含煤盆地深层“超饱和”煤层气形成条件

中国深层煤层气资源丰富,但总体勘探和认识程度较低,尚未形成较为系统的深层煤层气地质理论。通过解剖分析准噶尔盆地白家海凸起和鄂尔多斯盆地临兴区块深层"超饱和"煤层气井的试气/生产动态,估算原地游离气的含气量,分析了深层"超饱和"煤层气的形成条件。研究表明：①深层"超饱和"煤层气储层中除吸附气外,还含有原地游离气,用常规试气方法可直接获得气流,煤层气的产出不明显依赖于排水降压;②埋藏超过一定深度,在煤阶和温度的综合作用下,煤的吸附能力将随埋深的继续增加而降低,煤层中吸附气的饱和度有增加的趋势,在达到吸附饱和后,出现原地游离气并形成"超饱和"煤层气,盆地深层具有"超饱和"煤层气形成的优势条件;③由于地温梯度和压力梯度的不同,不同盆地"超饱和"煤层气出现的临界深度不同,异常高压和异常高热流可以降低深层"超饱和"煤层气形成的临界深度;④深层"超饱和"煤层气开发具有大大缩短见气时间、充分利用地层能量和累积产水量低等优势,有望成为未来煤层气勘探开发的一个重要领域。

Formation conditions for deep oversaturated coalbed methane in coal-bearing basins

China is rich in deep coalbed methane resources, but the overall degree of exploration and level of understanding is low; there is still lack of a relatively systematic geological theory for deep coalbed methane. Through thoroughly analyzing the gas testing/production performance of deep oversaturated coalbed methane wells in the Baijiahai uplift of Junggar Basin and the Linxing block of Ordos Basin, this paper estimates the gas content of the in-situ free gas, and explores the formation conditions of deep oversaturated coalbed methane. The study shows that:(1) the deep oversaturated coalbed methane reservoirs contain in-situ free gas in addition to adsorbed gas, which can be directly obtained by conventional gas testing methods and the production of coalbed methane is obviously independent of drainage and decompression; (2) when the burial depth exceeds a certain depth, under the combined effect of two major factors including coal rank and temperature, the adsorption capacity of coal will decrease with the increasing of burial depth, and the saturation of adsorbed gas in coal seams will increase, so in-situ free gas will appear and form oversaturated coalbed methane after reaching the adsorption saturation that the deep reservoirs in basins have the advantageous conditions for forming the oversaturated coalbed methane; (3) due to the difference in geothermal gradient and pressure gradient and the variation of critical depth of oversaturated coalbed methane with different basins, the critical depth of deep oversaturated coalbed methane can be reduced by abnormally high pressure and abnormally high heat flow; (4) the deep oversaturated coalbed methane development possesses such advantages as a sharp decrease in gas breakthrough time, making the best of formation energy, and low cumulative water production, and is expected to become an important field for coalbed methane exploration and development in the future.