In coal-bed methane (CBM) horizontal wells, coal particles generally deposit on the low side of the wellbore to form a particle bed, which increases well pressure loss and even blocks the well. The coal particle bed undergoes changes in height and geometry with the fluctuation of gas and water production. Besides, with a low water flow rate, the water layer in CBM well is extremely thin. Both the dynamically changing particle bed and thin water layer make the conventional gas/water interfacial friction factor (fi) prediction method for stratified flow in horizontal pipe not suitable for that of CBM well.

Based on the large-size multiphase complex flow experimental equipment, a gas/liquid/solid three- phase flow experiment in horizontal pipe was carried out. In the experiment, gas flow rate, water flow rate and coal particle size were the three main factors considered. During the experiment, the gas/water flow pattern, migration of coal particles and evolution of blocked section could be investigated through the transparent pipe. Furthermore, due to the significant effect of coal particle bed on the height of gas-water interface, a non-negligible gravitational pressure drop calculated using water level differences is taken into consideration when obtaining the friction pressure drop.

In the experiment, three gas/water flow patterns including slug flow, stratified smooth flow and stratified wavy flow were observed. The coal particles mainly transport by means of rolling and saltating in the thin water layer under stratified flow and migrate by way of dispersing in water under slug flow. Besides, for particles no larger than 20-30 mesh, the blocked section tended to be overall driven forward like fluid by local high differential pressure and then led to the discontinuity of coal particle bed. However, for blocked section with particles not less than 10-20 mesh, instead of being driven forward, the coal particles on the top surface of it would be carried away layer by layer until the raised part was eroded off. Furthermore, as with the situation in gas/liquid two-phase stratified flow in horizontal pipe, fi in the experiment increases with liquid Reynolds number (ReL). However, it is much larger than that of gas/liquid stratified flow under the same ReL,. The analysis of experimental data presents a closely negative correlation between fi and the equivalent diameter of effective flow channel. As for the coal particle size, it has some indirect effects on fi through the equivalent diameter of effective flow channel.

This work is helpful for coal particle management and productivity prediction during CBM development, which may provide guidance to particle-bailing operation and also serve as a basis for theoretical mechanistic models to predict the gas/liquid interfacial friction factor of gas/liquid/solid three-phase stratified flow.

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