关于煤层气甲烷碳同位素值对比的探讨

A discussion on the comparison of carbon isotope values of coalbed methane

煤层气甲烷碳同位素值有别于常规天然气,具有总体偏轻的特征,其δ13C1分布比较分散,与煤岩成熟度存在一定联系,低煤阶偏轻程度较大。煤层气解吸过程发生同位素分馏,导致δ13C1存在不确定性,给对比分析带来干扰。通过研究不同采集方式(密闭解吸、密闭粉碎解吸和井口采集)下δ13C1的变化特征,结合不同的煤层气成因类型和煤岩成熟度,建立不同尺度解吸之间δ13C1的联系。研究表明:δ13C1随解吸程度增加而升高,孔隙游离气影响早期解吸气特征,低煤阶影响表现更明显。采集方式和解吸时间对δ13C1对比结果影响很大,晚期解吸气或较长时间的井口采集气能够代表原地煤层气的特征。对比δ13C1对认识煤层气的运移和富集成藏规律具有重要意义。

Carbon isotopes in coalbed methane,different from those in coal-formed gas,are relatively lighter as a whole,and the δ13C1 values in scattered distribution have a certain relationship with coal maturity and are rather lighter than those in coal-formed gas at the same Ro especially for low rank coals.Carbon isotope fractionation occurs during CBM desorption,resulting in uncertainty of δ13C1 values and interference with comparative analysis.Based on analysis of variation of δ13C1 values by different sampling methods,including desorption gas from airtight containers,two-stage thermal desorption gas from airtight containers,desorption gas from wellhead,and in combination with the genetic types of CBM and the maturity of coals,we establish the relationship among the δ13C1 values at different levels of desorption.The δ13C1 values increase with the increasing of desorption level.The free gas in pores influences the features of early desorption,and the influence is more significant in low rank coal.Sampling methods and desorption time are critical to the comparison of δ13C1 values.The late desorption gas or gas sampled at wellhead in a relatively long time is the representative of in-situ coalbed methane gas.The comparison of δ13C1 values on the basis of this is of great significance for a better understanding of coalbed methane migration and accumulation.