高压条件下页岩对CH_4、CO_2气体吸附特征及其在二元混合气体中应用:以四川盆地焦石坝地区龙马溪组页岩为例

为了解高压条件下页岩对甲烷(CH4)、二氧化碳(CO2)及二元混合气体吸附行为,以四川盆地焦石坝地区下志留统龙马溪组页岩样品为研究对象,通过重量法等温吸附实验,研究了不同温度压力条件下CH4、CO2在页岩中的吸附行为。实验表明,在0.01～50 MPa, 40～100℃条件下,页岩对CH4、CO2过剩吸附量随压力增大而增加,直至达到最大值,然后随压力增大而减小;绝对吸附量随压力增大而增加,在40～43MPa之后,吸附量趋于稳定。在高压条件下,页岩对CO2吸附量大于CH4,约为其5倍。利用CH4、CO2单一气体Langmuir吸附量和Langmuir压力,通过扩展的(Extended) Langmuir模型进行拟合,对不同比例CH4/CO2二元混合气体吸附量进行模拟预测,研究表明,二元气体总吸附量随混合气中CO2比例增大而增加。在高压条件下, Langmuir过剩吸附模型能较好地拟合CH4、CO2在页岩中的吸附,扩展的Langmuir过剩吸附模型也能较好地拟合二元混合气体在页岩中的吸附。     

河北开滦矿区晚古生代煤对CH_4/CO_2二元气体等温解吸特性

研究了河北开滦矿区不同变质程度的煤对不同配比CH4/CO2二元气体等温解吸特性,并用扩展Langmuir方程的推论计算了CH4/CO2二元气体各组分在吸附相中的浓度,分析了其变化特征。结果表明:在开滦矿区煤对CH4/CO2二元气体解吸过程中,中等变质程度煤(Ro=1.21%)对混合气体的吸附能力大于低变质程度煤(Ro=0.58%),且混合气体中CO2浓度越大,总吸附量越多。吸附相中CH4的相对浓度是逐渐降低的,CO2的相对浓度是逐渐升高的。开滦矿区中等变质程度煤相对于低变质程度煤,用CO2气体置换煤层中CH4,可以获得较高的单位压降CH4解吸率,注入CO2的量越多、相对浓度越高,其置换效果就越好,更适于往煤层注入CO2提高煤层气产量技术的实施。     

草酸的稳定性及CO_2包裹体压力计实验研究

应用金刚石压腔(diamond-anvil cell)实验技术,对草酸的稳定性进行了研究。草酸在高压低温条件下可以稳定存在,而在低压高温条件下将分解为CO2、H2O等气体。当成矿流体遇到破裂带或裂隙而发生减压沸腾时,可以使成矿流体中的有机络合物迅速发生分解产生大量CO2,从而造成金属元素在有利的空间沉淀、富集成矿。同时,实验研究了高温高压条件下CO2的物理化学性质,得到了CO2包裹体压力计的测定方程:P(MPa)=271.517·(Δν1381.93-0.010987·ΔT)+0.1,式中Δν1381.93(cm-1)为待测包裹体中CO2的拉曼位移相对于常温常压下CO2的拉曼位移1381.93cm-1之差,ΔT(℃)为待测包裹体的温度与常温(23℃)之差,P(MPa)为待测包裹体的内压。由上式计算拉曼位移ν的标准偏差为±0.2cm-1,压力P的误差为±54MPa。该压力标定方程适用于在高压下温度范围为23℃≤T≤390℃的压力标定。     

混合气体在水溶液中的溶解度计算模型

该文将段振豪及合作者建立的单气体溶解度模型推广到混合气体系,建立了能够计算CO2-CH4-N2-C2H6-H2S混合气 体在电解质水溶液中溶解度的热力学模型。本模型将DMW92方程扩展到上述多组分混合气体系并使用其计算气体组分的 逸度系数,采用Pitzer活度系数模型描述液相并沿用段振豪及合作者以前确定的纯CO2、CH4、C2H6和H2S的溶解度模型参 数,而纯N2的溶解度模型参数由本研究确定。由于本模型不包含依赖混合气体溶解度实验数据确定的参数,因此对混合气 体溶解度的计算是预测性的。通过与实验数据的对比,证实了本模型能够在宽广的温度、压力范围内准确预测 CO2-CH4-N2-C2H6-H2S混合气体在水溶液中的溶解度（对于CO2和CH4的摩尔百分数超过90%的混合气体,本模型适用于 273~523 K和0~2000×105 Pa的温压范围）。本模型的计算表明,相对于纯CO2气相,少量CH4、N2或H2S的加入会降低CO2的溶解度。对于CO2-H2O-NaCl型流体包裹体,少量CH4的加入会增大流体包裹体的均一压力。相关的计算程序可从通讯作者 处获得。     

鄂尔多斯盆地石油中沥青的拉曼光谱特征

激光拉曼光谱技术具有测试简便和精确的特点,通过对石油中沥青的拉曼光谱探针分析,可以研究石油沥青的物质成分特征以及形成演化过程。鄂尔多斯盆地石油沥青中广泛存在CH4等还原性气体,部分样品中同时还含有CO2等氧化性气体,且氧化性气体与还原性气体存在着此消彼长的反相关关系,这种关系及其互为消长的幅度也反映了盆地后期改造作用程度的高低。     

笼状水合物拉曼光谱特征与结构水合数的耦合关系

为了探讨不同气体组分和环境介质对形成笼状结构类型水合物和水合数的影响, 开展了一元体系 (CH4、CO2、C3H8 )和二元体系(CH4 +CO2、CH4 +C3H8、CH4 +N2 )的水合物生成结晶充填过程、结晶构型和动力学特性分析, 并对生成的水合物进行了拉曼光谱分析。结果表明单组分甲烷充填小孔穴 512和大孔穴 512 62 形成Ⅰ型笼状结构水合物(SⅠ), 二氧化碳和丙烷只占据大孔穴 512 64 形成Ⅱ型笼状结构水合物 (SⅡ ); 而二元混合组分中小孔穴中只充填有甲烷, 而没有CO2、N2 和C3H8。应用反褶积的ν1 对称谱带测定了CH4 分子在Ⅰ型结构大孔穴和小孔穴中的相对占有率,并根据谱带的面积比(对应于小孔穴与大孔穴)计算了平衡条件下甲烷水合物孔穴占有率及其耦合的水合数, 认为气体分子的大小不仅影响它所充填的孔穴形态和类型, 而且影响水合物生成的结构类型和水合数。     

注CO2提高煤层气采收率的模拟实验研究

根据煤储层的吸附/解吸机理,模拟煤层气井"排采-注气-排采"的生产过程,进行CH4、CO2的单相气体吸附/解吸和CO2注入置换煤层CH4的实验,得到了CH4和CO2二元气体相组分变化数据和CH4和CO2混合气体的相分离图解.结果表明,在CH4和CO2二元气体的竞争吸附中,CO2组分的吸附速率是先快后慢,而CH4组分的吸附速率先慢后快,解吸时则相反.反映了CO2组分在与CH4组分的竞争吸附中占据优势,优先被吸附;同时发现注入气体数量越大,注入气体中CO2组分浓度越高,单位压降下的CH4解吸率和CO2吸附率越高.实验结论对工业规模的煤层气开发试验具有一定的指导意义.     

C-H-O-N体系挥发分的拉曼定量分析:压力、温度和流体组成的影响

拉曼光谱是定量分析流体包裹体中挥发分组成和压力的重要手段,而这又是恢复流体捕获温度、压力的前提。本文收集了CH_4-N_2、CH_4-N_2-H_2O和CH_4-N_2-CO_2-CO体系在21～350℃和1～60 MPa条件下的原位拉曼光谱,获取了CH_4、N_2、CO_2和CO特征拉曼光谱的峰位和峰面积等光谱参数,并以上述体系为例,探讨了应用拉曼光谱定量分析C-H-O-N体系挥发分组成的关键技术问题,即压力、温度和流体组成对分析结果的影响。首先,在温度恒定的条件下,v_1-CH_4、v_1-N_2、(v_1-2v_2)-CO_2高频峰和v_1-CO峰位随着压力的升高而发生负向偏移。在压力恒定的条件下,v_1-CH_4、(v_1-2v_2)-CO_2峰位随温度升高向高频偏移,但v_1-N_2和v_1-CO峰位随温度变化未发生明显偏移,因而可以应用这些组分的特征峰位来反映流体压力。其次,CH_4、CO_2和CO相对于N_2的拉曼定量因子随压力升高而增大,分别在大于10 MPa、10 MPa和2.5 MPa以上达到稳定值。温度对CH_4和CO的定量因子影响不显著,但CO_2的定量因子随温度升高明显降低。水对CH_4定量因子影响仅在低压条件下显著,在200℃以下的高压条件下影响较弱。最后,以含未知CH_4/N_2比例的CH_4-N_2混合气体的熔融毛细硅管合成包裹体为例,提出了应用拉曼光谱定量分析流体包裹体中复杂挥发分比例和内压的一般方法。受实验条件和仪器设置的影响,各个实验室测定的拉曼定量因子存在较大差异,在应用拉曼光谱定量分析包裹体挥发分组成时应予以注意。     

山东昌乐-临朐火山岩流体包裹体成分研究及其意义

山东昌乐-临朐地区火山岩内的橄榄石和辉石中舍有大量的流体包裹体,流体来源于地幔。采用激光拉曼光谱技术对部分火山岩流体包裹体样品进行分析,获得了包裹体中挥发分的成分和相对含量。结果表明,该区岩浆中舍有大量的CO2和水,CO2是气相包裹体中的主要成分。同时在包裹体内存在H2、CO、H2S、CH4等还原性气体,说明流体来源于还原环境。此外,少量低碳烷烃的出现为天然气无机成因提供了证据。     

吸附势理论在煤层气吸附/解吸中的应用

煤层气的吸附/解吸将导致煤层甲烷碳同位素以及煤层气多组分分馏,使得煤层气富集区预测成为可能;并为揭示注入CO2增强CH4产出提供依据。本文根据Polanyi吸附势理论和实测及收集的等温吸附试验数据,探讨煤层甲烷碳同位素和多组分气体的分馏。通过研究,得到如下两个结论：①13CH4在煤表面的吸附势普遍高于12CH4,也就是说13CH4与12CH4相比具有优先吸附、滞后解吸的特点。这种差异具有随压力增加而增加的特点。②煤层气吸附/解吸过程中CH4和CO2的分馏可归纳为以下3种情形： a. CO2和CH4的吸附/解吸等温线不相交,CO2的吸附势大于等于CH4,在CO2和CH4吸附势接近的中压阶段(1~2.5 MPa)不利于注CO2驱CH4,高压、低压阶段均有利; b. 因CH4的吸附/解吸等温线相交造成CH4和CO2的吸附特性曲线相交,在高压条件(>2.5 MPa)下利于注CO2驱CH4; c. 因CO2的吸附/解吸等温线相交造成CH4和CO2的吸附特性曲线相交,在高压条件(>2.5 MPa)下利于注CO2驱CH4。吸附势理论的引入为定量评价注入二氧化碳驱甲烷工艺参数和有利储层的选择提供了方法,并揭示了在高压条件(>2.5 MPa)下总是有利于向煤层注入CO2强化CH4产出。     

Selection of coals of different maturities for CO2 Storage by modelling of CH4 and CO2 adsorption isotherms

CO₂ injection in unmineable coal seams could be one interesting option for both storage and methane recovery processes. The objective of this study is to compare and model pure gas sorption isotherms (CO₂ and CH₄) for well-characterised coals of different maturities to determine the most suitable coal for CO₂ storage. Carbon dioxide and methane adsorption on several coals have been investigated using a gravimetric adsorption method. The experiments were carried out using both CO₂ and CH₄ pure gases at 25 °C from 0.1 to 5 MPa (1 to 50 bar). The experimental results were fitted using Temkin's approach but also with the corrected Langmuir's and the corrected Tóth's equations. The two last approaches are more accurate from a thermodynamical point of view, and have the advantage of taking into account the fact that experimental data (isotherms) correspond to excess adsorption capacities. These approaches allow better quantification of the adsorbed gas. Determined CO₂ adsorption capacities are from 0.5 to 2 mmol/g of dry coal. Modelling provides also the affinity parameters of the two gases for the different coals. We have shown these parameters determined with adsorption models could be used for classification and first selection of coals for CO₂ storage. The affinity ratio ranges from a value close to 1 for immature coals to 41 for high rank coals like anthracites. This ratio allows selecting coals having high CO₂ adsorption capacities. In our case, the modelling study of a significant number of coals from various ranks shows that anthracites seem to have the highest CO₂ storage capacities. Our study provides high quality affinity parameters and values of CO₂ and CH₄ adsorption capacities on various coals for the future modelling of CO₂ injection in coal seams.     

Flue gas and pure CO2 sorption properties of coal: A comparative study

Presently many research projects focus on the reduction of anthropogenic CO₂ emissions. It is intended to apply underground storage techniques such as flue gas injection in unminable coal seams. In this context, an experimental study has been performed on the adsorption of pure CO₂ and preferential sorption behavior of flue gas. A coal sample from the Silesian Basin in Poland (0.68% V Rr), measured in the dry and wet state at 353 K has been chosen for this approach. The flue gas used was a custom class industrial flue gas with 10.9% of CO₂, 0.01% of CO, 9% of H₂, 3.01% of CH₄, 3.0% of O₂, 0.106% of SO₂ and nitrogen as balance.Adsorption isotherms of CO₂ and flue gas were measured upto a maximum of 11 MPa using a volumetric method. Total excess sorption capacities for CO₂ on dry and wet Silesia coal ranged between 1.9 and 1.3 mmol/g, respectively. Flue gas sorption capacities on dry and wet Silesia coal were much lower and ranged between 0.45 and 0.2 mmol/g, respectively, at pressures of 8 MPa. The low sorption capacity of wet coal has resulted from water occupying some of the more active adsorption sites and hence reducing the heterogeneity of adsorption sites relative to that of dry coal. Desorption tests with flue gas were conducted to study the degree of preferential sorption of the individual components. These experiments indicate that CO₂ is by far the prefered sorbing component under both wet and dry conditions. This is followed by CH₄. N₂ adsorbs very little on the coal in the presence of CO₂ and CH₄. It is also observed that the adsorption of CO₂ onto coal is not significantly hindered by the addition of other gases, other than dilution effect of the pressure.In addition to the sorption experiments, the density of the flue gas mixture has been determined up to 18 MPa at 318 K. A very good precision of these measurements were documented by volumetric methods.     

Laboratory study of self-potential signals during releasing of CO2 and N2 plumes

Carbon Capture Sequestration (CCS) projects require, for safety reasons, monitoring programmes focused on surveying gas leakage on the surface. Generally, these programmes include detection of chemical tracers that, once on the surface, could be associated with CO₂ degassing. We take a different approach by analysing feasibility of applying electrical surface techniques, specifically Self-Potential. A laboratory-scale model, using water-sand, was built for simulating a leakage scenario being monitored with non-polarisable electrodes. Electrical potentials were measured before, during and after gas injection (CO₂ and N₂) to determine if gas leakage is detectable. Variations of settings were done for assessing how the electrical potentials changed according to size of electrodes, distance from electrodes to the gas source, and type of gas. Results indicated that a degassing event is indeed detectable on electrodes located above injection source. Although the amount of gas could not be quantified from signals, injection timespan and increasing of injection rate were identified. Even though conditions of experiments were highly controlled contrasting to those usually found at field scale, we project that Self-Potential is a promising tool for detecting CO₂ leakage if electrodes are properly placed.     

Characteristics of high heating rate biomass chars prepared under N2 and CO2 atmospheres

Partial substitution of coal by biomass in combustion systems in conjunction with advanced technologies for CO₂ capture and storage may result in a significant reduction of greenhouse gases emissions. This study investigates three biomass chars produced from rice husk, forest residuals and wood chips under N₂ and CO₂ atmospheres using a drop tube furnace (DTF) heated at 950 °C. The char constitutes an unburned residue which has been devolatilized under conditions resembling in thermal history those in full scale boilers. Higher weight losses were achieved under N₂ than under CO₂ for each type of biomass, and the highest weight loss was that of wood chips biomass, followed by forest residuals and then rice husk. The results indicate significant morphological differences between the biomass chars produced. The wood chips yielded thick-walled chars with a cenospheric shape very similar to those of low-rank vitrinite. The forest residual chars were angular in shape and often had a tenuinetwork structure, while the rice husk chars retained their vegetal structure. Overall, the studied biomass chars can be described as microporous solids. However, in the case of the rice husk, the silica associated to the char walls was essentially mesoporous, increasing the adsorption capacity of the rice husk chars. The atmosphere in the DTF affects the development of porosity in the chars. The pore volumes of the rice husk and forest residual chars prepared under a CO₂ atmosphere were higher than those of chars prepared under a N₂ atmosphere, whereas the opposite was the case with the wood chip chars. The chars that experienced the most drastic devolatilization were those with the lowest intrinsic reactivity. This indicates a more efficient reorganization of the chemical structure that reduces the number of active sites available for oxygen attack. Overall a similar morphology, optical texture, specific surface area and reactivity were found for the biomass chars generated under N₂ and CO₂, which is a similar result to that obtained for coal chars.     

Methane and CO2 sorption and desorption measurements on dry Argonne premium coals: pure components and mixtures

Sorption and desorption behaviour of methane, carbon dioxide, and mixtures of the two gases has been studied on a set of well-characterised coals from the Argonne Premium Coal Programme. The coal samples cover a maturity range from 0.25% to 1.68% vitrinite reflectance. The maceral compositions were dominated by vitrinite (85% to 91%). Inertinite contents ranged from 8% to 11% and liptinite contents around 1% with one exception (Illinois coal, 5%). All sorption experiments were performed on powdered (−100 mesh), dry coal samples.Single component sorption/desorption measurements were carried out at 22 °C up to final pressures around 51 bar (5.1 MPa) for CO₂ (subcritical state) and 110 bar (11 MPa) for methane.The ratios of the final sorption capacities for pure CO₂ and methane (in molar units) on the five coal samples vary between 1.15 and 3.16. The lowest ratio (1.15) was found for the North Dakota Beulah-Zap lignite (VR_r=0.25%) and the highest ratios (2.7 and 3.16) were encountered for the low-rank coals (VR_r 0.32% and 0.48%) while the ratio decreases to 1.6–1.7 for the highest rank coals in this series.Desorption isotherms for CH₄ and CO₂ were measured immediately after the corresponding sorption isotherms. They generally lie above the sorption isotherms. The degree of hysteresis, i.e. deviation of sorption and desorption isotherms, varies and shows no dependence on coal rank.Adsorption tests with CH₄/CO₂ mixtures were conducted to study the degree of preferential sorption of these two gases on coals of different rank. These experiments were performed on dry coals at 45 °C and pressures up to 180 bar (18 MPa). For the highest rank samples of this sequence preferential sorption behaviour was “as expected”, i.e. preferential adsorption of CO₂ and preferential desorption of CH₄ were observed. For the low rank samples, however, preferential adsorption of CH₄ was found in the low pressure range and preferential desorption of CO₂ over the entire pressure range.Follow-up tests for single gas CO₂ sorption measurements consistently showed a significant increase in sorption capacity for re-runs on the same sample. This phenomenon could be due to extraction of volatile coal components by CO₂ in the first experiment. Reproducibility tests with methane and CO₂ using fresh sample material in each experiment did not show this effect.     

CO2 solubility in dacitic melts equilibrated with H2O-CO2 fluids: Implications for modeling the solubility of CO2 in silicic melts

The solubility of CO₂ in dacitic melts equilibrated with H₂O-CO₂ fluids was experimentally investigated at 1250°C and 100 to 500 MPa. CO₂ is dissolved in dacitic glasses as molecular CO₂ and carbonate. The quantification of total CO₂ in the glasses by mid-infrared (MIR) spectroscopy is difficult because the weak carbonate bands at 1430 and 1530 cm⁻¹ can not be reliably separated from background features in the spectra. Furthermore, the ratio of CO_2,mol/carbonate in the quenched glasses strongly decreases with increasing water content. Due to the difficulties in quantifying CO₂ species concentrations from the MIR spectra we have measured total CO₂ contents of dacitic glasses by secondary ion mass spectrometry (SIMS).At all pressures, the dependence of CO₂ solubility in dacitic melts on x^fluid_CO2,total shows a strong positive deviation from linearity with almost constant CO₂ solubility at x_CO2fluid > 0.8 (maximum CO₂ solubility of 795 ± 41, 1376 ± 73 and 2949 ± 166 ppm at 100, 200 and 500 MPa, respectively), indicating that dissolved water strongly enhances the solubility of CO₂. A similar nonlinear variation of CO₂ solubility with x_CO2fluid has been observed for rhyolitic melts in which carbon dioxide is incorporated exclusively as molecular CO₂ (Tamic et al., 2001). We infer that water species in the melt do not only stabilize carbonate groups as has been suggested earlier but also CO₂ molecules.A thermodynamic model describing the dependence of the CO₂ solubility in hydrous rhyolitic and dacitic melts on T, P, f_CO2 and the mol fraction of water in the melt (x_water) has been developed. An exponential variation of the equilibrium constant K₁ with x_water is proposed to account for the nonlinear dependence of x_CO2,totalmelt on x_CO2fluid. The model reproduces the CO₂ solubility data for dacitic melts within ±14% relative and the data for rhyolitic melts within 10% relative in the pressure range 100-500 MPa (except for six outliers at low x_CO2fluid). Data obtained for rhyolitic melts at 75 MPa and 850°C show a stronger deviation from the model, suggesting a change in the solubility behavior of CO₂ at low pressures (a Henrian behavior of the CO₂ solubility is observed at low pressure and low H₂O concentrations in the melt). We recommend to use our model only in the pressure range 100-500 MPa and in the x_CO2fluid range 0.1-0.95. The thermodynamic modeling indicates that the partial molar volume of total CO₂ is much lower in rhyolitic melts (31.7 cm³/mol) than in dacitic melts (46.6 cm³/mol). The dissolution enthalpy for CO₂ in hydrous rhyolitic melts was found to be negligible. This result suggests that temperature is of minor importance for CO₂ solubility in silicic melts.     

Etude des inclusions fluides du système N2-CO2 de dolomites et de quartz de Tunisie septentrionale. Données de la microcryoscopie et de l'analyse à la microsonde à effet Raman

Optical and analytical studies were performed on 400 N₂ + CO₂ gas bearing inclusions in dolomites and quartz from Triassic outcrops in northern Tunisia. Other fluids present include brines (NaCl and KCl bearing inclusions) and rare liquid hydrocarbons. At the time of trapping, such fluids were heterogeneous gas + brine mixtures. In hydrocarbon free inclusions the $\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}$ mole ratio was determined using two different non-destructive and punctual techniques: Raman microprobe analysis, and optical estimation of the volume ratios of the different phases selected at low temperatures. In the observed range of compositions, the two methods agree reasonably well.The N₂ + CO₂ inclusions are divided into three classes of composition: (a) $\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}\text{> 0,57}$: Liquid nitrogen is always visible at very low temperature and homogenisation occurs in the range ?151°C to ? 147°C (nitrogen critical temperature) dry ice (solid CO₂) sublimates between ?75°C and ?60°C; (b) $\text{0,20 <}\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}\text{? 0,57}$: liquid nitrogen is visible at very low temperature but dry ice melts on heating; liquid and gas CO₂ homogenise to liquid phase between ?51°C to ?22°C; (c) $\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}\text{? 0,20}$: liquid nitrogen is not visible even at very low temperature (?195°C) and liquid and gas CO₂ homogenise to liquid phase between ?22°C and ?15°C. The observed phases changes are used to propose a preliminary phase diagram for the system CO₂-N₂ at low temperatures.Assuming additivity of partial pressures, isochores for the CO₂-N₂ inclusions have been computed. The intersection of these isochores with those for brine inclusions in the same samples may give the P and T of trapping of the fluids.     

N2CH4z.sbnd;CO2 fluids during formation of the Dôme de l'Agout,France

The Dôme de l'Agout consists of an ellipsoidal mass of gneisses and migmatites emerging from a large area of chlorite-sericite schists. Fluid inclusions in syn-metamorphic quartz segregations are typically one-phase at room temperature. Analyses by gas chromatography indicate that their main constituents are N₂, CH₄ and CO₂; such compositions are confirmed by freezing studies on individual inclusions. Nitrogen contents of the inclusions range from 2 to 72 mol% and tend to increase with increasing degree of metamorphism. CH₄ and CO₂ show an erratic distribution with the exception of the lowest grade samples, which invariably have very high CO₂ contents. The origin of the nitrogen remains unsolved; however, it seems likely that the nitrogen was not produced by mineral reactions in the presently exposed rocks but came from an external source and moved from the centre of the dome outwards.     

Oxygen isotope exchange between H2O and CO2 at elevated CO2 pressures: Implications for monitoring of geological CO2 storage

Traditionally, the application of stable isotopes in Carbon Capture and Storage (CCS) projects has focused on δ¹³C values of CO₂ to trace the migration of injected CO₂ in the subsurface. More recently the use of δ¹⁸O values of both CO₂ and reservoir fluids has been proposed as a method for quantifying in situ CO₂ reservoir saturations due to O isotope exchange between CO₂ and H₂O and subsequent changes in δ¹⁸O_H2O values in the presence of high concentrations of CO₂. To verify that O isotope exchange between CO₂ and H₂O reaches equilibrium within days, and that δ¹⁸O_H2O values indeed change predictably due to the presence of CO₂, a laboratory study was conducted during which the isotope composition of H₂O, CO₂, and dissolved inorganic C (DIC) was determined at representative reservoir conditions (50 °C and up to 19 MPa) and varying CO₂ pressures. Conditions typical for the Pembina Cardium CO₂ Monitoring Pilot in Alberta (Canada) were chosen for the experiments. Results obtained showed that δ¹⁸O values of CO₂ were on average 36.4 ± 2.2‰ (1σ, n = 15) higher than those of water at all pressures up to and including reservoir pressure (19 MPa), in excellent agreement with the theoretically predicted isotope enrichment factor of 35.5‰ for the experimental temperatures of 50 °C. By using ¹⁸O enriched water for the experiments it was demonstrated that changes in the δ¹⁸O values of water were predictably related to the fraction of O in the system sourced from CO₂ in excellent agreement with theoretical predictions. Since the fraction of O sourced from CO₂ is related to the total volumetric saturation of CO₂ and water as a fraction of the total volume of the system, it is concluded that changes in δ¹⁸O values of reservoir fluids can be used to calculate reservoir saturations of CO₂ in CCS settings given that the δ¹⁸O values of CO₂ and water are sufficiently distinct.     

Displacement behavior of CH4 adsorbed on coals by injecting pure CO2, N2, and CO2–N2 mixture

Gas adsorption isotherms of Akabira coals were established for pure carbon dioxide (CO₂), methane (CH₄), and nitrogen (N₂). Experimental data fit well into the Langmuir model. The ratio of sorption capacity of CO₂, CH₄, and N₂ is 8.5:3.5:1 at a lower pressure (1.2 MPa) regime and becomes 5.5:2:1 when gas pressure increases to 6.0 MPa. The difference in sorption capacity of these three gases is explained by differences in the density of the three gases with increasing pressure. A coal–methane system partially saturated with CH₄ at 2.4 MPa adsorption pressure was experimentally studied. Desorption behavior of CH₄ by injecting pure CO₂ (at 3.0, 4.0, 5.0, and 6.0 MPa), and by injecting the CO₂–N₂ mixture and pure N₂ (at 3.0 and 6.0 MPa) were evaluated. Results indicate that the preferential sorption property of coal for CO₂ is significantly higher than that for CH₄ or N₂. CO₂ injection can displace almost all of the CH₄ adsorbed on coal. When modeling the CH₄–CO₂ binary and CH₂–CO₂–N₂ ternary adsorption system by using the extended Langmuir (EL) equation, the EL model always over-predicted the sorbed CO₂ value with a lower error, while under-predicting the sorbed CH₄ with a higher error. A part of CO₂ may dissolve into the solid organic structure of coal, besides its competitive adsorption with other gases. According to this explanation, the EL coefficients of CO₂ in EL equation were revised. The revised EL model proved to be very accurate in predicting sorbed ratio of multi-component gases on coals.