The recovery of coalbed methane resources is generally composed of three processes. Generally the coal seam is filled with water, so in the first stage, water is drained out of the seam to decrease the seam pressure, which is a single-phase (water) flow process. In the second stage, with the depletion of pressure, the gas begins to desorb from the coal, leading to a water-gas two-phase flow stage. Due to the small amount of gas content, this process is generally a bubbly flow stage. In the last stage, when more gas desorbs from the coal particles, the gas phase forms a continuous flow and the gas resources are exploited in a large scale. In the present study, an experiment was conducted to investigate the bubbly flow characteristics in two artificially intersecting fractures, which are manufactured with glasses for visualization. The results show that:

  1. the distribution of gas into two fracture outlets was influenced by the location of the gas injection point, especially when the gas injection rate was in a small magnitude;

  2. when the gas injection rate was small, the gas might totally flow into one outlet because the transport of small bubbles was dominated by the water local flow field;

  3. when the gas injection rate was increased, the gas were more uniformly distributed to two outlets.

1. Introduction

Coal seams are extensively filled with underground water. In the exploitation of coalbed methane resources, the first step is water drainage, by which the coal seam pressure is released. With the depletion of pressure, gas starts to desorb from the surface of coal particles, leading to an initial stage of bubbly flow, in which process the gas phase is discontinuous. This is the second stage of coalbed methane exploitation. With more gas desorbing from the coal matrix, the gas content in free phase in the fracture increases and finally the flow of gas phase also becomes continuous. Generally the third stage is the main process of gas production because in this stage the gas output becomes stable. In these different stages, there is a process of transition from single phase flow of water to two-phase flow of water and gas and the transition of flow patterns with the increasing gas content in the fracture network. Consequently, the two-phase flow characteristics within the fracture network of coal seams are required to be understood with different water-gas ratios.

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