Recognition of Gas Hydrate Using AVO-Attribute Crossplots Based on the Porous Medium Theory

Gas hydrate is gradually considered as a potential energy resource. The presence of gas hydrate is commonly inferred from the appearance of “bottom simulating reflector”(BSR) on seismic section. Understanding the properties of hydrate-bearing sediments and studying the AVO characteristics of BSR are of great significance. Although more and more domestic and international studies have been conducted on the subjects mentioned above, they are still in the primary stage and need a long way to go to be appled in practice, especially in the field of gas hydrate. Aiming at the identification of gas hydrate, we studied the characteristics of the AVO attributes based on the Biot‘s theory when the sediments were bearing gas hydrate or free gas. The AVO attribute crossplots obtained from seismic sections with the forward simulation by means of staggered-grid finite-difference were compared with that of theoretic models. The coincidence shows that utilization of AVO attribute crossplots is an effective way to recognize gas hydrate and free gas.     

Seismic reflection and transmission coefficients at an air-water interface of saturated porous soil

Based on the modified Biot＇s theory of two-phase porous media, a study was presented on seismic reflection and transmission coefficients at an air-water interface of saturated porous soil media. The major differences between air-saturated soils and water-saturated soils were theoretically discussed, and the theoretical formulas of reflection and transmission coefficients at an air-water interface were derived. The characteristics of propagation and attenuation of elastic waves in air-saturated soils were given and the relations among the frequency, the angle of incidence and the reflection, transmission coefficients were analyzed by using numerical methods. Numerical results show that the propagation characteristic of the wave in air-saturated soils is great different from that in water-saturated soils. The frequency and the angle of incidence can have great influences on the reflection and transmission coefficients at interface. Some new cognition about the wave propagation is obtained and the study suggests that we may carefully pay attention to the influence of air on the dynamic analysis of seismic wave.     

Controlling factors for gas hydrate occurrence in Shenhu area on the northern slope of the South China Sea

Temperature and pressure on seafloor of the northern slope in the South China Sea are suitable for gas hydrate formation, but bottom simulation reflector (BSR), an indication of gas hydrate occurrence, only occurred in limited areas of the slope. Drillings in the BSR-distributed area (the District S) on the northern slope of the South China Sea suggested that gas hydrate only occurred at Sites SH2, SH3, and SH7 with high saturation (up to 20%-40%), and there is no hydrate at Sites SH1 and SH5 although the distance between SH1 to SH3 is only 500m. In this paper, we investigated seafloor gradient, fault development, temperature, and pressure in the District S on the northern slope of the South China Sea to understand the possible factors con- trolling BSR distribution and gas hydrate occurrence. The District S is a structurally fractured continental slope zone and its seafloor gradient varied greatly. The BSR-occurred areas have an average gradient of 19.89×10 2 whereas the BSR-free zone has the average gradient of 10.57×10 2 . The calculated relative structural intensities from fault densities and displacements show that the BSR-distributed areas tend to occur in the areas with a moderately high structural intensity, where faults frequently developed close to the seafloor that are possibly favored for lateral migration of gases. On the basis of temperatures and pressures at drilling sites, hydrate-occurred Sites SH2, SH3, and SH7 are located within the thermodynamically stable area for methane hydrate, and hydrate-absent Sites SH1 and SH5 are out of the thermodynamically stable area for methane hydrate formation, suggesting that both BSR and the thermodynamic stability are necessary for hydrate occurrence in the subsurface.     

Multi—transmitting formula for attenuating waves

The MTF is extended to case of attenuating incident wave by introducing an attenuation coefficient.The reflection coefficients of this modified MTF and MTF are evaluated and compared when an attenuating wave impinges on the boundary,and the results demonstrate that MTF can be used to absorb slightly attenuating waves and the modi-fied MTF is more capable of absorbing heavily attenuating waves than MTF.The accuracy of modified MTF is also tested by numerical examples of fluid saturated porous media.     

Effect of thermal stimulation on gas production from hydrate deposits in Shenhu area of the South China Sea

The Shenhu area on the northern continental slope of the South China Sea (SCS) is one of the promising fields for gas hydrate exploitation. The hydrate-bearing layer at drilling site SH2 is overlain and underlain by permeable zones of mobile water. In this study a vertical well was configured with a perforated Interval I for producing gas and a coiled Interval II for heating sediment. The hydrate is dissociated by a small depressurization at Interval I and a thermal stimulation at Interval II. The numerical simulations indicate that the thermal stimulation has a significant effect on gas release from the hydrates in the production duration and improves the gas production in the late period. The gas released by thermal stimulation cannot be produced as quickly as the production gets operated because of the hard pathway for fluids to flow in the sediments. The gas production is enhanced due to the heating for 7242 m 3 in the whole production. Increasing heating temperature at Interval II can improve gas production and restrain water output, and advance the arrival time of the gas flow from the zone at Interval II. The absolute criterion and relative criterion suggest that the thermal stimulation in the production schemes is pronounced for releasing gas from the hydrate deposit, but the production efficiency of gas is limited by the sediment of low permeability. The study provides an insight into the production potential of the hydrate accumulations by thermal stimulation with depressurization in two wells, and a basis for analyzing economic feasibility of gas production from the area.     

Analytical solutions for dynamic pressures of coupling fluid-solid-porous medium due to P wave incidence

Wave reflection and refraction in layered media is a topic closely related to seismology, acoustics, geophysics and earthquake engineering. Analytical solutions for wave reflection and refraction coefficients in multi-layered media subjected to P wave incidence from the elastic half-space are derived in terms of displacement potentials. The system is composed of ideal fluid, porous medium, and underlying elastic solid. By numerical examples, the effects of porous medium and the incident wave angle on the dynamic pressures of ideal fluid are analyzed. The results show that the existence of the porous medium, especially in the partially saturated case, may significantly affect the dynamic pressures of the overlying fluid.     

地球物理技术在天然气水合物预测中的应用

Based on the sensitivity of geophysical response to gas hydrates contained in sediments, we studied the prediction of gas hydrates with seismic techniques, including seismic attributes analysis, AVO, inverted velocity field construction for dipping formations, and pseudo-well constrained impedance inversion. We used an optimal integration of geophysical techniques results in a set of reliable and effective workflows to predict gas hydrates. The results show that the integrated analysis of the combination of reflectivity amplitude, instantaneous phase, interval velocity, relative impedance, absolute impedance, and AVO intercept is a valid combination of techniques for identifying the BSR （Bottom Simulated Reflector） from the lower boundary of the gas hydrates. Integration of seismic sections, relative and absolute impedance sections, and interval velocity sections can improve the validity of gas hydrates determination. The combination of instantaneous frequency, energy half attenuation time, interval velocity, AVO intercept, AVO product, and AVO fluid factor accurately locates the escaped gas beneath the BSR. With these conclusions, the combined techniques have been used to successfully predict the gas hydrates in the Dongsha Sea area.     

基于三相孔隙弹性理论的火山岩气层岩石物理响应特征分析

Unlike previous theories with velocity and/or elastic modulus averaging, we use a three-phase porous rock physics model developed by Santos for analyzing the seismic response of two immiscible fluids in saturated porous media. Considering reservoir reference pressure and coupling drag of two fluids in pores, the effects of frequency, porosity, and gas saturation on the phase velocities of the P-and S-waves are discussed in detail under field conditions. The effects of porosity and gas saturation on Vp/Vs are also provided. The data for our numerical experiments are from a sample of deep volcanic rock from Daqing. The numerical results show that the frequency dispersion effect can be ignored for deep volcanic rocks with low porosity and low permeability. It is concluded that for deep volcanic rocks the effect of gas content in pores on Vp/Vs is negligible but the effect of porosity is significant when there is a certain amount of water contained in the pores. The accurate estimate of lithology and porosity in this case is relatively more important.     

Characteristics of shallow gas hydrate in Okhotsk Sea

Multidisciplinary field investigations were carried out in Okhotsk Sea by R/V Akademik M.A. Lavrentyev (LV) of the Russian Academy of Sciences (RAS) in May 2006, supported by funding agencies from Ko- rea, Russia, Japan and China. Geophysical data including echo-sounder, bottom profile, side-scan- sonar, and gravity core sample were obtained aimed to understand the characteristics and formation mechanism of shallow gas hydrates. Based on the geophysical data, we found that the methane flare detected by echo-sounder was the evidence of free gas in the sediment, while the dome structure de- tected by side-scan sonar and bottom profile was the root of gas venting. Gas hydrate retrieved from core on top of the dome structure which was interbedded as thin lamination or lenses with thickness varying from a few millimeters to 3 cm. Gas hydrate content in hydrate-bearing intervals visually amounted to 5%―30% of the sediment volume. This paper argued that gases in the sediment core were not all from gas hydrate decomposition during the gravity core lifting process, free gases must existed in the gas hydrate stability zone, and tectonic structure like dome structure in this paper was free gas central, gas hydrate formed only when gases over-saturated in this gas central, away from these struc- tures, gas hydrate could not form due to low gas concentration.     

Effects of salinity on methane gas hydrate system

Using an approximately analytical formation, we extend the steady state model of the pure methane hydrate system to include the salinity based on the dynamic model of the methane hydrate system. The top and bottom boundaries of the methane hydrate stability zone (MHSZ) and the actual methane hy-drate zone (MHZ), and the top of free gas occurrence are determined by using numerical methods and the new steady state model developed in this paper. Numerical results show that the MHZ thickness becomes thinner with increasing the salinity, and the stability is lowered and the base of the MHSZ is shifted toward the seafloor in the presence of salts. As a result, the thickness of actual hydrate occur-rence becomes thinner compared with that of the pure water case. On the other hand, since lower solubility reduces the amount of gas needed to form methane hydrate, the existence of salts in sea-water can actually promote methane gas hydrate formation in the hydrate stability zone. Numerical modeling also demonstrates that for the salt-water case the presence of methane within the field of methane hydrate stability is not sufficient to ensure the occurrence of gas hydrate, which can only form when the methane concentration dissolved in solution with salts exceeds the local methane solubility in salt water and if the methane flux exceeds a critical value corresponding to the rate of diffusive methane transport. In order to maintain gas hydrate or to form methane gas hydrate in marine sedi-ments, a persistent supplied methane probably from biogenic or thermogenic processes, is required to overcome losses due to diffusion and advection.     

基于双相介质理论的AVO属性交汇图识别天然气水合物

天然气水合物(gas hydrates)作为一种新的潜在能源 是目前世界范围内研究的热点和难点之一。目前 普遍认为拟海底反射(简称BSR)是天然气水合物存 在的重要标志之一,但有关水合物的识别仍有很 多问题需要解决。例如世界各地已发现水合物的 例证表明在没有BSR的情况下仍然存在水合物,在 没有BSR的情况下如何识别水合物仍是没有解决 的研究课题。本文以天然气水合物的识别为研究 目的,以AVO属性交汇图为研究手段,通过双相介 质正演理论模型研究了当地层含有天然气水合物 或游离气时的AVO属性特征,并与由双相介质的 交错网格有限差分法得到的合成地震记录所反演 得出的AVO交汇图进行了对比,得到高度一致的 对比结果,表明利用AVO属性交汇图是识别天然 气水合物和游离气并能估测其含量的有效技术。     

海底天然气水合物OBS多分量地震正演模拟（英文）

根据Ecker的水合物沉积物的三种微观模式,计算含水合物沉积层和含游离气沉积物的弹性模量,分析对比了水合物的不同微观模式、不同水合物饱和度以及不同游离气饱和度对沉积物弹性模量的影响;从纵横波分离的弹性波动方程出发,采用交错网格空间有限差分方法模拟地震波在海底天然气水合物沉积地层的传播,得到纵、横波的海底地震（OBS）共接收点道集。数值算例表明,当水合物作为流体的一部分或胶结颗粒骨架时,仅纵波记录上存在BSR;当水合物胶结颗粒接触,纵、横波记录上均存在BSR。并且,OBS会接收到上行纵波和上行横波在海底界面形成的转换波,干扰横波记录上BSR的识别。     

南海北部神狐海域天然气水合物形成聚集的数值模拟研究

利用2D数值方法对南海北部陆坡神狐海域水合物形成聚集过程进行了模拟,对气烟囱、泥底辟与水合物成藏间的关系进行研究.模拟结果表明,来自深部的甲烷热解气在向上运移过程中以垂向运动为主,且局限在某一狭窄的范围内,故在地震剖面上显示为气烟囱及顶部BSR.只有当其越过水合物稳定带底界,才能形成水合物,此时BSR等于水合物稳定带底...     

Acoustic properties of sediments saturated with gas hydrate, free gas and water

We obtain the wave velocities and quality factors of gas‐hydrate‐bearing sediments as a function of pore pressure, temperature, frequency and partial saturation. The model is based on a Biot‐type three‐phase theory that considers the existence of two solids (grains and gas hydrate) and a fluid mixture. Attenuation is described with the constant‐Q model and viscodynamic functions to model the high‐frequency behaviour. We apply a uniform gas/water mixing law that satisfies Wood's and Voigt's averages at low and high frequencies, respectively. The acoustic model is calibrated to agree with the patchy‐saturation theory at high frequencies (White's model). Pressure effects are accounted by using an effective stress law for the dry‐rock moduli and permeabilities. The dry‐rock moduli of the sediment are calibrated with data from the Cascadia margin. Moreover, we calculate the depth of the bottom simulating reflector (BSR) below the sea floor as a function of sea‐floor depth, geothermal gradient below the sea floor, and temperature at the sea floor.     

含天然气水合物沉积物的岩石物性模型与似海底反射层的AVA特征

研究含天然气水合物沉积物的岩石物性模型与似海底反射层的振幅随入射角变化(AVA)特征. 基于时间平均-Wood加权方程、三相介质波传播理论模型和弹性模量模型,计算并阐述含天然气水合物岩石弹性参数与水合物饱和度、含游离气岩石弹性参数与游离气饱和度的关系;给出不同模型AVA特征. 结果表明,不同天然气水合物饱和度、不同游离气饱和度的各种组合呈现形态相似但反射系数值不同的AVA特征.     

尼日尔三角洲天然气水合物体系地震特征研究

天然气水合物体系一般由地震上的似海底反射层BSR和下伏强振幅带(解释为游离气FGZs)所指示,但并非所有BSR及下伏强振幅带都与水合物和游离气有关.本文通过提取多种属性剖面,优选出视极性属性来辅助判断与水合物有关的BSR和下伏FGZs.—般来说,BSR和FGZs的顶部反射层表现为负视极性、高振幅的强反射特征.研究发现,尼日尔三角洲南部的水合物主要分布在与重力作用有关的生长断层及伴生的滚动背斜地区,广泛发育的断层、气烟囱、不整合面,以及砂岩层都可作为流体运移通道.除为水合物体系提供气源,这些流体运移通道还在水合物分解或FGZs超压时发挥作用,使游离气沿通道在FGZs和含水合物地层间循环,部分游离气可运移至海底进入海水甚至大气中·总之,视极性是判断水合物相关地震指示标志的有效属性,流体运移通道对尼日尔三角洲的水合物成藏具有重要作用.     

含两项不混合/单项黏性流体孔隙介质分界面上反透射系数特征研究

弹性孔隙介质分界面上的反透射系数特征,在岩性划分、流体识别、储层边界判识等方面有重要的应用.本文研究上层为含两项不混合黏性流体孔隙介质、下层为含单项黏性流体孔隙介质分界面上的反透射理论.首先根据两种孔隙介质分界面上的能量守恒得到边界条件,再将波函数、位移、应力与应变关系代入边界条件,推导出完全连通孔隙情况下,第一类纵波入射到孔隙介质分界面上的反透射系数方程.通过建立砂岩孔隙介质模型,分别分析不同孔隙流体类型、不同含油饱和度及不同入射角情况下,各类波的反透射系数特征.研究表明,第二、三类纵波反透射系数数值比第一类纵波小多个数量级,且两者对入射角的变化不敏感,但对孔隙流体性质、含油饱和度的变化较敏感,而横波反透射系数特征恰好与此相反;第一类纵波反透射系数特征比较复杂,入射角、孔隙流体的性质及含油饱和度的变化都对其产生影响.不同孔隙流体弹性物性的差异、孔隙介质中含油饱和度的变化及不同入射角引起垂向和切向应力分量的变化都会影响各类波的反透射系数特征,分析这些特征可以为研究储层含油气性提供理论基础.     

海底天然气水合物层界面反射AVO数值模拟

本文采用AVO数值模拟方法，共选取水合物系统分层结构6个模型，对水合物、游离气和饱水沉积物接触界面的反射特征进行了数值模拟，研究了BSR及双BSR的存在条件与水合物体系垂向分布的关系，对一些现象从理论上进行了阐明.主要结论是：（1）强的似海底反射界面BSR与游离气体的存在密不可分，实际地震剖面中的“BSR”可能不对应水合物而只对应气体，无明显BSR的地方可能有水合物.（2）水合物顶部有可能存在游离气体，它可以在正常BSR（BSR1）之上形成另一个具有正极性的似海底反射界面BSR2.（3）正常BSR之下的双BSR（BSR0）其弹性机理有两种可能，一是水合物之下游离气上升迁移遇到某种阻隔层或不同气体组分的自然分层所造成的气体垂向分布的梯度性差异；二是当水合物之下的游离气体中存在残存的水合物时，同样会形成一定强度的似海底反射，在这种情形下BSR0的极性比较难以判断，取决于残存水合物上下游离气的饱和度和残存水合物的厚度.     

The impact of faulting on the stability conditions of gas hydrates in Lake Baikal sediments

The phase transition problem of methane hydrate in porous sediments is solved. Based on the obtained solution, the impact of faulting on the stability conditions of gas hydrates is investigated by the numerical modeling of the filtration and thermal regimes in the sedimentary cover of the Central Basin of Lake Baikal within the segment of the anomalous behavior of the bottom simulating reflector (BSR). It is assumed that such behavior is caused by the tectonic action. The calculations testify to the plausibility of the proposed model of formation of the anomalous area with total decomposition of the contained hydrates. It is shown that dissociation of gas hydrates in sediments due to faulting and the subsequent uplift of the products of these transformations along the incipient channel toward the bottom of the lake can result in the extensive accumulation of gas hydrates on this surface. It is also shown that if the total amount of the free gas, which left the hydrate dissociation zone, reached the level of the lake surface at normal pressure and temperature, its volume could be equivalent to the resources of a medium-size gas field. The results of numerical modeling the violation of the gas-hydrate stability conditions in Lake Baikal sediments can also be valid for the other regions with hydrate-bearing sediments if the case specific conditions and regional tectonic activity are taken into account.