页岩中丙烷的降解特征及对页岩气组成的影响

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

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C_3H_8、C_3H_8+页岩、C_3H_8+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C_3H_8、C_3H_8+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,C_3H_8+页岩体系中CH_4、C_2H_6生成量及CH_4/C_2H_6值比相应的C_3H_8对照实验高,且黏土矿物含量高的S1系列实验生成的CH_4、C_2H_6及CH_4/C_2H_6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,C_3H_8+页岩实验中的CH_4、C_2H_6产率均高于对照实验,且CH_4的产率高于C_2H_6。2个系列的模拟实验结果均说明黏土矿物能催化C_3H_8的裂解,且有利于CH_4的产生。含水体系中CH_4、C_2H_6生成量及CH_4/C_2H_6值比无水体系高,说明水能促进页岩中C_3H_8的裂解,且有利于CH_4的富集。页岩中黏土矿物和水对C_3H_8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。

Propane and shale samples taken from the same well and different depths in the Longmaxi Formation in theSichuan Basin were selected to carry out different series of simulation experiments in order to investigate the influenceof the degradation of small molecular hydrocarbons on the composition of shale gas in the shale during the over-maturestage. The constant temperature thermal simulation experiments of a gold tube-restricted system were carried out onC3H8, C3H8＋ shale, C3H8＋ shale ＋ water at 360 ℃ and 50 MPa for 72, 216, 360, 720 h. Other pyrolysis experimentswere performed using C3H8 and C3H8＋ shale samples at 400, 450, 500 and 550 ℃ with a pressure of 50 MPa for 72 h inorder to increase the degradation degree of C3H8. The results showed that the CH4, C2H6 yields and the CH4 / C2H6 ratioof C3H8＋ shale at 360 ℃ are higher than those of C3H8 alone. The experiments with S1 shale rock which contains moreclay minerals generated more CH4 and C2H6 than those with S2 rock sample, and generally showed higher CH4 / C2H6ratios. After increasing the simulation temperature, the conversion of C3H8 was significantly increased. The yields ofCH4 and C2H6 in the C3H8＋ shale experiment were all higher than those in the control experiment, and the yield ofCH4 was higher than that of C2H6. These results indicated that clay minerals can catalyze C3 H8 degradation andproduce more CH4. The yields of CH4 and C2H6, and the CH4 / C2H6 ratio from the hydrous experiments were higherthan those from anhydrous experiments, which indicated that water can promote the cracking of C3H8 to produce moreCH4. 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 gasparticularly in high- to over-mature shale rocks. It seems that water played a significant role and its effect on theevaluation of shale gas reserve should be thoroughly investigated especially for high- to over-mature shale rocks.