生物成因煤层气是一种清洁的、可再生的和环境友好型的非常规天然气资源。众所周知，微生物降解煤会生成一定数量的甲烷，同时，在生烃过程中会改造煤储层孔渗结构，为生物成因煤层气的运移和富集提供基础条件。但是对于微生物多级降解下低阶煤生烃速率、生烃潜力和储层孔隙结构变化规律未见报道，基于此，本文以黄陵矿区和彬长矿区两个低阶煤样品为研究对象，通过定期更换培养基的方式来模拟地下水流经煤层过程，在实验室条件下开展微生物多级（三次）降解煤生烃模拟实验。在实验过程中每隔7天对顶空瓶内气体组分进行测试分析，研究低阶煤在微生物多级降解作用下的甲烷产量及生成潜力。采用氮气吸附、核磁共振和孔隙度/渗透率测试等手段，对煤微纳米孔隙类型、大小、形态、连通性、孔隙度、比表面积、孔容、平均孔隙直径等进行表征，揭示微生物多级降解下孔隙结构演化规律。采用3D形貌仪、扫描电镜、CT扫描等手段，对微生物降解前后的煤岩表面形貌特征及煤岩内部孔隙分布情况进行全面表征，并探讨微生物降解作用对煤孔隙度和渗透率的影响。取得的主要认识如下：

在微生物模拟生烃方面，微生物降解煤会生成以甲烷为主的气体，三级降解的甲烷生成量分别为420~450μmol/g、300~490μmol/g、360~390μmol/g，并伴随有部分二氧化碳和氢气等产生。多级降解过程中甲烷生成量分别受煤中挥发分产率、镜质组含量和惰质组含量三种因素影响。一级降解过程中的甲烷生成速率与后两级差异明显，其中一级降解过程中甲烷生成速率为“缓慢增长-快速增长-缓慢增长-平衡”演化特征，二级降解和三级降解的生烃速率均为“快速增长-缓慢增长-平衡”特点。

（2）在煤表面形貌变化方面，微生物的降解作用会使煤表面形貌发生明显改变，扫描电镜下可观察到明显的扩孔和增孔效果，即在微生物降解作用下增加原有孔隙直径，并生成新的孔隙；煤表面的有机质被降解后，体积减小甚至消失，还可见大量的微生物及其活动痕迹。微生物对煤基质孔隙比较发育的区域具有更强的降解作用，使孔隙数量和孔隙度显著增加。

（3）在煤孔隙结构演化方面，微生物降解作用对煤储层孔渗结构具有明显的改造作用，在多级降解过程中，孔隙变化特征分别为“扩孔-增孔-扩孔”，其中黄陵煤主要以扩孔为主，而大佛寺煤以增孔为主，煤中孔径的增大还会增加煤与胞外酶的接触面积，促进微生物降解煤有机质，提高产气效率。随着孔隙直径不断扩大，孔隙之间开始相互连通，导致孔隙度和渗透率均有明显增加，为煤层气的渗流和运移提供了良好的通道，对煤层气的开采具有积极意义。

Biogenic coalbed methane is a clean, renewable, and environmentally friendly unconventional natural gas resource. It is well known that microbial degradation of coal can generate a certain amount of methane, and transform the porosity structure and permeability of coal reservoirs during the process of hydrocarbon generation, providing the basic conditions for the migration and enrichment of biogenic coalbed methane. However, there is no report on the hydrocarbon generation rate, hydrocarbon generation potential, and reservoir pore structure change law of low-rank coal under microbial multi-stage degradation. Based on this, the paper takes two low-rank coal samples from the Huangling mining area and the Binchang mining area as research objects. To simulate the process of groundwater flowing through the coal seam, the simulation experiment of microbial multi-stage (three times) degradation of coal to generate hydrocarbons was carried out under laboratory conditions by changing the medium regularly. During the experiment, the gas components in the headspace bottle were tested and analyzed every 7 days to study the methane production and generation potential of low-rank coal under the action of microbial multi-stage degradation. Tested based on the experiments of nitrogen adsorption, nuclear magnetic resonance, and porosity/permeability to determine the features of pore type, size, shape, connectivity, the porosity of coal micro-nano pores, specific surface area, pore-volume, average pore diameter. The evolution law of pore structure under microbial multi-stage degradation was revealed. The surface morphology of coal before and after microbial degradation and the distribution of coal pores were comprehensively characterized by the means of 3D topography instrument, scanning electron microscope, CT scanning. The effect of coal porosity and permeability by degradation was also discussed. The main understandings were as follows.

In terms of hydrocarbon generation during microbial degradation coal, the methane is the main gas for hydrocarbon generation and its generated amounts are 420-450μmol/g, 300-490μmol/g, and 360-390μmol/g in three stages. Carbon dioxide and hydrogen are also produced from microbial multistage coal degradation. The amount of produced methane during the process of multi-stage degradation is affected by three factors: the volatile yield, vitrinite content, and inertinite content in coal. The methane generation rate in the first-stage (0-28 days) is significantly slower than the second-and-third stages. The methane generation rate in the first-stage biodegradation is characterized of "slow-growth, fast-growth, slow-growth, and equilibrium". The methane generation rates of second-and-third stages are all characterized by "fast-growth, slow-growth, and equilibrium".

In terms of the change of coal surface morphology, the surface morphology of coal has a significantly change during biodegradation, enlarging and enhancing the pore structure observed under the scanning electron microscope. That is, the original pores and pore diameter are increased and generated some new pores under the action of microbial degradation. The pore volume decreases or even disappears, and a large number of microorganisms and their activity traces can be seen after the degradation of organic matter on the coal surface. Microorganisms have a stronger degradation on the regions of coal matrix with relatively developed pores, and can significantly increase the pores amount and porosity.

In terms of the pore structure evolution of coal, microbial degradation has an obvious effect on the pore structure and permeability of coal reservoirs. In the process of multistage degradation, the pore is characterized by "pore-expanding, pore-increasing, pore-expanding" respectively. Huangling coal is mainly expanding the pore, while Dafosi coal is mainly increasing the pore. The increase of pore size in coal will also increase the contact area between coal and extracellular enzyme, promote microbial degradation of coal organic matter and improve gas production efficiency. With the continuously increase of pore diameter, the pores begin to connect with each other, leading to a significant increase in porosity and permeability, which provides a good channel for the seepage and migration of coalbed methane. biodegradation coal has a positive significance for the exploitation of coalbed methane. 

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