川南地区龙马溪组页岩高压甲烷等温吸附特征

准确测定页岩吸附气含量对于页岩气储层的评价和开发都具有重要的意义,但目前国内外学者在页岩甲烷等温吸附实验中对模型选择、吸附模式及吸附特征参数的认识上存在着差异,并且对于高压等温甲烷吸附特性的研究较少。为此,在利用N_2/CO_2气体低压等温吸附实验对四川盆地南部地区下志留统龙马溪组页岩孔隙结构特征进行分析的基础上,采用重量法高压甲烷等温吸附实验,选取SDR、Langmuir、BET等3种不同的吸附模型对吸附态甲烷含量进行计算,并对样品甲烷吸附特征进行研究。研究结果表明:①页岩在0～50 nm孔径区间内比表面积分布具有双峰特征,孔体积分布具有三峰特征,较之于中孔,微孔比表面积发育较好,而其孔体积和非均质性均弱于中孔(D_1D_2);②3种模型中SDR和Langmuir模型的计算结果与实测值平均误差均小于6%,甲烷分子主要以单分子层与微孔充填吸附模式共存于页岩孔隙内;③在高压深埋藏情况下,温度是影响吸附态甲烷吸附量和密度值的主要因素,但热力学参数、孔隙结构、非均质性等也会对吸附态甲烷密度造成一定的影响;④低压阶段甲烷分子优先以单分子层形式吸附于吸附能较高、比表面积较大的孔径介于0.4～0.8 nm的微孔中,随后大部分甲烷分子以微孔充填与单分子层共存的形式吸附于孔径介于1.4～8.0 nm的微孔与中孔中,高压阶段极少部分甲烷以多分子层形式吸附于中孔及宏孔中。

High-pressure isothermal methane adsorption characteristic of Longmaxi Formation shale in the southern Sichuan Basin

Accurate determination of gas content in shale is of great significance to the evaluation and the exploitation of shale gas reservoirs. At present, however, domestic and foreign scholars have different opinions on the model selection, adsorption mode and adsorption characteristics parameter in the isothermal adsorption experiment of shale methane, and the high-pressure isothermal methane adsorption characteristics are less researched. In this paper, the structural characteristics of the pores in the shale of Lower Silurian Longmaxi Formation were analyzed by conducting N2/CO2 low-pressure isothermal adsorption experiments. Then, gravimetric high-pressure isothermal methane adsorption experiments were carried out, and three different adsorption models (SDR, Langmuir and BET) were used to calculate the content of adsorbed methane. Finally, the methane adsorption characteristics of the samples were researched. And the following research results were obtained. First, as for the pores with a size of 0–50 nm, the distribution of specific surface area and pore volume is bimodal and trimodal, respectively. Compared with mesopores, micropores are higher in specific surface area and lower in pore volume and heterogeneity (D1