煤体结构与甲烷吸附/解吸规律相关性实验研究及启示

甲烷(CH_4)在煤体中的流动包含"渗流—扩散—吸附/解吸"3个环节,相比粉状煤,采用块状煤体进行CH_4吸附/解吸实验能够更有效地表征煤层中气体的流动状态。为此,依托渭北煤田韩城矿区煤样,利用自行设计的块煤吸附/解吸实验装置,研究了低压下块状同体积原生结构煤、碎裂煤和糜棱煤的CH_4等温吸附/解吸特性;采用显微CT和扫描电镜分析了3种煤样的孔裂隙结构和显微构造,探讨了煤体结构对CH_4吸附/解吸的影响。结果表明:不同煤体结构煤的CH_4吸附/解吸特性有显著差异。结构致密的原生结构煤,孔隙度较低,导致CH_4吸附/解吸平衡时间长,吸附量低,解吸率低;相比原生结构煤,脆性变形碎裂煤张裂隙发育且相互贯通,孔隙度变大,连通性好,导致CH_4吸附/解吸平衡时间变短,吸附量升高,解吸率增大;韧性变形糜棱煤孔隙数量虽增多,但裂隙被揉皱闭合,形成孤立分布的孔隙结构,渗透性变差,导致CH_4吸附/解吸平衡时间最短,解吸速率最快,说明大多数CH_4仅吸附在块煤内构造变形作用下形成的粒间孔隙中。可知,碎裂煤储层是煤层气开发的有利区域;而致密原生结构煤和糜棱煤储层可尝试通过多尺度压裂、注热等技术手段实施储层改造以增加煤体裂隙通道,达到气井增产增效的目的。

Experimental investigation of methane adsorption/desorption behavior in coals with different coal-body structure and its revelation

The adsorption/desorption experiment of bulk coal sample can be more effective to characterize the flow state of methane (CH4) in coal seams compared to powder coal sample,since CH4 flow through a coal seam consists of fluid flow,diffusion and adsorption/desorption process.Thus,the low-pressure CH4 adsorption/desorption properties of bulk primary coal (PC),cataclastic coal (CC) and mylonitic coal (MC) with identical volume collected from Hancheng Mine,Weibei coalfield were analyzed using self-designed adsorption/desorption apparatus of bulk coal.In combination with micro-focus CT and scanning electron microscope images,the influence of coal-body structure on CH4 adsorption/desorption behavior was discussed.The results indicate that the obvious difference can be observed in coals with different coal-body structures.Long CH4 adsorption/desorption equilibrium time,small adsorption capacity and low desorption rate were recorded in PC due to its dense structure and low porosity.In comparison with PC,a shorter adsorption/desorption equilibrium time,higher adsorption capacity and faster desorption rate were occurred owing to developed and connected fracture system and increased porosity in CC under brittle tectonic stress.Under ductile deformation,the coal composition of MC was folded and concomitant with increased pore number and closed fracture,which induced isolated pore structure and poor permeability.These characteristics result in the shortest adsorption/desorption equilibrium time and the fastest desorption rate,indicating most of CH4 in bulk MC may merely adsorb on inter-granular pores formed during deformation process.Therefore,the CC reservoir is a favorable area for CBM development.Nonetheless,for both PC and MC reservoirs,the increase in the well yield and efficiency can be achieved through reservoir reformation by means of multi-scale fracturing,heat injection and other techniques which can increase the channels within coal fracture.