陆相页岩微观孔隙结构特征及对甲烷吸附性能的影响

Characteristics of Microscopic Pore Structures and Their Effect Impacts on Methane Adsorption Capacity in Continental Shales

页岩的微观孔隙结构对其甲烷吸附性能及页岩油气潜力具有重要影响，前人研究主要集中在海相页岩。该文以四川盆地川西坳陷上三叠统须家河组五段为例，开展了陆相页岩的探索研究。首先通过低温氮气吸附实验对页岩样品的微观孔隙结构特征进行了研究，计算了页岩的比表面积、孔径分布、孔体积和平均孔径等孔隙结构参数；然后通过高压甲烷等温吸附实验，研究了页岩样品的甲烷吸附特征；最后探讨了页岩微观孔隙结构特征对甲烷吸附性能的影响。结果表明，须五段页岩平均孔径为7.81~9.49 nm，主体孔隙为中孔，也含有一定量的微孔和大孔，孔隙形状以平行板状孔为主，含有少量墨水瓶形孔。页岩比表面积高出常规储层岩石许多，有利于气体在页岩表面吸附存储，孔径在2~50 nm的中孔提供了主要的孔体积，构成了页岩中气体赋存的主要空间。在85℃条件下，页岩甲烷吸附的兰氏体积为1.21~4.99 m3/t，不同页岩样品之间的吸附性能差异明显。页岩的兰氏体积与比表面积之间呈现良好的正相关关系，比表面积与黏土矿物含量呈正相关，而与总有机碳含量关系不明显。页岩的兰氏体积与微孔和中孔体积之间都具有良好的正相关关系，微孔体积和中孔体积与总有机碳含量之间存在一定的正相关关系，但是正相关性的程度没有微孔体积和中孔体积与黏土矿物含量之间的关系强烈。陆相页岩有机质热演化程度相对较低，因此有机孔发育有限：但另一方面同时黏土矿物含量较高，所以其内部发育大量微孔和中孔，从而构成可观的比表面，影响甲烷吸附能力。

Characteristics of microscopic pore structures of shales have important impacts on the assessment of gas and associated oil/gaspotential adsorption capacity of the shales, and previous studies have mainly focused on marine shales. Here, we conduct a pilot studyof continental shales based on a case study of the fifth member of the Upper Triassic Xujiahe Formation in the western depression,Sichuan Basin. The microscopic pore structures of the shales were investigated by using low-temperature nitrogen gas adsorptionmethod. Multiple structural parameters of the shales were calculated, including the specific surface area, pore size distribution, pore volume and average pore diameter. Then, high-pressure methane adsorption analysis was conducted to obtain the parameters ofLangmuir volume and Langmuir pressure. Finally, we address the effect of microscopic pore structure characteristics on the methaneadsorption capacity of the shales. Results show that the average pore diameter of the shales ranges from 7.81 to 9.49 nm. Mesoporesdominate in the shales, while some micro- and macro-pores also exist. Slit-like pores are the main pore types in shales, and a smallamount of ink-bottle-like pores are also present. The specific surface area of the shales are much greater than that of conventionalreservoir rocks; this is good for gas adsorption in shales. The mesopores with the pore diameter between 2 and 50 nm provide thedominating pore volume, which constitutes the major space for gas adsorption and storage in the shales. Under the temperaturecondition of 85 ℃, the Langmuir volume of methane adsorption in shales ranges from 1.21 to 4.99 m3/t, and methane adsorptioncapacity varies largely within different shales. There are positive correlations between Langmuir volume and specific surface area ofshales, and between specific surface area and clay minerals content of shales. However, no correlation exists between specific surfacearea and TOC of shales. There are positive correlations between Langmuir volume and micropore and mesopore volume. Organic matteris an important control on micro- and meso-pores of shales, evidenced by the positive correlations between micropore and mesoporevolume and TOC. Comparatively, clay minerals play a much more important role on the pore volume of micro-and meso-pores of shales.For continental shales, organic-matter thermal evolution is relatively low and organic pores are thus not well developed. In contrast,abundance of clay minerals is relatively high in shales, with well development of micropores and mesopores, which constitute aconsiderable specific surface area of shales and thus affect the methane adsorption capacity of shales.