中煤阶煤层气井排采阶段划分及渗透率变化

煤层渗透率动态变化规律是煤层气开发所面临的重点问题之一。根据无因次产气率划分煤层气井排采阶段,结合等温吸附实验下煤层气的解吸过程确定排采阶段分界点位置。通过物质能量动态平衡理论建立中煤阶煤储层渗透率评价模型,从渗透率变化趋势、主导机制、产能动态等方面,阐释了中煤阶煤层气井不同排采阶段煤储层渗透率动态变化特征与控制机理。结果表明,排采过程中,煤储层绝对渗透率发生“先降低-后回返-再上升”的动态变化。排水阶段水相有效渗透率迅速下降,气相有效渗透率为0。储层压力降低至临界解吸压力后进入产气阶段,气相有效渗透率迅速增加,水相有效渗透率缓慢降低。产气量衰减阶段绝对渗透率开始下降,在滑脱效应影响下,气相有效渗透率仍然保持缓慢上升,水相有效渗透率降低。

Division of Different Drainage and Production Stages of Medium-rank Coalbed Methane Wells and the Change in Permeability

Understanding the dynamic changing patterns of the permeability of coal seams is one of the key issues in coalbed methane development. In this study, we divided the drainage and production stages of coalbed methane wells according to the non-dimensionalized gas production rate. By combining these figures with the coalbed methane desorption process identified in isothermal adsorption experiments, we further determined the interfacial location between the drainage and production stages. A model for evaluating the permeability of medium-rank coal reservoirs was also developed using the dynamic balance theory of material and energy. Finally, the dynamic characteristics and control mechanisms of coal reservoir permeability in different drainage and production stages of medium-rank coalbed methane wells were explained from multiple perspectives, including the trend of permeability variation, the dominant mechanism, and the productivity dynamics. The results show that the absolute permeability of the coal reservoir experiences a dynamic change of"first decreasing, then reverting, and finally increasing" during the drainage and production processes. A rapid reduction in the effective permeability of water phase and a zero effective permeability of gas phase are observed during the drainage stage. The reservoir enters the gas production stage once the reservoir pressure drops to the critical desorption pressure. During this stage, the effective permeability of gas phase increases rapidly while the effective permeability of the water phase drops slowly. The absolute permeability begins to decrease during the gas production reduction phase. Influenced by the slippage effect, the effective permeability of the gas phase continues to increase slowly while the effective permeability of water phase reduces.