Characterization Of Bituminous Coal Fracture Structure And Permeability Evolution By Liquid Nitrogen Fracturing

Coalbed methane（CBM）is an important coal-associated resource,with abundant resource reserves and significant development and utilization value.How to enhance the recoverability of coal seam and realize the efficient extraction of CBM has been one of the research hotspots and difficulties in the field of CBM exploitation.Liquid nitrogen（LN₂）fracturing is an effective technology for enhancing the permeability of coal seam,which has a great prospect of application.Coal seam permeability is an important parameter for evaluating the recoverability of coal seams,while the permeability is mainly determined by the development of internal fractures.Considering the bituminous coal in the Xutuan Coal Mine of Anhui Province,China,the single and cyclic LN₂ fracturing experiments were carried out,and the evolution law of pore-fracture stracture and permeability characteristics of coal samples were studied.Firstly,the mechanism of LN₂ fracturing was analyzed,and the feasibility and present situations of LN₂ fracturing technology were summarized.Secondly,the mineral composition of experimental coal samples was determined by X-ray diffraction（XRD）and semi-quantitative analysis was performed.The mercury intrusion pore measurement（MIP）and computerized tomography（CT）technology were used to quantitatively characterize the changes of coal pores and fractures before and after LN₂fracturing.The fractal geometry and pore network model（PNM）were applied to analyze the fractal characteristics,distribution of spatiality and connectivity of micro-fractures.Finally,the seepage experiment was carried out by the Gas Flow and Displacement Testing Apparatuus（GFDTA）,which revealed the change law of coal permeability under single and cyclic LN₂ fracturing with different stress state.The main conclusions of this thesis include:（1）Under the combined effect of the water-ice phase transition expansion force and the thermal stress caused by the uneven shrinkage of the coal matrix,the original fractures expand and extend,accompanied by the formation of new fractures and interpenetration between the cracks,leading to an increase in permeability.LN₂fracturing,as a non-water fracuring technology,has the advantages of clean and pollution-free,saving water resources,low price,and LN₂ injection which can displace and increase gas production in coal seams.However,the current LN₂ fracturing technology is still in the initial research stage,and it is faraway before it can be commercialized.（2）The results of mineral composition analysis showed that the main minerals in the experimental coal samples were kaolinite（60.8%）,pyrite（16.2%）,quartz（14.4%）,and calcite（8.6%）.The results of MIP test show that the LN₂ treatment can significantly enhance the development of pores,and the porosity of coal bodies after LN₂ fracturing increased from 2.95%to 4.77%.The mercury injection curve of coal samples is"S",and dividing the curve into rapid rise stage,slow rise stage,and re-rapid rise stage.After LN₂ fracturing,the pore size distribution in coal body changed significantly.The proportion of micropores,small pores,and mesopores in the coal body after LN₂fracturing decreased,while the proportion of macropores increased significantly.The proportion of macropores increased from 19.11%to 50.32%,which the higher the proportion of large pores,the better the fluid transportation.（3）LN₂ fracturing can effectively promote the formation of macroscopic fractures in coal bodies,and the degree of coal body damage increases with the number of cycles.The micro-fractures were accurately segmented by combining DTM threshold segmentation and deep learning models.According to the results of fractal analysis,the fractal dimension of micro-fracture increased significantly after LN₂ fracturing,from0.9462 to 1.3023 in the horizontal direction and from 1.0153 to 1.3585 in the vertical direction.The results of 3D reconstruction by using Dragonfly software showed that the size and number of micro-fractures in coal body after LN₂ fracturing were significantly higher than before treatment.The total volume of fractures increased from306.48 mm³ to 967.55 mm³ and the porosity increased from 0.38%to 1.22%.The results of the PNM analysis showed that the number and size of pores and throats after LN₂ fracturing were significantly increased.The relatively large pore fissures（>1000μm in diameter）play a decisive role in the permeability of the coal body,although their proportion is small.（4）The permeability of coal samples was tested under the conditions of single and cyclic LN₂ fracturing.The results of the single LN₂ treatment showed that at a constant confining pressure,the permeability increased exponentially with increase in gas pressure.The permeability increased significantly after LN₂ fracturing,and the permeability increment increased exponentially with inlet pressure.The permeability of cyclic LN₂ fracture increased with the gas pressure,showing a trend first of decrease and then of increase,which is consistent with a quadratic function distribution.Under the same stress conditions,the permeability of coal increases with the number of cycles.LN₂ cyclic fracturing has higher cracking efficiency than that of a single LN₂ treatment.In the case of the same absolute cracking time,cyclic fracturing had a more efficient fracturing effect than a single treatment.Therefore,in the practical application of the LN₂ fracturing technology,it is appropriate to implement a"less time and more cycles"strategy to obtain a better effect on subsequent CBM production.This thesis contains 48 figures,15 tables,and 116 references...