Study On The Mechanism Of Coal Cracking And Engineering Response Under The Action Of Liquid Nitrogen Leaching

Coalbed methane,commonly known as "gas" in coal mines,is an independent mineral associated with coal and belongs to unconventional natural gas mineral resources.Practice has proved that the development and utilization of coalbed methane has multiple values,which can not only greatly reduce the occurrence of mine gas accidents and avoid the greenhouse effect caused by the direct discharge of mine gas into the atmosphere,but also produce huge economic effect as a clean energy and effectively improve China's energy structure,which is of great significance for ensuring China's energy security and reducing the dependence of natural gas on foreign countries.Most of the coal seams in China are of low permeability,so the measures of increasing permeability must be taken in the process of CBM exploitation.The existing CBM stimulation technology has some shortcomings,such as unsatisfactory fracturing effect,short stable production cycle,easy to cause waste of water resources,environmental pollution,complex fracturing fluid formula,difficult recovery and flowback.The temperature of liquid nitrogen is very low under normal pressure.Freezing and thawing treatment of coal with liquid nitrogen will cause damage to coal,greatly increase the permeability of coal seam,and achieve the purpose of increasing coalbed methane production.In this paper,the low temperature gas adsorption method and mercury intrusion method were used to test the pore structure of the original coal sample and the coal sample after liquid nitrogen freeze-thaw treatment,to explore the evolution law of coal pore structure under the action of liquid nitrogen;the metallographic microscope was used to observe the evolution law of coal surface cracks after liquid nitrogen freeze-thaw of coal samples with different moisture content,and the CT scanning was used to observe the evolution law of coal surface cracks under the action of liquid nitrogen Through the preparation of coal samples with different particle sizes for circulating liquid nitrogen freeze-thaw,the methane isothermal adsorption experiments of coal samples with different freeze-thaw cycles were carried out to study the effects of particle size and liquid nitrogen freeze-thaw cycles on the adsorption performance of coal.The main conclusions are as follows.(1)After liquid nitrogen freezing and thawing,the pore volume and average pore size of coal samples are increased,and the freezing and cracking of liquid nitrogen have the greatest effect on the increase of mesopore content and micropore content in coal;for macropores and transition pores,the effect of liquid nitrogen on them is less obvious than that of micropores and micropores.The evolution characteristics of pore structure of coal body caused by liquid nitrogen freezing and thawing are as follows: under the action of liquid nitrogen at low temperature,micropores in coal are promoted to develop into transition pores and mesopores,and mesopores and transition pores are further developed into macropores and fissures,which will change the pore structure characteristics of coal and effectively promote the desorption and seepage of gas in coal.(2)Through the observation of CT technology and metallographic microscope,obvious meso damage will occur on the surface and inside of coal after liquid nitrogen freezing and thawing;the expansion degree of coal surface cracks is closely related to the water content of coal and the times of liquid nitrogen freezing and thawing,the higher the water content of coal,the higher the expansion rate of coal surface cracks after liquid nitrogen freezing and thawing,and the more times of liquid nitrogen freezing and thawing,the higher the expansion rate of coal surface cracks.(3)The experimental results of methane adsorption isotherm of column coal samples by liquid nitrogen freeze-thaw drying show that liquid nitrogen freeze-thaw treatment can effectively enhance the adsorption performance of coal.The Langmuir adsorption model was used to fit the adsorption data of different times of liquid nitrogen freeze-thaw coal samples.The results showed that: after liquid nitrogen freeze-thaw,the Langmuir volume VL and PL of coal samples were improved;compared with the original untreated coal samples,the adsorption parameters VL and PL of coal samples after two freeze-thaw cycles were improved most obviously;after four freeze-thaw cycles,the adsorption parameters VL and PL of coal samples were improved on the basis of two freeze-thaw cycles,but not as good as the original ones The adsorption parameters VL and PL of six freeze-thaw cycles coal samples were slightly improved on the basis of four freeze-thaw cycles,indicating that the influence of six liquid nitrogen freeze-thaw cycles on the adsorption performance of coal samples was basically saturated.(4)The experimental results show that the adsorption performance of columnar coal is improved most obviously by the freeze-thaw effect of liquid nitrogen cycle.Under the limited number of freeze-thaw cycles of liquid nitrogen in this experiment,the final adsorption capacity increases with the increase of the number of cycles of liquid nitrogen;under the effect of liquid nitrogen,the final adsorption capacity of granular coal increases It reaches the maximum value after twice freezing and thawing,and then decreases with the increase of liquid nitrogen freezing and thawing times.(5)Using the lagergren model of adsorption kinetics to fit the experimental data of adsorption,the changes of adsorption rate constant k of coal samples with different particle sizes and freeze-thaw times of liquid nitrogen were studied.The results show that the improved lagergren model considering fractal characteristics can better describe the changes of adsorption rate constant K in the process of coal adsorption under different freeze-thaw times of liquid nitrogen;the adsorption rate constant of columnar coal samples presents a trend of change With the increase of freeze-thaw times of liquid nitrogen,the adsorption capacity of small-size coal sample increases first and then decreases.After reaching the freeze-thaw saturation,it is not conducive to the increase of adsorption capacity to continue freeze-thaw.However,the adsorption rate constant increases with the increase of freeze-thaw times.(6)The cracking effect of liquid nitrogen on coal is mainly caused by the thermal stress caused by cold shrinkage of coal body,expansion stress caused by water freezing in coal and high energy gas impact caused by hot vaporization of liquid nitrogen,which damage coal body and change the pore fracture structure of coal.Liquid nitrogen as fracturing fluid is not only environmental protection and resource saving,but also has the advantage that conventional fracturing fluid does not have.