下保护层开采上覆煤层产气机理及规律

通过理论研究分析了采动区煤层裂隙特征、煤层气赋存特征和煤层气运移特征,揭示了煤矿采动影响下上覆被保护煤层产气机理。分析认为采动区煤层产气机理与常规煤层气产气机理不同,一是采动区煤层基质孔隙大、内部裂隙多、离层裂隙发育。二是采动区煤层气解吸快、解吸量大、水中溶解气少。三是采动区煤层气运移以气体单向流为主,评价渗透性指标为透气性系数。在收集大量采动区煤层气井产气数据的基础上,研究了产气机理对产气规律的影响。采动区煤层气井产气规律与常规煤层气井相比,具有产量提升快、产气峰值高的特点,整体呈现先迅速增大后逐渐减小趋于平稳的规律。结合采动区煤层产气机理研究,分析认为气产量提升速度快是由于煤层裂隙迅速增多且气体在裂隙中以气相单相流运移为主,产气峰值高是由于储层压力快速下降使煤层气加速解吸,短时间内在井筒附近聚集大量游离气。

Overlying Coal SeamGas Production Mechanism and Pattern during Lower Protective Seam Extraction

Through theoretical study analyzed mining district coal fissure features,CBM hosting and migration features have revealed coalmine mining impact on overlying protected coal seam gas production mechanism.The analysis has considered that mining district CBM production mechanism is different from conventional CBM production mechanism.The first is mining district has larger bulk material pores,more interior fissures and developed abscission layer fissures;the second is CBM in mining district has quicker desorption,larger desorbed quantity and less water dissolved gas;the third is mining district CBM migration mainly gas one-way flow,permeability assessment index is gas permeability coefficient.On the basis of massive mining district CBM well production data acquired have studied CBM production pattern impacted from CBM production mechanism.Mining district CBM well production pattern compared with conventional CBM well has peculiarities of quicker output rising,higher peak CBM output;as a whole,presented quick increasing first,then gradually decreasing and leveling off.Combined with mining district CBM production mechanism study,analysis has considered that the quicker output increasing is owing to coal seam fissure quickly increased and gas in fissure mostly one-way flow migration;higher peak CBM output is a result of reservoir pressure quickly deceased and CBM desorption accelerated,around the well shaft accumulated a mass of free gas in a short time.