【作者】 周枫；

【导师】 徐鸣洁；

【作者基本信息】 南京大学 ， 构造地质学， 2014， 博士

【摘要】 煤层气是与煤伴生并储藏在煤层中的一种气体,是一种以吸附状态的形式自生自储在煤层中的非常规天然气。沁水盆地是全国煤层气赋存最为富集的地区之一,是目前国内勘探程度最高、储量条件稳定、开发潜力巨大、商业化程度较高的煤层气气田。目前,沁水盆地共施工各类煤层气井已达1000余口,盆地南部煤层气年产能已达5亿~6亿m3。由于煤层气包括基质表面的吸附气、煤层裂缝与割理中的游离气、煤层水中的溶解气和煤层间夹层的游离气4部分。它既不同于呈液态赋存的石油,又有别于主要呈游离态存在于地层中的常规天然气。煤的吸附性导致煤层气成藏的机制和开发技术与常规天然气截然不同。对煤层气藏的成藏规律研究国内外都比较薄弱。20世纪80年代早期,美国开始进行地面开发煤层气的试验和研究,提出了通过排水降压实现煤层气开采的理论。我国学者仅做了一些初步工作,没有建立起系统的煤层气成藏理论,不能具体指导煤层气勘探、资源／储量计算等生产实践。本文针对煤层气地震勘探理论基础薄弱,煤层气岩石物理和地震响应特征的认识不清的问题,从煤岩弹性参数、裂隙结构、各向异性、非均质、衰减等方面对沁水盆地主力煤层开展了系统的研究,以期寻找煤层气富集层的岩石物理特征。首先,本文对沁水盆地主要煤层的岩石物理特征开展了系统的实验测试。在沁水盆地典型的中高阶煤层采集有代表性的煤岩样本,并加工成标准的地震岩石物理测试标本,完成煤岩的各种物理参数的测试。通过对煤样的实验测试结果的分析,初步获得了中高煤阶煤岩岩石物理特征：煤样具有低密度、低速度、低泊松比特点；无论是纵波还是横波,在温度不变的情况下,其速度均随围压的增大而增加,但P波的增加幅度大于s波,波速比Vp/Vs随围压的增大而增大。总结了煤岩微观结构上和在弹性特征上的各向异性特征,及其地震波衰减等特性的变化规律,对煤储层的各向异性进行了定量描述；这些岩石物理特征为煤层气地震技术的应用提供了依据。其次,开展煤岩样品等温吸附以及煤样吸附过程的超声波速度测试研究,获得煤样在不同压力点下的气体吸附量,并通过分析吸附时间与吸附量的关系来确定煤样的吸附饱和周期,从而建立了等温吸附的测试流程,在此基础上利用理论模型如Langmuir模型、Freundlich经验公式、2参数BET模型以及Dubinin-Radushkevich体积填充模型(D-R)对测试的压力和吸附量数据进行拟合,优选出煤岩吸附气体的理论模型。通过利用煤岩三轴测试系统实时采集声波波形,获得煤岩在不同吸附时间和吸附量状态下的速度,并在此基础上分析煤岩速度在不同吸附时间和吸附量下的变化规律,从而获得煤岩吸附量和速度之间的经验关系,为煤层气富集区地震预测提供基础。第三,根据沁水盆地主要煤层的典型构造特征,开展了煤储层地震物理模型研究。针对煤储层中薄煤层、小地层、陷落柱进行了物理模型地震数据采集和分析,揭示其地震响应特征。通过研究发现薄互层及宏观结构存在变化的煤层反射波特征比简单均一煤层的反射波要复杂。其受煤层厚度,煤层层数及分层间距等诸多因素影响且往往以复合波形式出现,正确认识复合波的特征是准确解释煤层形态的前提。由于煤层速度较砂岩低,其视波长较小,在相同的厚度范围内利用频率调谐曲线可分辨的厚度也就比较薄。因此,在同等条件下利用振幅和频率特征对煤层的分辨率比砂岩要高。当单层煤层厚度小于λ/8时,用地震波振幅预测复合厚度是可行的,但要预测单层厚度是困难的。更多还原

【Abstract】 Coal bed Methane(CBM) is a form of natural gas extracted from coal beds. In recent decades it has become an important source of energy in the United States, Canada, Australia, and other countries.The term refers to methane adsorbed into the solid matrix of the coal. Coalbed methane is distinct from a typical sandstone or other conventional gas reservoir, as the methane is stored within the coal by a process called adsorption. The Qingshui basin is one of the most concentration zones of Coal bed Methane in China. It is also a large coal seam gas field with higher commercialization level, great exploitation potential and higher exploration degree. There are more than1000CBM wells in the basin, and the productivity of the southern basin achieve to500,000,000to600,000,000m3per year.CBM is adsorbed to the surface of the coal. The adsorption sites can store commercial quantities of gas as part of the coal matrix. It is different to conventional petroleum resource, because in conventional reservoirs, oil or gas is made in the source rock, migrates to the reservoir rock, and its migration is stopped by cap rock. The reservoir-forming mechanism and development technology are important for the exploration and development of the coal bed gas which are quite different to the conventional gas.The research on reservoir genetic feature of coal bed methane is not very much. Domestic researchers have done some work, but there is no reservoir genetic theory to guide coal bed methane exploration.This study is dedicated to find the rock physical characteristics of coal bed methane enrichment layer. The author completed a lot of work on rock physical tests, which explain the characteristics of main coal beds in the Qingshui basin in terms of elastic parameters, fracture structure, anisotropy, heterogeneity and attenuation. This work is helpful to solve the problem in seismic exploration in coal bed methane.First, a series of tests are completed to obtain the characteristics of coal in the Qingshui basin. The variation range of coal metamorphic grade covers from low rank coal to high rank coal. The results show that coal samples are characterized with low density, low seismic velocity and low Poisson’s ratio. Velocities of compressional wave and shear wave are increased with the increase of confining pressure under a stable temperature. However, the increasing amplitude of compressional wave is higher than that of shear wave. Vp/Vs ratio increased with increasing of confining pressure up to25MPa. Micro-structure and anisotropic characteristics are measured to study the seismic wave attenuation.Second, we develop a series of isothermal adsorption tests to build a test process for bulk samples. Then, the author evaluate four theoretical models for bulk coal adsorption, such as Langmuir model, Freundlich empirical formula, BET model and Dubinin-Radushkevich mode. The results are used to fit a curve to quantify pressure vs adsorption quantity. As a result, the Langmuir model is the most fitable model for adsorption quantity calculation. Ultrasonic wave velocity tests are completed to study relationship between adsorption quantity, then the velocity of samples are built.Third, several seismic physical models are built for simulating the typical structure of coal bed in the Qingshui basin. Analysis for the seismic response of thin seam, small layer and collapse column are carried out based on the seismic data of physical model. The reflection wave of thin interbed is more complex than uniform coal seam. Because the reflection wave is usually a composite wave subjected to many factors like thickness of coal seam, number of layers. Understanding of the characteristics of reflected wave is premise for coal seam seismic interpretation. Resolution of coal seam thickness by using frequency tuning curves is higher, because of the low velocity and apparent wavelength of coal seam. When the thickness of single coal layer is smaller than A/8(λ is the wavelength), it is practicable to predict total thickness of coal seam by seismic wave amplitude, although it is difficult to identify thickness of single layer.更多还原