主要能源消费国的非常规油气资源及其对天然气水合物的展望——考虑到页岩气所带来的天然气价格降低和政寤进口财政负担的减少各国有必要加大在非常规能源技术方面的投资

文章调研并总结了主要能源消费国美国、中国、印度和日本的非常规油气资源开发情况，讨论了其勘探和开发的方向：依赖于常规能源、煤层气、进口页岩气的中国、印度和日本很难实现能源自给，且中国和印度的页岩气资源还有待进一步评估；美国得益于其本土页岩资源和煤层气的商业化开采，有望在未来实现能源独立，同时由于天然气价格低廉、储量丰富，在短期内没有开采国内天然气水合物的需求：为降低能源对外依存度，印度和日本需要加大天然气水合物的开发力度；在全球范围内，考虑到页岩气革命所带来的天然气价格的降低和政府进口财政负担的减少，各国有必要加大在非常规能源技术方面的合理投资。

Using 4C OBS to reveal the distribution and velocity attributes of gas hydrates at the northern continental slope of South China Sea

To investigate the distribution and velocity attributes of gas hydrates in the northern continental slope of South China Sea, Guangzhou Marine Geological Survey conducted four-component （4C） ocean-bottom seismometer （OBS） surveys. A case study is presented to show the results of acquiring and processing OBS data for detecting gas hydrates. Key processing steps such as repositioning, reorientation, PZ summation, and mirror imaging are discussed. Repositioning and reorientation find the correct location and direction of nodes. PZ summation matches P- and Z-components and sums them to separate upgoing and downgoing waves. Upgoing waves are used in conventional imaging, whereas downgoing waves are used in mirror imaging. Mirror imaging uses the energy of the receiver ghost reflection to improve the illumination of shallow structures, where gas hydrates and the associated bottom-simulating reflections （BSRs） are located. We developed a new method of velocity analysis using mirror imaging. The proposed method is based on velocity scanning and iterative prestack time migration. The final imaging results are promising. When combined with the derived velocity field, we can characterize the BSR and shallow structures; hence, we conclude that using 4C OBS can reveal the distribution and velocity attributes of gas hydrates.

Multicomponent seismic forward modeling of gas hydrates beneath the seafloor

We investigated the effect of microscopic distribution modes of hydrates in porous sediments, and the saturation of hydrates and free gas on the elastic properties of saturated sediments. We simulated the propagation of seismic waves in gas hydrate-bearing sediments beneath the seafloor, and obtained the common receiver gathers of compressional waves（P-waves） and shear waves（S-waves）. The numerical results suggest that the interface between sediments containing gas hydrates and free gas produces a large-amplitude bottomsimulating reflector. The analysis of multicomponent common receiver data suggests that ocean-bottom seismometers receive the converted waves of upgoing P- and S-waves, which increases the complexity of the wavefield record.

Relation between relative permeability and hydrate saturation in Shenhu area, South China Sea

Nuclear magnetic resonance measurements in hydrate-bearing sandstone samples from the Shenhu area, South China Sea were used to study the effect of gas hydrates on the sandstone permeability. The hydrate-bearing samples contain pore-filling hydrates. The data show that the pore-filling hydrates greatly affect the formation permeability while depending on many factors that also bear on permeability; furthermore, with increasing hydrate saturation, the formation permeability decreases. We used the Masuda model and an exponent N = 7.9718 to formulate the empirical equation that describes the relation between relative permeability and hydrate saturation for the Shenhu area samples.

Detection of gas hydrate sediments using prestack seismic AVA inversion

Bottom-simulating reflectors (BSRs) in seismic profile always indicate the bottom of gas hydrate stability zone, but is difficult to determine the distribution and features of gas hydrate sediments (GHS). In this study, based on AVA forward modeling and angle-domain common-image gathers we use prestack AVA parameters consistency inversion in predicting gas hydrate sediments in the Shenhu area at northern slope of South China Sea, and obtain the vertical and lateral features and saturation of GHS.

Characteristics of Gas Hydrate Stability Zone and Resource Evaluation in Okinawa Trough

According to the processing and interpretation of multichannel seismic reflection data in the area of Okinawa Trough, the BSR （bottom simulating reflector） was identified in 16 seismic profiles. By means of special processing technologies such as AVO and waveform inversion, the authors, for the first time, directly used the BSR to outline the distribution tendency of thickness of gas hydrate stability zone in the Trough and thought that the largest stability zone thickness was in the south and the smallest in the north. Then through calculation the authors got the thickness of hydrate stability zone and resource of the hydrate. This would be useful to the future hydrate exploration and resource evaluation in the Okinawa Trough.

The Relationship of Sulfate-methane Interface,the Methane Flux and the Underlying Gas Hydrate

The sulfate-methane interface is an important biogeochemical identification interface for the areas with high methane flux and containing gas hydrate. Above the sulfate-methane interface, the sulfate concentration in the sediment is consumed progressively for the decomposition of the organic matter and anaerobic methane oxidation. Below the sulfate-methane interface, the methane concentration increases continuously with the depth. Based on the variation characters of the sulfate and methane concentration around the sulfate-methane interface, it is feasible to estimate the intensity of the methane flux, and thereafter to infer the possible occurrence of gas hydrate. The geochemical data of the pore water taken from the northern slope of the South China Sea show the sulfate-methane interface is relatively shallow, which indicates that this area has the high methane flux. It is considered that the high methane flux is most probably caused by the occurrence of underlying gas hydrate in the northern slope of the South China Sea.

Contrast of Seismic Reflection Characteristics of Gas Hydrate Distribution between Northeast SCS Slope and Hengchun Ridge

By analyzing and interpreting the newly acquired seismic profile supported by the national 973 Program and synthesizing the data with other geologic ＆ geographic information, we draw conclusions as follows, a） Two seismic reflections located at the northeast South China Sea （SCS） slope and the Hengchun ridge are the Bottom Simulated Reflections （BSRs）. Yet, the genesis and process of the gas hydrate in these two areas are different because of different regional tectonics and geological environments; b） The genesis of gas hydrate located at the northeast SCS slope area is related to the broadly existing fracture zones, slumping tectosomes, and the distinctive shielding environment of pressure masking field formed by them. But the genesis of the gas hydrate at the Hengchun ridge is associated with the thrust nappe structures and accretionary wedges formed along the Manila subduction zone and the related sub-floor fluid channel system built by them; c） Since the analogous geologic bodies are broadly distributed at slope areas around SCS and the temperature-press environment is very suitable to the formation and conservation of the gas hydrate, we suggest that much more of this resource should be stored in these areas.

3D topographic correction of the BSR heat flow and detection of focused fluid flow

The bottom-simulating reflector（BSR） is a seismic indicator of the bottom of a gas hydrate stability zone. Its depth can be used to calculate the seafloor surface heat flow. The calculated BSR heat flow variations include disturbances from two important factors：（1） seafloor topography, which focuses the heat flow over regions of concave topography and defocuses it over regions of convex topography, and（2） the focused warm fluid flow within the accretionary prism coming from depths deeper than BSR. The focused fluid flow can be detected if the contribution of the topography to the BSR heat flow is removed. However, the analytical equation cannot solve the topographic effect at complex seafloor regions. We prove that 3D finite element method can model the topographic effect on the regional background heat flow with high accuracy, which can then be used to correct the topographic effect and obtain the BSR heat flow under the condition of perfectly flat topography. By comparing the corrected BSR heat flow with the regional background heat flow, focused fluid flow regions can be detected that are originally too small and cannot be detected using present-day equipment. This method was successfully applied to the midslope region of northern Cascadia subducting margin. The results suggest that the Cucumber Ridge and its neighboring area are positive heat flow anomalies, about 10%–20% higher than the background heat flow after 3D topographic correction. Moreover, the seismic imaging associated the positive heat flow anomaly areas with seabed fracture–cavity systems. This suggests flow of warm gas-carrying fluids along these high-permeability pathways, which could result in higher gas hydrate concentrations.

Identification Signs and Prospects of Hydrate Gas

Gas hydrate is a kind of icy crystal body formed by water and natural gas in special conditions. The discovery of gas hydrates provides a wide sphere and a new way of thinking for finding clean and effective energy resources to replace increasingly exhausted traditional energy resources. Moreover, in our country there are a wide realm and bright prospect in the exploration of gas hydrate. This paper has summarized the progress on the study of gas hydrate. And based on the former research about gas hydrates, the integrative identification signs of gas hydrates were summarized in the aspects of seismic data, geophysical well logging, sedimentary and rock, geochemistry, topography and morphology. In the end, the author hopes it may provide some useful clues to the exploration of gas hydrate.

1-D Controlled source electromagnetic forward modeling for marine gas hydrates studies

We discuss the feasibility of using controlled-source electromagnetic （CSEM） in the frequency domain for prospecting marine gas hydrates. Based on the Ocean Drilling Program （ODP） Leg 164 log data, we have established several 1-D resistivity models which have different gas hydrate concentrations. Meanwhile, we analyzed the electromagnetic response of marine gas hydrates in the frequency domain based on these models. We also studied the relationship between electrical field magnitude or phase and parameters such as receiver-transmitter distance and frequency. Our numerical modeling results provide us with a quantitative reference for exploration and resource evaluation of marine gas hydrates.

The seismic reflecting characteristics of gas hydrate bearing strata and its possible distribution in the South China Sea

The research on gas hydrate is one of the topics of general interest in the field of energy resource and environment. The South China Sea has favorable conditions for the occurrence and formation of gas hydrate. The presence of gas hydrate changes acoustic properties of the sedimentary strata and results in the occurrence of bottom simulating reflectors, which makes the multi-channel seismic investigation an important method to identify gas hydrates. First, the paper, based on results of seismic reflection imaging, analyzes the qualitative seismic reflection characteristics of sedimentary strata containing gas hydrate. Some key seismic imaging techniques are also discussed. Next, a pseudo-well is constructed to perform an impedance inversion to get the quantitative velocity structure of the strata since there is no well in the study area. Finally, the velocity field from geophysical inversion is integrated with the geochemical and geophysical data acquired on the Ocean Drilling Program 184 cruise. All information confirms the presence of gas hydrate and shows its spatial distribution.

Focused fluid flow in the Baiyun Sag, northern South China Sea: implications for the source of gas in hydrate reservoirs

The origin and migration of natural gas and the accumulation of gas hydrates within the Pearl River Mouth Basin of the northern South China Sea are poorly understood. Based on high-resolution 2D/3D seismic data, three environments of focused fluid flow： gas chimneys, mud diapirs and active faults have been identified. Widespread gas chimneys that act as important conduits for fluid flow are located below bottom simulating reflections and above basal uplifts. The occurrence and evolution of gas chimneys can be divided into a violent eruptive stage and a quiet seepage stage. For most gas chimneys, the strong eruptions are deduced to have happened during the Dongsha Movement in the latest Miocene, which are observed below Pliocene strata and few active faults develop above the top of the Miocene. The formation pressures of the Baiyun Sag currently are considered to be normal, based on these terms： 1） Borehole pressure tests with pressure coefficients of 1.043-l.047; 2） The distribution of gas chimneys is limited to strata older than the Pliocene; 3） Disseminated methane hydrates, rather than fractured hydrates, are found in the hydrate samples; 4） The gas hydrate is mainly charged with biogenic gas rather than thermogenic gas based on the chemical tests from gas hydrates cores. However, periods of quiet focused fluid flow also enable the establishment of good conduits for the migration of abundant biogenic gas and lesser volumes ofthermogenic gas. A geological model goveming fluid flow has been proposed to interpret the release of overpressure, the migration of fluids and the formation of gas hydrates, in an integrated manner. This model suggests that gas chimneys positioned above basal uplifts were caused by the Dongsha Movement at about 5.5 Ma. Biogenic gas occupies the strata above the base of the middle Miocene and migrates slowly into the gas chimney columns. Some of the biogenic gas and small volumes ofthermogenic gas eventually contribute to the formation of the gas hydrates.

Sponge Effect on Coal Mine Methane Separation Based on Clathrate Hydrate Method

The findings were presented from laboratory investigations on the hydrate formation and dissociation processes employed to recover methane from coal mine gas.The separation process of coal mine methane(CMM) was carried out at 273.15K under 4.00 MPa.The key process variables of gas formation rate,gas volume stored in hydrate and separation concentration were closely investigated in twelve THF-SDS-sponge-gas systems to verify the sponge effect in these hydrate-based separation processes.The gas volume stored in hydrate is calculated based on the measured gas pressure.The CH4 mole fraction in hydrate phase is measured by gas chromatography to confirm the separation efficiency.Through close examination of the overall results,it was clearly verified that sponges with volumes of 40,60 and 80 cm 3 significantly increase gas hydrate formation rate and the gas volume stored in hydrate,and have little effect on the CH4 mole fraction in hydrate phase.The present study provides references for the application of the kinetic effect of porous sponge media in hydrate-based technology.This will contribute to CMM utilization and to benefit for local and global environment.

Effect of surfactant Tween on induction time of gas hydrate formation

Acceleration of gas hydrate formation is important in preventing coal and gas outbursts and is based on a hydration mechanism. It becomes therefore necessary to investigate the effect of surfactants, acting as accelerants for hydrate formation, on induction time. We experimented with three types of a Tween solution with equal concentrations of 0.001 mol/L (T40, T40/T80 (1:1), T40/T80 (4:1)). By means of visual experimental equipment, developed by us, we measured generalized induction time using a Direct Observation Method. The experimental data were analyzed combined with a mass transfer model and a hydrate crystal nuclei growth model. Our major conclusions are as follows: 1) solubilization of surfactants produces supersaturated gas molecules, which promotes the mass transfer from a bulk phase to hydrates and provides the driving force for the complexation between host molecules (water) and guest molecules in a gas hydrate formation process; 2) when the solution of the surfactant concentration exceeds the critical micelle concentration (CMC), the surfactant in an aqueous solution will transform to micelles. Most of the gas molecules are bound to form clusters with water molecules, which promotes the formation of crystal nuclei of gas hydrates; 3) the surfactant T40 proved to have more notable effects on the promotion of crystal nuclei formation and on shortening the induction time, compared with T40/T80 (1:1) and T40/T80 (4:1).

Progress in Research of Gas Hydrate

It is of great significance to study gas hydrate because of following reasons. （1） Most organic carbon in the earth reserves in the form of natural gas hydrate, which is considered as a potential energy resource for the survival of human being in the future. （2） A series of novel technologies are based on gas hydrate. （3） Gas hydrate may lead to many hazards including plugging of oil/gas pipelines, accelerating global warming up, etc. In this paper, the latest progresses in exploration and exploitation of natural gas hydrate, the development of hydrate-based technologies including gas separation, gas storage, CO2 sequestration via forming hydrate, as well as the prevention of hydrate hazards are reviewed. Additionally, the progresses in the fundamental study of gas hydrate, including the thermodynamics and kinetics are also reviewed. A prospect to the future of gas hydrate research and application is given.

Resistivity in Formation and Decomposition of Natural Gas Hydrate in Porous Medium

A new one-dimensional system for resistivity measurement for natural gas hydrate(NGH)exploitation is designed,which is used to study the formation and decomposition processes of NGH.The experimental results verify the feasibility of the measurement method,especially in monitoring the nucleation and growth of the NGH. Isovolumetric formation experiment of NGH is performed at 2°C and 7.8 MPa.Before the NGH formation,the initial resistivity is measured to be 4-7Ω·m,which declines to the minimum value of 2-3Ω·m when NGH begins to nucleate after the pressure is reduced to 3.3 MPa.As the NGH grows,the resistivity increases to a great extent,and finally it keeps at 11-13Ω·m,indicating the completion of the formation process.The NGH decomposition experiment is then performed.When the outlet pressure decreases,NGH begins to decompose,accordingly,the resistivity declines gradually,and is at 5-9Ω·m when the decomposition process ends,which is slightly higher than the resistivity value before the formation of NGH.The occurrence and distribution uniformity of NGH are determined by the distribution and magnitude of the resistivity measured on an one-dimensional sand-packed model.This study tackles the accurate estimation for the distribution of NGH in porous medium,and provides an experimental basis for further study on NGH exploitation in the future.

Three-phase flow of submarine gas hydrate pipe transport

In the hydraulic transporting process of cutter-suction mining natural gas hydrate, when the temperature-pressure equilibrium of gas hydrate is broken, gas hydrates dissociate into gas. As a result, solid-liquid two-phase flow(hydrate and water) transforms into gas-solid-liquid three-phase flow(methane, hydrate and water) inside the pipeline. The Euler model and CFD-PBM model were used to simulate gas-solid-liquid three-phase flow. Numerical simulation results show that the gas and solid phase gradually accumulate to the center of the pipe. Flow velocity decreases from center to boundary of the pipe along the radial direction. Comparison of numerical simulation results of two models reveals that the flow state simulated by CFD-PBM model is more uniform than that simulated by Euler model, and the main behavior of the bubble is small bubbles coalescence to large one. Comparison of numerical simulation and experimental investigation shows that the values of flow velocity and gas fraction in CFD-PBM model agree with experimental data better than those in Euler model. The proposed PBM model provides a more accurate and effective way to estimate three-phase flow state of transporting gas hydrate within the submarine pipeline.

Accumulation and exploration of gas hydrate in deep-sea sediments of northern South China Sea

The large deep-sea area from the southwestern Qiongdongnan Basin to the eastern Dongsha Islands,within the continental margin of northern South China Sea,is a frontier of natural gas hydrate exploration in China.Multiform of deep-sea sedimentations have been occurred since late Miocene,and sediment waves as a potential quality reservoir of natural gas hydrate is an most important style of them.Based on abundant available data of seismic,gravity sampling and drilling core,we analyzed the characteristics of seismic reflection and sedimentation of sediment waves and the occurrence of natural gas hydrate hosted in it,and discussed the control factors on natural gas hydrate accumulation.The former findings revealed the deep sea of the northern South China Sea have superior geological conditions on natural gas hydrate accumulation.Therefore,it will be of great significance in deep-sea natural gas hydrate exploration with the study on the relationship between deep-sea sedimentation and natural gas hydrate accumulation.

Similarity theory for the physical simulation of natural gas hydrate reservoir development

In order to apply physical simulation results to natural gas hydrate reservoir parameters to provide a theoretical framework for the design of a development plan,an analytical equation method was used to obtain the similarity criteria of natural gas hydrate reservoir development by physical simulation,based on a mathematical model of natural gas hydrate development.Given the approach of numerical simulation,a sensitivity analysis for all parameters was carried out,which specifically demonstrated that initial temperature is the most important parameter.Parameters of thermal conductivity coefficients are not necessary for conducting the NGH dissociation process,which will fundamentally simplify the design and establishment of the model.The analysis provides a sound theoretical basis and design principles for particular similarity.