Study On Multi-component Competitive Adsorption Mechanism Of Shale Gas

Researches on competitive adsorption of multicomponent in shale gas reservoir possess profound implications for the evaluation of gas-bearing capability,the investigation of gas transport and the injection of CO₂ for enhanced gas recovery.Based on the black shale cores from Wufeng Formation?Upper Ordovician?and Longmaxi Formation?Lower Silurian?in Sichuan Basin,pore structure characteristic and physical and chemical characteristics were identified and analyzed by X-ray diffraction analysis,rock pyrolysis analysis,total organic matter content determination,and low temperature N₂/CO₂ adsorption-desorption methods.In addition,with the conductions of CH₄isotherm adsorption experiment in the sample with equilibrium water,and single component and multicomponent isotherm adsorption experiments of CH₄/CO₂ in dry sample,the competitive adsorption behaviors of CH₄/H₂O and CH₄/CO₂in high pressure and temperature conditions were further investigated.Based on the experimental researches,single component and multicomponent shale gas adsorption models were both constructed.Gas-bearing capability prediction model of shale gas reservoir was established with the consideration of moisture and nonmethane gases.The main conclusions and recognitions are as below.Analyze the composition and pore structure of gas shale and establish an experimental equipment for the isotherm adsorptions of single component and multicomponent shale gas.Experimental cores are mainly consisted of quartz and clay,with a TOC higher than 1.5%generally.The majority of pores are micropores and macropores.Specific surface area of micropores possess positive relationship with TOC.The experimental equipment was improved by the adding of middle zone for realizing gas sampling and the function of one injection for multiple samples adsorption.Experiment procedure was also simplified in case of the unnecessary mixture in gas sampling with the help of middle zone.Investigate the competitive adsorption of CH₄/H₂O,and discuss the impact of water vapor pressure on adsorption.The moisture adsorptions on montmorillonite,illite and the whole gas shale are all consistent with the type III adsorption curve,indicating a multilayer adsorption feature.The number of moisture adsorption layers is mainly among the range of 3?7.After the equilibrium water treatment,the Langmuir adsorption value of montmorillonite decreases by 97.4%,which almost loses its entire adsorption ability,while the ones of illite and the whole gas shale decrease by 33.7%?78.9%.The adsorption value of CH₄ would be undervalued by 26.4%considering no water vapor pressure impact.Therefore,the isosteric heat of adsorption corrected by water vapor pressure matches the thermodynamics theory more consistently.Study the competitive adsorption of CH₄/CO₂,and construct the adsorption model of supercritical CO₂ and the swelling model caused by adsorption.The adsorption behavior of each component in the mixture gas is not only related to the adsorption capacity of this component,but also related to the partial pressure of this component.CO₂ takes the predominance in the competitive adsorption and the advantage grows with the increasings of temperature and the content of CO₂ in the mixture gas.Establish the adsorption model for single component and multicomponent shale gas in terms of simplified local density model,and analyze the gas displacement in shale pore.The results indicate that the adsorbed gas only appears around the wall in large pore.The adsorbed gas starts to appear in the middle of pore only when the pore getting smaller enough and all the gas in the pore is in adsorption state then.The adsorption phase density is the comprehensive function of temperature,pressure and pore size.Lower temperature,higher pressure and smaller pore size all result in a larger adsorption phase density.Excess adsorption is approximately equal to absolute adsorption in low pressure condition and gets larger than absolute adsorption in a growing disparity with pressure increasing.Construct the gas-bearing capability prediction model of shale gas reservoir with the consideration of the volume of adsorption phase,moisture and nonmethane gases.Gas-bearing capability of shale gas is overestimated by 27%?38%without the consideration of the volume of adsorption phase and is reduced by 1.17%?3.12%with the consideration of moisture.Specially,multicomponent gas possess not only an impact on the occurrence state of shale gas,i.e.the distribution of shale gas in adsorption phase and separate phase,but also an impact on the total gas-bearing capability of shale gas.In the depth range of 2384.18 m?2414.56 m,gas-bearing capability of shale gas is overestimated by 5.6%?31.8%without the consideration of multicomponent gas.