| Gas hydrate is a potential new energy and has the following characteristics:high energy density, wide distribution, shallow depth and other advantages. Compared with the sea gas hydrate, Terrestrial gas hydrate is good at exploration and development. In China terrestrial gas hydrate is mainly distributed in the Qinghai-Tibet Plateau permafrost zone.The preliminary geological survey indicates that gas hydrate resources is huge and have great development prospects in the area based on the survey of seismic exploration and logging. Seismic modeling and response characteristics of hydrate is the base of delineation of the exploration target areas and make for providing a reference, promoting the development of hydrate resources, alleviating the contradictions of economic development and energy shortages, to make a significant contribution to national economic development. This will undoubtedly have a great significance for the national resources security, economic development and technological innovation.On account of the geology and physical data of terrestrial permafrost regions, the physical parameters of hydrate layers are analyzed by rock physical methods. We set up models including the levels layers, the wedge layers and layer-groups in line with actual situation. We simulate a great number of models by a viscoelastic medium finite difference method of wave equation and analyze the response characteristics of hydrates, and finally through the physical model of ultrasonic in order to verify the correctness of the numerical simulation.The main work of this paper and the achievements are as follows:(1) The velocity-density characteristics of actual hydrate layers, and the physical parameters of the hydrate rock skeleton. First,’high velosity, low density’ feature is analyzed in view of different logs and different lithologies of hydrate compared differences between hydrate layers and layers without hydrate. Comparing the different lithologies of hydrate layers, we research the hydrate siltstone of DK-1 borehole and DK-4 borehole. On the base of the porosity and saturation of DK-4 borehole hydrate layers,in view of the velocity ratio (vp/vs) of DK-1 borehole hydrate,we caculate the vp,vs and p, the bulk modulus K and shear modulus G of matrix by the time average formula, to provide the basis for the K-T equations and the elastic model equations. The filling modes of hydrate are speculated in the pore of hydrate layers, and methods are selected for the physical properties of numerical model. According to the microstructure theory of hydrate, the filling modes of hydrate are as follows:Mode-1, hydrate particles are in the pore as part of the fluid; Mode-2, hydrate particles are in contact with the rock matrix grains or cemented together as a part of the rock matrix. The section of 166.35m-165.80m hydrate layer in DK-4 borehole is more likely to belong to the Mode-2. We calculate the physical properties of numerical model by the elastic model equations,which can be used for numerical modeling of hydrate layers.(2) For the models of level layers, the methods of characteristics analysis are the time domain and frequency domain of waveform parameters. We set up models of layer, which contain the hydrate layer, water layer and free gas layer, and calculate the waveform parameters of time domain and frequency domain,then analyze variation of three different layers on the waveform parameters characteristic.In view of the increasing saturation(S) (respectively Sh, Sw and Sg), the waveform parameters as follows:For the hydrate layer, the mean-value is increased, the first zero-point value and the second zero-point value is gradually reduced as well, the main frequency value is gradually increased; For free gas layer, the mean-value is decreased, the first zero-point value and the second zero-point value is increased, the main frequency value is gradually reduced.For the water layer, the four waveform parameters is not very obvious.lt can identify the hydrate layer parameters according to the mean, the first value of zero-point, the second value of zero-point and the main frequency.(3)Extraction of waveform parameter about Mode-1 and Mode-2 of hydrate layers data and the analysis of the waveform parameters with respect to different filling modes. With the increase of the saturation of the hydrate,we analyze the variation of time domain and frequency domain of waveform parameters. Comparing with time domain and frequency domain parameters, the method can be applied to determine the filling mode of hydrate layers.(4)We research the seismic response characteristics based on different models of hydrate with the presence of free gas beneath them or not. For the five parameters of the time-domain,the tatol energy of waveform segments, variance, maximum peak, half area values of the maximum peak, the ratio of extreme amplitude and the main frequency,the value of hydrate layers with free gas layer is higher than the value of hydrate layers without free gas layers. The variation of the different waveform parameters can be used as the means to determine whether the presence of free gas below the hydrate layers.(5) To analyze time-frequency characteristics of simulation data,we make a wedge model of hydrate,which are constructed with hydrate and free gas respectively filled in the pore of rock. The maximum amplitude of the hydrate is significantly higher than that of the free gas.At the same time, the waveform parameters of wedge-shaped formation also are significantly different each other. According to two sets of parameters (g= 1.3, p= 0.9 and g= 0.9, p= 1.3), datas of numerical simulation are generalized by S transform.Two wedge-shaped formations in the the energy group of frequency distribution are obvious on the map. When the thickness of wedge is equal to 14m, high-frequency components of hydrate wedge model are significantly higher than the low-frequency component of the energy, and the energy of high frequency components of free gas wedge model is significantly lower than the low-frequency component of energy.(6) On the basis of the single wedge formation model of hydrate (Mode-2), using the ideas of previous level formation, we set up a variable thickness adjacent strata in which the bottom of the wedge formation, filling with the free gas and hydrate (Mode-1) respectively. Two different underlying strata make wedge overlying strata differences in the seismic response. For forward modeling of multiple models of thickness variation, we analyze the variation in thickness of the underlying strata impacting on top and bottom boundary amplitudes of the wedge formation. By comparison, for the wedge formation of underlying free gas, the wedge formation reflection performance shows obvious strong amplitude characteristics, and its amplitude is higher than roof reflection amplitude of the wedge,which is impacted by the wedge formation and underlying layer of free gas. The strength amplitude characteristics is an important indicator to determine whether there is a layer free gas below the wedge formation.(7) In view of the vertical hydrate layers, based on geological theory of hydrate, models of hydrate layers group are constructed to analyze factors of the weak amplitude characteristic of the interface of layers. Based on the porosity and saturation of layers, the three combined modes of layers are made to obtain the interface amplitude characteristics of layers.These three kinds modes of layers are as follows:Model (Ⅰ), constant porosity; Model (Ⅱ),constant saturation; Model (Ⅲ), multiple porosity and multiple saturation. Each mode has a single shot records and the reflection coefficients of AVO method. We get the following conclusions:For Model (Ⅰ) the weak amplitude phenomenons of hydrate layers are most obvious.For Model (Ⅱ) the weak amplitude phenomenons are not very obvious. In addition, for the conditions of low porosity, it is indicat that the porosity of the layers is lower and water saturation is lower, the weak amplitude characteristics are more obvious. Under the same situation in other conditions, weak amplitude characteristics can be applied to analyze the mode of porosity and saturation of the hydrate layer group.The main innovations of this paper are as follows:(1) Based on the logging data of actual hydrate layers, rock physics equations are applied to estimate the density, velocity and elastic modulus of the skeleton of actual hydrate formations, and are the basis of building the numerical model. The prediction method of hydrated filling mode is created in view of rock physics.(2) The time-domain waveform parameters are extracted by the auto-correlation for hydrate simulation data and the frequency-domain waveform parameters are extracted in the use of the power spectrum. On the basis of analyzing and summarizing multiple variations of waveform parameters which are applied to identify hydrate, determine hydrated filling mode and infer the presence or absence of free gas below the hydrate layers.The waveform parameter is a important tool for land identification of hydrate.(3) The kinds of methods are utilized to analyze the seismic response of the hydrate. According to the time-frequency characteristics of hydrate, the amplitude intensity of presence or absence of free gas and the interface features of low amplitude between the hydrate layers group such as response characteristics, it is concluded that the application of the several method is an effective way to research the terrestrial hydrate. |
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