深部低阶煤三相态含气量建模及勘探启示——以准噶尔盆地侏罗纪煤层为例

准噶尔盆地低阶煤煤层气资源量巨大,其勘探开发突破是提高我国煤层气产量的重要途径之一。针对该区低阶煤浅部含气性差、深部含气量测试数据匮乏且现有技术手段难以准确测试的状况,从煤层气三相态含气量基本构成出发,开展了低阶煤较高温度和压力条件下甲烷的等温吸附、溶解度、密度和孔隙率等一系列实验,分析了影响甲烷吸附气量、溶解气量和游离气量的相关因素,构建了深部低阶煤吸附气、溶解气和游离气三相态含气量的计算模型。以准噶尔盆地低阶煤储层为例,利用该模型计算的结果显示:三相态含气量构成以吸附气量和游离气量为主,溶解气量所占比例较少;随埋深增加和含水饱和度降低,含气量构成序列由吸附气量高于游离气量转换为游离气量高于吸附气量;含水饱和度越低,其相对含气量转换深度越浅。综合分析后认为,该区深部低阶煤气体构成以游离气为主,煤层气勘探的思路则要以寻找圈闭和局部构造高点为主。

Three-phase gas content model of deep low-rank coals and its implication for CBM exploration : A case study from the Jurassic coal in the Junggar Basin

The Junggar Basin has huge coal-bed methane (CBM) resource potentials in the low-rank coals and a breakthrough in exploration and development of these CBM resources may contribute greatly to the CBM production in China. The gas potential of the shallow low-rank coals in this basin is low, the gas content data of the deep low-rank coals are rare, and it is very difficult to accurately test the gas content by using the available techniques. According to the basic components of three-phase gas content of CBM, a series of experiments were performed on methane isothermal adsorption, dissolution, density and porosity of low-rank coals at relatively high temperature and pressure. Based on the results of these experiments, we analyzed the factors influencing methane adsorption gas content, solution gas content and free gas content, and built a three-phase gas content model for calculating the adsorption gas content, dissolution gas content and free gas content in the deep low-rank coals. The low-rank coals in the Junggar Basin were taken as a case study. The model results show that adsorption gas content and free gas content are predominant, while the dissolution gas content is less. Along with the increasing burial depth and lowering water saturation, the gas content sequence with adsorption gas content being larger than free gas content transforms to a gas content sequence with free gas content being higher than adsorption gas content. The lower the water saturation is, the shallower the relative gas content transformation depth is. A comprehensive analysis reveals that the deep low-rank coals in this basin feature the predominance of free gas, thus CBM exploration should focus on searching for traps and local structural highs.