed and released. Most of the sulfide minerals are resistant to HCl; galena is the only mineral that partially reacts with the acid forming lead chloride.

Introduction

The physical and chemical conditions and the origin of the fluids in a geothermal reservoir play an important role in scale deposition in wells and pipelines. Scaling in the production pipe-lines has an important effect on production, costs of cleaning and maintenance activities and even abandoning a producing well.

In the Cerro Prieto geothermal field, two types of scales are formed. One, is composed of amorphous silica, calcite and anhy-drite; the other by metallic sulfides (chalcopyrite, bornite, cubanite, sphalerite and galena) and iron oxides (magnetite and hematite).

Efforts to maintain the required production of steam and reduce in production decline include mechanical and chemical cleaning. Mechanical cleaning requires reaching scales and breaking them into small chips that are recovered at the surface.

Diluted solutions of HCl are used to dissolve calcite; other mixtures of chemicals are needed in order to remove some other minerals from tube pipes.

Scaling is not the only process that reduces productivity. As a result of water-rock interactions, formation of secondary miner-als may decrease production by deposition in pores and fractures in the rock.

To minimize or eliminate the effects of scale deposition as well as to restore or induce permeability, several methodologies

Chemical and Mineralogical Characterization of Sulfide Scales in Well 624 From Cerro Prieto Geothermal Field and Its Reactivity in HCl

Georgina Izquierdo Montalvo1, Alfonso Aragón Aguilar1, Enrique Portugal Marin1, Jesús Porcayo Calderón1, Daniel Díaz Mojica2, and Julio Alvarez Rosales3

1Instituto de Investigaciones Eléctricas, Cuernavaca Morelos, CP2Universidad Autónoma de Guerrero, Unidad Académica Ciencias de la Tierra, Taxco El Viejo, GRO

3Residencia General de Cerro Prieto Comisión Federal de Electricidad, Mexicali, BCgim@iie.org.mx

Figure 1. The Cerro Prieto geothermal field and well location. The star indicates location of well 624.

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have been implemented in geothermal systems. These include: Matrix acidizing, Hydraulic fracturing, Thermal fracturing and Acid fracturing.

Matrix stimulation is an old methodology used to enhance well productivity in petroleum reservoirs systems. It has now been extended successfully to the geothermal industry. HCl and mixtures of HF and HCl are the common acid solutions used in acidizing operations. HCl is s elected to treat limestone and calcite in veins, pores and scales. A mixture of 12% HCl–3% HF (called regular mud acid) is also commonly used to dissolve silicates and silica (Malate et al., 1998).

Other chemicals and acid mixtures, in varying proportions, have been used for specific purposes.

The objectives of this work have been to characterize the scale samples from a high salinity and high enthalpy well and to investigate the behavior of sulfide scales interacting with one of the acid solutions used during the stimulation of wells.

The Cerro Prieto Geothermal FieldThe Cerro Prieto Geothermal Field (CPGF) is a liquid-

dominated field located in the northwest part of Mexico close to the United States border (Figure 1). It is the largest producing field within Mexico with an installed capacity of 720 MWe of installed capacity. It is the second largest producer in the world. At present more than 300 wells have been drilled, but not all of them are in use.

For administrative purposes the entire field has been divided into five areas: CP-I, CP-II, CP-III, CP-IV and CP-V. The later two are the shortest production histories.

The geothermal reservoir extends underneath the entire area of the CPGF at depths between 1500 and 2800 m, with tempera-tures higher than 350 °C. It is hosted in high porosity, permeable sandstones underlying a low porosity, relatively impermeable shale unit.

Wells in the eastern part of the field produce a mixture of steam and liquid at the wellhead. The liquid fraction has a chemi-cal composition characteristic of geothermal brine classified as sodium-chloride type. The salt concentration in the wellhead brine varies depending on the water/steam ratio; it is between 1 and 5 weight %.

The northeastern Cerro Prieto reservoir has shown unusual features and a different response to exploitation compared to other sectors of the field. Compared to the rest field, the CP-IV and CP-V areas have the highest average production enthalpy. Also the chemistry of the fluids shows differs from the other areas of the field. Some wells have high H2S (g) and relatively high B, Fe, Mn and Zn in the liquid phase.

The hydrological model developed by Halfman et al. (1984, 1986) for the entire field indicates that the geothermal fluids cir-culate horizontally through permeable stratum from east to west as well as ascension along faults.

Recharge of the reservoir has been considered to occur later-ally along the edges of the geothermal reservoir (Truesdell and Lippmann, 1990), but there is also evidence of inflow of descend-ing cooler groundwater.

From the distribution of hydrothermal minerals, Elders et al. (1984) developed a similar model of fluid circulation pattern and suggested that the heat source was located to the east of the field.

Izquierdo et al. (2000, 2001) carried out detailed mineralogical studies in drill cuttings from the producing strata from wells throughout the field. It was found that hydrothermal mineralogy is the result of the interaction between the reservoir rocks and neutral to alkaline pH fluids. Sulfide minerals like pyrite and pyrrhotite are in very low proportion.

Recently, some of the CP IV wells have shown the effects of corrosion in their casings; black solids have been recovered. By X-ray diffraction, magnetite and minor amounts of halite and calcite have been identified. The pH of samples collected at the wellhead does not show low values (Portugal et al., 2006). However the effects of corrosive fluids and most of the differences found in fluid chemistry of CP IV wells may be associated with the processes occurring in the reservoir in response to exploitation of the field or be related to different sources of fluid recharge, which may be controlled by geologic structures.

Well 624Well 624 is located in the sector CP II, south of CP IV and west

of CP V (Figure 1). It was drilled at depth of 2801.5 m. When repaired at the end of 2010, it showed circulation losses between 2493 and 2801 m. After 51 hours of quiescence temperature at the bottom of the well was 277 °C, (CFE, internal register).

The well produces a mixture of liquid and vapor. The liquid fraction is sodium chloride in composition. CO2 is the major component of the vapor phase followed by H2S and CH4 and N2.

Results

Four scale samples from well 624 were recovered from depths of 551 m, 1765 m, 2155 m and 2355 m. Hand samples show botryoidal textures, most are gray-greenish to yellowish in color and some fragments reflect pink, green and blue colors typical of metallic sulfides (Figure 2).

Reflected light microscopy, X-ray diffraction, scanning elec-tron microscopy and microprobe analyses were carried out in order to characterize the four scale samples.

Figure 2. Scale samples of well 624. The sample at left sample is from 551 m, top right from 1765 m, bottom left from 2155 m and bottom right from 2355 m.

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A portion of each sample was prepared as a plug with an ep-oxy resin. The surface was polished and observed using Olympus transmitted light microscope. Figure 3 (a and b) show images from samples at 1765 m and 2355 m. Minerals are randomly distrib-uted in a chalcopyrite (CuFeS2) matrix. Sulfide minerals include: sphalerite (ZnS), galena (PbS, pyrite (FeS2), bornite (Cu2Fe2S2), chalcocite (CuS2) and covellite (CuS).

Another fraction of each specimen was finely powdered and analyzed by X-ray diffraction in an Ital Structures, APDO 2000 diffractometer. Minerals are the same as those identified by re-flected light microscopy. Wurtzite (ZnS), which is compositionally the same as sphalerite (ZnS), was additionally identified by XRD.

Semi quantitative analyses by XRD and point counting under the reflected light indicate that chalcopyrite is the main phase, followed by sphalerite and galena, bornite, cubanite, chalcocite and covellite are in minor amounts. Scale mineral abundance is relatively homogeneous throughout the well. As samples come from depths between 551 m and 2355 m, it was expected that mineralogy will show some dependence on temperature.

(a) (b)

Figure 3a. Sample 624/1765 m in reflected light. Chalcopyrite (yellow), random intergrowths of sphalerite (dark gray), scarce galena (light gray) and at the right, low corner, traces of chalcocite (blue).Figure 3b. Sample 624/2355 m in reflected light. At the left side galena (light gray), chalcopyrite (yellow), sphalerite (dark gray) and bornite (pink-ish).

Scanning electron microscope images were obtained using a Zeiss DSM 960 electron microscope. Microanalyses were performed using an EDS6533 microprobe from Oxford Instru-ments. Most of the minerals are anhedral in shape. Minerals occur as intergrowths or “fused masses” which were distinguished by their color, from white to gray shades. There were no boundaries between each mineral, they occur as spots randomly distributed in the chalcopyrite matrix. Galena appears as white spots or occupies holes and micro fractures. Microanalysis allowed identification of chalcopyrite, sphalerite, galena and smectite. In samples treated with HCl, cottunite was additionally identified. As most of the sulfide phases are solid solutions, the approximate formula was calculated from the punctual chemical analyses.

Figure 4a shows, a SEM image of an untreated sample from 624/551 m. A non metallic mineral is indicated by the numbers 1 and 2, whose chemistry corresponds to an iron silicate. We assume that it could be a residue of the drilling mud attached to the casing and could have served as a core for sulfide deposition. Galena is the white phase occupying borders and empty spaces. Figure 5a presents a SEM image of an untreated sample from 624/2355 m, chalcopyrite is the main phase, sphalerite is randomly distributed and galena is the white phase filling holes and occurring along borders.

In order to understand the behavior of the scale samples in acid solutions, samples were treated in 10% HCl at 110 °C for 3 hours. The acid concentration, the temperature and the time are parameters of a well acidizing test; pressure was not considered. As soon as the acid solution contacts the samples, H2S is formed. When the solution is cooling to 94 °C, fine white needles begin to form. After the acid treatment, X-ray diffraction shows the same original mineralogical phases. Needles were identified as cotunnite (PbCl2). Galena is the only phase that partially reacts with HCl forming lead chloride. In some specimens galena is completely removed, leaving empty pores and micro fractures.

The SEM images in Figures 4b and 5b show samples 624/551 m and 624/2355 m respectively after treatment with HCl. In Figure 4b open pores and open conduits were formed by the reaction of galena with the HCl; only chalcopyrite and sphalerite were identified.

In Figure 5b chalcopyrite and sphalerite were also recognized and some of the empty pores are occupied by the new phase cot-tunite.

Additional work on the formation of sulfide minerals is re-quired; as there is no evidence of secondary sulfide minerals in the Cerro Prieto reservoir exist. Only scarce pyrrhotite and pyrite, and very rare chalcopyrite have been reported in some wells. No copper, lead or zinc minerals have been recorded. The source of metals and sulfur could be magmatic and transported either by H2S or as a chloride complex as seen in other high salinity /high enthalpy fluids (Andritsos and Karabelas, 1991).

(a) (b)

Figure 4. (a) SEM image of untreated sample 624/551 m,(500X), numbers 1 and 2 correspond to a silicate (identified by XRD as nontronite), 3 = galena and 4 = chalcopyrite. (b) The same sample after HCl (100X), 1, 2, 3 = chalcopyrite, 4 = sphalerite.

(a) (b)

Figure 5. (a) SEM images of sample 624/2355 m. At the left the specimen untreated (100X), 1 = galena, 2 and 4 = chalcopyrite, 3 = sphalerite. (b) The same specimen after HCl (100X), 1= cotunite, 2 = chalcopyrite 3= sphalerite.

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Conclusions

In well 624 sulfide scales consist of chalcopyrite as the main phase, followed by sphalerite and galena. Other sulfides include bornite, cubanite, chalcocite and covellite in minor amounts.

Sulfides do not show any relationship to the temperature within the well. Its deposition seems to be homogeneous through the well casing. Additional work has to be done in order to evaluate other parameters such as pH, pressure and processes as boiling etc.

SEM observations indicate that sulfide scales are not formed by a sequential precipitation of layers of a particular composi-tion. Chalcopyrite dominates and show intergrowths of galena and sphalerite.

According to the mineralogy of the reservoir rocks and the fluids composition, in Cerro Prieto the source of sulfur and metals may be magmatic. Metals may come from reaction between of casing steel and acid fluids formed as a consequence of extraction of deep fluids. Metals may be transported by H2S or as chloride complexes.

Laboratory experiments indicate that only galena reacts with HCl forming cotunnite and leaving a micro porous material. Cotunnite crystals are insoluble in water and as seen in SEM the tiny crystals occupy voids.

Wells in which sulfide scales have been formed, particularly galena; should be evaluated before acid stimulation is conducted. Galena will react with HCl and HF and will form other second-ary products.

All sulfides and new formed phases are insoluble in water, hard to destroy and do not affect the environment.

Acknowledgements

The authors express their gratitude to the authorities of the Residencia General de Cerro Prieto, from the Comisión Federal de Electricidad of Mexico (CFE), for authorization to present this article related to the contract number 800529404 between Instituto de Investigaciones Eléctricas and the Comisión Federal de Electricidad. Also we thank to Dr. Joseph Moore for reviewing and improving this manuscript.

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