煤层中流体地震可探测性的模拟分析

作为一种典型的强阻抗差低阻抗薄层，煤层中孔隙含流体时是否会引起地震反射产生明显的异常是回答地震检测流体是否可行的根本.为此，本文针对强阻抗差薄层模型，基于Biot双相介质理论，通过弹性波有限差分法数值模拟，与各向同性单相介质假设的煤层反射对比，探讨了反射复合波受煤层孔隙度及流体性质变化的影响程度.模拟分析发现：由于薄层孔隙度和孔隙流体属性的变化在Biot理论中表现为纵波速度的变化，PP波反射AVO（Amplitude Versus Offset，振幅随偏移距变化）特征对薄层是否含流体相对敏感；综合使用PP与PS波对比有利于薄层中流体的预测；孔隙度一定时，PP波反射振幅随着含气饱和度的增加而增大；受薄层调谐作用的影响，孔隙和流体变化对煤层反射的频谱特征影响不大，近似于单相介质时的情况.

Simulation analysis of fluid seismic detectability in coal seams

The coal seam is a typical thin bed characterized by low elastic impedance and high-impedance compared to surrounding rocks. It is essential that whether the existence of fluid in the coal seam can cause an obvious anomaly of seismic reflected waves, which answers if seismic exploration can be used in thin-bed fluid detection. To address this issue, this work uses a thin-bed model with strong impedance contrasts to simulate its seismic response by an elastic-wave finite difference modeling method based on the Biot's dual-phase theory and compares with the isotropic single-phase coal seam. Then this paper discusses the influence of porosity and fluid properties in the reflected composite waves of coal seam. Numerical simulations show that PP-wave reflected AVO (Amplitude Versus Offset) characteristics are relatively sensitive to the fluid in coal seams, because the variations of the porosity and the fluid properties in the thin-bed model causes a relative obviously change of the P-wave velocity. Combined use of PP-and PS-waves can facilitate the fluid prediction in thin coal seams. With a fixed porosity, a higher gas saturation results in higher PP-wave reflected amplitude. Meanwhile, affected by the thin-bed tuning effect, the existence of fluid has a slight influence on amplitude spectrum of seismic response, which is almost the same with the case of single-phase coal seams.