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1.
Geologic carbon sequestration (GCS) is being considered as a climate change mitigation option in many future energy scenarios. Mathematical modeling is routinely used to predict subsurface CO2 and resident brine migration for the design of injection operations, to demonstrate the permanence of CO2 storage, and to show that other subsurface resources will not be degraded. Many processes impact the migration of CO2 and brine, including multiphase flow dynamics, geochemistry, and geomechanics, along with the spatial distribution of parameters such as porosity and permeability. In this article, we review a set of multiphase modeling approaches with different levels of conceptual complexity that have been used to model GCS. Model complexity ranges from coupled multiprocess models to simplified vertical equilibrium (VE) models and macroscopic invasion percolation models. The goal of this article is to give a framework of conceptual model complexity, and to show the types of modeling approaches that have been used to address specific GCS questions. Application of the modeling approaches is shown using five ongoing or proposed CO2 injection sites. For the selected sites, the majority of GCS models follow a simplified multiphase approach, especially for questions related to injection and local‐scale heterogeneity. Coupled multiprocess models are only applied in one case where geomechanics have a strong impact on the flow. Owing to their computational efficiency, VE models tend to be applied at large scales. A macroscopic invasion percolation approach was used to predict the CO2 migration at one site to examine details of CO2 migration under the caprock.  相似文献   

2.
Particular attention is paid to the risk of carbon dioxide (CO2) leakage in geologic carbon sequestration (GCS) operations, as it might lead to the failure of sequestration efforts and to the contamination of underground sources of drinking water. As carbon dioxide would eventually reach shallower formations under its gaseous state, understanding its multiphase flow behavior is essential. To this aim, a hypothetical gaseous leak of carbon dioxide resulting from a well integrity failure of the GCS system in operation at Hellisheiði (CarbFix2) is here modeled. Simulations show that migration of gaseous carbon dioxide is largely affected by formation stratigraphy, intrinsic permeability, and retention properties, whereas the initial groundwater hydraulic gradient (0.0284) has practically no effect. In two different scenarios, about 18.3 and 30.6% of the CO2 that would have been injected by the GCS system for 3 days could be potentially released again into the atmosphere due to a sustained leakage of the same duration. As the gaseous leak occurs, the aquifer experiences high pressure buildups, and the presence of a less conductive layer further magnifies these. Strikingly, the dimensional analysis showed that buoyant and viscous forces can be comparable over time within the predicted gaseous plumes, even far from the leakage source. Local pressure gradients, buoyant, viscous, and capillary forces all play an important role during leakage. Therefore, neglecting one or more of these contributions might lead to a partial prediction of gaseous CO2 flow behavior in the subsurface, giving space to incorrect interpretations and wrong operational choices.  相似文献   

3.
This study presents the impact of fractures on CO2 transport, capillary pressure and storage capacity by conducting both experimental and numerical studies. A series of laboratory experiment tests was designed with both a homogeneous and a fractured core under CO2 storage conditions. The experimental results reveal a piston-like brine displacement with gravity override effects in the homogeneous core regardless of CO2 injection rates. In the fractured core, however, two distinctive types of brine displacements were observed; one showing brine displacement only in the fracture whereas the other shows brine displacement both in the fracture and matrix with different rates, which were dependent on the magnitude of the pressure build-up in the matrix. The injectivity in the fractured core was twice of the homogeneous core, while the amount of calculated CO2 in the homogeneous core was over 1.5 times greater than the fractured core. Salt precipitation, which is likely to occur near injection wells, was observed in the experiments; X-ray images enabled the observation of salt-precipitation during CO2-flooding tests. Finally, numerical simulations predict free-phase CO2 transfer between fracture and matrix in a fracture-matrix system. Pressure gradients between the fracture and matrix enforced CO2 to transfer from the fracture into matrix at the front of the CO2 plume, whereas, the reversal of pressure gradients at the rear zone of the CO2 plume reversed the transfer process. The variation of CO2 saturation within the fracture was caused by fracture aperture variations, and local variations of fracture permeability control the free-phase CO2 transfer between the fracture and matrix.  相似文献   

4.
CO2 injection and storage in deep saline aquifers involves many coupled processes, including multiphase flow, heat and mass transport, rock deformation and mineral precipitation and dissolution. Coupling is especially critical in carbonate aquifers, where minerals will tend to dissolve in response to the dissolution of CO2 into the brine. The resulting neutralization will drive further dissolution of both CO2 and calcite. This suggests that large cavities may be formed and that proper simulation may require full coupling of reactive transport and multiphase flow. We show that solving the latter may suffice whenever two requirements are met: (1) all reactions can be assumed to occur in equilibrium and (2) the chemical system can be calculated as a function of the state variables of the multiphase flow model (i.e., liquid and gas pressure, and temperature). We redefine the components of multiphase flow codes (traditionally, water and CO2), so that they are conservative for all reactions of the chemical system. This requires modifying the traditional constitutive relationships of the multiphase flow codes, but yields the concentrations of all species and all reaction rates by simply performing speciation and mass balance calculations at the end of each time step. We applied this method to the H2O–CO2–Na–Cl–CaCO3 system, so as to model CO2 injection into a carbonate aquifer containing brine. Results were very similar to those obtained with traditional formulations, which implies that full coupling of reactive transport and multi-phase flow is not really needed for this kind of systems, but the resulting simplifications may make it advisable even for cases where the above requirements are not met. Regarding the behavior of carbonate rocks, we find that porosity development near the injection well is small because of the low solubility of calcite. Moreover, dissolution concentrates at the front of the advancing CO2 plume because the brine below the plume tends to reach high CO2 concentrations quite rapidly. We conclude that carbonate dissolution needs not to be feared.  相似文献   

5.
Between the years 2008 and 2013, approximately 67 kilotons of CO2 have been injected at the Ketzin site, Germany. As part of the geophysical monitoring programme, time‐lapse electrical resistivity tomography has been applied using crosshole and surface‐downhole measurements of electrical resistivity tomography. The data collection of electrical resistivity tomography is partly based on electrodes that are permanently installed in three wells at the site (one injection well and two observation wells). Both types of ERT measurements consistently show the build‐up of a CO2‐related resistivity signature near the injection point. Based on the imaged resistivity changes and a petrophysical model, CO2 saturation levels are estimated. These CO2 saturations are interpreted in conjunction with CO2 saturations inferred from neutron‐gamma loggings. Apart from the CO2–brine substitution response in the observed resistivity changes, significant imprints from the dynamic behaviour of the CO2 in the reservoir are observed.  相似文献   

6.
The expanding use of horizontal drilling and hydraulic fracturing technology to produce oil and gas from tight rock formations has increased public concern about potential impacts on the environment, especially on shallow drinking water aquifers. In eastern Kentucky, horizontal drilling and hydraulic fracturing have been used to develop the Berea Sandstone and the Rogersville Shale. To assess baseline groundwater chemistry and evaluate methane detected in groundwater overlying the Berea and Rogersville plays, we sampled 51 water wells and analyzed the samples for concentrations of major cations and anions, metals, dissolved methane, and other light hydrocarbon gases. In addition, the stable carbon and hydrogen isotopic composition of methane (δ13C‐CH4 and δ2H‐CH4) was analyzed for samples with methane concentration exceeding 1 mg/L. Our study indicates that methane is a relatively common constituent in shallow groundwater in eastern Kentucky, where methane was detected in 78% of the sampled wells (40 of 51 wells) with 51% of wells (26 of 51 wells) exhibiting methane concentrations above 1 mg/L. The δ13C‐CH4 and δ2H‐CH4 ranged from ?84.0‰ to ?58.3‰ and from ?246.5‰ to ?146.0‰, respectively. Isotopic analysis indicated that dissolved methane was primarily microbial in origin formed through CO2 reduction pathway. Results from this study provide a first assessment of methane in the shallow aquifers in the Berea and Rogersville play areas and can be used as a reference to evaluate potential impacts of future horizontal drilling and hydraulic fracturing activities on groundwater quality in the region.  相似文献   

7.
One of the main concerns of geological carbon storage (GCS) systems is the risk of leakage through “weak” permeable areas of the sealing formation or caprock. Since the fluid pressure pulse travels faster than the carbon dioxide (CO2) plume across the storage reservoir, the fluid overpressure transmitted into overlying permeable formations through caprock discontinuities is potentially detectable sooner than actual CO2 leakage occurs. In this work, an inverse modeling method based on fluid pressure measurements collected in strata above the target CO2 storage formation is proposed, which aims at identifying the presence, the location, and the extent of possible leakage pathways through the caprock. We combine a three-dimensional subsurface multiphase flow model with ensemble-based data assimilation algorithms to recognize potential caprock discontinuities that could undermine the long-term safety of GCS. The goal of this work is to examine and compare the capabilities of data assimilation algorithms such as the ensemble smoother (ES) and the restart ensemble Kalman filter (REnKF) to detect the presence of brine and/or CO2 leakage pathways, potentially in real-time during GCS operations. For the purpose of this study, changes in fluid pressure in the brine aquifer overlying to CO2 storage formation aquifer are hypothetically observed in monitoring boreholes, or provided by time-lapse seismic surveys. Caprock discontinuities are typically characterized locally by higher values of permeability, so that the permeability distribution tends to fit to a non-Gaussian bimodal process, which hardly complies with the requirements of the ES and REnKF algorithms. Here, issues related to the non-Gaussianity of the caprock permeability field are investigated by developing and applying a normal score transform procedure. Results suggest that the REnKF is more effective than the ES in characterizing caprock discontinuities.  相似文献   

8.
The aim of this study was to assess whether acoustic emission (AE) could carry information on preferential sorption/desorption of CH4 or CO2 in coal. AE and expansion/contraction of two nearly identical cylindrical coal samples were continuously monitored during displacement sorption experiments. One sample was subjected to presorption of CH4, followed by sorption of CH4/CO2 mixture. With the other one, presorption of CO2 preceded sorption of the mixture. The results obtained are the following: first, AE and stain kinetics show that the affinity of the coal tested is higher for CO2 than for CH4; second, methane is preferentially desorbed after presorption of CH4 — sorption of mixture of CH4 and CO2; third, during displacement sorption, kinetics of AE and sample swelling/shrinkage bring out the importance of presorption and the sorbate used. It matters whether the coal is first exposed to CH4 or to CO2. The present study has demonstrated that injection of CO2 into the coal previously exposed to CH4 causes considerable swelling of the coal. On desorption after CH4/CO2 exchange sorption, initial shrinkage is followed by swelling of the coal. These results could have implications for the sequestration of CO2 in coal seams and CH4 recovery from coalbeds (ECBM). Swelling/shrinkage of the coal matrix should be included in models used to predict coal permeability and gas flow rates. They also show that the AE technique can give more insights into coal matrix-gas interactions.  相似文献   

9.
Gaining streams can provide an integrated signal of relatively large groundwater capture areas. In contrast to the point‐specific nature of monitoring wells, gaining streams coalesce multiple flow paths. Impacts on groundwater quality from unconventional gas development may be evaluated at the watershed scale by the sampling of dissolved methane (CH4) along such streams. This paper describes a method for using stream CH4 concentrations, along with measurements of groundwater inflow and gas transfer velocity interpreted by 1‐D stream transport modeling, to determine groundwater methane fluxes. While dissolved ionic tracers remain in the stream for long distances, the persistence of methane is not well documented. To test this method and evaluate CH4 persistence in a stream, a combined bromide (Br) and CH4 tracer injection was conducted on Nine‐Mile Creek, a gaining stream in a gas development area in central Utah. A 35% gain in streamflow was determined from dilution of the Br tracer. The injected CH4 resulted in a fivefold increase in stream CH4 immediately below the injection site. CH4 and δ13CCH4 sampling showed it was not immediately lost to the atmosphere, but remained in the stream for more than 2000 m. A 1‐D stream transport model simulating the decline in CH4 yielded an apparent gas transfer velocity of 4.5 m/d, describing the rate of loss to the atmosphere (possibly including some microbial consumption). The transport model was then calibrated to background stream CH4 in Nine‐Mile Creek (prior to CH4 injection) in order to evaluate groundwater CH4 contributions. The total estimated CH4 load discharging to the stream along the study reach was 190 g/d, although using geochemical fingerprinting to determine its source was beyond the scope of the current study. This demonstrates the utility of stream‐gas sampling as a reconnaissance tool for evaluating both natural and anthropogenic CH4 leakage from gas reservoirs into groundwater and surface water.  相似文献   

10.
Spatial and seasonal variations in CO2 and CH4 concentrations in streamwater and adjacent soils were studied at three sites on Brocky Burn, a headwater stream draining a peatland catchment in upland Britain. Concentrations of both gases in the soil atmosphere were significantly higher in peat and riparian soils than in mineral soils. Peat and riparian soil CO2 concentrations varied seasonally, showing a positive correlation with air and soil temperature. Streamwater CO2 concentrations at the upper sampling site, which mostly drained deep peats, varied from 2·8 to 9·8 mg l?1 (2·5 to 11·9 times atmospheric saturation) and decreased markedly downstream. Temperature‐related seasonal variations in peat and riparian soil CO2 were reflected in the stream at the upper site, where 77% of biweekly variation was explained by an autoregressive model based on: (i) a negative log‐linear relationship with stream flow; (ii) a positive linear relationship with soil CO2 concentrations in the shallow riparian wells; and (iii) a negative linear relationship with soil CO2 concentrations in the shallow peat wells, with a significant 2‐week lag term. These relationships changed markedly downstream, with an apparent decrease in the soil–stream linkage and a switch to a positive relationship between stream flow and stream CO2. Streamwater CH4 concentrations also declined sharply downstream, but were much lower (<0·01 to 0·12 mg l?1) than those of CO2 and showed no seasonal variation, nor any relationship with soil atmospheric CH4 concentrations. However, stream CH4 was significantly correlated with stream flow at the upper site, which explained 57% of biweekly variations in dissolved concentrations. We conclude that stream CO2 can be a useful integrative measure of whole catchment respiration, but only at sites where the soil–stream linkage is strong. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

11.
The Okinawa trough is a spreading back-are basin featuring emitting hydrothermal solutions (black chimney type) and modem sulfide precipitation on the sea floor. The study of fluid inclusions in water-rock interaction products in the Jade hydrothermal field indicates that the deep hydrothermal system beneath the sea floor is fairly rich in gas and there are two independent and coexisting fluids-CO2-hydrocarbon fluid and salt aqueous fluid. On the whole, the composition of CO2-hydrocarbon fluid inclusions is similar to that of the fluid inclusions in natural gas fields. The dominant composition of the inclusions in aqueous fluid is H2O with CO2 and CH4 being oversaturated. The salt aqueous fluid of the Jade hydrothermal system might be emitted through a black chimney, whereas CO2-rich fluids discharge CO2 bubbles and CO2 hydrate through fissures. Hydrocarbons in gas phase or in fluid might be enclosed somewhere under the sea. Large storage of CO2-CH4-H2S gas or fluid and reaction of this gas or fluid with salt water will lead to commercial sulfide deposits.  相似文献   

12.
Baseline characterization of concentrations and isotopic values of dissolved natural gases is needed to identify contamination caused by the leakage of fugitive gases from oil and gas activities. Methods to collect and analyze baseline concentration‐depth profiles of dissolved CH4 and C2H6 and δ13C‐CH4 in shales and Quaternary clayey tills were assessed at two sites in the Williston Basin, Canada. Core and cuttings samples were stored in Isojars® in a low O2 headspace prior to analysis. Measurements and multiphase diffusion modeling show that the gas concentrations in core samples yield well‐defined and reproducible depth profiles after 31‐d equilibration. No measurable oxidative loss or production during core sample storage was observed. Concentrations from cuttings and mud gas logging (including IsoTubes®) were much lower than from cores, but correlated well. Simulations suggest the lower concentrations from cuttings can be attributed to drilling time, and therefore their use to define gas concentration profiles may have inherent limitations. Calculations based on mud gas logging show the method can provide estimates of core concentrations if operational parameters for the mud gas capture cylinder are quantified. The δ13C‐CH4 measured from mud gas, IsoTubes®, cuttings, and core samples are consistent, exhibiting slight variations that should not alter the implications of the results in identifying the sources of the gases. This study shows core and mud gas techniques and, to a lesser extent, cuttings, can generate high‐resolution depth profiles of dissolved hydrocarbon gas concentrations and their isotopes.  相似文献   

13.
Systematic analysis of major and minor elements in groundwaters from springs and wells on the slopes of Mt. Etna in 1995–1998 provides a detailed geochemical mapping of the aquifer of the volcano and of the interactions between magmatic gas, water bodies and their host rocks. Strong spatial correlations between the largest anomalies in pCO2 (pH and alkalinity) K, Rb, Mg, Ca and Sr suggest a dominating control by magmatic gas (CO2) and consequent basalt leaching by acidified waters of the shallow (meteoric) Etnean aquifer. Most groundwaters displaying this magmatic-type interaction discharge within active faulted zones on the S–SW and E lower flanks of the volcanic pile, but also in a newly recognised area on the northern flank, possibly tracking a main N–S volcano-tectonic structure. In the same time, the spatial distribution of T°C, TDS, Na, Li, Cl and B allows us to identify the existence of a deeper thermal brine with high salinity, high content of B, Cl and gases (CO2, H2S, CH4) and low K/Na ratio, which is likely hosted in the sedimentary basement. This hot brine reaches the surface only at the periphery of the volcano near the Village of Paternò, where it gives rise to mud volcanoes called “Salinelle di Paternò”. However, the contribution of similar brines to shallower groundwaters is also detected in other sectors to the W (Bronte, Maletto), SW (Adrano) and SE (Acireale), suggesting its possible widespread occurrence beneath Etna. This thermal brine is also closely associated with hydrocarbon fields all around the volcano and its rise, generally masked by the high outflow of the shallow aquifer, may be driven by the ascent of mixed sedimentary–magmatic gases through the main faults cutting the sedimentary basement.  相似文献   

14.
Understanding multiphase transport within saline aquifers is necessary for safe and efficient CO2 sequestration. To that end, numerous full‐physics codes exist for rigorously modeling multiphase flow within porous and permeable rock formations. High‐fidelity simulation with such codes is data‐ and computation‐intensive, and may not be suitable for screening‐level calculations. Alternatively, under conditions of vertical equilibrium, a class of sharp‐interface models result in simplified relationships that can be solved with limited computing resources and geologic/fluidic data. In this study, the sharp‐interface model of Nordbotten and Celia (2006a,2006b) is evaluated against results from a commercial full‐physics simulator for a semi‐confined system with vertical permeability heterogeneity. In general, significant differences were observed between the simulator and the sharp‐interface model results. A variety of adjustments were made to the sharp‐interface model including modifications to the fluid saturation and effective viscosity in the two‐phase region behind the CO2‐brine interface. These adaptations significantly improved the predictive ability of the sharp interface model while maintaining overall tractability.  相似文献   

15.
M. F. Billett  T. R. Moore 《水文研究》2008,22(12):2044-2054
Carbon dioxide (CO2) and methane (CH4) concentrations and evasion rates were measured in surface waters draining Mer Bleue peatland (Ontario, Canada) between spring and autumn 2005. All sites exhibit a consistent pattern of supersaturation throughout the year, which is broadly related to hydrological and temperature changes between spring snowmelt and autumn freezing. Both measurements and estimates of CO2 and CH4 evasion from open water to the atmosphere suggest that parts of the catchment (including beaver dams) are significant degassing hot spots. We present data showing how vertical gaseous carbon fluxes compare with lateral carbon fluxes and make an initial estimate of the importance to the overall carbon budget of CO2 and CH4 evasion to the atmosphere from water surfaces at Mer Bleue. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

16.
Surface waters associated with peatlands, supersaturated with CO2 and CH4 with respect to the atmosphere, act as important pathways linking a large and potentially unstable global repository of C to the atmosphere. Understanding the drivers and mechanisms which control C release from peatland systems to the atmosphere will contribute to better management and modelling of terrestrial C pools. We used non‐dispersive infra‐red (NDIR) CO2 sensors to continuously measure gas concentrations in a beaver pond at Mer Bleue peatland (Canada); measurements were made between July and August 2007. Concentrations of CO2 in the surface water (10 cm) reached 13 mg C l?1 (epCO2 72), and 26 mg C l?1 (epCO2 133) at depth (60 cm). The study also showed large diurnal fluctuations in dissolved CO2 which ranged in amplitude from ～1·6 mg C l?1 at 10 cm to ～0·2 mg C l?1 at 60 cm depth. CH4 concentration and supersaturation (epCH4) measured using headspace analysis averaged 1·47 mg C l?1 and 3252, respectively; diurnal cycling was also evident in CH4 concentrations. Mean estimated evasion rates of CO2 and CH4 over the summer period were 44·92 ± 7·86 and 0·44 ± 0·25 µg C m?2s?1, respectively. Open water at Mer Bleue is a significant summer hotspot for greenhouse gas emissions within the catchment. Our results suggest that CO2 concentrations during the summer in beaver ponds at Mer Bleue are strongly influenced by biological processes within the water column involving aquatic plants and algae (in situ photosynthesis and respiration). In terms of carbon cycling, soil‐stream connectivity at this time of year is therefore relatively weak. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

17.
The injection of CO2 at the Ketzin pilot CO2 storage site started in June 2008 and ended in August 2013. During the 62 months of injection, a total amount of about 67 kt of CO2 was injected into a saline aquifer. A third repeat three‐dimensional seismic survey, serving as the first post‐injection survey, was acquired in 2015, aiming to investigate the recent movement of the injected CO2. Consistent with the previous two time‐lapse surveys, a predominantly west–northwest migration of the gaseous CO2 plume in the up‐dip direction within the reservoir is inferred in this first post‐injection survey. No systematic anomalies are detected through the reservoir overburden. The extent of the CO2 plume west of the injection site is almost identical to that found in the 2012 second repeat survey (after injection of 61 kt); however, there is a significant decrease in its size east of the injection site. Assessment of the CO2 plume distribution suggests that the decrease in the size of the anomaly may be due to multiple factors, such as limited vertical resolution, CO2 dissolution, and CO2 migration into thin layers, in addition to the effects of ambient noise. Four‐dimensional seismic modelling based on dynamic flow simulations indicates that a dynamic balance between the newly injected CO2 after the second repeat survey and the CO2 migrating into thin layers and being dissolved was reached by the time of the first post‐injection survey. In view of the significant uncertainties in CO2 mass estimation, both patchy and non‐patchy saturation models for the Ketzin site were taken into consideration.  相似文献   

18.
Although riparian zones are well known to reduce nitrogen (N) and phosphorus (P) runoff to streams, they also have the potential to affect greenhouse gas (CO2, N2O, and CH4) fluxes to the atmosphere. Following large storms, soil biogeochemical conditions often become more reduced, especially in oxbow depressions and side channels, which can lead to hot moments of greenhouse gas production. Here, we investigate the impact of the remnants of Hurricane Irene and Tropical Storm Lee on riparian zone hydrology (water table: WT), and biogeochemistry (oxidation‐reduction potential [ORP], dissolved oxygen [DO], NO3?, PO43?, CO2, N2O, CH4). Results indicate that large storms have the potential to reset WT levels for weeks to months. Overbank flooding at our site following Irene and Lee led to the infiltration of well‐oxygenated water at depth (higher DO and ORP) while promoting the development of anoxic conditions within soil aggregates near the soil surface (increased N2O and CH4 fluxes). A short‐term increase in CO2 emission was observed following Irene at our site where aerobic respiration was water‐limited. Over a 2‐year period, an oxbow depression exhibited higher WT, higher N2O and CH4 fluxes (hot moment), higher CO2 fluxes (seasonal), and lower NO3? concentrations (seasonal) than the rest of the riparian zone. However, neither Irene, nor Lee, nor the oxbow depression significantly impacted PO43?. Dissolved organic carbon, ORP, and DO data illustrate the time‐lag (>20 years) between the creation of an oxbow depression and the development of reducing conditions despite clear differences in riparian zone and oxbow WT dynamics.  相似文献   

19.
Steam samples from six wells (Colombaia, Pineta, Larderello 57, Larderello 155, Gabbro 6, and Gabbro 1) in a south to north section across the Larderello geothermal field have been analyzed for inorganic and hydrocarbon gases and for oxygen-18 and deuterium of steam. The wells generally decrease in depth and increase in age toward the south. The steam samples are generally characterized by
  1. Total gas contents increasing south to north from 0.003 to 0.05 mole fraction;
  2. Constant CO2 (95±2 percent); near constant H2S (1.6±0.8), N2 (1.2±0.8), H2 (2±1), CH4 (1.2±1), and no O2 in the dry gas;
  3. Presence of numerous, straight chain and branched C2 to C6 hydrocarbons plus benzene in amounts independent of CH4 contents with highest concentrations in the deeper wells;
  4. Oxygen-18 contents of steam increasing south to north from ?5.0‰ to ?0.4‰ with little change in deuterium (?42±2‰).
These observations are interpreted as showing:
  1. Decreasing gas contents with amount of production because the proportion of steam boiled from liquid water increases with production;
  2. Synthesis of CH4 from H2 and CO2 with CO2 and H2 produced by thermal metamorphism and rock-water reactions;
  3. Extraction of C2 to C6 hydrocarbons from rock organic matter;
  4. Either oxygen isotope exchange followed by distillation of steam from the north toward the south (2 plates at ～220°C) or mixture of deeper more-exchange waters from the north with shallow, less-exchanged recharging waters from the south.
  相似文献   

20.
Gas variations of many mud volcanoes and hot springs distributed along the tectonic sutures in southwestern Taiwan are considered to be sensitive to the earthquake activity. Therefore, a multi-parameter automatic gas station was built on the bank of one of the largest mud-pools at an active fault zone of southwestern Taiwan, for continuous monitoring of CO2, CH4, N2 and H2O, the major constituents of its bubbling gases. During the year round monitoring from October 2001 to October 2002, the gas composition, especially, CH4 and CO2, of the mud pool showed significant variations. Taking the CO2/CH4 ratio as the main indicator, anomalous variations can be recognized from a few days to a few weeks before earthquakes and correlated well with those with a local magnitude >4.0 and local intensities >2. It is concluded that the gas composition in the area is sensitive to the local crustal stress/strain and is worthy to conduct real-time monitoring for the seismo-geochemical precursors.  相似文献   

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