| Gas extraction is the most basic technical means for gas control in coal mines.It plays a decisive role in ensuring safe production of gas and mines.With the gradual development of coal mining to deep coal seam,the gas content and gas pressure in coal seam also increase gradually,and the structure of coal bearing strata is more complex.Especially in deep and ultra thick coal seam,in addition to the coal seam itself as the gas source rock and reservoir,other strata adjacent to the coal seam(such as mudstone,shale,etc.)also contain a considerable amount of gas gas.However,the theoretical research and practical application of gas extraction are mostly concentrated in the coal seam,and the gas reserves in the adjacent strata and its influence on the coal seam gas extraction are ignored.In this paper,the rule and mechanism of gas drainage and migration in coal seam and its upper and lower adjacent gas bearing strata(such as mudstone)as a whole is studied.The basic characteristics and gas storage mechanism of composite reservoirs,as well as the flow characteristics of gas in composite reservoirs are expounded.A mathematical model for gas extraction andmigration in composite reservoirs is established.Through numerical simulation of COMSOL Multiphysics multi physics field numerical software,the gas drainage and migration in the seam of the composite reservoir were numerically simulated.The main conclusions are as follows:(1)Composite reservoir is composed of coal seam and its adjacent gas bearing strata.Gas and gas exist mainly in adsorbed and free state in reservoir.When gas extraction is carried out,with the gradual extraction of gas in the coal seam,the gas in the upper and lower layers of the coal seam is directed to flow under the interlayer pressure difference,and the gas will flow from the mudstone layer to the coal seam.It is called the gas interlayer channeling.(2)A mathematical model of gas drainage for composite reservoirs under natural pressure lowering was established.The differential equation of gas migration in composite reservoir was derivated and simplify solved.Combined with the occurrence characteristics of 8# coal seam in the Dong Qu coal mine,the numerical simulation of gas drainage is carried out.Simulation results show that simulation gas extraction time continues 10 days,30 days,60 days and 90 days,the gas pressure around the boreholes in the reservoir is increased by 0.8%,1.8%,3.2% and 3.9%.(3)For the cis-seam drilling under the action of non gas interlayer channeling in the composite reservoir,with the continuous extraction of gas,the gas pressure around the borehole is decreasing.But there are some differences in gas pressure changes between gas layers.With the prolongation of the drainagetime,the gas pressure drop around the borehole increases gradually.It is the main reason for the gas flow across the reservoir.(4)The interlayer channeling effect of gas in composite reservoir presents a trend of first increase and then decrease.That is,the interflow between gas layers is rather weak at the early stage of extraction.With the extraction,gas in the mudstone began to flow through interlayer channeling to coal seams,the phenomenon of channeling was obvious,the effect of channeling increased,and the gas pressure around the coal seam increased rapidly.When the gas extraction time continues to extend,the gas supply in the adjacent mudstone layer of the coal seam is reduced,that is,the flow rate of gas interlayer decreases,and the gas pressure around the coal seam is gradually reduced.As a result,the influence of interlayer channeling on the gas drainage is decreasing. |
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