Abstract A correlation between methanogenesis and dolomite formation has been reported; however, the mechanism underlying this association is not fully understood. In this study, we conducted forced carbonate precipitation experiments at room temperature in calcite-seeded Ca/Mg carbonate solutions containing either purified non-living biomass or bound extracellular polymeric substances (EPS) of the methanogen Methanosarcina barkeri. Purified non-living biomass and bound EPS was used so as to avoid the possible influence of the complex components of the growing microbial culture on carbonate crystallization. Our results demonstrated that non-living biomass of M. Barkeri can enhance the Mg incorporation into calcitic structure and induce the crystallization of disordered dolomite. In the presence of ~113 mg L–1 of non-living biomass, disordered dolomite with ~41 and 45 mol% of MgCO3 was precipitated in solutions with initial Mg:Ca ratios of 5:1 and 8:1, respectively. A systematic increase in the MgCO3 contents of the precipitated Ca-Mg carbonates was also observed with the increased non-living biomass concentration. Bound EPS was shown to be the component of non-living biomass that catalyzed the precipitation of disordered dolomite. At only ~25 mg L–1 of bound EPS, disordered dolomite with ~47 and 48 mol% of MgCO3 was precipitated in solutions with initial Mg:Ca ratios of 5:1 and 8:1, respectively. We propose that adsorption of bound EPS to growing carbonate surfaces through hydrogen bonding is the key to catalyzing disordered dolomite crystallization, and that this mechanism is also applicable to natural EPS-induced dolomite formation. This study provides significant insight into the formation mechanism of microbial-induced dolomite with high δ13C values.
Because of its rare occurrence in modern sediments, as well as the difficulty in synthesizing it under low-temperature conditions in the laboratory, the origin of sedimentary dolomite has remained a long-standing enigma, often referred to as the “dolomite problem.” Recently, anaerobic microorganisms, such as sulfate-reducing bacteria and methanogens, have been recognized for mediating dolomite precipitation. However, the exact role of microorganisms in dolomite crystallization is still under debate and the possible involvement of anaerobic fermenting bacteria has not been studied. In this study, we characterized the effect of purified non-metabolizing biomass and bound extracellular polymeric substances (EPS) of a natural consortium of anaerobic microorganisms dominated by fermenting bacteria and sulfate-reducing bacteria on Ca-Mg carbonate precipitation. This natural consortium was enriched from sediments of Deep Springs Lake, California, where dolomite is still precipitating. Our data show that disordered dolomite, a precursor of some sedimentary stoichiometric ordered dolomite, can be precipitated in calcite-seeded Ca-Mg carbonate solutions containing purified non-metabolizing consortium biomass. Bound EPS extracted from the consortium culture were shown to be the active component that triggered the crystallization of disordered dolomite. Further experiments show that purified non-metabolizing biomass from pure cultures of both anaerobic fermenting and sulfate-reducing bacteria closely related to those organisms present in the consortium could also catalyze the precipitation of disordered dolomite. This study contributes to the understanding of the “dolomite problem” by revealing (1) the catalytic effect of bound EPS on Ca-Mg carbonate crystallization and (2) the possible involvement of anaerobic fermenting bacteria in sedimentary dolomite formation, which has not been reported previously.
Summary As the oil and gas industry is making firm strides in deepwater and shale exploration and development, possible thermal degradation of scale-inhibitor molecules poses a great challenge for scaling control and flow assurance for high-temperature reservoirs. Although extensive research has been conducted to test thermal stability of scale inhibitors, little work has been devoted to study the thermodynamics/kinetics of thermal degradation of scale inhibitors. In this work, a novel and efficient testing approach based on inhibition kinetics has been developed and successfully applied to determine the fraction of the active inhibitor molecules in preheated samples of scale inhibitors with various generic chemistries. Moreover, for the first time, we have modeled the kinetics of inhibitor thermal degradation on the basis of the integrated first-order rate equation and Arrhenius equation, with good agreements between the model predictions and experimental data. The preheated scale inhibitors have been analyzed by nuclear-magnetic-resonance (NMR) spectroscopy for organic-compound characterization. Our results and predictions based on inhibition testing assay are consistent with the 31P/1H NMR analyses. This work has enabled an in-depth understanding of the time and temperature dependence of thermal degradation of scale inhibitors, and facilitates the rational selection and deployment of scale inhibitors for high-temperature oil and gas production.
Summary Barite (BaSO4) is one of the common scales in oil-and-gas production. Extensive work has been conducted to study barite nucleation and inhibition at temperatures below 100°C. However, with the advance in deepwater exploration and production (E&P) which can encounter high-temperature (HT) conditions, a better understanding of barite-scaling risk at HT (e.g., > 150°C) becomes essential. In this paper, a systematic study was conducted to explore barite nucleation kinetics from 70 to 200°C in synthetic brines containing phosphonate (0–10 ppm) or polymeric (5–10 ppm) scale inhibitors. A 2-hour protection time with or without any detectable barite nucleation was used to define the scaling risk. To detect barite nucleation, two novel apparatuses were developed—a modified dynamic flow loop and a batch reactor. The modified dynamic flow loop has a retention time of up to 4 hours and is ideal to carry out experiments at higher than 100°C. Ba concentrations in the effluents were monitored to determine barite nucleation more precisely compared with traditional “tube blocking” technique. The new batch reactor uses our newly developed laser-detection method, a transparent pressure tube, and an oil bath. The transparent pressure tube allows laser light to pass through and can withstand 150-psi pressure at 175°C, therefore providing an efficient approach to study the precipitation kinetics of scales and to evaluate inhibition efficiency of inhibitors at HT. Constant inhibitor-concentration isopleths of diethylenetriamine pentamethylene phosphonic acid (DTPMP) for barite inhibition were constructed on the basis of our experimental data. Finally, a semiempirical model that is based on data of barite nucleation and inhibition kinetics from this study and previous work was built to predict scaling risk of barite at different physicochemical conditions. This model covers a wide range of temperature (from 4 to 200°C) and brine compositions. It also covers the effect of Ba2+–SO42− ratio in solution, common cations (e.g., Ca2+), and thermodynamic hydrate inhibitors on barite precipitation. Model precipitations were found to be consistent with field observations. The results of this study can guide the design of barite-scale treatment for HT oil-and-gas production.
Significance Magnesium-bearing carbonate minerals play critical roles in the health and function of the Earth system because they constitute a significant fraction of lithosphere carbon reservoir and build skeletal structures for the majority of marine invertebrate organisms. Despite wide occurrence, high-Mg and sole-Mg phases such as dolomite ([Ca,Mg]CO 3 ) and magnesite (MgCO 3 ) prove virtually impossible to be crystallized under ambient conditions. It has long been believed that Mg 2+ hydration is the cause for such a geological mystery. Here, we probe this hypothesis by investigating Ca–Mg–CO 3 precipitation in the absence of water and find direct proof suggesting the existence of a more intrinsic crystallization barrier. These findings provide a perspective augmenting our understanding in carbonate mineralogy, biomineralization, and mineral-carbonation processes.
Summary This work reports a reliable and systematic study of barite-nucleation kinetics in the presence of scale inhibitors from 4 to 90°C and at various conditions. In this study, we designed and developed an apparatus to study the nucleation kinetics of barite-scale formation by monitoring the change of photocurrent created by a 5-mW, 635-nm red laser. The photodetector has a wide wavelength range in which sensitivity has a peak at 960 nm. A set of convex and concave lenses was used to control the beam diameter so that it can pass through more particles and increase the sensitivity. Temperature and mixing procedure were precisely controlled by an external waterbath and magnetic stirplate, respectively. The photocurrent output was constant when the laser is shining through a clear solution before scale formation. After scale occurred, laser was scattered by scale particles, which causes the decrease of photocurrent. One can expand this method to study nucleation kinetics of other scales such as carbonates, other sulfates, and sulfide scales. In addition, one can customize it to perform study under high temperature, high pressure, and anoxic conditions. With this newly developed “laser” method, we successfully measured the nucleation kinetics of barite in synthetic brine (1 M NaCl, 0.1 M CaCl2) under various combinations of reaction parameters including temperature (T), pH, saturation index (SI), and Ba2+/SO42− ratio (R). Furthermore, the inhibition efficiency of various scale inhibitors including sulfonated polycarboxylic acid, polyvinyl sulfonate, and inulin on barite precipitation was also investigated. On the basis of the experimental results, the relationship of precipitation kinetics of barite as a function of T, pH, SI, and R was established. Results of this study will be incorporated into scale-prediction software to predict the risk of scale formation and the efficiency of scale inhibitors.
The origin of dolomite is a long-standing enigma in sedimentary geology. It has been proposed that microorganisms, especially anaerobic microorganisms, can overcome kinetic barriers to facilitate dolomite precipitation, although their specific role in dolomite formation is still unclear. Our experimental results demonstrate that disordered dolomite can be synthesized at room temperature abiotically from solutions containing polysaccharides such as carboxymethyl cellulose or agar. We propose that when dissolved in solution, polysaccharides can be strongly adsorbed on Ca-Mg carbonate surfaces through hydrogen bonding. The adsorbed polysaccharides may help weaken the chemical bonding between surface Mg2+ ions and water molecules, which can lower the energy barrier to the desolvation of surface Mg2+-water complexes, enhance Mg2+ incorporation into the precipitating carbonate, and thereby promote disordered dolomite formation. In natural environments, it is possible that polysaccharides produced by microorganisms, e.g., extracellular polysaccharides, may play a key role in promoting disordered dolomite nucleation and crystallization. In marine sediments, the accumulated dissolved carbohydrates produced from organic matter degradation during early diagenesis may also serve as catalysts for disordered dolomite formation.
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