含吸附气煤体的弹性波响应特征及其机理研究

To investigate elastic wave response characteristics and mechanism of unconventional coalbed-adsorbed methane is the theoretical basis for effectively evaluating coalbed methane (CBM) resources by using seismic exploration technique. And it has become the key scientific problem that needs to be solved in CBM seismic exploration field. In view of this, coals respectively bearing adsorbed gas (methane) and non-adsorbed gas (helium) are to be taken as the research objects in this project. Experimental study on ultrasonic (P and S) wave test in the process of pore pressure increasing is to be carried out based on triaxial mechanics experiment system. The elastic wave response characteristics of free gas and adsorbed gas and their change rules with pore pressure could be found out. Based on the specific fluid-solid coupling, the expansion deformation of coal matrix skeleton caused by gas adsorption, the influence rule and control mechanism of gas adsorption on coal’s elastic wave response characteristics could be revealed. Then we can make clear the internal relations between coal’s gas-bearing properties and elastic wave response characteristics, and extract the elastic wave sensitive parameters to coal’s gas-bearing properties. We can further establish the evaluation method and prediction model based on elastic wave response characteristics for coal reservoir’s gas-bearing properties. This study will provide scientific theoretical foundation for objective evaluation and prediction on coal reservoir’s gas-bearing properties with elastic wave technology such as seismic.

探明非常规煤层吸附气的弹性波响应特征及机理是利用地震勘探技术有效评价煤层气资源的理论基础，已成为煤层气地震勘探领域中急需解决的关键科学问题。鉴于此，项目以含吸附气（甲烷）、非吸附气（氦气）煤体为研究对象，基于三轴力学实验系统分别对其开展孔隙压力升高过程中的超声纵、横波测试实验研究，探明煤体内游离气、吸附气的弹性波响应特征及其随孔隙压力的变化规律；以煤储层内特有的流固耦合作用-“气体吸附引起煤基质骨架的膨胀变形”为理论依据，揭示气体吸附作用对煤体弹性波响应特征的影响规律及控制机理；进而明确煤体含气性与弹性波响应特征之间的内在关系，提取反映煤体含气性的弹性波敏感参数，建立基于弹性波响应特征的煤储层含气性的评价方法与预测模型。这项研究为利用地震等弹性波技术客观评价与预测煤储层含气性提供科学的基础理论依据。

为了加强应用地震等弹性波对主要以吸附方式赋存于煤层中煤层气的含气性勘探，项目主要开展了含吸附气煤体的弹性波响应特征的基础实验研究。实验设计模拟了含四种不同吸附强度的气体（氦气、氮气、甲烷、二氧化碳）煤岩体在气体产出过程（气体压力下降）中的超声波测试，获得不同稳定含气状态（以含气种类、气体压力表征）下弹性波响应（诸如超声波波形、速度、衰减、频谱等）的动态变化特征。结果显示同种气体作用下，含气煤体随着气体压力的下降弹性波速度、品质因子呈升高趋势。随着所含气体吸附能力的增强（即由氦气、氮气、甲烷至二氧化碳）煤体相应波速、品质因子依次降低。气-煤耦合作用能够在一定程度上显著改变含气煤体的动静态弹性力学性质，并相应表现出一些针对于含气性的弹性波响应特征。并且含气煤体的弹性波响应表现出一定的各向异性特征。基于气体压力、吸附膨胀等气固耦合作用机制、煤体的双重孔裂隙结构模型以及频谱分析方法等对含气煤体的弹性波响应的控制机理进行了全面分析。认为多孔介质结构及其气-固耦合机制是控制含气煤岩体弹性波响应特征的内在本质原因。表征多孔介质结构力学性质的主控裂隙系统控制着煤体动静态弹性力学性质。基于等效介质理论及弹性波实际实验结果构建含气煤体总有效应力模型和弹性波参数模型。更加明确了煤体含气性与弹性波响应特征之间的内在关系，进一步建立基于弹性波响应特征的煤储层含气性的评价方法与预测模型。本项研究为实际煤层含气性的地震等弹性波勘探提供基础理论依据。