Experimental study on the change of reservoir characteristics of different lithotypes of lignite after dehydration and improvement of seepage capacity

摘要

Find out the changes in lignite properties accompanying dehydration will not only benefit the development of lignite coalbed methane (CBM), but also play a guiding role in the underground gasification, combustion, thermal fragmentation, coal cleaning, and carbon dioxide sequestration. Dehydration can significantly improve the gas flow capacity in the lignite reservoir with originally low permeability. Through nuclear magnetic resonance (NMR) tests, imaging experiments, and permeability tests, the changes in reservoir properties of different lithotypes of lignite during dehydration were comprehensively summarized. In general, the effect of dehydration on lignite reservoir structure is shown in two aspects: the rapid expansion of large fractures, and the shrinkage of relatively small pores. On this basis, different types of lignite exhibit different modes of fracture expansion. Imaging results show that the matrix lignite sample can quickly generate evenly distributed fractures with drying, but these fractures are short in length and poor in orientation. The xylite lignite sample has a lower dehydration efficiency, but can eventually form an extended and well-oriented fracture network. Fractures caused by matrix shrinkage significantly increase reservoir permeability. The results of permeability tests show that dehydration significantly improves the permeability of lignite reservoirs. Since coal permeability is highly stress-sensitive, the effect of stress on the permeability of these samples before and after dehydration was further analyzed. Although the effect of stress on permeability after drying has increased, the effective permeability after drying is always orders of magnitude higher than before drying. In sum, dehydration is an effective measure to improve the seepage capacity of lignite reservoirs.