In Coalbed Methane (CBM) reservoir, especially the thin low permeability CBM reservoirs, undulating horizontal wells are often drilled to maximize contact area with the reservoir for higher CBM production rates. Coal fines, however, may accumulate in the wellbore, making pressure along the wellbore trajectory more complex, affecting gas production behavior. This paper presents experimental work to study the coal fines-water-gas three-phase flow in transparent pipes to mimic different undulation angles that may occur in horizontal wells. Coal fines with different particle sizes are studied. Observed from the experiments, four flow patterns for coal fines migration were identified: stratification, saltation, moving dunes, and dispersed flow. Moreover, relations between coal fines critical moving velocity and discharge velocity and the size of coal fines are investigated. This paper further investigated the settlement and plugging of coal fines at the curvature section of the pipes under four flow regimes: elongated bubble, transition, slug, and intermittent flows. Using the experimental data, we estimated the boundaries for five flow regimes under five different undulation angles. The analyses indicated that the pressure fluctuation magnitude increases when flow regime evolves from bubble flow to intermittent flow. The largest pressure fluctuation and the lowest fluctuation frequency take place under intermittent flow. Based on the experimental data, this paper derived the critical velocity correlation for coal fines in undulating CBM wellbore. The findings of this study is useful for CBM development in the field, which may help to optimize wellbore geometry to minimize coal fine settlement/plugging, act as a guide in designing well flushing operation to effectively circulate coal fines out of bottomhole.

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