Influence of coal lithotypes on adsorption/desorption characteristics in coal reservoirs and its practical significance: A case study in Baode Block

The difference of coal lithotypes is an important factor affecting the adsorption/desorption characteristics of coalbed methane (CBM) and the productivity of CBM wells. According to four coal samples of different lithotypes from No. 8+9 Coal Seam in Well BX-2, proximate analysis, maceral, wettability and isothermal adsorption/desorption experiments are carried out, and the influence of coal lithotypes on CBM adsorption/desorption characteristics and its mechanism is discussed. The results show that vitrinite content in dull, semi-dark, semi-bright and bright coal samples increases gradually, water and ash content decreases gradually, hydrophilicity decreases gradually, and the Langmuir volume increases gradually. Bright and semi-bright coal samples have stronger adsorption capacity, higher starting pressure, transition pressure and sensitive pressure, and the corresponding effective stage interval is wider in the desorption process, which is more conducive to CBM development. The influence mechanism of coal lithotypes on adsorption/desorption characteristics is mainly reflected in the differences in composition and wettability. The parameters of desorption characteristics of Well BX-2 are calculated by weighted average based on proportion parameters of the coal lithotypes thickness ratio. The CBM desorption stages of Well BX-2 are divided into slow, fast and sensitive desorption stages. The drainage stages are divided into drainage depressurization, unstable gas production, stable gas production and gas production attenuation stages. In the drainage depressurization stage, the drainage speed should be controlled to reduce the damage of stress sensitive effect to permeability. In the unstable gas production stage, the casing pressure should be properly controlled to enlarge the desorption radius as much as possible. The production pressure difference should be appropriately increased in the stable gas production and gas attenuation stages, in order to extend the peak and stable gas production period with the advantage of high desorption efficiency.