Fundamental Study Of Enhancing Coalbed Methane In Coal Measure Strata By Gas Injection Strata And Pressuer Relief Technology

The gas source in coal-bearing strata is mainly composed of enriched and dispersed organic rock,including coal,mudstone and shale.The best choice for the combination of environmental protection,gas accident prevention and clean energy efficient was to combine the characteristics of coalbed methane storage with its extraction.However,CBM exploitation was not a simple act.Limited by the reservoir physical parameters and mining technology conditions,the mining rate is low at present.Existing coalbed methane exploration research mainly focused on coal-based,few considering coal strata as a whole comprehensive system.In this paper,the technological foundation as to effectively improve coalbed methane recovery rate of coalbed methane was studied in-depth.In this paper,the gas storage characteristics and coalbed gas migration laws of the main pore fissure gas source rocks in the coal strata were expounded firstly.The method of gas injection was proposed in no relief area to improve the extraction rate of CBM,while in the coal measures strata pressure relief area the gas is drainaged directly by borehole.Secondly,the main gas source bed-coal seam and mudstone layer in coal-bearing strata were sampled,and the adsorption characteristics and adsorption models under different temperature,pressure and different gas conditions were studied and their suitability was tested.Furthermore,the comprehensive parameter a-mass transfer coefficient a,which characterizes the magnitude of desorption resistance and mass exchange capacity,was experimentally studied in different gas source media with different gases.Finally,a physical mathematical model of CBM extraction contained coal measure strata natural step-down and gas injecton was developed,which was based on the inter bedding effect and mass transfer mechanism.Combining the actual engineering conditions and experimental data with Comsol multiphysics software,which was simulated numerically.Through the deep analysis of the simulation results,the paper provided a theoretical basis and technical guidance for the coalbed methane extraction technology in coal-bearing strata.The main contents and conclusions were as follows:1:Based on the theory of adsorption/desorption and diffusion of coalbed methane in the reservoir,the coal-bearing strata were considered as a whole system in combination with the characteristics of main gas source parameters in the coal-bearing strata.Based on the characteristics of low extraction rate in the unconsolidated zone of coal-bearing strata,the method of gas injection can be adopted to improve oil recovery in the absence of pressure relief in the downhole.The pressure relief zone could optimize the fracture zone extraction according to the spatial position of annular fracture circle process.2.According to the fundamental theory of mining field around layer movement,ways of calculating the spatial position parameters of the annular fracture circle under the pressure relief condition were concluded,and the coalbed methane extraction technology in the fracture zone was optimized.The height of caving zone,which had the greatest influence on the drainage effect of inclined boreholes,was measured by borehole backwater measurement method.3.By examining the suitability of the three main adsorption models（Langmuir model,DR model and BET model）for the main gas source-coal and mudstone at different temperatures and with different gases in coal-bearing strata,it is found that adsorption characteristics of coal and mudstone to CO₂ conforms to BET model,while those to CH₄ or N₂ conforms to Langmuir model and DR model.The experiment reflects a negative correlation was found between the amount of CH₄ adsorbed and the temperature in different gas source media.In the actual production process,we could use the method of injecting heat from coal seam,Joule heating effect of electric field,heat generated by alternating electromagnetic field or ultrasonic frictional heat generation to increase the temperature of the reservoir in order to improve the adsorption coal bed methane desorption rate,and so the coalbed methane extraction rate then raised.4.The relationship between the mass transfer coefficient（a）,the characterization of the desorption resistance and the mass exchange capacity,and the geological conditions of the coalbed methane and the coalbed methane desorption velocity were determined by the volumetric method.The experimental results showed that the adsorption matrix concentration in different gas source media was proportional to a;For different gas source rock medium,the pace of change of αCO₂ over time is relatively slow than αCO₂,and for the same concentration,αCH₄ was always larger than αCO₂.That means the adsorption ability of different gas source rock media for CH₄ and CO₂ is consistent and the desorption of CH₄ is lager than C02.Therefore the CBM production rate can be improved by the way of CO₂ injection.5.In the process of coal measures strata of CBM extraction,the gas pressure difference between the source rock layers and the source rock layers would be caused by the drilling of the downhole coal seam and the drilling of the coal bed gas,which led to the phenomenon of interlayer flow between the gas source beds.Besides,the different pumping modes were different,the difference of the pressure between adjacent source rocks was different,and finally the difference of the interfacial channeling flow was different.6.On the basis of research and based on the basic theory of adsorption,diffusion and percolation in porous media,the physical mathematics model of coalbed gas in coal seam formation was set up.7.Combined with characteristics of Jin Cheng 3#coal seam、Ma Lad 8#coal seam which based on mathematical model,the process was simulated by Comsol multiphysics.The simulation results show that:（1）In the process of natural decompression extraction of coalbed gas,comparing with the date without considering interlayer channeling,the pressure around extraction borehole is higher than that without considering conditions.That is to say the interlayer channeling supplies slow down the energy attenuation,which is useful to extend the stable period of extraction well.（2）When considering fluid channeling in bedding drilling of underground pressure relief zone and comparing CH₄ pressure within the same extraction time and distance,we can discover that the quantity of extraction under the condition of gas injection is lager than no gas injection,which indicates that it is not obvious between the pressure of gas supercharging size and the kinds of gas injection in a certain time in terms of energized drive.