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《Chinese Journal of Geophysics》 2013-08
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The relation between CBM content and the elastic parameters of CBM reservoirs: Reasoning and initial probing

CHEN Xin-Ping1,HUO Quan-Ming2,LIN Jian-Dong2,HU Chao-Yuan2,WANG Yang2, SUN Fen-Jin3,ZHAO Qing-Bo3,LI Wu-Zhong3,LI Gui-Zhong3 1 Geomodulus Energy Tech Co.Ltd.,Beijing 102208,China 2 Research Institute of Coal Geophysical Exploration,National Administration of Coal Geology,Hebei Zhuozhou 072750,China 3 Langfang branch,Research Institute of Petroleum Exploration and Development,Hebei Langfang 065007,China  
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.
【Fund】: 国家重大科技攻关项目(2008ZX05062、2009ZX05062和2011ZX05033)资助
【CateGory Index】: P631.4;P618.11
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