煤层气的两种地震AVO响应对比研究

Research shows that the acoustic impedance difference between sandstone reservoirs and surrounding rock, as well as the Poisson's ratio difference between gas-saturated sandstone and water-saturated sandstone, is the petrophysical basis of using AVO probe the natural gas..The similar key points by AVO technology to predict CBM is describe as follows: acoustic impedance difference between coal and surrounding rock, Poisson's ratio difference between coal bed saturated with CBM and coal bed saturated with CBM saturated with water..According to the CBM geology, the actual coal seam must contain CBM (from the perspective of the mine disasters are called Gas). This means that the water-saturated coal bed are not widely distributed, so the key point mentioned as before is changed as follows: acoustic impedance difference between coal and surrounding rock, Poisson's ratio difference between coal bed saturated with difference CBM content..Petrophysical tests show that not only large differences in acoustic impedance between coal and surrounding rock, but also in Poisson's ratio. Gas Geological studies show that the Poisson's ratio of the different coal structure are quite different. Coal structure changes are ubiquitous, it occur not only in the lateral direction, but also in the longitudinal direction..Similarly, the Poisson's ratio difference of coal structure is also the basis for the coal AVO detection. The result presents a new problem:are the two AVO response of CBM the same.Is there a link between CBM content and coal structure?.To answer above questions, the most appropriate method is by petrophysical testing to get Poisson's ratio under different CBM or coal structure. It is so difficult that it isn't recommended for implementation. The mathematic model of coal containing fracture is used for AVO response analysis, such as Hudson model, Cheng model. The CBM is set as free state in these models, therefore, it cannot answer on the CBM adsorption problem..Therefore, we assume that the nature of the two AVO response of CBM is unity for the area of seismic exploration in coalmine. The main research measures are as follows: Sihe coalmine and Zhao coalmine in Qinshui basin is set as the study area, 63 samples of coal are for p-wave velocity and s-wave velocity test by low-frequency systems, and density test by densitometer, 189 samples corresponding to former coal samples are for CBM content measurements. A quantitatively describe method is established for coal structure, AVO response for coal structure longitudinal change is analyzed, the relationship between pressure and the elastic parameters of different coal structure is studied. Based on the petrophysical results, the two AVO response of CBM is analyzed, so as to verify assumptions. The two AVO response of CBM from Seismic data is obtained, the two AVO response of CBM in the same study area is made comparative analysis, the same AVO response in different research areas is done too.

煤层气的AVO响应有两种观点：一是以煤层含气量为预测目标的AVO分析方法，称为煤层含气量的AVO响应，二是以煤体结构为预测目标的AVO分析方法，称为煤体结构的AVO响应，它们统称为煤层气的两种AVO响应。上述从不同角度出发的研究工作，在实践中都见到了效果。因此，两者之间是对立或统一？地质上认为，煤体结构与煤层气富集区关系密切，因此，假设两种响应本质是统一的。选取沁水盆地寺河煤矿和赵庄煤矿为例，取煤岩样进行密度、地震频带内纵横波速度、含气量测量。基于测试成果，研究煤体结构的定量描述方法，分析煤体结构纵向变化对AVO的影响；分析煤岩在不同压力下的动弹性参数，研究埋深对AVO响应的影响；基于测试参数分析煤层气的两种AVO响应差异，从实际地震资料中获得煤层气的两种AVO响应，对比分析同一研究区的两种煤层气AVO响应，对比分析同一AVO响应在不同研究区的特征，验证两种AVO响应是否内在统一。

本项目通过三个关系的研究：煤体结构与弹性参数的关系、煤岩成分与弹性参数关系、吸附性瓦斯对煤岩弹性参数的影响，分析探讨了煤层气勘探的岩石物理基础。通过分析了煤体结构与纵横波速度、密度等参数之间的统计规律。研究结果表明：总体上，煤体结构破坏程度与纵横波速度、杨氏模量和剪切模量呈负相关关系，而与体积模量、泊松比、密度等参数没有明显规律。不同煤体结构类型具有一定纵横波速度区间，但是区间有一定的重叠性。.通过分析煤岩工业成分对煤岩弹性属性的影响，结果表明：①煤的弹性模量及体密度与灰分成正相关关系，与成反相关关系。在孔隙度相近的情况下，灰分中矿物的变化会极大的影响煤岩体密度；②煤岩弹性模量、纵横波速度和体密度随着灰分的增加而增加，横波速度、弹性模量、体密度随着孔隙度的增大而减小，纵波速度随着孔隙度增大而增大；③灰分与实测剪切模量具有较高相关关系（R2=0.75），说明煤岩中的灰分对于煤岩的剪切模量是主要的控制因素，其原因主要是由于灰分的剪切模量要远大于有机质的剪切模量，并且灰分增加引起煤岩剪切模量的变化量更大。.根据Langmuir方程，构建了物理吸附性甲烷的密度函数，进而可以求得物理吸附性甲烷的密度和体积模量。结果表明，物理吸附性甲烷的密度要大于游离态甲烷的密度。物理吸附性甲烷的体积模量与甲烷压力、Langmuir压力有关，与Langmuir体积无关。煤岩含有水和游离甲烷、吸附态甲烷时，且游离甲烷饱和度、吸附态甲烷饱和度两者之和超过5%（第二种混合流体），则饱和第二种混合流体的煤岩体积模量变化要远小于饱含第一种混合流体（含水饱和度大于95%）的煤岩体积模量。大部分煤岩中甲烷的饱和度大于5%，因此，实际工作中饱和流体煤岩体积模量变化较小。.上述研究表明，煤体结构的AVO响应，与煤层气的AVO响应是对立的，煤层气没有直接的AVO响应特征。