| Shale gas resources are abundant in China,and the technically recoverable resources are among the highest in the world.Accelerating the efficient development of shale gas reservoirs is of great significance for alleviating the contradiction between the supply and demand of natural gas and guaranteeing national energy security.Gas adsorption plays an important role in the storage and transportation of shale gas,and it is very important to study the adsorption law of shale gas for the efficient development of shale gas reservoirs.Shale gas is a mixture of various light hydrocarbon gases.In addition to CH4,C2H6 also presents a large proportion in some geological conditions,it is the second-largest component in shale gas reservoirs and cannot be ignored.Different light hydrocarbon gases have different adsorption capacities,and the multi-component competitive adsorption phenomenon occurs when they co-exist in shale reservoirs.However,there is still a lack of research on the multi-component adsorption mechanism of shale gas.In this dissertation,the adsorption of CH4,C2H6 pure component gases and their binary mixtures in shale were studied experimentally and theoretically,which can deepen the understanding of shale gas multi-component adsorption mechanism,and thus expand and improve the adsorption theory of shale gas.Firstly,the geochemical characteristics,mineral composition,and pore structure characteristics of shale samples were analyzed by conducting the rock pyrolysis analysis experiment,X-ray diffraction experiment,and N2 adsorption experiment at 77 K,which deepened the understanding of the physical properties of shale reservoir.Based on the volumetric method,the isothermal adsorption experiments of CH4,C2H6 pure component gases and CH4/C2H6 binary gas mixtures with different gas compositions in shale were carried out by using a multi-component gas isothermal adsorption apparatus.The experimental data of gas adsorption amounts were obtained and the isothermal adsorption characteristics of shale gas were analyzed.On the basis of the experimental study,three engineering adsorption models for pure component gas considering the temperature dependence were established and their applicability was analyzed.The multi-temperature Langmuir-Freundlich adsorption equation was selected to characterize the adsorption behavior of pure component gas in shale samples at different temperatures.In order to more accurately model the adsorption behavior of multi-component mixed gas,a high-pressure 3-D ideal adsorption solution model IAS-f based on absolute adsorption amount was established,and the multi-component competitive adsorption mechanism of shale gas under high pressure was studied by using this model.Compared with the E-L model commonly used in engineering,the proposed model improved the prediction accuracy of isothermal adsorption behavior of mixed gas and had significant advantages in characterizing the competitive adsorption among components of mixed gas.By introducing the simplified local density functional theory and considering the influence of different pore geometry,the SLD models of slit and cylindrical pore were established,and the simulation ability of these two models for shale gas adsorption behavior was compared and analyzed.The results show that the cylindrical pore SLD model can capture the curved characteristics of the shale pore surface and thus can improve the accuracy of shale gas adsorption behavior simulation.Based on the cylindrical pore SLD model,the existing state of adsorbed phase in shale nanoscale pores was studied,and the effects of gas composition,temperature,pressure,and pore size on the density profiles and composition distribution of adsorbed phase were also analyzed.Based on the adsorption data of light hydrocarbon gases in actual shale samples measured experimentally in this dissertation,the vapor-liquid two-phase flash algorithm was improved by combining the multi-temperature Langmuir-Freundlich adsorption equation and IAS-f model established in this dissertation with capillary pressure equation.The effect of adsorption on the phase behavior of CH4,C2H6 pure fluids and CH4/C2H6mixtures in shale nano-pore space were studied using the modified flash algorithm.The results show that the adsorption enhances the confinement effect,and further shrinks the phase envelope of the confined hydrocarbon fluid.The smaller the pore size,the greater the influence of the adsorption film.If the influence of adsorption is neglected,the capillary pressure will be underestimated and the bubble point pressure will be overestimated.A comprehensive evaluation model for shale gas content was established by using the isothermal adsorption method considering the influence of several key factors such as TOC,adsorbed phase volume,moisture content,and multi-component adsorption.The predicted values of shale gas content by the proposed model are in good agreement with the measured results by the field desorption method.Thus the established model can provide data reference for the initial evaluation of shale gas content.The research results in this dissertation can provide basic data support and theoretical guidance for the shale gas-in-place evaluation and the formulation of efficient development strategies for shale gas reservoirs. |
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