China is vigorously promoting the development of coalbed methane (CBM) resources because CBM is cleaner than coal and a hazardous gas in coal mining. However, the CBM production in China is significantly lower than that in other coal‐rich countries, such as Australia and the United States. In this paper, the main problems hindering CBM development in China are discussed to provide management suggestions for the government. These key problems include exploration and development techniques, the overlap of mining rights, management, financial supporting policy, investment environment, regulations, and market conditions. The results of this study indicate that to improve the development of the CBM industry in China, national and local governments must enact specific and exercisable regulations to increase financial support, improve technology, increase investment in CBM exploration, strengthen public project establishment, and improve the reform of CBM exploration and development policies.
The Qinshui basin is the most successful area for coalbed methane (CBM) development in China. Since 2008, 45 multibranched horizontal wells (MBHWs) have been put into production in the Fanzhuang block, of which 10 wells have an average daily gas rate (DGR) of >10 000 m3/d. Well 4‐5 with the total gas production of 1.054 × 108 m3 achieved the China's highest DGR of 63 744 m3/d. Compared with vertical wells, MBHW is an effective way to develop CBM in low permeability reservoirs in China. However, there are still 23 wells have an average DGR of <3000 m3/d, of which 18 wells are below 1000 m3/d. The results show that shallow depth (16 m3/t) and saturation (>80%), long coal seam interval (>3000 m), reasonable branch spread, and stable wellbore are the basis for achieving high production. The encountered faults connected to the aquifer will cause a high water production and an extremely low gas rate (<1000 m3/d). In the inclined coal seam, the up‐dip MBHW is more favorable for the formation water drainage and the pressure drop funnel expansion than the down‐dip MBHW, and the horizontal well should be drilled perpendicular to the high‐permeability direction to achieve a higher recovery ratio. MBHW is more sensitivity to drainage rate due to the coal fine migration in the horizontal segments, and the high pressure drop rate will lead to high frequency of stunk pump and eventually affect the gas production. Currently, the biggest challenge for CBM development with MBHW is wellbore stability. 57% of inefficient wells are caused by wellbore collapse during drilling and production processes. Here, a new type of “Roof Drilling‐Tree Shaped” horizontal well structure was presented in this work. The first “Roof Drilling‐Tree Shaped” horizontal well with a total length of 12 288 m and a coal seam interval of 9408 m was successfully applied without drilling accidents.
Coal samples were cut into cores to perform a flow velocity sensitivity (FVS) analysis under varying initial permeability, confining pressures, injection volumes, and injection intensities. The results show that the permeability and the output of the coal fines decrease with an increase in confining pressures at a constant displacement velocity. A critical flow velocity exists for the migration of relatively large coal fines. Below this critical flow velocity, very small coal fines can be transported out of the coal by the fluid, slightly increasing the coal permeability. However, larger coal fines are transported at a higher flow velocity, which may block the effective seepage paths and reduce coal permeability, inducing FVS. Moreover, as the flow velocity and the injection volume increase, the permeability damage rate increases, but the rate of increase in the permeability damage decreases. The damage to the permeability due to FVS mainly occurs in the early stage of coal fines migration, and an abrupt increase in the flow velocity can damage reservoirs and induce substantial coal fines generation. Thus, maintaining a stable effective strength and a controlled depressurization rate during drainage can effectively constrain coal fines output and decrease permeability damage within coal reservoirs.