Mechanism And Experimental Research On Coalbed Methane Displacement By Continuous And Intermittent Air Injection

Coal bed methane?CBM?is generated and stored in the coal and there are abundant reserves,the efficient extraction of CBM is of great significance to mine safety,energy and environment.It is hard to achieve a satisfactory result by using conventional mining methods and stimulation measures for coalbed methane in high metamorphic,high adsorption and low permeability coal seams,and it is one of the main research directions to improve the recovery rate of CBM by injecting gaseous adsorption and improving permeability in recent years.Based on the basic principles of material structure,surface physical chemistry,diffusion mass transfer,seepage mechanics and rock mechanics,this paper makes an in-depth theoretical analysis on the adsorption relationship and displacement mechanism between the pores of"Polar and non-polar"coal porous media and injection media polarity.Laboratory studies the adsorption characteristics of No.3 fat coal and its mudstone roof in Shuangliu Coal seam,and the adsorption desorption capacity of CH₄,N₂ and O₂ were determined by isothermal adsorption constant apparatus,according to the analysis of experimental results the adaptability between different adsorbate and adsorbent adsorption model;Finally,according to the basic parameters measured and the basic law of diffusion seepage,the physical mathematical model of gas injection displacement process is established,and the numerical simulation and industrial test of air injection for coalbed methane are carried out.Meanwhile,the influence of O₂ in the air composition on the spontaneous combustion of coal seam is investigated.The main achievements can be drawn as follows:1)Adsorption displacement mechanism of the molecular sieve of polar moleculesCoal is a macromolecular organic rock,with the characteristics of?Polar groups are hydrophilic,and non-polar groups behave as pro-oil?.Coal matrix surface consists of the polar groups of the surface of the polar and nonpolar groups composed of nonpolar together form the surface,the adsorption mechanism of gas injection are differences from coal matrix surface varies due to the polarity of the gas molecules.Matrix polarity surface preferentially adsorption of polar molecules,its main force is the orientation force;non-polar surface adsorption nonpolar molecules,its main force is dispersive force;These two adsorption referred to as the paired adsorption.The remaining polar surface adsorption of non-polar molecules of the main force is induced force.Coal matrix within the pore structure is mainly composed of micro hole,hole,large hole and pore structure of the matrix pore and pore diameter into the pore adsorption to gas molecule sieve filter,gas molecules polarity or not,only the small molecules can pass hole sieve filter are likely to be within the pore surface adsorption?sieve filtration adsorption?;When injecting non-polar molecules N₂,O₂ and CH₄under the above adsorption mechanism,competitive adsorption and displacement of CBM,which promotes the diffusion of coal bed methane and increases the production of coal bed methane.2)The adsorption characteristics and model applicability of different particle size fertilizer coal and mudstoneFour millimeters of small particles of fat coal samples?0.075⁰.125?0.125⁰.200?0.200⁰.250?0.250⁰.500mm?,Under the condition of 4 temperature classes and adsorption equilibrium pressure of 0².5MPa in the range of 25⁴0?below the critical autothermal temperature?60⁸5??,the results of adsorption experiments show that:The small particles of fat coal samples adsorption capacity of different gases is CH₄>N₂>O₂,the adsorption test data of three kinds of gases under the experimental conditions are in line with the Langmuir equation.Large-scale spherical fat coal samples containing certain cracks and centimeter-level macropores??=40 mm?,at 25?and 0².5MPa adsorption equilibrium pressure conditions,the experimental data showed that the fitting effect of BET model is better for long time adsorption,adsorption of multi molecular layer adsorption,the measured data are more close to the original coal seam conditions and the pore fissure effect on adsorption results.Mudstone is one of the main rocks in the coal layer of coal,and a certain amount of coalbed methane is stored,in the same experimental conditions of adsorption of small particles of coal samples,the adsorption of mudstone and O₂ and CH₄ performance analysis,the results show that the BET model to describe the adsorption performance of O₂ is better;D-R model for CH₄ adsorption performance fitting data than it is good.These experimental results provide the basis for reservoir evaluation,gas injection mechanism analysis and Select the type of gas injection.3)Permeability variation law of coal and rock under three axis stressA fat coal permeability and confining pressure in the process of load growth law of negative exponent,experimental results:k?28?1.2281e^-0.458?;while the relationship between permeability and confining pressure conforms to the power equation of unloading process,experimental results:k^?28?0.7677?^-1.151;When the confining pressure is the same,the permeability of the loading process is larger than the permeability of the unloading process.Experiments show that the unloading process and the loading process are irreversible processes.Under the condition of constant pore pressure,the permeability experiment of the reloading and unloading of fat coal samples results are as follows:Confining pressure from 3MPa loaded into 4 MPa when the permeability of coal sample suddenly increased from 0.2243 to0.3528 mD,which show that coal sample permanent damage appeared in the process of loading,create new and irreversible cracks,increased infiltration volume,permeability exploded.The permeability decreased by 18.76%from 4MPa to 5MPa,indicating that the new crack was closed under confining pressure.In the process of unloading confining pressure from 5 MPa down to 4 MPa permeability increases 3.7%,this shows that the process of unloading and loading are not reversible,and in the process of confining pressure from 4MPa dropped to 3MPa,permeability sudden increase to 0.4195 from 0.2971 mD,as the coal body is under tensile stress,and the unloading damage of coal is formed in the new fissure,forming a larger seepage channel,and the permeability is significantly improved.Under the same confining pressure of 3MPa,the permeability after unloading was 0.4195mD and the permeability before loading was 0.2243mD.The comparison showed that the permeability after unloading increased 87%than that before loading.4)Study on permeability change of coal sample before and after raw coal displacementUnder the condition of 6MPa of axial pressure and 5MPa of confining pressure,the relation between permeability and pore pressure satisfies the exponential type with air as the medium,k?28?.0114e^0.460p;and the original sample permeability of 0.01512mD is obtained when the pore pressure is 0.6MPa.After continuous injection of coalbed methane to displace coal samples?coal samples after continuous injection?,the same axial compression and confining pressure conditions were used to test the permeability of 0.01679mD again,which was 11.04%more than the original coal samples.Intermittent gas injection after the replacement of coalbed methane?coal gas after intermittent gas injection?,after the vacuum with the original coal sample using the same axial pressure,confining pressure conditions again measured permeability of 0.01312mD,compared with the original coal sample decreased by 13.23%.After continuous gas injection,the increase of permeability of coal sample is due to the increase of pore pressure,the decrease of effective stress and the expansion of macro fracture system,thus increasing the permeability of coal seam.The reason for the decrease of the permeability of intermittent gas injection is that the coal seam skeleton collapses after many alternating loads,and the seepage channel of pore fissure is blocked,so the permeability is reduced as it is.Compared with intermittent gas injection,the former increases the permeability of coal,and the latter reduces the permeability of coal seam.The results show that the effect of continuous gas injection for coalbed methane extraction is better than that of intermittent gas injection from the perspective of increasing energy and increasing permeability.5)The effect of fat coal desorption and continuous and intermittent gas displacement for CBMAfter the adsorption equilibrium of large scale spherical fat coal,the 24 hour free release was 0.24 mL/g,and the average desorption rate was 0.010ml/?g.h?.Under the condition of constant stress field,the air displacement 24 hours continuously,produced 125.12 ml CBM,the average recovery was 0.01961 ml/?g.h?,unit time and continuous quality coal gas injection flooding is 96.10%higher than free release.Under the condition of constant stress field,intermittent gas injection displacement experiment using note 12 hours in a first,and then note 6 hours for the second time,and then note 6 hours for the third time,three times in total recovery of CBM 73.48 ml,24 hours of release 0.2765 ml/g,intermittent gas injection displacement than the freedom to release at the same time increased by 15.20%.Compared with the intermittent injection method,the continuous displacement method has the advantages of long injection time,competitive adsorption and displacement time,and large displacement,but due to the too short extraction time,the total extraction volume increased by competitive displacement.It is still less than the continuous gas injection to increase the pumping capacity under the same injection pressure for a long time.The continuous gas injection increases the permeability of the original coal seam while the latter is opposite,so continuous gas injection has a good pumping effect.6)Numerical simulation of single hole continuous gas injection of fat coal without supply boundary conditionsBased on the experiments of adsorption and displacement,the mathematic model of air displacing CBM was constructed,and the permeability of CBM under different injection pressures and injection times was simulated by COMSOL software.The simulation found that with the air on the methane displacement process,the adsorption air concentration increases with time and the adsorption methane concentration decreases with time.Gas injection 5d,injection pressure of 0.6MPa,for example,the recovery rate increased 18.34%,and the experimental results are basically consistent.7)Industrial Test of 3#Coal Seam in Shuangliu Coal MineThe continuous and intermittent injection of air to displace CBM industrial test was carried out in No.3 coal seam after the suction at 139 d of negative pressure in Shuangliu Coal Mine.Compared with the natural gas drainage in the same period,continuous gas injection can improve the average gas production by 44.52%and intermittent gas injection by 17.53%.During the gas injection experiment,CO,C₂H₄,and other indicators of coal spontaneous combustion in the mine air were simultaneously detected.No abnormalities were found in the indicators.The industrial test was consistent with the results from the continuous and intermittent gas injection laboratories.The technology of injecting air to displace CBM was moderate Deterioration of the implementation of fat coal is safe and feasible.