| 其他摘要 | 本文研究目标区为四川盆地东南缘，大部位于贵州省境内，具有较大的页岩气资源潜力。通过对下古生界两套富有机质泥页岩进行的以纳米孔隙特征为中心的页岩气储层研究，探索了多种纳米孔隙表征方法在页岩气储层研究中的应用；分析了牛蹄塘组页岩样品在进行低压N2吸附实验时合适的测试条件；探讨了牛蹄塘组页岩气储层纳米孔隙特征及影响因素；对比研究了下古生界龙马溪组和牛蹄塘组页岩孔隙分布特征；讨论了页岩气储层孔隙连通性的表征及孔隙连通性对页岩气勘探开发的意义。具体如下： 首先，通过对多种纳米孔隙表征方法的实践探索，分析和总结了各种方法的优缺点及适用性，并将页岩气储层孔隙表征方法初步划分为局域分析技术和统计分析技术两类。局域分析技术主要包括原子力显微镜、场发射扫描电子显微镜、纳米X射线断层成像技术等；统计分析技术主要有气体吸附法、压汞法、小角中子散射及小角X射线散射等。目前这些技术方法多数都已较为成熟地应用于页岩气储层的各方面研究中，但仍有一些材料科学领域应用广泛的技术方法（如SANS、SAXS等）在页岩气方面的应用还比较少。因此，我们不仅要不断探索应用新的技术方法来完善页岩气储层纳米孔隙的表征，也要综合利用多种技术方法，取长补短，为页岩气储层孔隙评价提供丰富而准确的数据资料。其次，通过对牛蹄塘组页岩样品进行不同粒径及不同脱气温度条件下的低压N2吸附研究，确定了牛蹄塘组页岩合适的粒径和温度条件。粒径对测试结果的影响表明粒径过大不利于获取样品中的真实孔隙特征，粒径减小能够增加样品的孔隙连通性，有利于获取样品的真实孔隙特征，建议在相同条件下选择<0.074 mm的粒级对牛蹄塘组页岩进行低压N2吸附实验。不同脱气温度对测试结果的影响表明脱气温度过低不能完全去除样品中的自由水，脱气温度过高又会造成样品中物质（尤其是有机质）的改变。因此，为保证样品能够完全脱去自由水及易挥发份并且不改变样品自身的物质组成和性质，建议在150 ℃左右条件下进行脱气。第三，通过分析牛蹄塘组页岩钻孔样品的沉积环境、物质组成、演化历史及其与孔隙特征的关系，探讨了页岩气储层纳米孔隙特征的影响因素及影响规律。研究结果显示贵州省牛蹄塘组页岩均存在微孔、介孔、大孔这三类孔隙，其中微孔主要存在于有机质中，介孔主要存在于石英和伊利石（或有机粘土聚集体）中，并且介孔贡献了纳米孔体积的60–70 %，是纳米孔隙的主要组成部分。平均孔径随埋深增加有减小趋势，可能是TOC含量的增加造成的。另外，有机质孔隙在牛蹄塘组中上部发育较下部更为广泛，而有机粘土聚集体中的孔隙则在牛蹄塘组下部更为发育，且具有一定的方向性。这种孔隙分布的垂向差异可以归因于沉积环境、物质组成的非均质性、成岩演化过程、构造运动等多个因素。第四，通过对牛蹄塘组和龙马溪组页岩储层的孔隙分布特征对比研究发现：龙马溪组页岩储层中纳米孔隙主要存在于有机质中，而牛蹄塘组页岩中纳米孔隙主要存在于粘土矿物或有机粘土聚集体中，有机质孔隙发育较少。这两套页岩孔隙分布位置的差异很大程度上是由储层的热演化过程和构造发育情况造成的，也是两套地层含气性差异的主要原因。在牛蹄塘组页岩和龙马溪组页岩储层中，介孔均是最重要的孔隙组成部分；在微孔和介孔范围，小于20 nm的孔隙比20–50 nm的孔隙具有更强的含气能力。龙马溪组页岩由于有机质孔隙较好的连通性，在压裂时这些孔隙能够被较好的连通，从而获得较好的页岩气产量。而牛蹄塘组页岩中大量的小于20 nm的孔隙存在于有机粘土聚集体中，由于粘土片层的保护并且孔隙孔径较小，很难通过水力压裂来连通这些孔隙。因此，我们认为孔隙分布特征的差异是龙马溪组页岩的页岩气产率好于牛蹄塘组页岩的主要原因。最后，通过采用低压N2吸附来表征页岩气储层孔隙连通性，揭示了孔隙连通性对页岩气勘探开发的意义。不同粒径样品的孔体积变化特征表明低压N2吸附能够初步定性表征页岩气储层的孔隙连通性；龙马溪组页岩比牛蹄塘组页岩具有更好的孔隙连通性，这也是目前牛蹄塘组页岩产气性不如龙马溪组页岩的原因之一。本论文通过对页岩气储层纳米孔隙特征的系统研究，揭示了页岩气储层中各类纳米孔隙的形成机理和演化过程，分析了纳米孔隙结构的主要控制因素及影响规律，归纳了纳米孔隙对页岩气的生成、储集、运移及勘探开发等方面影响，为中国页岩气的勘探开发提供了一定的科学依据。; This study takes southeast of Sichuan basin as the research target zone, which has a large potential of shale gas resource and most of the land locate in Guizhou province. Focusing on nanoscale pore characteristics, we carry out the research on two shale gas reservoirs of Lower Paleozoic shale (Niutitang formation and Longmaxi formation). We have discussed the applications of various characterization methods of nanoscale pores during the study of shale gas reservoir, and analyzed the test conditions of low pressure N2 adsorption for Niutitang shale, and studied nanoscale pore characteristics and influential factors of Niutitang formation shale gas reservoir, and compared nanoscale pore distribution characteristics of gas shales between Niutitang formation and Longmaxi formation, and explored pore connectivity characterization of shale gas reservoir and the implications for exploration and development of shale gas. Specifics as follows:Firstly, through the exploration of various characterization methods of nanoscale pores, the strengths and limitations of each method as well as its application perspective are analyzed and summarized primarily, and the methods are preliminarily separated into two types: local analysis technologies and statistical analysis technologies. Local analysis technologies mainly include atomic force microscope, field emission scanning electron microscope and nano X-ray tomography imaging technology, etc. Statistical analysis technologies mainly include gas adsorption, mercury intrusion porosimetry and small angle scattering, etc. At present, most of the methods have been applied to study shale gas reservoir extensively, however, some methods (e.g., SANS, SAXS, etc.), which have been applied in the study of materials science successfully, are still less used in shale gas reservoir. Therefore, we should not only continue to explore new technology to consummate nanoscale pore characterization of shale gas reservoir, but also take advantage of multiple complementary characterization methods and provide abundant and accurate data for the assessment of pore characteristics shale gas reservoir.Secondly, suitable particle size and outgas temperature of low pressure N2 adsorption have been confirmed for Niutitang shale according to the study on the effects of particle size and outgas temperature on pore characteristics. The effects of particle size on test results show that it isn’t conducive to obtain real pore characteristics of shale samples with oversized particle size, while the decrease of particle size could improve pore connectivity and benefit the acquirement of real pore characteristics. We suggest that<0.074 mm should be selected as a suitable particle size for Niutitang shale to conduct low pressure N2 adsorption. The effects of outgas temperature on test results indicate that the free water is unlikely to be removed completely if the outgas temperature is too low. But an excessive outgas temperature maybe causes the decomposition of organic matter or hydrocarbons. Therefore, to guarantee the complete remove of free water and volatile, and keep the material composition and property of samples unchanged, 150 ℃ or nearly should be a suitable outgas temperature for low pressure N2 adsorption.Thirdly, combining material composition, sedimentary environment, evolution history and pore characteristics of Niutitang formation shale core samples, we explore influential factors and regularity of nanoscale pore characteristics in shale gas reservoir. The results indicate the existence of all micropores, mesopores and macropores in Niutitang shale. Organic matter appears to be the major contributor of the micropores. As the most important components of nanoscale pores, mesopores account for 60–70 % of total nanoscale pore volume, and are likely contributed by quartz and illite (or organic -clay aggregates). The decrease of average pore size with increasing burial depth appears to be closely associated with the increase of organic matter content. In addition, organic matter pores develop more common in the middle-upper Niutitang formation than the bottom of Niutitang formation. While the lower Niutitang formation seems to develop substantial amount of pores in organic-clay aggregates, which preferentially lie parallel to the shale bedding. The perpendicular variation of pore structure features is explained with multiple mechanisms including sedimentary environment, heterogeneity of material composition, diagenetic evolution, tectonic movement thermal, etc.Fourthly, according to the comparison of nanoscale pore distribution characteristics between Niutitang shale and Longmaxi shale, it can be found that nanoscale pores mostly exist in organic matter for Longmaxi shale, while for ?1n shale, nanoscale pores in clay minerals or OM-clay aggregates are most common and organic matter pores could hardly been found. The distribution differences depend largely on thermal evolution and tectonic development of reservoirs, and may be the main reason for the difference in gas-bearing capacity between Niutitang shale and Longmaxi shale. For both Niutitang shale and Longmaxi shale, mesopores are the most important components of pores. In the range of micropores and mesopores, the pores less than 20 nm have a stronger storage capability greater exploitation potential than that in the pores between 20–50 nm. Because of the good connectivity of OM pores, most of OM pores could be accessed by fracturing in Longmaxi shale and a good productivity could be obtained. However, in Niutitang shale, large amounts of the pores less than 20 nm in organic-clay aggregates are difficult to be accessed through current hydraulic fracturing technology because of the protection of clay layers and the small size of pores. Therefore, we suggest that pore distribution characteristics differences may be one of the main reasons why production efficiency of Longmaxi shale is much better than Niutitang shale.Lastly, based on low pressure N2 adsorption, we characterize the pore connectivity of shale gas reservoir and reveal the implications of pore connectivity on exploration and development of shale gas. The variation characteristics of pore volume with different particle size indicate that low pressure N2 adsorption could qualitative characterize pore connectivity of shale gas reservoir preliminarily. The better pore connectivity of Longmaxi shale than Niutitang shale should also be one of the main reasons that production of Longmaxi shale is much better than Niutitang shale.Based on the systematic study of nanoscale pore characteristics in shale gas reservoir, the thesis reveals formation mechanism and evolution process of various nanoscale pores, analyzes the main control factors and regularity of nanoscale pore structure characteristics, conclude the effects of nanoscale pore on the generation, storage, migration and exploitation of shale gas, and provides certain scientific basis for exploration and development of shale gas in China. |
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