Study On Rock-physics, Seismic And Electromagnetic Responses For Natural Gas Hydrate In Continental Permafrost

Natural gas hydrate is a clean energy for sustainable development,but re-stricted to mining technology and theoretical research, the global hydrate devel-opment, is basically at t.he same trial stage of indoor tests and outdoor resource-investigation. The exploitation of natural gas hydrate in continental permafrost seems to be easier than that in marine sediments, but up to now Messoyakha Gas Field in continental permafrost is still the only one reservoir which can be used for industrial exploitation . As the third largest country for frozen soil in the world,the amount of natural gas hydrate in continental permafrost of China is quite promising and inspiring, but due to the late start, theoretical researches on accu=mulation pattern of natural gas hydrate and exploration investigation are far left behind ,so far, only Muli area in northern Qinghai-Tibet Plateau has been drilled with hydrate samples. This paper starts from effective electrical and elastic prop-erties of complex multi-phase media, and establishes mathematical models for gas hydrate in continental permafrost. Based on the characteristics of rock-physics,this paper does numerical simulation on electromagnetic and seismic responses to search the geophysical exploration-identification for natural gas hydrate reservoir in continental permafrost of China.Rock-physics is the basis of geophysical exploration. The currently rock-physics modelling for natural gas hydrate is designed for marine sediments with high porosities, and only suitable to spherical pores. This paper focuses on the low-porosity property of continental permafrost, applies Generalized Mixture Rule(GMR) to acoustics and electrical properties of multiphase medium with ar-bitrary pore shapes, and obtains the relation between GMR parameter and the microstructure, it turns out that GMR has a better universality than the widely used Differential Effective Medium (DEM) and Self-Consistent (SC) models. And a large number of experimental data supports the results. GMR can solve the problems of the widely-used Archie' s law : It has as many as three parameters,can only be used for "Water-wet grains" which contains only one conducting phase, and is not available for "Oil-wet grains” . For the first time,this paper derives GMR parameter for " Oil-wet grains" . Through numerical simulation on several kinds of natural gas hydrate models and the real data, the results suggest that the conducting mechanism of natural gas hydrate is that the conducting fluid surrounds solid grains, and proposes the occurrence mode of natural gas hydrate is the un-cementing type.Natural gas hydrate reservoirs are characterized by high sonic velocity and high resistivity, which is the basis for the exploration and identification of nat-ural gas hydrate in continental permafrost. Based on geological , geophysical ,and petro-physical materials, the gas hydrate modes in continental permafrost are established.This paper does numerical simulation on seismic and electromag-netic responses . The study on seismic modeling of Amplitude versus Offset(AVO)shows that the amplitudes of direct P-wave phase from vertical seismic profiling data and primary reflection of P-wave phase at the near offset are weaker than those of sediments bearing no hydrate reservoir. The currently Controlled Source Electromagnetics (CSEM) on the exploration for metal deposits with low resistiv-ity has achieved obvious success.As gas hydrate reservoir exhibits high resistivity CSEM method has not yet achieved good results. To solve this problem, for the first time the principle and methods of marine CSEM is employed for gas hydrate exploration in continental permafrost, the numerical simulation results show that the magnitude of electrical field increases at reservoir position.Therefore, the com-bination of the relative weak amplitude of seismic AVO and the strong amplitude of CSEM may be the indicative feature for comprehensive geophysical exploration on natural gas hydrate in continental permafrost.Direct Current(DC) resistivity method is rarely used in geological exploration in continental permafrost, because the current is difficult to penetrate through the high resistivitiy permafrost layer. We have used the Current Density Allocation(CDA) formula for long electrode sources, improved numerical simulation based on CDA of 3D DC resistivity modeling with arbitrary long electrode sources using finite element method and we learned from the linear electrodes model, used the wire electrode to replace cylindrical electrode, finally formed 3D long electrodes modeling with linear electrodes based on CDA for power-supply vs linear electrode for measuring . In this paper, the method is applied to the exploration of natural gas hydrate in continental permafrost. The results show that when the long electrodes passing near hydrate reservoir, the apparent resistivity will increase.This paper studies the effect of cracks on seismic wave velocity for hydrate exploration in continental permafrost, from rock-physics model, experimental data and logging response, crack density along the depth is obtained,the results show the occurrence of gas hydrate is closely related to crack. Numerical simulation on seismic wave. propagation in cracks shows that when the fluid (gas or liquid) in the cracks is not connected, the seismic wave velocity is independent of the frequency,and is affected by the shape of the crack. When the fluid in cracks is connected,the seismic wave velocity is not affected by the shape of the cracks, but is related to the frequency. For the case the fluid in cracks is connected through cracks ,the seismic response is: high frequency with low P wave velocity, low frequency with high P wave velocity. For the case the fluid in cracks is connected through porous matrix, it shows low frequency with low velocity , high frequency with high velocity. According to the characteristics of continental permafrost zone a crack model of hydrate reservoir is established to study the AVO features. Comparing with the actual seismic exploration response, it shows that the crack model with fluid passing through cracks fits well with the natural gas hydrate exploration,and it may provide some new knowledge on formation the mechanism of hydrate reservoir.