深部煤层气成藏条件特殊性及其临界深度探讨

深煤层概念及其评价指标的科学界定，是推动深部煤层气基础理论研究的基础。基于对地应力、含气性、储层物性及岩石力学性质等随煤层埋深变化特征的分析结果显示:相对浅部，深部煤孔隙结构变化小，中孔-微孔比例趋于均一；煤层含气量与埋深之间存在一个“临界深度”，超过此埋深之后含气量随埋深进一步增大而趋于降低；渗透率的常用对数与埋深呈线性负相关关系，暗示深部煤储层趋于致密化；深部围压正效应和温度负效应强弱相互转换，煤岩弹性模量随着埋深增加存在“拐点”。构建了基于地应力、饱和含气量、渗透率等深煤层界定指标体系。以沁水盆地为例，将该盆地深煤层界定在750m以深。即在此深度以深，煤层气成藏特征开始发生转换，其开发须针对储层特性变化采取相应的措施。 

Properties of Deep Coalbed Methane Reservoir-forming Conditions and Critical Depth Discussion

Concept and evaluation indices of deep coalbed are essential for motivating deep coalbed methane research.Based on the analysis of ground stress,pore properties,gas content,reservoir physical and mechanical properties,it has shown that variations of proportion of different pore types of deep coal are very small,especially for mesopore to micropore.With the increase of coal burial depth,gas content increases firstly and then decreases.The inflexion point from gas content increasing to decreasing is defined as “critical depth”,and that is,the gas content decreases when the burial depth is deeper than the critical depth.Decimal logarithm of permeability is linearly negatively related to burial depth,which means that permeability of deep coal is probably very low.Elasticity modulus of coal are positively related to confining pressure and negatively related to temperature,and it also suggests that there is an “inflexion point” in the relation curve between elasticity modulus and burial depth.In further,indices of ground stress,saturated gas content,permeability were constructed to define the critical depth.Taking the Qinshui Basin as an example,the critical burial depth of deep coalbed is at 750m through calculations of the indices.It is suggested that reservoir condition changes a lot compared to shallow coalbed,and corresponding measures must be taken in coalbed methane development.