| Adsorption phase is one of the main occurrence states in shale gas reservoir,and the key to shale gas accumulation-enrichment mechanism.Taking the organic enriched Longmaxi shale in Southeast Chongqing as research target,the thesis elucidates systematically the distribution character and formation mechanism of the effective adsorbents in target shale.Then by combining molecular simulation and experimental verification,the thesis reveals the basic mechanism of dynamic adsorption and adsorption induced effects under reservoir condition.Based on the basic mechanism,the gas storage capacity during geological evolution and various current reservoir condition are carefully investigated.The main conclusions are as follows.(1)The enrichment of organic matters in Longmaxi shale was controlled by continental and biological sedimentation comprehensively.The bottom of Longmaxi Formation(LM1)formed under euxinic reductive condition,with a high total organic carbon(TOC)content but thin in thickness because of low sedimentary rate.During LM2-LM4,the boosting diversity of graptolite species implied the recovery of ecosystem which led to an increasing bio-productivity,while the euxinic water retained.Along with appropreciate sedimentary rate formed the carbonaceous sillica shale with high TOC content(>2.0 %)and promising thichness,which is the main duration of profitable shale formation.In the upper LM5-LM9,the normal sea condition led to an increasing bio-productivity,but negative for the preservation of organic matters,meantime the high sedimentary rate kept diluting TOC content.In the interval sedimented the argillaceous,calcareous shale in great thickness with low TOC content.(2)The simulation results of main components in shale indicated that the methane surface excess of inorganic minerals,organic graphite/kerogen are of the same scale,and the excess was contributed by the fluids near the surface(<0.5nm).Under the same condition graphite has a higher excess than clays,but the difference minimises with the increasing pressure.The excess of kerogen is the lowest among the structures studied because of the weak surface along the margin.The combined organicinorganic structure also implies that the weak surfaces are of negative contribution of total result.Under supercritical condition,the pore structure is the key to a higher adsorption excess per gram shale.(3)The reversed surface excess in experiment is related with the different density incremental rate of total and bulk phase in the system,but more prominently,the error during helium expansion.The smaller diameter of helium than methane will lead to a higher accessible volume gained in helum expansion than the real volume accessible for methane in adsorption.Meanwhile,the weak amount of helium adsored during expansion are taken as the accessible volume in calculating the surface excess.Structures with high affinity surface,or micropores would show a more distinctive reversed isotherm because of a higher solid-fluid attraction during helium expansion which lead to a higher error.By reproducing the experimental procedure in simulation,an adjustment methodology of surface excess basing on pore structure is proposed.(4)The methane adsorption will lead to the expansion of slit pores among graphene or clay aggregates under supercritical condition.The relation of expansion rate and pressure depends on the mobility of slit structure.If only the inner solids are flexible,the expansion rate increases with pressure linearly at high pressure.If all solids are movable,the slit will expand as well,but the rate will decrease during high pressure(>40 MPa).Due to the weak interacting energy of methane and TOT sheets,which can not compensate the energy lost during expansion of sheets,methane can not enter the layers between TOT sheets automatically.(5)The gas storage capacity is controlled by the depth(T,P)and reservoir pore structure,the important stage of gas accumulation is middle-late Yanshan movement and the following uplifting.During middle-late Yanshan,the pore structure is more contract because the deep burial depth,but meantime it's the main gas generating period as well.The limited capacity along with the high gas generation would form an over saturated reservoir.During the uplifting,the gas storage capacity increases because of decompression.Adsorption induced deformation will gain more pore volume in shale,which is the key to a higher gas storage capacity in high pressure index reservoirs.(6)The gas storage capacity of Southeast Chongqing indicates that the highest excess exists within 2 km if ignore the compaction with depth,shallower or deeper burial depth would lead to a lower excess cpapcity.While the capacity of free gas keeps increasing with depth,and the increment is able to compensate the decrease of surface excess in deep.In research area,Wulong-Pengshui has higher capacity because of a deeper burial depth and better pore character. |
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