Changes In Fluid Chemistry Research Articles

Mineralization of carbon dioxide sequestered in volcanogenic sandstone reservoir rocks

Geological storage of carbon dioxide in deep saline formations can decrease the accumulation of CO2 in the atmosphere, and thus slow down global warming. Most CO2 injected into subsurface rock formations is expected to remain for a long time as either a separate supercritical phase or in solution in brine; both forms present the possibility of leakage back to the surface or other environmental impacts. Mineralogical trapping of injected CO2 is more secure but usually thought to be too slow to add significantly to sequestration security. For quartz-rich sandstones (quartzarenite and arkose), only ca. 5% CO2 mineralization is achieved over 1000–10,000 years (Audigane et al., 2007). However, if volcanogenic and other sandstones that have larger amounts of reactive minerals were used for storage, there could be a larger fraction of CO2 mineralized in a shorter time. The limitation is that porosity and permeability tend to decrease with increase of volcanic rock fragments (VRF), which limits the rate at which CO2 can be injected. We evaluate these tradeoffs to assess the feasibility of using volcanogenic sandstone to achieve secure CO2 storage. Using relationships between VRF percent, porosity and permeability from available geological data, the reactive transport code TOUGHREACT was used to model the flow, transport, mineral reactions, changes in fluid chemistry, and the rate and extent of CO2 mineralization over 1000 years during and after CO2 injection into a sandstone reservoir. We use the models specifically to evaluate the expected trade-off between higher reactivity and lower porosity and permeability. A model volcanic fragment mineralogy is used (pyroxene and feldspar mainly for which kinetic data are available) along with conservative estimates for silicate and oxide mineral dissolution kinetics and reactive surface area. Substitution of other more common reactive minerals such as chlorite and amphibole would not significantly change the results. The simulations show that in rocks with 10–20% reactive minerals, as much as 80% CO2 mineralization could occur in 1000 years and still allow sufficient injectivity so that 1Mt of CO2 could be injected per year per well. The calculated mineralized fraction depends on several factors, most notably the kinetics and reactive surface area of dissolving silicates and the detailed relationship of reactive mineral content to effective permeability and injectivity.

From serpentinization to carbonation: New insights from a CO2 injection experiment

We injected a CO2-rich hydrous fluid of seawater chlorinity into an ongoing, mildly reducing (H2(aq)≈3 mmol/kg) serpentinization experiment at 230 °C and 35 MPa to examine the changes in fluid chemistry and mineralogy during mineral carbonation. The chemistry of 11 fluid samples was measured, speciated, and compared with MgO–SiO2–H2O–CO2 (MSHC) phase equilibria to approximate the reaction pathway from serpentinization to carbonation. Although the overall system was in apparent disequilibrium, the speciated activities of dissolved silica (aSiO2(aq)) and carbon dioxide (aCO2(aq)) evolved roughly along MSHC equilibrium phase boundaries, indicative of 4 distinct mineral assemblages over time: (1) serpentine–brucite (± magnesite) before the injection, to (2) serpentine–talc–magnesite 2 h after the injection, to (3) quartz–magnesite (48 h after injection), and (4) metastable olivine–magnesite (623 h after injection) until the experiment was terminated. Inspection of the solid reaction products revealed the presence of serpentine, magnesite, minor talc, and magnetite, in addition to relict olivine. Although quartz was saturated over a short segment of the experiment, it was not found in the solid reaction products. A marked and rapid change in fluid chemistry suggests that serpentinization ceased and precipitation of magnesite initiated immediately after the injection. A sharp decrease in pH after the injection promoted the dissolution of brucite and olivine, which liberated SiO2(aq) and dissolved Mg. Dissolved Mg was efficiently removed from the solution via magnesite precipitation, whereas the formation of talc was relatively sluggish. This process accounts for an increase in aSiO2(aq) to quartz saturation shortly after the injection of the CO2-rich fluid. Molecular dihydrogen (H2(aq)) was generated during serpentinization of olivine by oxidation of ferrous iron before the injection; however, no additional H2(aq) was generated after the injection. Speciation calculations suggest a strong affinity for the formation of methane (CH4(aq)) at the expense of CO2(aq) and H2(aq) after the injection, but increased CH4(aq) formation was not observed. These findings suggest that kinetically fast mineral carbonation dominates over sluggish CH4(aq) formation in mildly reducing serpentinization systems affected by injection of CO2-rich fluids.

HYDROTHERMAL ALTERATION AND CO2 METASOMATISM (NATURAL CARBON SEQUESTRATION) OF KOMATIITES IN THE SOUTH-WESTERN ABITIBI GREENSTONE BELT

Secondary mineral assemblages in komatiites of the south-western Abitibi greenstone belt reflect the tectonic evolution of the region. Changes in mineralogy were accompanied by changes in fluid chemistry over time. Hot, plume-generated komatiites (2719–2704 Ma) were hydrated by hot fluids (200–300 oC), replacing the primary olivine and pyroxenes by serpentine + magnetite. Accretion and thrusting between neighbouring subprovinces was followed by pulses of granitic magmatism between 2692 Ma and 2677 Ma and by the subsequent devolatilization of the crust. Large volumes of oxidized, CO 2 -bearing fluids (250–300 oC) discharging through faults gave rise to two regionally widespread carbonate-bearing assemblages. Quartz + carbonate rocks formed from fluids having a high CO 2 :H 2 O ratio and talc + carbonate rocks from fluids of lower CO 2 :H 2 O ratio. The quartz + carbonate assemblage commonly hosts gold deposits and probably represents the site of CO 2 discharge where fluid build-up and hydraulic fracturing promoted the development of mineralized crack-seal veins. The quartz + carbonate and talc + carbonate assemblages along the Destor Porcupine fault are by-products of natural carbon sequestration of ultramafic rocks where the carbonates provided permanent storage for CO 2 . Experimental studies on carbon sequestration support earlier conclusions, based on fluid inclusion microthermometry, that the quartz + carbonate formed from fluids with high CO 2 :H 2 O ratios. The crystallization of the mineral assemblage was accompanied by volume increase, loss of porosity, and reduced permeability. These effects were ultimately reversed as the build-up of CO 2 gave way to fracturing in the rocks, thereby increasing porosity and permeability. The periodically fractured rocks (crack-seal veins) would have been ideal hosts for mineralizing fluids. CO 2 metasomatism of ultramafic rocks in the Abitibi belt was followed by partial replacement of the carbonate-rich assemblages by chromian muscovite ± chlorite ± rutile at 2633-2617 Ma. The localized low pH metamorphic fluids were associated with folding and the development and re-activation of shear zones in the region.

Geochemically induced shifts in catabolic energy yields explain past ecological changes of diffuse vents in the East Pacific Rise 9°50'N area

The East Pacific Rise (EPR) at 9°50'N hosts a hydrothermal vent field (Bio9) where the change in fluid chemistry is believed to have caused the demise of a tubeworm colony. We test this hypothesis and expand on it by providing a thermodynamic perspective in calculating free energies for a range of catabolic reactions from published compositional data. The energy calculations show that there was excess H2S in the fluids and that oxygen was the limiting reactant from 1991 to 1997. Energy levels are generally high, although they declined in that time span. In 1997, sulfide availability decreased substantially and H2S was the limiting reactant. Energy availability dropped by a factor of 10 to 20 from what it had been between 1991 and 1995. The perishing of the tubeworm colonies began in 1995 and coincided with the timing of energy decrease for sulfide oxidizers. In the same time interval, energy availability for iron oxidizers increased by a factor of 6 to 8, and, in 1997, there was 25 times more energy per transferred electron in iron oxidation than in sulfide oxidation. This change coincides with a massive spread of red staining (putative colonization by Fe-oxidizing bacteria) between 1995 and 1997.For a different cluster of vents from the EPR 9°50'N area (Tube Worm Pillar), thermodynamic modeling is used to examine changes in subseafloor catabolic metabolism between 1992 and 2000. These reactions are deduced from deviations in diffuse fluid compositions from conservative behavior of redox-sensitive species. We show that hydrogen is significantly reduced relative to values expected from conservative mixing. While H2 concentrations of the hydrothermal endmember fluids were constant between 1992 and 1995, the affinities for hydrogenotrophic reactions in the diffuse fluids decreased by a factor of 15 and then remained constant between 1995 and 2000. Previously, these fluids have been shown to support subseafloor methanogenesis. Our calculation results corroborate these findings and indicate that the 1992-1995 period was one of active growth of hydrogenotrophic communities, while the system was more or less at steady state between 1995 and 2000.

Optimizing a Wind Turbine Oil Condition Monitoring Program

Abstract The logistics and expense of obtaining oil samples from wind farms are among the conditions that make it challenging to establish a valuable condition monitoring program. Optimizing the oil condition monitoring program for a wind turbine is a critical step towards achieving forecasted ROI for the wind farm. To advance the current oil condition monitoring practices for most wind turbines, 150 identical wind turbine systems in the field for up to four years were studied. Several oil analysis tests were performed to understand the depletion of additives and changes in fluid chemistry. Basic statistical analysis was obtained on the data set to identify correlations in the data. The result of the analysis was a better understanding of the wind turbine oil’s formulation and the relationship to fluid degradation and gearbox condition. The statistical methodology that we performed can be applied to other wind turbine farms to improve the knowledge derived from their oil analysis tests, both for gear as well hydraulic oils. In addition, other innovative tests were suggested which may augment a current oil analysis test slate to improve the value of their program.

Geochemical properties of saline geothermal fluids from the in-situ geothermal laboratory Groß Schönebeck (Germany)

Fluid samples have been collected between 2001 and 2009 from the two geothermal wells at the Groß Schönebeck site (North German basin). The samples were taken at 4100–4235 m depth (∼150 °C/∼45 MPa) and analyzed for their chemical composition, with special focus on parameters relevant to scaling and corrosion. The fluids have very high salinity (193–265 g/L total dissolved solids) and, typical for fluids from lower Permian (Rotliegend) sandstones, contain a complex mixture of aqueous compounds and dissolved gases. The aqueous phase is nearly saturated with respect to CaCl 2 and NaCl, which together represent 98% of its salt content. Due to high chloride concentrations, the fluids can be considered highly corrosive. Various minerals, such as sulfates of the alkaline earths, are theoretically oversaturated and could precipitate due to changes in pressure and temperature conditions. Furthermore, even a slight change in pH and/or redox conditions could result in the formation of iron, manganese or lead scales. The fluid contains high amounts of dissolved gases (volume ratio of gas:liquid is about 1:1 at standard conditions), mainly consisting of nitrogen (∼80 vol%) and methane (∼15 vol%). Additional gases (hydrogen, carbon dioxide, helium, C 2+) approach 5 vol%. This study compares changes in fluid chemistry (of gas and solution), measured over eight years, with the history of geothermal well/reservoir activities (e.g. stimulation). It was found that changes occur mainly due to dilution of the formation fluid with the solutions used for stimulations (e.g. water-fracs) consisting mainly of shallow groundwater of low ionic strength. Some solutes (iron, lead, barium, sulfate, and bicarbonate), however, showed a different behavior as compared to the bulk (sodium, calcium, etc.), indicating that precipitation-, dissolution-, and/or redox reactions took place. Since the composition of the Groß Schönebeck fluid is a good example of many saline geothermal fluids worldwide, the insights gained into potential geochemical reactions will be crucial for general corrosion/scaling risk assessments.

Extremely high solubility of rutile in chloride and fluoride-bearing metamorphic fluids: An experimental investigation

Rutile (TiO 2 ) is an important high field strength element sequestering phase, and thus understanding its solubility in metamorphic fluids is important if we are to understand terrestrial trace element cycling. Titanium is often assumed to be fluid immobile, and previous experimental data suggest that rutile has very low solubility in H 2 O. However, fluids in high-temperature metamorphic rocks are known to be more complex in composition. The solubility of rutile in chloride and fluoride-rich hydrous fluids was determined experimentally at 0.5 GPa and high temperatures. The experimental results indicate much higher rutile solubility in brines compared to pure H 2 O. In chloride brines solubility was 2–4 times higher than in H 2 O, and in fluoride brines solubility was elevated 20–100 times. The results imply that titanium may be much more mobile in metamorphic fluids than previously anticipated, as solubility is critically dependent on fluid chemistry. We also present a possible mechanism for rutile precipitation in metamorphic veins due to a change in fluid chemistry, following the precipitation of halogen-rich minerals such as apatite.

Long-term Petrophysical Investigations on Geothermal Reservoir Rocks at Simulated In Situ Conditions

In the course of stimulation and fluid production, the chemical fluid–rock equilibrium of a geothermal reservoir may become disturbed by either temperature changes and/or an alteration of the fluid chemistry. Consequently, dissolution and precipitation reactions might be induced that result in permeability damage. In connection with the field investigations at a deep geothermal doublet, complementary laboratory-based research is performed to address these effects. The reservoir is located at a depth of 4100 to 4200 m near Groß Schönebeck within the Northeast German Basin, 50 km north of Berlin, Germany. Within the reservoir horizon, an effective pressure of approximately 45 MPa and a temperature of 150°C are encountered. Furthermore, the Lower Permian (Rotliegend) reservoir rock is saturated with a highly saline Ca–Na–Cl type formation fluid (TDS ≈ 255 g/l). Under these conditions we performed two sets of long-term flow-through experiments. The pore fluid used during the first and the second experiment was a 0.1 molar NaCl-solution and a synthetic Ca–Na–Cl type fluid with the specifications as above, respectively. The maximum run duration was 186 days. In detail, we experimentally addressed: (1) the effect of long-term flow on rock permeability in connection with possible changes in fluid chemistry and saturation; (2) the occurrence and consequences of baryte precipitation; and (3) potential precipitations related to oxygen-rich well water invasion during water-frac stimulation. In all substudies petrophysical experiments related to the evolution of rock permeability and electrical conductivity were complemented with microstructural investigations and a chemical fluid analysis. We also report the technical challenges encountered when corrosive fluids are used in long-term in situ petrophysical experiments. After it was assured that experimental artifacts can be excluded, it is demonstrated that the sample permeability remained approximately constant within margins of ±50 % for nearly six months. Furthermore, an effect of baryte precipitation on the rock permeability was not observed. Finally, the fluid exchange procedure did not alter the rock transport properties. The results of the chemical fluid analysis are in support of these observations. In both experiments the electrical conductivity of the samples remained unchanged for a given fluid composition and constant p-T conditions. This emphasizes its valuable complementary character in determining changes in rock transport properties during long-term flow-through experiments when the risk of experimental artifacts is high.

A Case Study in Using Compositional Grading to Improve Reservoir Characterization

Abstract This paper presents a case study of how compositional grading was used as a tool to greatly affect the subsurface interpretation of a deepwater field. Originally, the main reservoir at the field was viewed as being compartmentalized. There were also two viscous low API gravity oil reservoirs that were of little economic interest. A new interpretation based on compositional grading suggests that the main reservoir is, in fact, much better connected. There is also a risk of a viscous asphaltene-enriched zone at the oil-water contact that was not previously considered. Fortunately, if it exists, it would likely only delay aquifer drive. The two low API gravity oil reservoirs of little economic interest likely have much better quality oil updip making them good appraisal candidates. The different interpretation improves the overall view of the field and has implications for optimal development well locations, initial production rates and facility gas capacity. Compositional grading is particularly useful in aiding subsurface characterization in areas where grading is strong, and in deepwater offshore developments where uncertainty is great and development and appraisal costs are high. Fundamental geology and geochemistry are critical to being able to recognize a priori areas where compositional grading is strong. Introduction Compositional grading of hydrocarbons within a reservoir refers to the gradual and continuous change in fluid chemistry and physical properties such as density, gas-oil ratio and viscosity within the reservoir as a function of spatial location; in particular changes in elevation. Experience has shown that compositional grading is a common phenomenon(1–3). This paper demonstrates how compositional grading was used at a deepwater field as a valuable tool to aid in subsurface characterization. Such a tool is particularly useful in deepwater where development costs can be on the order of a billion dollars, uncertainty is great and appraisal costs are high. Yet, despite its common occurrence and potential use in subsurface characterization, compositional grading is poorly recognized in general and little used in practice to aid in characterization. For example, in the recent past, often only a single fluid sample would be taken in a reservoir. The fluid properties from this single sample would then be applied uniformly throughout a reservoir. Even when grading is recognized, there is often a belief among many practicing reservoir engineers that grading can be easily predicted using commercially available thermodynamic software. We discuss below how this can be misleading. Whilst grading occurs in both gas condensate and oil fields, in this paper we focus on grading in black oil columns; in particular in gas-oil ratio and in situ density. We first present background information on the theory of compositional grading and its practical manifestation including one of its by-products; namely asphaltene-enriched zones. We then discuss the case history. We feel that one of the keys to understanding compositional grading and using it in subsurface characterization is in the integration of a wide range of information, particularly from reservoir engineering, geochemistry and geology.

Fluid Inclusion Evidence for Magmatic-Hydrothermal Fluid Evolution in the Porphyry Copper-Molybdenum Deposit at Butte, Montana

The porphyry Cu-Mo deposit in Butte, Montana, formed where magmatic hydrothermal fluids, introduced with injections of porphyrytic dikes, fractured and permeated the Butte Quartz Monzonite. These fluids formed a stockwork of quartz and quartz-sulfide veinlets with a variety of styles of potassic and sericitic alteration envelopes. The distribution of vein and alteration types and the distribution of fluid inclusions in these veins record the progressive pressure, temperature, and compositional evolution of the hydrothermal fluids that formed this world-class deposit. Deep drilling and 1,300 m of offset along the Continental fault provide a vertical view of almost 3 km through the Butte deposit. Deep veins within and below the highest Mo grades are quartz dominated with thin K-feldspar or, less commonly, biotitic alteration rims. Fluid inclusions in deep veins trapped a single phase aqueous fluid containing 2 to 5 wt percent NaCl equiv and 2 to 8 mol percent CO2 at temperatures between 575° and 650°C and pressures between 200 and 250 MPa, corresponding to depths between 6 and 9 km. Although Cu grades are low in this region, abundant chalcopyrite daughter minerals in fluid inclusions indicate that the fluids were Cu rich. Fluids that formed these veins transported Cu from the magma below, upward into the region of Cu mineralization with only minor Cu precipitation. Over a kilometer above the bulk of deep quartz and quartz-molybdenite veins, the highest Cu grades are in and around chalcopyrite-bearing quartz-sulfide veins with biotitic alteration (early dark micaceous veins), and their upward, equivalent magnetite-chalcopyrite-pyrite-quartz veins with wide K-feldspar, green sericite, and chlorite alteration (pale-green sericitic veins). These veins contain more evidence for brine-vapor unmixing than any other vein type. The upward progression of early dark micaceous veins to pale-green sericitic veins formed where low salinity, CO2-bearing fluids, similar to those trapped in deep quartz veins, ascended, de-pressurized, sometimes unmixed, and cooled from ~650°C at 90 MPa to ~475°C at ~50 MPa. As low salinity, CO2-bearing, aqueous fluids, similar in composition to fluids trapped in deep quartz veins, cooled at shallow depths, they formed late pyrite-quartz veins with sericitic alteration. These veins formed from fluid cooling at temperatures between 370° and 450°C at transiently hydrostatic pressures between 40 and 70 MPa, corresponding to depths of 4 to 7 km. Most pyrite-quartz veins formed at pressures and temperatures above the H2O-NaCl-CO2 solvus, but evidence for brine-vapor unmixing is also present. Pyrite-quartz veins formed at progressively greater depths as the hydrothermal system cooled, overprinting much previous mineralization. Late Cu-Pb-Zn-Ag-As-rich Main stage veins formed from dilute fluids containing <3 wt percent NaCl equiv and <2 mol percent CO2. These fluids were trapped between 230° and 400°C under hydrostatic pressures between 20 and 60 MPa and depths of 2 to 6 km. No evidence of boiling is observed in Main stage veins. Fluid inclusion phase relationships indicate that the Butte porphyry Cu-Mo deposit formed at 5 to 9 km depth, greater than any other porphyry-type deposit. At Butte, the similarity in bulk composition of fluids trapped in early quartz-rich veins with potassic alteration and late pyrite-quartz veins with sericitic alteration implies that an underlying magma continually provided low salinity, CO2-bearing fluids of relatively constant composition during the entire life of the hydrothermal system. We hypothesize that rather than resulting from changes in fluid chemistry due to magma crystallization, the entire suite of vein and alteration types and the ore metal distribution reflect the path of cooling, depressurization, and wall-rock interaction of a parental magmatic-derived fluid of relatively constant initial composition. Fluid inclusions, vein and alteration relations, and ore metal distribution indicate that Cu and Mo were introduced into the hydrothermal system by the same fluids, but that the mechanisms of precipitation of these metals were decoupled. Early dark micaceous and, to a greater extent, pale-green sericitic, veins have wide alteration envelopes and contain more evidence for fluid unmixing than any other vein type, which suggests that chalcopyrite precipitation was driven by a combination of fluid unmixing, fluid-rock reaction, and fluid cooling between 650° and 475°C. Most molybdenite mineralization, however, is in quartz-dominated veins with little or no alteration that are dominated by low salinity inclusions. These veins formed in response to pressure decrease rather than cooling. After chalcopyrite and molybdenite precipitation, low salinity fluids cooled, usually at temperatures and pressures above the H2O-NaCl-CO2 solvus, to produce significant acid and voluminous sericitic alteration accompanied by pyrite-quartz vein formation that overprints much of the deposit and contains anomalous but noneconomic Cu.

Temporal Changes in Fluid Chemistry and Energy Profiles in the Vulcano Island Hydrothermal System

In June 2003, the geochemical composition of geothermal fluids was determined at 9 sites in the Vulcano hydrothermal system, including sediment seeps, geothermal wells, and submarine vents. Compositional data were combined with standard state reaction properties to determine the overall Gibbs free energy (DeltaG(r) ) for 120 potential lithotrophic and heterotrophic reactions. Lithotrophic reactions in the H-O-N-S-C-Fe system were considered, and exergonic reactions yielded up to 120 kJ per mole of electrons transferred. The potential for heterotrophy was characterized by energy yields from the complete oxidation of 6 carboxylic acids- formic, acetic, propanoic, lactic, pyruvic, and succinic-with the following redox pairs: O(2)/H(2)O, SO(4) (2)/H(2)S, NO(3) ()/NH(4) (+), S(0)/H(2)S, and Fe(3)O(4)/Fe(2+). Heterotrophic reactions yielded 6-111 kJ/mol e(). Energy yields from both lithotrophic and heterotrophic reactions were highly dependent on the terminal electron acceptor (TEA); reactions with O(2) yielded the most energy, followed by those with NO(3) (), Fe(III), SO(4) (2), and S(0). When only reactions with complete TEA reduction were included, the exergonic lithotrophic reactions followed a similar electron tower. Spatial variability in DeltaG(r) was significant for iron redox reactions, owing largely to the wide range in Fe(2+) and H(+) concentrations. Energy yields were compared to those obtained for samples collected in June 2001. The temporal variations in geochemical composition and energy yields observed in the Vulcano hydrothermal system between 2001 and 2003 were moderate. The largest differences in DeltaG(r) over the 2 years were from iron redox reactions, due to temporal changes in the Fe(2+) and H(+) concentrations. The observed variations in fluid composition across the Vulcano hydrothermal system have the potential to influence not only microbial diversity but also the metabolic strategies of the resident microbial communities.

Redox evolution and mass transfer during serpentinization: An experimental and theoretical study at 200 °C, 500 bar with implications for ultramafic-hosted hydrothermal systems at Mid-Ocean Ridges

Highly reducing and high-pH vent fluids characterize moderately low temperature ultramafic-hosted hydrothermal systems, such as the recently discovered Lost City hydrothermal field at 30°N Mid-Atlantic Ridge Ridge (MAR). To better understand the role of mineral reaction rates on changes in fluid chemistry and mineralization processes in these and similar systems, we conducted an experimental study involving seawater and peridotite at 200 °C, 500 bar. Time series changes in fluid chemistry were monitored and compared with analogous data predicted using experimental and theoretical data for mineral dissolution rates. Although there was qualitative agreement between predicted and measured changes in the chemical evolution of the fluid for some species, the rate and magnitude of increase in pH, dissolved chloride and H 2 did not agree well with predictions based on theoretical modeling results. Experimental data indicate that dissolved H 2 abruptly and intermittently increased, reaching a value only approximately 20% of that predicted assuming magnetite as the primary Fe-bearing alteration phase. The distribution and valence of Fe in primary and secondary minerals reveal that the most abundant secondary mineral, serpentine, contained significant amounts of both ferric and ferrous Fe, with the less abundant brucite, also being Fe-rich ( X Fe = 0.3). Surprisingly, magnetite was present in only trace amounts, indicating that H 2 generation was largely accommodated by the formation of Fe–chrysotile. Accordingly, the diversity of Fe-bearing secondary minerals together with rates of serpentinization less than theoretically predicted, account best for the relatively low dissolved H 2 concentrations produced. Thus, the experimental data can be used to obtain provisional estimates of thermodynamic data for Fe-bearing minerals, enhancing the application of reaction path models depicting mass transfer processes during serpentinization at mid-ocean ridges. Similarly, the observed differences between theoretically predicted and experimentally measured pH values result from constraints imposed by complex patterns of mass transfer inherent to the experimental system. In particular, the experimental observation of a late stage increase in Na/Cl ratio likely results from the dissolution of a Na 2O component of clinopyroxene, which causes pH to increase sufficiently to induce precipitation of a Ca-bearing phase, perhaps portlandite. As with the redox variability observed during the experiment, this event could not be predicted, underscoring the need to use caution when modeling alteration processes in the chemically complex ultramafic-hosted hydrothermal systems at elevated temperatures and pressures.

Continuous enrichment culturing of thermophiles under sulfate and nitrate-reducing conditions and at deep-sea hydrostatic pressures

A continuous culture bioreactor was developed to enrich for nitrate and sulfate reducing thermophiles under in situ deep-sea pressures. The ultimate objective of this experimental design was to be able to study microbial activities at chemical and physical conditions relevant to seafloor hydrothermal vents. Sulfide, sulfate and oxide minerals from sampled seafloor vent-chimney structures [East Pacific Rise (9 degrees 46'N)] served as source mineral and microbial inoculum for enrichment culturing using nitrate and sulfate-enriched media at 70 and 90 degrees C and 250 bars. Changes in microbial diversity during the continuous reaction flow were monitored using denaturing gradient gel electrophoresis (DGGE) of PCR amplified 16S rRNA gene fragments. Time series changes in fluid chemistry were also monitored throughout the experiment to assess the feedback between mineral-fluid reaction and metabolic processes. Data indicate a shift from the dominance of epsilon Proteobacteria in the initial inoculum to the several Aquificales-like phylotypes in nitrate-reducing enrichment media and Thermodesulfobacteriales in the sulfate-reducing enrichment media. Methanogens were detected in the original sulfide sample and grew in selected sulfate-enriched experiments. Microbial interactions with anhydrite and pyrrhotite in the chimney material resulted in measurable changes in fluid chemistry despite a fluid residence time only 75 min in the reactor. Changes in temperature rather than source material resulted in greater differences in microbial enrichments and mediated geochemical reactions.

Microchemical evidence for episodic growth of antitaxial veins during fracture-controlled fluid flow

The mechanism by which syntectonic hydrothermal veins form is widely debated, with some workers suggesting that certain vein textures are related to specific fluid flow regimes. Central to the debate is whether vein formation involves advective fluid flow, or occurs by local diffusion of material from the surrounding wall rock. To address this issue, we integrated textural observations and microchemical analyses of a hydrothermal vein from the Lachlan Orogen, southeast Australia, to reveal information about vein growth history, changes in fluid chemistry, and the evolution of fluid flow pathways during vein growth. The study area is part of a regional-scale fault–fracture network in an interbedded limestone–shale sequence, which formed at depths of ∼ 5–10 km (∼ 150–200 °C) during late Devonian crustal shortening. This integrated approach demonstrates that the zonation of textures, Sr isotopes, stable isotopes (C, O), and trace and rare earth elements is distinctly asymmetrical about the median growth-line of the vein. δ 18O values in vein calcite (17.0–18.8‰, VSMOW) are lower than those in surrounding unaltered limestones (23–25‰, VSMOW), and vary systematically across the vein. In contrast, δ 13C values are relatively constant across most of the vein, but become markedly depleted ( ca. 4‰) immediately adjacent to the wall rock. This strong depletion in δ 13C was probably caused by the influx of more oxidised fluids during the latest stages of vein growth. Strontium isotope ratios ( 87Sr/ 86Sr) vary between 0.70912 and 0.70931 across the vein. Abrubt changes in 87Sr/ 86Sr, δ 18O, Ce/Ce ⁎, Eu/Eu ⁎ and trace element concentrations indicate that vein growth was accompanied by stepwise changes in the fluid flow pathway and consequent variations in fluid chemistry. Taken together, our findings are not consistent with growth of fibrous antitaxial veins by diffusional transfer of material from the surrounding wall rock. Instead, they suggest that externally sourced fluids migrated along episodically changing fracture-controlled flow pathways. This has implications for the dynamics of crustal permeability and mineralisation.

Characteristics of a hydrothermal vent assemblage on a volcanically active segment of Juan de Fuca Ridge, northeast Pacific

An eruption on Cleft segment, Juan de Fuca Ridge, in 1986 provided an opportunity to observe potential successional patterns in vent-animal colonisation. Other objectives were to describe the Cleft fauna, examine the distribution and abundance of selected taxa, and determine if the fauna reflected changes in water chemistry. Biological samples were tubeworm grabs collected by submersible, and visual data were still photographs and videos. Two years post eruption, there were extensive diffuse vents and 23 of the 44 species constituting the Cleft species pool were present. Five years post eruption, most low-temperature vents were extinct. High-temperature venting was maintained, and biological communities were reduced in visual extent. Four of the 44 species in samples from 1988-1994 accounted for over 90% of the individuals. Cluster analyses of species collected on tubes did not distinguish year or substratum differences, suggesting that a study of less than a year is necessary to document successional patterns at new vents. The Cleft subset of the Juan de Fuca Ridge species pool is likely adapted to episodic eruptive events on the decadal scale. Major changes in fluid chemistry did not result in detectable community changes other than habitat loss due to a decrease in dissolved sulphide availability.

Fault rocks and past to recent fluid characteristics from the borehole survey of the Nojima fault ruptured in the 1995 Kobe earthquake, southwest Japan

The mineralogy, fluid inclusions, and distribution of fault rocks of the Nojima fault were examined in the core recovered from a borehole drilled by the Geological Survey of Japan (GSJ) 12 months after the 1995 Kobe (Hyogo‐ken Nanbu) earthquake (MJMA = 7.2) in southwest Japan. The borehole was drilled across a slipped portion of the fault to a depth of 746.7 m. Nearly continuous coring between 152.2 and 746.7 m recovered granodiorite protolith, porphyry dikes, and fault‐related rocks. The fault zone was intersected at 426.2 m and is characterized by a greater intensity of brittle deformation and/or hydrothermal alteration than typical host granodiorite. The fault core consists of three types of fault gouge and occurs at the depth range of 623.1 to 625.3 m. The fault‐normal thicknesses of the fault core and the fault zone are 0.3 m and &gt;46.5 m, respectively. Three types of hydrothermal alteration are recorded by mineral assemblages and fluid inclusions. The first type is characterized by chloritization of mafic minerals at &gt;200°C and occurred prior to the fault activity during the intrusion and cooling of the granodiorite. The second type occurred during faulting and is recorded by zeolite mineralization at &lt;200°C. The third type is recorded by carbonate mineralization related to recent fluid flow. Although most of the second type of alteration occurred prior to the third type, repeated mineralization is recorded by mutually crosscutting relationships between zeolite and carbonate veins and between zeolite vein and carbonate‐precipitated fault gouge. This may record repeated changes in fluid chemistry within the fault zone in connection with the seismic cycle. Although the Nojima fault slipped in the 1995 earthquake, ancient fault‐related textures and mineral alteration are well preserved in the fault rocks.

Dissolution and time-dependent compaction of albite sand: experiments at 100°C and 160°C in pH-buffered organic acids and distilled water

Aqueous fluids are important in the diagenesis and deformation of crustal rocks. Both chemical and physical interactions are involved and often they are strongly coupled. For example, pore waters not only dissolve, transport, and precipitate chemical species, but they also substantially affect the mechanical behavior of the rocks that contain them. Stresses magnified at grain contacts by differences in pore-fluid pressure ( P p) and confining pressure ( P c) can, in turn, influence the rate and extent of chemical exchange. To begin investigation of these coupled systems, compaction experiments were conducted using albite sand (250-500 μm) and distilled water (pH 5.8), 0.07 M acetate (pH 4.7), and 0.07 M acetate + 0.005 M citrate (pH 4.4) solutions in a hydrothermal flow-through system at conditions that simulate diagenesis. Pore-fluid chemistry and pore-volume loss were monitored to quantify the effects of organic acids on time-dependent compaction rates. The effects of stress and fluid chemistry on the dissolution kinetics were also examined. Albite dissolution rates, monitored by steady-state fluid chemistry, increased when an effective pressure ( P e= P c− P p) was applied, probably due to increases in total surface area caused by grain breakage at contacts. These effects were transient in distilled water, however, Si and Al concentrations remained elevated in the acetate pore fluid. The average Si-based release rates indicate ≈35% increase in reactive surface area by application of P e=34.5 MPa. At 100°C with P e=34.5 MPa, steady-state Si concentrations were ≈2.3 times higher in 0.07 M acetate and 5.8 times higher in 0.07 M acetate + 0.005 M citrate than in distilled water. Al increased by even larger factors (3× in the acetate buffer and 10× in the citrate solution). These changes in fluid chemistry are attributed to both pH and ligand-enhanced reactions. Albite dissolution appears to be controlled by surface complexation reactions at Al sites. Rapid dissolution of albite in the organic acid solutions is probably due to the ability of organic acid ligands to selectively complex with aluminum. Time-dependent compaction was observed at 100 and 160°C with P e=34.5 MPa. Strain rates increased with temperature from ≈10 −9 s −l at 100°C to ≈10 −8 s −l at 160°C and decreased with strain in all pore fluids, especially at 100°C. Compaction rates in distilled water and in the acetate solution had similar magnitudes and strain dependencies; however, small amounts of citrate species apparently enhance compaction compared to the other fluids at similar strains. Textural data indicate that time-dependent compaction of the albite sand occurred primarily by brittle mechanisms at these temperatures. However, the deformation is clearly thermally activated and may be chemically assisted by the aqueous pore fluid.

Modeling fluid–rock interaction during greisenization at the East Kemptville tin deposit: implications for mineralization

Interaction between a tin-bearing greisenizing fluid and a leucogranitic rock was modeled in order to evaluate the possible role of alteration induced changes in fluid chemistry in cassiterite deposition. The interaction was modeled using simple titration and multiple-pass fluid flow models with the program CHILLER, modified to permit calculations at 450°C and 4.0 kb. The compositions of the mineralizing fluid and the unaltered leucogranite were based, respectively, on analyses of fluid inclusions and whole-rock chemistry, of samples from the East Kemptville greisen-hosted tin deposit, Nova Scotia. The starting fluid contained 4.85 M Cl, 3.7 M Na, 0.53 M Fe, and minor K, Si, F and Sn, and was at a pH of 3.0, while the leucogranite contained 35 wt.% quartz, 29 wt.% albite, 19 wt.% muscovite and 17 wt.% K-feldspar. In both models, the calculations show that with increasing alteration, K-feldspar was the first mineral consumed, and that this was followed by replacement of albite by muscovite. With further alteration, muscovite was replaced by topaz and pyrrhotite, and cassiterite precipitated in quartz–topaz rocks. In the most altered rocks, only topaz and quartz are present. These changes accurately reproduce those determined for natural samples at East Kemptville. However, only the multiple-pass fluid flow model predicts the changes in fluid inclusion composition. The multiple-pass fluid flow model quantified changes in fluid composition as a function of its interaction with the wall rock and thus enabled us to assess better the factors that controlled cassiterite precipitation. Important conclusions of the model are that the fluid and the rock equilibrated at each stage of the alteration and the water/rock ratio remained constant across the alteration halo. We further concluded that phases precipitated close to the vein in an earlier stage of the alteration were re-dissolved with evolution of the mineralizing system. In particular, re-dissolution of cassiterite caused an increase in the concentration of SnCl + in the fluid. As a result, cassiterite saturated at a lower pH, and the chemical potential of SnCl + increased away from the vein. This could, in turn, explain the low tin grades at East Kemptville.

The Potential Role of Pyrite Oxidation In Corrosion And Reservoir Souring

Abstract Pyrite (FeS2) is a common and ubiquitous constituent of reservoir rocks and forms geologically under extremely reducing conditions. Pyrite is easily oxidized by exposure to moderately oxidizing conditions, including de-oxygenated injection or process water. When pyrite is oxidized in water, the resulting products include sulphuric acid (H2SO4) and sulphate ion (SO42-). In the near well bore, the resulting waters are far from equilibrium with Portland type well cements. A small amount of pyrite oxidation can result in total removal of well cement and the replacement of the cement by structurally less stable products, such as calcium sulphate. In addition, the sulphuric acid may result in corrosion of well tubing or above ground facilities. Casing leaks, particularly in injection wells, may lead to failures of casing due to cement collapse, tubing corrosion, or both. In the reservoir, during water flooding or steam injection, pyrite oxidation may release sulphate ion (SO42 -) which can promote the activity of sulphate reducing bacteria, resulting in "souring" and the production of the acids gases H2S and CO2, where no production of acid gas was noted before injection. The results of pyrite oxidation are examined using reaction path modeling, in which the effects on reservoir fluids and rocks of these complex chemical reactions, including corrosion and scaling, can be determined. Introduction Pyrite (FeS2) is an ubiquitous constituent of carbonate and clastic reservoir rocks. Pyrite is particularly abundant in shales, and may be an unrecognized cause of well bore or casing failures in shaly intervals. Pyrite is only stable at very low oxidation states, partly depending on the pH and temperature, and consequently is very susceptible to oxidation during injection of fluids of relatively high oxidation state, such as steam or water from water floods. The objective of this paper is to show the possible consequences of pyrite oxidation for well bore cement stability and the effect on the composition of formation fluids. Chemical models are used to determine whether the changes in fluid chemistry can have significant effects on the stability of well cement (portlandite) and common reservoir minerals, such as calcite. One of the consequences of oxidation of a sulphide mineral is the release of sulphate to the aqueous fluid. In the typical hydrocarbon reservoir environment this provides a sulphate source for anaerobes that reduce sulphate to sulphide, resulting in the creation of sour gas in water flooded reservoirs that previously were not subject to sour production. Limited data from steam and water flood operations suggest that pyrite oxidation may occur, leading to the possibility that such pilots may become subject to sour production as production proceeds. Pyrite Oxidation and Cement Dissolution The oxidation of pyrite does not require a source of "free" oxygen, but can utilize the oxygen in water. Observation of Reaction (1) below shows that, in addition to the release of sulphate, oxidation of pyrite will release hydrogen ions, producing acidic conditions. Effectively, the product of pyrite oxidation and dissolution is sulphuric acid. Equation 1 (available in full paper)

Effect of Variable-pH Landfill Leachate on a Carbonate Rock Aggregate

Carbonate rock aggregate is commonly used as drainage stone in leachate collection systems of RCRA Subtitle D landfills. U.S. EPA technical guidance for waste containment facilities states that excessive carbonate in drainage stone may result in dissolution and clogging at sites of reprecipitation (Daniel and Koerner, 1993, p. 197). Unfortunately, this theoretical concern does not appear to be confirmed by any experimental evidence with typical landfill leachate or documented instances of clogging at typical Subtitle D landfills. This paper describes a practical laboratory experiment that measured the effects of landfill leachate—maintained at three distinct pH levels—on a mixed limestone and dolomitic limestone aggregate. Equilibrium concentrations of dissolved calcium carbonate in dilute aqueous solutions can be calculated readily for various pH levels. However, such calculations do not predict the kinetics of such a reaction. Moreover, formation of complexes, consisting of calcium and magnesium ions derived from the aggregate and anions from organic acids contained in the leachate, is a complicated process not easily accounted for by simple equilibrium chemistry of dilute aqueous solutions. The experimental approach employed for this project circumvents these theoretical obstacles to predicting the rate and extent of dissolution. Four column-type laboratory models were constructed of plexiglass and filled with carbonate aggregate and leachate from an operating RCRA Subtitle D sanitary landfill to simulate actual conditions within a leachate collection system. Fluids in the cylinders were continuously recirculated and maintained at different target pH values of: a) as close to 3.0 as possible, to simulate a worst-case dissolution scenario; b) 6.0 to 6.5, to simulate typical landfill conditions; c) equilibrium pH; and d) distilled water at 6.0 to 6.5, as an experimental control. The cylinders were sealed and subjected to an anaerobic atmosphere of carbon dioxide mixed with nitrogen. Trends in the chemistry of the cylinder fluids were measured over a 20-week period, during which time the cylinder fluids were sampled 12 times for alkalinity, total dissolved solids, specific conductance, total and dissolved calcium, and total and dissolved magnesium. The cylinders functioned essentially as batch reactors, with fluids removed only for sampling and fluids added only to replace the sampled volumes. After five and 17 weeks, the cylinders were opened, the fluids emptied, fresh fluids added, and the cylinders resealed with anaerobic atmosphere for subsequent test intervals. Results of the experiment demonstrated negligible weight loss in the aggregate sample with leachate maintained at a pH of 6.0–6.5, conditions typical of a RCRA Subtitle D landfill. Leachate maintained at equilibrium pH, and distilled water maintained at pH 6.0–6.5, also experienced negligible weight loss. The aggregate sample maintained with leachate at approximately pH 3.0 experienced a weight reduction of 12 percent. Consistent with the weight loss, leachate samples collected from the same cylinder showed a sharp decrease in alkalinity, and sharp increases in TDS and dissolved calcium, indicating significant dissolution of the aggregate material. Fluid chemistry changes in the three cylinders maintained at higher pH were judged to be insignificant. Based on the results of the completed experiment, 100-percent carbonate rock aggregate is suitably dissolution-resistant for use in leachate collection systems containing typical landfill leachate (pH = 6.0–6.5). The experimental results further suggest that the exact chemical\mineralogical composition of carbonate drainage stone and minor changes in leachate pH may be much less important in controlling dissolution than factors such as particle size.