高-过成熟海相页岩有机质力学性质热演变特征及对页岩宏观力学性质的影响

Lower Paleozoic marine shales are the main producing layers of shale gas in China, and are basically in the high-over mature stage. As an essential component of shale reservoirs, organic matter (OM) is not only responsible for the generation of shale gas, but also strongly influence shale mechanical properties. Limited by testing techniques, previous studies rarely involved the OM mechanical properties as well as its thermal evolution characteristics, which however may be the dominant factors that cause the uncertainty of shale mechanical properties in the gas sweat spots. In this project, the mature kerogen and soluble OM separated from the artificially matured shale sample, were used to thermally simulate the change of mechanical properties of kerogen and solid bitumen in the high mature stage, respectively. Meanwhile, the relationship between the mechanical properties of OM and its chemical composition and organic pore development during thermal evolution were also discussed and further quantified. On this basis, the obtained mechanical properties of kerogen and solid bitumen are subsequently applied to the actual high-over mature marine formation shales, including the Lower Silurian Longmaxi Shale and the Lower Cambrian Niutitang Shale. Combined with the nanomechanical testing, the general level of mechanical properties of total OM as well as the OM composition (i.e., relative content of solid bitumen and kerogen) in the above marine shales are quantitatively determined. Finally, based on the macromechanical properties of shale reservoir obtained by in-situ nanoindentation technology, the effect of OM composition and OM mechanical properties on the macromechanical properties of high-over marine shales are comprehensively explored. Overall, the revelation of OM mechanical properties and OM composition not only provide basis for the study of macromechanical properties of high-over mature marine shales in gas sweat spots, but also offer a new perspective for the study of the organic pore development and gas-bearing characteristics in shale.

有机质作为页岩储层的重要组成部分，不仅是页岩气的生烃母质，同时其自身力学性质也强烈影响着页岩力学性质的表达。高-过成熟海相页岩是我国页岩气的主力产层，受技术手段的限制，之前的研究甚少涉及页岩基质有机质力学性质及其热演变特征。项目通过实验设计模拟了干酪根和固体沥青的力学性质热演变特征，并量化了其与有机质化学组构改变和有机孔发育之间的关系。继而将有机质力学性质热演变差异应用于实际高过熟海相页岩地层，结合纳米压痕技术在页岩力学性质测试中的应用，明确了页岩有机质力学性质并量化了基质有机质组成。在此基础上，结合页岩原位纳米压痕技术获得的页岩整体力学性质，综合探讨页岩基质有机质组成以及有机质力学性质热演变对高-过熟海相页岩整体力学性质的影响。海相页岩干酪根和固体沥青力学性质的揭示以及相对含量的确定，不仅为海相页岩气甜点区宏观力学性质的变化研究提供依据，同时也为页岩有机孔发育和含气性的研究提供新的视角。

非常规油气的兴起使得人们开始重新审视页岩基质有机质的原位生烃转化过程、自身物理化学性质的热演变等特征，这些与页岩气的生成、储集以及开发密切相关。本项工作针对中国海相龙马溪组页岩系统开展了有机岩石学和微观力学特征研究。有机岩石学分析表明龙马溪组页岩有机质以焦沥青和笔石为主，而不是以往认为的以原生干酪根组分为主，此结论为龙马溪组页岩的生烃转化过程研究提供了基础认识。另外，以往的宏观力学测试技术不能对微观材料进行力学测试，故本研究将材料学中广泛应用的纳米力学测试技术应用到页岩及其基质组成相的力学性质表征上，并摸索建立了一套页岩微观力学性质测试的方法技术体系。在此基础上，对中国广泛发育的龙马溪组页岩基质有机质进行了力学表征，测得杨氏模量值在4.5-10GPa，硬度在0.35-1.3GPa。上述力学值的变化受热演化成熟度的控制，本质上与有机质的化学结构热演变有关。有机质力学形变方面同样显示出随热演化成熟度的变化，随热演化程度的提高，弹性增强，至变质作用阶段，几乎为纯弹性体，类似石墨的压痕响应特征。这种高弹性使得焦沥青中的有机孔在后期构造变动中能够得以完整保存。同时，有机质随热演化程度的提高脆性程度增大，可压裂性增强。实测有机质可被压裂的成熟度节点约在3.0%Ro。特别地，涪陵、长宁页岩气田龙马溪组页岩的成熟度在3.2-3.45%Ro，有机质可以被压裂，连通矿物诱导缝形成基质全网络缝，利于页岩气导流，这可能是这些页岩气田单井天然气日产量非常高的另外一个原因（好的保存条件）。再者，通过页岩力学性质测试，探讨了有机质对页岩整体力学性质的影响，得出有机质主要以与黏土混合形成有机黏土的形式来充当基质承压骨架，影响页岩的力学性质以及在开发过程中的表现，未来应加强此方面的研究。以上认识为中国页岩气的勘探开发提供了新的地质理论认识和方向指导。