页岩中丙烷的降解特征及对页岩气组成的影响

为探讨高过成熟阶段页岩中小分子烃类的降解对页岩气组成的影响，选取了丙烷和页岩（取自四川盆地龙马溪组同一钻井、不同深度）开展不同系列的模拟实验。对C₃H₈、C₃H₈+页岩、C₃H₈+页岩+水在360℃、50 MPa条件下进行黄金管限定体系恒温热模拟实验，恒温时间包括72，216，360，720 h；同时为探讨更高演化程度条件下页岩中小分子烃类的降解特征，对C₃H₈、C₃H₈+页岩分别在400，450，500，550℃和50 MPa条件下恒温热模拟72 h。结果显示，360℃恒温实验条件下，C₃H₈+页岩体系中CH₄、C₂H₆生成量及CH₄/C₂H₆值比相应的C₃H₈对照实验高，且黏土矿物含量高的S1系列实验生成的CH₄、C₂H₆及CH₄/C₂H₆值基本较S2系列高。提高模拟实验温度后，丙烷的转化率显著提高，C₃H₈+页岩实验中的CH₄、C₂H₆产率均高于对照实验，且CH₄的产率高于C₂H₆。2个系列的模拟实验结果均说明黏土矿物能催化C₃H₈的裂解，且有利于CH₄的产生。含水体系中CH₄、C₂H₆生成量及CH₄/C₂H₆值比无水体系高，说明水能促进页岩中C₃H₈的裂解，且有利于CH₄的富集。页岩中黏土矿物和水对C₃H₈裂解的促进作用导致页岩气向干燥系数高的方向演化，页岩中小分子烃类的降解对高过成熟阶段页岩气的组成具有重要影响，在此过程中水分子起到重要作用，其对高过成熟页岩气资源的评价值得更多关注。

Degradation characteristics of propane in shale rocks and its implication to shale gas composition

Propane and shale samples taken from the same well and different depths in the Longmaxi Formation in the Sichuan Basin were selected to carry out different series of simulation experiments in order to investigate the influence of the degradation of small molecular hydrocarbons on the composition of shale gas in the shale during the over-mature stage. The constant temperature thermal simulation experiments of a gold tube-restricted system were carried out on C₃H₈, C₃H₈ + shale, C₃H₈ + shale + water at 360℃ and 50 MPa for 72, 216, 360, 720 h. Other pyrolysis experiments were performed using C₃H₈ and C₃H₈ + shale samples at 400, 450, 500 and 550℃ with a pressure of 50 MPa for 72 h in order to increase the degradation degree of C₃H₈. The results showed that the CH₄, C₂H₆ yields and the CH₄/C₂H₆ ratio of C₃H₈ + shale at 360℃ are higher than those of C₃H₈ alone. The experiments with S1 shale rock which contains more clay minerals generated more CH₄ and C₂H₆ than those with S2 rock sample, and generally showed higher CH₄/C₂H₆ ratios. After increasing the simulation temperature, the conversion of C₃H₈ was significantly increased. The yields of CH₄ and C₂H₆ in the C₃H₈ + shale experiment were all higher than those in the control experiment, and the yield of CH₄ was higher than that of C₂H₆. These results indicated that clay minerals can catalyze C₃H₈ degradation and produce more CH₄. The yields of CH₄ and C₂H₆, and the CH₄/C₂H₆ ratio from the hydrous experiments were higher than those from anhydrous experiments, which indicated that water can promote the cracking of C₃H₈ to produce more CH₄. The positive effect of clay mineral and water in shale on C₃H₈ cracking increased the dryness index of shale gas. The degradation of small hydrocarbon molecules had an important influence on the composition of shale gas particularly in high-to over-mature shale rocks. It seems that water played a significant role and its effect on the evaluation of shale gas reserve should be thoroughly investigated especially for high-to over-mature shale rocks.