天然气汞含量作为煤型气和油型气判识指标的探讨

Discussion of Natural Gas Mercury Content as an Identification Index of Coal Type Gas and Oil Type Gas

天然气汞含量作为判识煤型气和油型气的一项重要指标已经被很多学者所接受,但在勘探实践中应用的并不多,究其原因是对该指标的认识还不够深入。探讨该指标的前提是搞清天然气中汞的形成机制,通过对美国和中国不同产煤区煤中汞含量和不同煤阶下煤的产气率进行统计分析,认为煤系有机质具备形成高含汞天然气的物质基础。但很多检测数据显示并不是所有的煤型气均具有较高的汞含量,有的甚至接近于0。为解释这一现象,开展了煤的吸汞和热释汞实验,发现煤在低温下不仅不能够释汞反而会从周围环境中吸收汞,只有当温度升高到一定程度煤中的汞才会在热力的作用下大量释放出来,因此说气源岩类型和地层温度共同决定了天然气汞含量的高低。全国八大盆地500多口气井的汞含量数据统计分析表明,当天然气汞含量大于30 000ng/m3时,可基本判断该天然气类型为煤型气。当天然气汞含量介于10 000～30 000ng/m3之间时,其为煤型气的几率较大,在结合其他地质资料的情况下也可比较容易得出合理的结论。但当天然气汞含量介于5 000～10 000ng/m3之间甚至更低时,天然气汞含量只能作为判识煤型气和油型气的辅助参数。

Natural gas mercury content used as an important identification index of coal type gas and oil type as has been generally accepted. But in the practice,it was used not so much. The reason lies in the inaccur- ance illustrating of the index. Before discussing this index,it is necessary to clarify the formation mecha- nism of mercury in natural gas. The paper did some statistics and analysis of mercury content of coal in dif- ferent coal production area of America and China, and also gas production ratio of coal with different coal rank. It was suggested that coal can supply enough mercury to form high mercury content natural gas. But lots of mercury content detection data shows that not all coal type gas has high mercury content. To illus- trate this phenomenon,the paper carried out coal adsorption and thermal desorption experiment. It shows that coal can＇t release but adsorb mercury at low temperature, mercury can be released enormously only at high temperature. So the source rock type and the formation temperature together determine the natural gas mercury content. More than 500 gas well＇s mercury content data shows that when the natural gas has more than 30 000ng/m3 mercury content,it can be generally judged as coal type gas,when mercury content is between 10 000-30 000ng/m3 ,it is much posgible to be coal type gas and it is easy to get an reasonable conclusion. Otherwise,the mercury content in natural gas can only be used as an auxiliary identification pa- rameter when it varied from 5 000 to 10 000ng/m3 or even more lower.