摘要
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.
The paper first probes into the applicability of the rock physics foundation （i. e.Gassmann equation and Biot theory, hereafter ＂Gassmann-Biot theory＂） of AVO for C-gas （conventional gas） reservoirs to AVO for CBM reservoirs. The paper reasons, based on hypotheses assumed by both Gassmann and Biot as well as on dual porosity systems of CBM reservoirs and dual storage states of CBM therein, that Gassmann-Biot theory, as a whole, is not fully applicable to CBM reservoirs because pores within matrix porosity system of coal are of poor connectivity and because fracture system of coal is heterogeneous and anisotropic. Furthermore, Gassmann-Biot theory cannot predict the tendency of possible variation of coal＇ s elastic parameters, even if the elastic parameters do vary, when methane content changes but water- saturation of coal seam keeps unchanged. Thus, the rock physics foundation of AVO for C-gas reservoirs does not apply to AVO for CBM reservoirs. It is found that there exist inverse correlations between methane content and elastic parameters of CBM reservoirs, i. e. higher methane content goes with smaller density, lower P- and S-wave velocity while lower methane content goes with bigger density, higher P- and S-wave velocity. However, current rock physics theory cannot predict such inverse correlations, nor can it explain the existence of the inverse correlations. Based on CBM geological theory and on consistency between current rock physics theory and the inverse correlations, this paper offers an explanation to the inverse correlations between methane content and elastic parameters of Coal Seam A of a certain CBM prospect. And thus, the paper claims that the inverse correlations between methane content and elastic parameters of CBM reservoirs may be the intrinsic, inherent and regular relationship possessed by CBM reservoirs. The paper shows that the inverse correlations between methane content and elastic parameters can serve as rock physics foundation of CBM AVO technology. And intensity of AVO anomaly is related to CBM reservoirs＇ ＂sweet-spots＂ （i. e. locally-rich-methane and locally-high permeability positions） and the former can be employed to delineate the latter.
出处
《地球物理学报》
SCIE
EI
CAS
CSCD
北大核心
2013年第8期2837-2848,共12页
Chinese Journal of Geophysics
基金
国家重大科技攻关项目(2008ZX05062
2009ZX05062和2011ZX05033)资助
关键词
煤层气
含气量
弹性参数
负相关关系
AVO
Coalbed methane, Methane content, Elastic parameters, Inverse correlation, AVO
