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首先研究了天然气AVO技术的岩石物理基础———Gassmann方程和Biot理论(以下统称为“Gassmann-Biot理论”)———对煤层气AVO技术的适用性.根据该理论创始人在建立其理论时的假设条件、煤层气储层的双相孔隙特征、煤层气的双相赋存特征,本文推理认为,由于煤层气储层的基质型孔隙体系的连通性差,裂隙性孔隙在宏观上是非均质的,并且是各向异性的,因此,总体上来说,Gassmann-Biot理论不完全适用于煤层气储层.特别地,当煤层的水饱和状态保持不变而煤层的含气量变化时,因为煤层中孔(裂)隙中流体的体积压缩模量不变或基本不变,即使煤层的弹性参数发生变化,Gassmann-Biot理论也不可能预测其变化趋势.因此,天然气AVO技术的岩石物理基础完全地不适用于水饱和煤层气储层.研究发现,煤层气储层含气量与其密度、纵波速度、横波速度之间存在负相关关系,即含气量高,密度小、纵波速度小、横波速度小;含气量低,密度大、纵波速度大、横波速度大.但是,这些负相关关系不是现有岩石物理理论能够预测或能够解释的.依据煤层气地质学理论以及负相关关系与现有岩石物理理论的一致性,本文提供了对某勘探区A号煤层含气量与其密度、纵波速度、横波速度之间负相关关系的解释,并认为煤层气储层的含气量与其弹性参数之间的负相关关系可能是这类储层内在的固有的规律性的关系.本文证明含气量与其密度、纵波速度、横波速度之间负相关关系可以作为煤层气AVO技术的岩石物理基础,建立了AVO异常与煤层气储层“甜点(即富集高渗部位)”之间的关系,从而能够使用AVO异常探测煤层气富集高渗部位.
First of all, the authors studied the petrophysical basis of the AVO technology of natural gas --- the applicability of the Gassmann equation and the Biot theory (hereinafter referred to as “Gassmann-Biot theory”) to the AVO technology of CBM. According to this theory, The theoretical assumption, biphasic pore characteristics of coalbed methane reservoirs and the dual-phase coal-bed methane occurrence feature suggest that due to the poor connectivity of the matrix-type pore system in coalbed methane reservoirs, the fractured pores are macroscopically Is heterogeneous and anisotropic, so Gassmann-Biot theory is not entirely suitable for CBM reservoirs in general.In particular, when the water saturation of the coal seam remains unchanged and the gas content of the coal seam changes , The Gassmann-Biot theory can not predict the change trend because the compressive modulus of the fluid in the hole (cleft) gap in the coal seam remains unchanged or almost unchanged, even if the elastic parameters of the coal seam change. Therefore, the natural gas AVO rock The physical basis is completely unsuitable for water-saturated coalbed methane reservoirs. It is found that there is a negative correlation between gas content of CBM reservoirs and its density, longitudinal wave velocity and shear wave velocity, ie, high gas content and dense However, these negative correlations are not predictable or can be explained by existing petrophysical theories.According to the theory of coalbed gas geology As well as the consistency between the negative correlation and the existing petrophysical theory, this paper provides an explanation of the negative correlation between the gas content of coal seam A in a prospecting area and its density, longitudinal wave velocity and shear wave velocity. The negative correlation between gas volume and its elastic parameters may be inherent inherent regularity of this kind of reservoir.This paper proves that the negative correlation between gas content and its density, P-wave velocity and S-wave velocity can be used as the rock for the AVO technology of CBM Physical basis, the relationship between AVO anomaly and CBM reservoirs “sweet spots (that is, enrichment of hypertonic sites ”) was established so that the AVO anomaly could be used to detect CBM enrichment in the hypertonic parts.