| 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. |
