Research On The Geophysical Well-logging Evaluation Of The Gas Hydrate Bearing Sediment Using The Modified Three-phase Biot Equation

As future human ideal alternative energy source, the gas hydrates resource evaluation and exploration has been the hotspot and difficult point.The key to solve this problem is to establish a suitable rock physics model of gas hydrate bearing sediment. On the basis of previous studies, this paper aims to build a more optimized physical model of gas hydrate, which has the theoretical significance and practical value.Based on the well-logging data published by the Ocean Drilling Program(ODP) Leg 204, this paper specifically choosed the three-phase Biot equation as the research object, which has a solid theoretical basis and is used widely. We found that the three-phase Biot equation is the expresstion of the self-consistency between the six physical quantities, which are the compress wave velocity, shear wave velocity, density, porosity, hydrate saturation and consolidation coefficient Based on this, we modified the order parameter, r, and proposed a modified three-phase Biot equation.The modified three-phase Biot equation can directly solve the consolidation coefficient and hydrate saturation without fitting parameters, which avoid the error brought by the traditional three-phase Biot equation. The practical calculation results shows that the consolidation coefficient with 20% deviation will bring the 40% or even more deviation of the hydrate saturation. These indicate the necessity and the rationality of the modified model. Further, with the nuclear magnetic resonance(NMR)- porosity model and classic Archie formula, comparing the calculation results of hydrate saturation, modified three-phase Biot equation shows better application effect.The Modified three-phase Biot equation extends the application range of the traditional three-phase Biot equation. In addition to calculate the saturation of gas hydrate, it also can predict and evaluate the logging data of compress wave velocity, shear wave velocity, density and porosity data. This paper discussed the principle and method of predictions or evaluation, and practically applied in evaluating the logging data of Leg 204. Results show that modified three-phase Biot equation has better prediction and evaluation ability within the gas hydrate stability zone, the calculation precision of the results depends on the accuracy of the data model used...