Petrophysical modeling for offshore natural gas hydrates
ZHANG Jinqiang1,2, HAN Lei1,2,3, LIU Junzhou1,2, LIU Xiwu1,2
1. Sinopec Key Laboratory of Shale Oil/Gas Exploration and Production Technology, Beijing 102206, China; 2. Sinopec Petroleum Exploration and Production Research Institute, Beijing 102206, China; 3. Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou, Guangdong 511458, China
Abstract:As offshore natural gas hydrates occur in unconsolidated sediments,the petrophysical model of unconsolidated sediments should be built before the petrophysical modeling of natural gas hydrates. The traditional petrophysical model of unconsolidated sediments,however,faces the pro-blems of a high error in the estimation of shear wave velocity and a non-self-consistent modeling process. Taking account of the lubrication effect of brine on sediment particles,this study introduces the modified Hertz-Mindlin equation in the mode-ling process to improve accuracy. During the mo-deling of unconsolidated sediments,the modified Hertz-Mindlin equation is used to calculate the elastic moduli of sediments under critical porosity,and then Hashine-Shtrikman bounds are applied to calculate the elastic moduli of the skeleton of unconsolidated sediments. Furthermore,this paper discusses the influence of different occurrence forms of hydrates on the elastic properties of sediments,as well as the applicable models and discrimination criteria. On this basis,for natural hydrates existing as supporting grains and pore-filling fluids,the petrophysical modeling method and process that suits the two occurrence forms are designed. The modeling results are highly consistent with the measured curves of shallow drilling,which verifies the applicability of this method.
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