Physical Simulation of Water Dissolved Gas (WDG)During Migration

J. China Univ, of Mining Tech (English EditionVoL 15 NPhysical Simulation of Water Dissolved Gas(WDG)During migrationLIU Zhao-Iu. LI-Jian. FANG Jia-bu'Guangahou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou2 Lang Fang Branch of Research institute of Petroleum Exploration and Development,, Langfang, Hebei 065007, ChinaSchool of Mineral Resources and Safery Engineering, China University of Mining Technology, Beijing 100083, ChinaAbstract: By using the equipment designed and developed by ourselves, experiment of investigating the influence ofdissolutiongeo-chemical parameters(such as 8C, 6D, and AC,nC, in water-dissolved gas(WDG)duringnon-hydrocarbon(CO2)and hydrocarbon(CH,) increase while the relative abundance of hydrocarbon(Cr+)decreases(the relative abundance of hydrocarbon (Cs+)can be basically negligible); 2) the relative abundance of benzene andmethylbenzene increase in the initial time and then decrease. The carbon and hydrogen isotopes of methane varyslightly, which can be regarded as indicators of gas dissolved in water formationKey words: Gas dissolved in water; Migration and formation; Physical simulationCLC numher: x 52is relatively late. Sun Y x11-123investigated theSince the early 1960s, many researches for effect of groundwater on water dissolved gas forma-measuring the solubility of hydrocarbon in water-tion: Later, Hao s studied the soluble character ofhave been carried out. The result shows that it is pos- natural gas in groundwater, and Fu X T also prosible for forming water dissolved gas pools. In 1970, posed two primary soluble mechanisms of natural gasPrice investigated the primary migration of petro-ater. The above mentioned studileum by means of solubility of hydrocarbon in water. basically based on the PVT cell at differentFurthermore, researchers in Italy, Hungary, Philip- tures and pressures. The primary purposepines, Nepal, Iran, Japan,etc, have found such natu- studies was to investigate some physical solubilityral gas pools and subsequently begun to exploit them parameters of natural gas in groundwater. So far, dat[l, which, in turn, promoted the research orabout compositions of natural gas and its stable caity of natural gas in groundwater中国煤化工 Isotope compositons, especiallyachievements have been obtained and-f relative abundance of benzenequantitative assessment during naturalCN MH Gure hard to be found in literatureand accumulation at preserIn this paper, the authors intend to investigate theCompared with the overseas researches, the re- influence of dissolution on some geochemistry pa-search on the water dissolved gas formation in China rameters(such as 8C, 8D, iCnC et al. )ofReceived 18 July 2004: accepted 10 October 200Projeet/s 2001CB20913 supported by 973 of ChinaCorrespoodingauthorTel+862085290913E-mailaddressiZ680101g26.comLIUZhao-luetal Experimental Research on Physical Simulation of Water-125WDG during gas migrationthe potential methane release from groundwater mayresult in a variation of geological conditions (along2 Geological Factors on the Release of the fault slope vertical migration)or the crust uplift,WDGleading to an uplift of aquifer or a descending ofAccording to the experiment performed by Fu s groundwater level regionally or locally. The decreaseH 13), the solubility of natural gas in groundwater in both temperature and pressure makes the naturalis to combine water gas dissolved in water change from unsaturatedlecule with natural gas molecule to form crystal saturated and over-saturated, and at last the waterline lens molecule and the other is that the pore space dissolved gas is released out and accumulated in aof water molecule is tilled by natural gas molecule. favorable trap(low potential energy).In both mechanisms, the solubility of natural gas isaffected by temperature and pressure, with the in- 3 Samples and Experimental Procedurerease of pressure, the whole solubility of natural gasemperature effect is relatively complex. When theRock sample: The real cores used in this extemperature is lower than 80C, with the rise of tem- periment were two dark gray compacted sandstonesature, the solubility decreases; when the tempera collected from well Su-6 in Ordos basin with a po-ture is higher than 80C, with the rise of temperature, rosity of 0.379-0. 75 %, a permeability of 0.0024-0.002 7 md, a length of 30. 14-30.37 m, and an IDrom the above mentioned analysis, it can be of 2. 49 cmseen that there exists a close relationship among soluGas sample: The source gas used in this exbility of natural gas, temperature, and pressure. periment was collected from North-China gas field.Korsenshtejn14-1 and Zorkin (16-I7I also discussed The composition of natural gas is shown inTablethe hydrogeology of aquifer and its role in hydrocarWater sample: 100 mg/L NaCl solution to takebon distribution in Sibrea basin. They suggested that the place of groundwater in Ordos basinTable 1 coral and isotopic data of simulation for gas dissolved in waterHydrocarbon /%△DC% a anrofMorsCt C, C iC, nCanCr C+ Co NaCI C, C, Cr Cr88158.712530280.27003000011030351041185-408-282-258-2167-1776as14MPa288308.552560260260.030020021.3504311170406-280-256-2134-1765sings cup39051320490050.050108195406-280-25.8-2145-1755498071510310040030中国煤化工3021-Gas dis.97731910280030020CNMHGR254-2146-175.3 solved it696892770280030020000115301.53.11-402-275-253-213.8-1749796393.160400030.02000108015361403-276-25.7-2149-17523.2 Procedure4 Results and Discussionsolved gas during migration is shown in Fig Iexperiment, five samples were collected during theexperiment. So altogether seven samples were col-起事lected. The amount of the sample is 30 mL, the in-terval is 15 min, and the experimental results are pre-sented in Table 14.1 Carbon and hyThe carbon and hydrogen isotopes of hydrocarThe device is composed of a high-pressure sys- bons in this experiment were determined by the de-tem(pump, container), a migration system(flow cell, vice Delta" XL produced by Finnigan company. Asand a sampling section. The main body of the flow shown in Table 1, the stable carbon isotope ofcell consists of a porous stainless discs with a length methane( 5"C) does not show a clear trend. Theof 800 cm, an internal diameter of 2.5 cm. And its difference in 8C, value between source gas andpressure may reach 70 Mpa, its temperature ranges water dissolved gas is quite small with the averagerom +20 C to +120 C. It may also be positioned values ranging from -40.8 %e for source gas toboth vertically and horizontally. The flow scheme of 40.6 %o for the gas in the upper part and fromthe experiment is also shown in Fig. 1-40.2 to 40.6 %o for water dissolved gas. As forBefore the experiment, 1 000 mL of stainless Czthydrocarbon, the carbon isotope change is evencontainer with a piston is filled with 600 mL of negligibly small and has a different changing ten-groundwater, and equipped with a valve at each end. dency. In addition, as shown in Fig. 1, stable hydro-The container is vacuumized and then filled with gen isotopes of methane(6 DCi) does not show anatural gas and vacuumized again, this process is clear trend either( Table 1)with the average valuesrepeated two or three times. The pressure is regulated eing.7 for source gas. -213.4 %e for gasto a constant value by means of a manual fluid pumpin the upper part, and from.9 %o to. 8%o for water dissolved gas. The difference in 8 DC,value between the source gas and the gas dissolvedrium. the pressure is held constant for at least 120 h. in water is also quite small. Its trend is almost con-During this time the container is kept vertically. After sistent with that of the stable carbon isotope of120 h, the container is overturned and the pressure is methane, Nearly no change has taken place in thekept constant for another 120hexpenmentFirst, the flow cell is equipped with real core 4.2 Compositions characteristics of hydro.samples(5 MPa) and is flushed with helium(I MPa)carbon and non-hydrocarbonfor 10 min to take the place of air inand the equipment is kept airtight. ThYH中国煤化工吗小可ACNMHGof methane increases quicklyintroduced into the flow cell, 1 min late. the bottomsolved gas correspondingly. The relative abundanceof Cz+hydrocarbons decreases from 11.85 to 1.93of gas dissolved in water is accumulated in a water- % It might be correlated with different abilities ofsaturated pool, and the set-up is removed until gas absorbing hydrocarbon and the difference in diffu-sion rate among different carbons. In addition, aZhao-ly et alcommon way of determining the migration distance dissolved in water. It is also one of the typical charof natural gas is the plot of isobutane against acteristics from high-pressure gas dissolved in waterN-butane. And the longer the path of migration is, the formation. Whereas the relative atgreater the iC,nCa values will be. As shown in Table creases from 0.35 %o for source gas to nothing for gasI, the values of iCnCa also increase from 1. 04 to dissolved in water.1. 50, it might show that the migration of natural gas 4.3 Characteristics of light hydrocarbonThe compositions of light hydrocarbon are ana-N-butane loss. Non-hydrocarbon gases also vary lyzed with the device 5890A gas chromatographwidely. The relative abundance of CO2 increases produced by Finnigan company. The analysis result isfrom 1.03 from source gas to 26.7 from gas shown in Fig. 2.4050C30405060030n4050的0(e)watcr-disy ved(n wader dissolved sFig 2 The experiment results of gas dissolved in water of light hydrocarbon during migrationIt can be seen from Fig. 2 that the relative abun-As described above. comparison of relativedance of natural gas has been greatly affected by abundance for water dissolved gas with that fordissolution reaction in water. Because different source gas indicates that the methane carbon and hycompositions of light hydrocarbon have different drogen isotope have not a clear trend (only a slightlydissolution abilities in groundwater (aromatic positive properties)and that the relative abundance ofnaphthene >chain alkane), the Co Cs of light hy- Cx+hydrocarbon and non-hydrocarbons of N2 is verydrocarbon in natural gas changes greatly. The relatlow while the values of iCanC, increase with theabundance of aromatic hydrocarbon, especially the increase of distance, especially, the relative abun-relative abundance of benzene and methylbenzene is dance of benzene and methylbenzene can be regardedvery low in the source gas, after dissolution in water, as an effective indicator of researching and identifythe relative abundance of benzene and methylben-ng water dissolved gas formationzene increase sharply first. And a while later, due to5 Conclusionsbenzene and methylbenzene are released from waterquickly, and the relative abundance中国煤化工 ween water dissolved gasshows that a) there is notance and percolation. This regularCNMH Methane carbon and hydrogensignificant for studying the migration of natural gas isotope(but with a slight positive character), b)thend its history of deposition. It is also one of effec. trend of the relative abundance of non-hydrocarbonstive indicators to identify the dissolved gas in such as Carbon dioxide( CO %)and nitrogen (N%)are in good agreement with previous litcratureJ. China Univ of Mining& Tech (English Edition)voLI5 No. 2advantageous indicators for identifying water dis-2)As for the case of iC/nC4, our results are sig- solved gas formationnificantly different from that of previous literature 9.The values of iC/n C4 in our study increase from 1. 04to 1.5, while that of iCwn C in Chengs study(bascoon the Pvt cell) is less than 1, 0.5-0.7 on average.The authors thank Dai JX academician for his guidanceThe author deduces that migration of natural gas caand some leaders of Laresult in isobune accumulation and N-butane lossof Petroleum Exploration and Development for financial sup3)The variation characteristic of relative abun- port.dance of benzene and methylbenzene is one of theMcAuliffe C. Solubility in water of Cr Cs hydrocarbons. Nature, 1963, 2002): 1092-1093[2 White D.E. Geothermal brine well: Mile-deep drill hole may tap ore-bearing magmatic water and rocks undergoing meta-morphism Science, 1963. 139: 919-922.[3] Price L C. 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