Numerical simulation of coalbed methane generation, dissipation and retention in SE edge of Ordos Basin, China

摘要

This paper presents a numerical study on the formation history of coalbed methane (CBM) reservoir in the southeast edge of Ordos Basin, China. The coal seams studied belong to the Late Palaeozoic coal-bearing series. These coal seams have a burial history and experienced the process of subsidence, rapid subsidence alternated with uplift and then uplift, sequentially, and underwent the geothermal actions at normal, extremely high, and then normal temperatures, respectively. Coal organic matter of the coal seams matured in the Triassic Period and in the Late Jurassic to Early Cretaceous Period. The results from numerical simulation reveal that CBM reservoir evolution history can be classified into five stages, namely primary, initial, stagnant, active and dissipative stages. In the first (primary) stage, coal rank was very low and there was little methane generated and stored in the coal seams. In the second (initial) stage, the coal was converted to middle-high volatile bituminous coal. As a result, a certain amount of methane was generated and began to accumulate in coal seams except part of it escaped from coal seams by diffusion and cap outburst. In the third (stagnant) stage, generation of methane was almost stagnant due to the temperature of the coal seam that dropped slightly and the maturation of coal organic matter stopped. Meanwhile CBM would keep dissipating through diffusion. In the fourth (active) stage, coal rank varied from high volatile bituminous coal A to semianthracite which accelerated pyrolysis gas formation and resulted in a large amount of methane generated at a high speed. During this period, CBM was increasingly accumulated in coal seams although there would be considerable amounts of gas dissipated from the coal seams. In the last (dissipative) stage, due to coal seams uplifted at various rates and no more methane generated, CBM was continuously dissipated by diffusion throughout the whole coal seams and by permeation at some local areas. The simulation provides insights for further interpretation of how many factors that control or affect the CBM reservoir formation history and CBM accumulation. These factors include features of coal-bearing series, characteristics of coal seams, physical properties of coal reservoir, tectonic evolution history, burial history and geothermal conditions, etc. In particular, tectonic evolution history and gas generation are critical. Under given conditions, CBM reservoirs in the study area were developed in different ways and the CBM was accumulated in the reservoirs at different levels. For example, the west part of study area is favourable for CBM accumulation. As a result, the gas content of the main coal seams in this region has a maximum of about 28 m3/t at depths of 900-1100 m, and generally increases with the increasing of burial depth from the east to the west. The coal reservoir is under-saturated in the east part where the burial depth is shallower than about 500 m while the west part is saturated. There is a close correlation of the lateral distribution of both gas content and saturation to the gas generation in the geological history.