煤岩类型对煤储层吸附/解吸特征影响及实践意义

宏观煤岩类型差异是影响煤层气吸附/解吸特征及煤层气井产能的重要因素之一。针对保德区块BX-2井8+9号煤4种不同宏观煤岩类型样品，开展工业分析、显微组分、润湿性及等温吸附/解吸实验研究，探讨了煤岩类型对煤层气吸附/解吸特征的影响及其机制。结果表明:暗淡煤、半暗煤、半亮煤和光亮煤的镜质组含量逐渐增大，水分和灰分含量逐渐降低，亲水性逐渐减弱，Langmuir体积逐渐增大；光亮煤和半亮煤具有更强的吸附能力，同时具有更高的启动压力、转折压力和敏感压力，解吸过程中对应的有效阶段区间宽度更大，更有利于煤层气开发。宏观煤岩类型对吸附/解吸特征的影响机制主要体现在不同煤岩类型煤组分和润湿性的差异。基于宏观煤岩类型分层厚度占比参数，对BX-2井解吸特征参数进行了加权平均计算，并将该井煤层气解吸阶段划分为缓慢解吸、快速解吸和敏感解吸3个阶段，将排采阶段划分为排水降压、不稳定产气、稳定产气和产气衰减4个阶段。排水降压阶段应控制排水速度，减少应力敏感效应对渗透性的伤害；不稳定产气阶段应适当控制套压，尽量扩大解吸半径；稳定产气和产气衰减阶段应适当增大生产压差，利用解吸效率高的优势延长产气高峰期和稳产期。 

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.