Study On The Deep Transfer And Storage Mechanism Of Mine Water In The Eastern Margin Of Ordos Basin

The research of mine water transfer and storage in deep formation at the eastern margin of Ordos basin was firstly conducted and proposed to intensify the internal relationship between the frail ecological environment,mine water drainage and water-preserved coal mining in the western coal mining region,China.The hydrogeological characteristics and recharge-runoff-drainage conditions of Baotashan sandstone in the bottom of the Middle and Lower Yan'an formation in Jurassic and Liujiagou formation in the Lower Triassic were both systematically studied.The data of cores and logging for MC-1 well was collected.The series of rock physical mechanics,X diffraction,SEM,mercury injection,permeability experiment,groundwater quality and mine water injection tests were all carried out,and the potential of Liujiagou formation as the mine water transfer and storage aquifer in the deep of Ordos basin was qualitatively and quantitatively evaluated.?1?The geological background in regional and local mining area for Baotashan sandstone,as well as rock compositions,hydrogeologic and sedimentary conditions,were systematically studied.The petrological characteristics,pore structures,physical properties,rock physical mechanics and permeability regularity were obtained by a series of microscopic analysis tests.Baotashan sandstone was located at the bottom of Yan'an formation,its main lithology was grayish-white pebbly coarse sandstone.The sedimentary environment was the channel sand dam interacting with washland in the braid-shaped river delta sedimentary system.Due to the master status of channel sand dam,its main compositions were quartz and feldspar,showed the weak cementation,loose structure and 14%⁴8%porosity,mainly developed intergranular pores.The natural compressive strength of Baotashan sandstone was 33.90MPa and water absorption was 5.53%.The total porosity was 19.89%,the main pore size ranged from66nm to 8.48?m and the relatively large nanometer to micrometer pores were relatively developed and indicated the considerable potential storage space for mine water.The groundwater quality of Baotashan sandstone aquifer was strong alkali of CO₃-Na and CO₃·Cl-Na type for this reason of limited recharge-runoff-drainage condition and evaporation-concentration-crystallization influence.In the evolution of permeability,long-term drainage could enhance its permeability,which was consistent with the characteristics of weak cemented sandstone in the western coal mining region.In the micro-condition,the influence of water-rock interaction was powerful and the permeability coefficient was rapidly reduced in the saturated permeability stage.After in the stable permeability stage,the influence of water-rock interaction was weak,and the permeability coefficient was stabilized from 2.5152×10^-7m/d⁵.6493×10^-6m/d.?2?The lithology of Liujiagou formation in the Lower Triassic was included purple mudstone,gray-white medium sandstone,gray-white and flesh red fine sandstone,with local horizontal and vertical fractures developed.It was a set of sedimentary formation of river-delta in the hot climate and strong oxidation environment,and formed a complete sedimentary cycle with the overlying Heshanggou formation.The petrology,pore throat structure,diagenesis,permeability property and vertical heterogeneity of Liujiagou formation were firstly studied comprehensively and described particularly,the potential capacity of mine water transfer and storage was evaluated.The sandstone of Liujiagou formation was mainly composed of quartz and feldspar,accounting for 40.1%and 31.1%respectively.The quartz content in the upper section was lower than that in the lower section,and the feldspar content in the upper section was higher than that in the lower section.As a whole,the content of grain skeleton that composed of quartz,feldspar and calcite in the upper section was higher than that in the lower section.The clay minerals content was 17.8%,mainly including cillite,chlorite/smectite and illite/smectite formations.Among them,the content of chlorite/smectite and illite/smectite formations were obviously more than cillite,and the intergranular pores,dissolved pores and micro fissures were all developed,with pore size ranging from 295.3nm to 19.01?m,72.09nm to 9.09?m and 77.7nm to 4.86?m,respectively.The intergranular pores were dominant.According to the mercury injection test,the median pore diameter of throat was 4.439×10²nm,the average was 48.39nm,the total porosity was 7.50%,the porosity with pore diameter less than 10?m was 5.26%and the effective pore diameter ranged from 6.33 to 12.09?m.The void ratio ranged from 3.32%to 6.48%,with an average of 5.03%.It belonged to low porosity dense sandstone.However,its vertical fractures were developed and the lithology changes were obviously higher than other strata,and was characterised with strong vertical heterogeneity according to the deep/shallow lateral resistivity logging curves.There were 38 aquifers in Liujiagou formation with a total thickness of 177.1m,accounting of 36.1%in the formation.The coarse sandstone,medium sandstone and fine sandstone were dominant.In terms of physical and mechanical characteristics,the strength of sandstone at the bottom of Liujiagou formation was lower than that the top,but the physical and mechanical parameters of sandstone in each group had no obvious correlation with the burial depth,therefore,the regularity was not obvious,and the vertical mechanical characteristics showed heterogeneity.?3?Under the influence of structure,sedimentary diagenesis,temperature and the very poor recharge-runoff-drainage condition,the original groundwater quality of Liujiagou formation aquifer was showed acidity and very high salinity of Cl-Ca·Na type.Rock salt,witherite and fluorite were in the state of dissolution,and the dissolution potential decreased successively.Barite,calcite,anhydrite,dolomite,gypsum and aragonite were all in the precipitation state,and the potential of precipitation gradually decreased.For the mixed groundwater quality of Liujiagou formation,Shiqianfeng formation and mine water,rock salt,barium carbonate,fluorite,gypsum and anhydrite were in the state of dissolution.Dolomite,calcite,aragonite and barite were in the state of precipitation.?4?In this study,hydraulic fracturing was proposed to enhance the permeability properties of Liujiagou formation by the expand of migration channels,the increase of porosity and the connectivity properties of fractures.On the basis of simulation and the study of the fracturing mechanism for sandstone,the minimum fracture pressure of Liujiagou formation sandstone was 31.5MPa,and the hydraulic fracturing was easy to create the artificial fractures connecting with the original fractures and result in formation leakage.In the process of mine water reinjection,deep Liujiagou formation sandstone would be affected and controlled by mine water pressure,ground stress,temperature and water-rock interaction.In-situ stress of sandstone controlled the expansion and extension direction of natural fractures.Mine water pressure promoted the expansion and extension of induced fractures.The effect of thermal expansion and cold contraction formed by temperature reduction was only effective for the shallow surface of fractures and particles.Acid-alkali water was neutralized.The dissolution action of these soluble and hydrophilic minerals changed pore structure and porosity.The vertical fractures of thick sandstone were developed in Liujiagou formation,and so the horizontal fractures of lithologic interface were.The induced fracture formed by artificial mine water reinjection pressure would expand and extend along the NE and NS directions of natural fractures to form the main permeability channels.?5?The natural water level recovery test and multiple mine water reinjection tests had been carried out successively.After the data fitting of natural water level recovery test by various wiring methods,the hydrogeological characteristics of Liujiagou formation sandstone aquifer were shown its weak permeability,poor water abundance and yield.The hydraulic conductivity ranged from 5.31×10^-6m/d to 6.19×10^-6m/d.After the high pressure water injection,the mine water transfer and storage aquifer was fractured,then the permeability and water storage performance were greatly increased.The hydraulic conductivity was from 0.0111m/d to 0.0146m/d,the stable reinjection flow was 103.3m³/d,and the stable pressure of wellhead was 6.8MPa.According to multi-stages pressure and flow monitoring,the continuous reinjection of high-pressure and low-temperature mine water would jointly promote the potential transfer and storage capacity of the formation.Among them,high pressure was able to fracture sandstone fractures,relatively low salinity mine water could dissolve rock salt,gypsum and other mineral components in the fractures to dredge the pores.The low temperature water would absorbed the heat from the surrounding rock to cause weak thermal expansion and contraction,and then promote the development of fractures again.The interaction of the above three features enhanced the potential transfer and storage capacity of mine water in the fracture network space of deep reservoir.The ultimate storage capacity was 1.318×10⁶m³ by the volume method,and the effective storage capacity was about 0.806×10⁶ to 1.047×10⁶m³ by the cone mathematical model.Based on the scenario simulation of normal long-term reinjection,the permeability difference of aquifers with different lithology would lead to different horizontal diffusion migration distance of mine water.The longest migration distance was in coarse sandstone,which was difficult to build the pressure in the well wall.The shortest migration distance was in siltstone,which was easy to build the pressure in the well wall.In the early stage of reinjection,the diffusion rate of mine water near the transfer and storage layer was fast,and the influence radius was fast increased with the present as an exponential increasing function.In the middle and late stage,the diffusion rate was slow in relatively far area,and the influence radius was also increased slowly,showing a linear increasing trend.?6?Mine water transfer and storage improved and deepen the category of water perserved coal mining and made an important practical significance to the western coal mining region.The regional and local groundwater flow field,temperature field and chemical field simulations were conducted.It was found that an inverted U-shaped groundwater mound and a funnel-shaped low-temperature zone were formed after the long-term mine water reinjection.The groundwater mound would make a certain artificial influence on the deep groundwater circulation of the region and hindered the upstream groundwater to force it to flow around.A large number of mine water was re-injected into the deep sandstone layer and formed three types of transition areas:mine water,mixed water and primary stratum water.The water quality types are SO₄-Na,SO₄·Cl-Na and Cl-Ca·Na type,respectively.The long-term reinjection of mine water would cause the transformation from groundwater rich in calcium&sodium type to sodium type.The dissolution and precipitation of minerals had always existed,including the dissolution state of rock salt,barium carbonate,fluorite,gypsum,anhydrite and other minerals,as well as dolomite,calcite,aragonite and barite were gradually tended from dissolution to precipitation.In this dissertation,there are 126 figures,14 tables and 276 references.