基于等效介质理论的煤层气储层岩石物理建模与应用

Rock physics model for coal-bed methane reservoir and its application based on equivalent medium theory

地震岩石物理建模作为表征油气储层物性参数与地震参数间映射关系的主流工具,鲜有应用于煤层气储层,关键制约因素在于煤层气储层特有的吸附气和双重孔隙的等效计算问题尚未有效解决.为此,本文将吸附气视为类似煤基质的固相,将双重孔隙分解为基质孔隙和裂隙两部分;尝试利用自相容近似模型计算煤基质、吸附气和基质孔隙混合后煤基质干骨架的等效纵、横波速度,通过Mori-Tanaka模型和Brown-Korringa各向异性流体替换理论加入裂隙和流体,以此构建煤层气储层岩石物理模型.在此基础之上,通过正演模拟分析基质孔隙参数、吸附气含量以及裂隙参数的等效纵、横波速度响应;基于模型反演基质孔隙和裂隙参数,并将基于模型预测的纵、横波速度与实测数据对比,论证所构建的煤层气储层岩石物理模型的合理性.进一步通过制作岩石物理量版,探讨煤层气“甜点区”界定的两个关键参数——吸附气含量和脆性指数与储层物性参数(基质孔隙度、裂隙孔隙度)以及地震参数间的关系.结果表明:吸附气含量的变化引起的纵、横波速度、纵横波速度比和纵波阻抗变化微弱,引起的流体因子参数(λρ和μρ)变化略显著;基质孔隙度变化引起的地震参数响应显著强于吸附气含量;裂隙孔隙度与两种脆性指数间均具有明显的负相关性,可认为是煤层气储层脆性的主要影响因素.

Rock physics modeling serves as a mainstream tool to describe the relationship between the physical parameters of an oil-gas reservoir and seismic corresponding,and has seldom been applied in coal-bed methane(CBM)reservoirs.The key issue is that an effective calculation method for the specific properties of a CBM reservoir has not been resolved,including adsorbed-gas and dual-pore systems.Therefore,in this study,we regarded the adsorbed gas as the solid phase,which is similar to the coal matrix,and divided the dual-pore system into two parts:matrix pore and fracture.The self-consistent model was then used to calculate the effective P-wave and S-wave velocities of a dry coal matrix skeleton mix comprised of coal matrix,adsorbed gas,and matrix pores.We then added fracture and fluid parameters to the rock physics model of the CBM reservoir using the Mori-Tanaka model and the Brown-Korringa anisotropic fluid substitution equation,respectively.Using the model,we analyzed the variations in effective P-wave and S-wave velocities with respect to the matrix pore parameters,adsorbed-gas content,and fracture parameters by forward modeling.The reasonability of the rock physics model was proved by comparing the predicted and measured P-wave and S-wave velocities with the dual-pore parameters inversed simultaneously.Finally,rock physics templates were constructed and used to describe the response of reservoir physics parameters(matrix and fracture porosity)and seismic parameters to the two key factors(adsorbed-gas and brittleness index)in the CBM sweet spot definition.It shows that the P-wave and S-wave velocity responses and velocity ratio,and the P-wave impedance caused by the adsorbed gas content variation are weakened.The change in fluid factors(λρandμρ)caused by the adsorbed gas content was only slightly significant.The fracture porosity showed a significantly negative correlation with both the brittleness indices of the CBM reservoir and can be regarded as the main influencing factor.