Hydrate Cores Research Articles

Numerical study on gas production via a horizontal well from hydrate reservoirs with different slope angles in the South China Sea

AbstractIt is important to study the effect of hydrate production on the physical and mechanical properties of low‐permeability clayey–silty reservoirs for the large‐scale exploitation of hydrate reservoirs in the South China Sea. In this study, a multiphysical‐field coupling model, combined with actual exploration drilling data and the mechanical experimental data of hydrate cores in the laboratory, was established to investigate the physical and mechanical properties of low‐permeability reservoirs with different slope angles during 5‐year hydrate production by the depressurization method via a horizontal well. The result shows that the permeability of reservoirs severely affects gas production rate, and the maximum gas production amount of a 20‐m‐long horizontal well can reach 186.8 m3/day during the 5‐year hydrate production. Reservoirs with smaller slope angles show higher gas production rates. The depressurization propagation and hydrate dissociation mainly develop along the direction parallel to the slope. Besides, the mean effective stress of reservoirs is concentrated in the near‐wellbore area with the on‐going hydrate production, and gradually decreases with the increase of the slope angle. Different from the effective stress distribution law, the total reservoir settlement amount first decreases and then increases with the increase of the slope angle. The maximum settlement of reservoirs with a 0° slope angle is up to 3.4 m, and the displacement in the near‐wellbore area is as high as 2.2 m after 5 years of hydrate production. It is concluded that the pore pressure drop region of low‐permeability reservoirs in the South China Sea is limited, and various slope angles further lead to differences in effective stress and strain of reservoirs during hydrate production, resulting in severe uneven settlement of reservoirs.

In-situ ultrasonic measurement for hydrate pressure cores sampled by deep sea drilling

Deep sea drilling core detection and analysis is a convenient technology deployed by drilling platforms or geological exploration vessels. This paper develops an in-situ ultrasonic detection unit which can be combined with long distance pressure-holding transfer of hydrate cores, based on the original Pressure-core On-board Transfer and Analysis System (POTAS). The screw-in design of transducers allows for rapid adjustments in measurement distance during hydrate core transfer, while effectively solving the problems of high-pressure resistance. The drilling depth and core diameter are firstly used as parameters for wave velocity calculation. The method simplifies the solution and error analysis of in-situ ultrasonic measurement of hydrate pressure cores under complex drilling operation conditions. The introduction of two engineering-related parameters takes full account of the influence of sedimental burial depths and drilling tool selection on in-situ ultrasonic detection, and provides important support for ultrasonic measurements of hydrate pressure cores.

A new study of multi-phase mass and heat transfer in natural gas hydrate reservoir with an embedded discrete fracture model

Studies of the hydrate cores have shown that natural fractures can be frequently observed in hydrate reservoirs, resulting in a fracture-filled hydrate. Therefore, it is highly necessary for industries to predict the gas well productivity of fracture-filled hydrate reservoirs. In this work, an embedded discrete fracture model is applied to characterize the natural fractures of fracture-filled gas-hydrate reservoirs. The non-linear mass and energy conservation equations which are discretized with the finite-difference method are solved by the fully implicit approach, and the proposed model is justified by a commercial simulator. On the basis of the proposed model, we investigate the influences of natural fractures, fracture conductivity, and hydrate dissociation rate on the gas well productivity and the distributions of pressure, temperature, and hydrate saturation. The simulation results show that hydraulic and natural fractures exert significant impacts on the gas well productivity of the fracture-filled hydrate reservoirs, and the cumulative gas production is increased by 45.6% due to the existence of the connected natural fractures. The connected natural fractures can impose a more important influence on the gas well productivity than the unconnected natural fractures. The cumulative gas production is increased by 6.48% as Nnf is increased from 2 to 50, whereas the increase is 43.38% as Nf_con is increased from 0 to 4. In addition, A higher hydraulic fracture conductivity can be more favorable than a higher natural fracture conductivity for improving the gas well productivity, and a higher hydrate dissociation rate can lead to a lower temperature along fractures due to a more noticeable reduction of solid hydrate. This study provides a theoretical basis for developing fracture-filled hydrate reservoirs efficiently in the future.

Application of a biomimetic wellbore stabilizer with strong adhesion performance for hydrate reservoir exploitation

Natural gas hydrate (NGH) in marine is usually reserved in the pores or cracks of weakly cemented argillaceous siltstones. Moreover, the hydrates are susceptible to variations in temperature and pressure conditions when water-based drilling fluids (WBDFs) are drilled into hydrate reservoirs, leading to the hydrate in-situ decomposition and severe sand production. In this work, an inexpensive and facilely prepared biomimetic wellbore stabilizer with strong adsorption of multiple hydrogen bonds was prepared by fusing catechol-rich tannic acid (TA) and polyvinyl alcohol (PVA) with polyol hydroxyl adsorption function, referred to as TA-PVA complex (TA-PVA). TA-PVA was added to WBDFs to strengthen the NGH reservoir wellbore for ensuring the safety of drilling process in a wide temperature range (2-60℃). The microscopic characterization of TA-PVA and its cohesion mechanism with argillaceous siltstone was investigated by Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The mechanical properties of skeleton of the artificial hydrate cores were evaluated by compressive strength test, with maximum compressive strength reaching 0.45 MPa. And the adhesion force of TA-PVA to the probe modified by silica microsphere (PSM) was further studied by AFM. Meanwhile, the adhesion forces between TA-PVA and cyclopentane hydrate particles were measured at 1.5℃. Adhesion test results demonstrate that TA-PVA can bond clay particles, quartz sand and hydrate particles together. In detail, the adhesion force between TA-PVA and PSM reaches more than 164.46 mN/m, the adhesion force between TA-PVA droplet and cyclopentane hydrate particle reaches 43.19 mN/m. The differential scanning calorimetry (DSC)curves demonstrate that TA-PVA releases no heat between 0 and 60 °C, indicating that TA-PVA has no impact on the decomposition of NGH, showing the potential to maintain the integrity of NGH reservoir.

Routes to hydrate formation from water dissolved in gas and impact of mineral surfaces

Mineral surfaces adsorb water to extreme densities and corresponding low chemical potentials. This results in a dual effect in terms of hydrate. Water and slightly polar components adsorb directly on mineral surfaces and generate efficient conditions for hydrate nucleation. But due to the extremely low chemical potential of adsorbed water the hydrate nuclei formed towards mineral surfaces have to either detach from the vicinity of mineral surfaces, or be bridged by structured water in a dynamic attachment of hydrate cores some few nm outside mineral surfaces. During transport of gas (CH 4 , gas mixtures, CO 2 ) the conventional water dew-point analysis will typically result in a substantially higher acceptable water concentration as compared to the concentration for adsorption of water from gas to rust surface. Direct formation of hydrate from water dissolved in gas is thermodynamically feasible, as discussed in open literature. In this work we demonstrate that it is also feasible in terms of mass transport. A new theory for enthalpy of hydrate dissociation has been extended to also direct hydrate formation from water dissolved in gas. The remaining question is whether direct hydrate formation from gas is also feasible in terms of transporting the hydrate formation heat away through a heat insulating medium. We propose further research strategies to enlighten this issue. Addition of glycols to critical points in processing of gas or transport is already in use by companies like for instance EQUINOR. There is, however, a need for more work on how efficient it is and if it can also be used for multiphase transport of hydrocarbons with significant water cut. Some research activities are in progress and briefly outlined here.

Reservoir characteristics and critical influencing factors on gas hydrate accumulations in the Shenhu area, South China Sea

The Shenhu area is the key target area for marine gas hydrate industrialization in China. Significant progress has been made with respect to gas hydrate exploration and production in this area in recent years. However, a comprehensive understanding of the reservoir characteristics and hydrate accumulation mechanism are still lacking; in particular, the hydrate enrichment factors are not, as yet, completely clear. In this study, we systemically characterize the geological controls on gas hydrate accumulation in the Shenhu area, including the sedimentary environment, gas sources, fluid and gas pathways, and reservoir-caprock system. We subsequently provide a quantitative evaluation of some critical factors influencing hydrate enrichment. The depositional environment in the study area was characterized by low energy (sedimentary hydrodynamic conditions), and anoxic environment, and the gas hydrate accumulations were closely associated with turbidites. The methane captured in the gas hydrates in the GMGS1 area and at the SHSC-4 and SHSC2-6 production test sites was mainly derived from the microbial reduction of CO2, while thermogenic gas was a more important source of hydrocarbon gases in the GMGS3 and GMGS4 areas. In addition, we used a combination of 8-slice spiral X-ray computed tomography (CT) and micro-CT techniques for the first time for the in-situ investigations of the microfractures and foraminifera in the hydrate cores. The results indicate that microfractures and foraminiferal shells are crucial to the occurrence and accumulation of gas hydrates in fine-grained sediments of the Shenhu area. Interestingly, microfractures were found to be more developed in the hydrate layers where most sediment particles were relatively small in size. Hydrate saturation increased with i) a decrease in the specific surface area of sediment grain, ii) an increase in the median pore size, and iii) increased foraminifera abundance in the reservoir. Our findings could help locate high-saturation gas hydrate layers (‘sweet spot’) in fine-grained marine sediments of the Shenhu area, South China Sea.

Numerical analysis of the gas recovery performance in hydrate reservoirs with various parameters by stepwise depressurization

Natural gas hydrates have drawn worldwide attention as a significant clean energy source in the 21st century. Depressurization is known to be the most effective method for hydrate exploitation owing to its feasibility and economic benefits. Recently, stepwise depressurization has received extensive attention owing to its advantages of effective prevention of ice generation and hydrate reformation during gas recovery. In this work, we optimized the 2-D simulator to model the evolution of temperature, pressure, hydrate saturation, and gas production behavior in hydrate sediments during decomposition process via stepwise depressurization. The effect of various reservoir characteristics on gas production was analyzed by introducing the sensitivity factor. The results demonstrated that higher specific heat capacity of the reservoir would promote gas production in the early stage of the reaction; however, this effect is reversed in the later stage. A higher thermal conductivity of the hydrate cores can dramatically enhance the gas production rate; however, this advantage is weakened with increasing thermal conductivity. Reducing the gas relative permeability hinders the hydrate decomposition process, and the water relative permeability has minimal effect on gas production. Based on the sensitivity analysis, the thermal conductivity and gas relative permeability are the main factors affecting hydrate decomposition. • Numerical simulation of gas production by step-wise depressurization is presented. • Higher thermal conductivity enhanced gas production via stepwise depressurization. • Gas permeability and thermal conductivity are main factors of gas recovery.

Numerical investigation on abnormally elevated pressure in laboratory-scale porous media caused by depressurized hydrate dissociation

Hydrate dissociation induced by artificial or environmental factors in submarine sediments may lead to drilling risks, submarine landslides, and the collapse of offshore platforms—all of which are associated with suddenly increased pore pressure. This study establishes a coupled model for the two-phase flow of fluids in porous media in which the pore pressure of water and gas are used as dependent variables and heat transfer and hydrate dissociation equations are combined. The simulation results of the model are consistent with Masuda’s experimental dataset. A quantifiable controlling hydrate dissociation specific reaction surface area (SRSA) model and an absolute permeability model are proposed to conduct numerical simulations of laboratory-scale hydrate cores, with the aim to investigate the mechanism of hydrate dissociation-induced geological hazard initiation and a law of their evolution in sediments. The results show that i) suddenly depressurized hydrate dissociation in porous media can be divided into three phases—pressure induction, transition, and temperature-controlled—and that the first two are much shorter than the third. Moreover, the lower initial permeability results in a shorter holding period for the first stage, and the transition phase becomes indistinguishable; conversely, the larger initial permeability makes the three stages more distinguishable; ii) hardly increasing the core permeability during the intermediate stage of hydrate dissociation makes the pore pressure rebound. However, the steep increase in SRSA does not result in a significant pore gas pressure rebound; conversely, a larger SRSA does facilitate pore pressure dissipation. This work provides theoretical and laboratory-scale model analyses for the safe development of natural gas hydrates bearing in marine sediments.

Description of Gas Hydrate Using Digital Core Technology

Abstract As a new energy source with abundant resources, clean combustion, and high calorific value, gas hydrates have received much attention in recent years. However, the sampling cost is relatively high because the gas hydrates exist in deep seas and frozen soils. Digital core technology can reconstruct hydrate cores without destroying rock samples. In this paper, the advanced image processing technology is used to process the gas hydrate computed tomography (CT) scan image, and a three-dimensional hydrate digital core model is constructed, which can depict the sample's pore structure features. avizo software is used for filtering and image segmentation; the porosity is calculated as 35.90%, the hydrate saturation is 36.92%, and the pore network model is established. The pore radius is mostly distributed in 0–1 × 102 µm, and the average pore radius is 168.131 µm; the throat radius is mostly distributed in 0.5–1×102 µm. The seepage simulation on the pore scale is carried out, and the absolute permeability is calculated to be 76.8 µm2. Compared with conventional physical experiments, the digital core technology can obtain the true distribution of the pores inside the hydrate core, which is very helpful for analyzing the physical parameters of the hydrate core. The digital core technology is of great significance in the study of hydrate reservoirs.

Research on pressure‐stabilizing system for transfer device for natural gas hydrate cores

AbstractTo broaden the analysis means for natural gas hydrate (NGH)‐pressured cores, long‐columnar cores are usually cut into smaller segments with a pressure‐retaining transfer device for NGH. The pressure‐stabilizing system is the core part of a transfer device for NGH, providing a stable high‐pressure environment for the transfer process to avoid NGH decomposition. In this study, the characteristics of the pressure‐stabilizing system were thoroughly studied, and the factors affecting the pressure fluctuations in the transfer device were discussed. Accordingly, the pressure‐stabilizing system was modeled with AMESim software, and the model parameters were set according to the actual working conditions. The simulated pressure fluctuations in the transfer process of transfer device for NGH confirm the theoretical results, which indicate that the precharge pressure of the accumulator should be 0.8‐0.9 times the working pressure. In addition, an increased accumulator volume decelerates the response of the system. Experiments were conducted to test the reliability of the pressure‐stabilizing system. During the transfer process of the NGH sample, the operating pressure can reach 20 MPa, and the fluctuations of the inner pressure are below 20%.

Stages in the Dynamics of Hydrate Formation and Consequences for Design of Experiments for Hydrate Formation in Sediments

Natural gas hydrates in sediments can never reach thermodynamic equilibrium. Every section of any hydrate-filled reservoir is unique and resides in a stationary balance that depends on many factors. Fluxes of hydrocarbons from below support formation of new hydrate, and inflow of water through fracture systems leads to hydrate dissociation. Mineral/fluid/hydrate interaction and geochemistry are some of the many other factors that determine local hydrate saturation in the pores. Even when using real sediments from coring it is impossible to reproduce in the laboratory a natural gas hydrate reservoir which has developed over geological time-scales. In this work we discuss the various stages of hydrate formation, with a focus on dynamic rate limiting processes which can lead to trapped pockets of gas and trapped liquid water inside hydrate. Heterogeneous hydrate nucleation on the interface between liquid water and the phase containing the hydrate former rapidly leads to mass transport limiting films of hydrate. These hydrate films can delay the onset of massive, and visible, hydrate growth by several hours. Heat transport in systems of liquid water and hydrate is orders of magnitude faster than mass transport. We demonstrate that a simple mass transport model is able to predict induction times for selective available experimental data for CO2 hydrate formation and CH4 hydrate formation. Another route to hydrate nucleation is towards mineral surfaces. CH4 cannot adsorb directly but can get trapped in water structures as a secondary adsorption. H2S has a significant dipole moment and can adsorb directly on mineral surfaces. The quadropole-moment in CO2 also plays a significant role in adsorption on minerals. Hydrate that nucleates toward minerals cannot stick to the mineral surfaces so the role of these nucleation sites is to produce hydrate cores for further growth elsewhere in the system. Various ways to overcome these obstacles and create realistic hydrate saturation in laboratory sediment are also discussed.

Investigation on the dissociation flow of methane hydrate cores: Numerical modeling and experimental verification

Methane hydrate has become one of the important topics in recent years as there are more and more reports on possible actual production systems. The most challenging problem for stable methane hydrate production from sea beds or under permafrost regions lies in the complex flow and transportation process, which usually occurs inside the unconsolidated porous layers. The current study is focused on laboratory-scale explorations of the basic dissociation behaviors of methane hydrate. A high-pressure experimental system for the methane hydrate synthesis and dissociation process has been established in this study. The experimental system is specially designed to form and store the methane hydrate under high pressure and low temperature conditions. The mixing of sand with the formed methane hydrate makes it possible to control the initial saturation for dissociation in the current study. A numerical simulation model for core-scale dissociation flow has also been set up, and good agreement with the experimental data was found. It is found that the dissociation on a core scale is more heat-transfer controlled. Through comparisons with previous experimental and numerical results, it is known that the operation strategy will significantly affect the dissociation parameter behaviors. The effects of the initial temperature and permeability on the dissociation process are also shown in this study. Future considerations of core-scale models and reservoir-scale production strategies are also discussed in detail.

Hydrate phase transition kinetics from Phase Field Theory with implicit hydrodynamics and heat transport

Most hydrate that forms or dissociates are in situations of constant non-equilibrium. This is due to the boundary conditions and Gibbs Phase rule. At a minimum this leaves a hydrate with two adsorbed phases in addition to hydrate and fluids. One adsorbed phase is governed by the mineral surfaces and the other by the hydrate surface. With pressure and temperature defined by local conditions, hydrate formation will never be able to reach any state of equilibrium. The kinetics of hydrate formation and dissociation are a complex function of competing phase transitions. This requires kinetic theories that include minimization of free energy under constraints of mass and energy transport. Since phase transitions also change density, further constraints are given by fluid dynamics. In this work, we describe a new approach for non-equilibrium theory of hydrates together with a Phase Field Theory for simulation of phase transition kinetics. We choose a three component system of water, methane and carbon dioxide for illustration. Conversion of methane hydrate into carbon dioxide hydrate is a win–win situation of energy production combined with safe long term storage of carbon dioxide. Carbon dioxide is able to induce and proceed with a solid-state exchange, but is slow due to mass transport limitations. A faster process is the formation of new hydrate from injected carbon dioxide and residual pore water. This formation releases substantial heat. This assists in dissociating in situ methane hydrate, making the conversion progress substantially faster, because heat transport is very rapid in these systems. But conversion of liquid water into carbon dioxide hydrate, in the vicinity of the hydrate core will increase temperatures to some portions of the surface. The dissociating regions of the methane hydrate core will show a local decrease in temperature, due to extraction of heat for methane hydrate dissociation from surroundings. Another reason for heat transport implementation is that regions of the system that contains non-polar gas phase will have low heat conductivity and low heat convection. At this stage we apply a simplified heat transport model in which “lumped” efficient heat conductivity is used. We illustrate the theory on the conversion of methane hydrate to mix methane–carbon dioxide hydrate using three initial hydrate sizes: 150Å×150Å, 500Å×500Å and 5000Å×5000Å. The hydrate cores used are spherical because it makes it easier to illustrate the impact of curvature. Symmetrical aspects simplifies the dependency to a two dimensional problem – although there are no such limitations in theory. The mineral surfaces are considered to be water wetting in these examples. It was observed that the smaller sizes convert to a more unstable mix hydrate for some periods of the simulation time, during which there were significant losses of the initial methane hydrate core. These instabilities are caused by local under saturated fluid phases around the hydrate core. Eventually a steady state progress was observed. The largest size system appeared to reach a steady state situation comparable faster than the two smaller systems.

Dissociation Behavior of Methane Hydrate in Sandy Porous Media below the Quadruple Point

To investigate the effect of ice formation on gas production from gas-hydrate-bearing sandy porous media, we conducted dissociation experiments using artificial methane hydrate cores by depressurizing them to below the quadruple point. We prepared water- and gas-saturated hydrate cores to evaluate the influence of water content on ice formation. The experiments showed that gas production under the ice formation regime had a unique high-rate period in the early stage of production, whereas under the water generation regime, the high-rate period was not observed. During ice formation, the gas production rates of the water-saturated cores exhibited greater acceleration than those of the gas-saturated cores. We conducted numerical simulations using the hydrate reservoir simulator MH21-HYDRES for quantitative analyses. The results showed that ice forms faster in a water-saturated core because of the availability of pore water for ice formation. This further enhances the gas production rate of a water-saturated core. Sensitivity analyses indicated that the rate of ice formation and the permeability reduction by ice formation are key model parameters affecting gas production behavior. From the experimental and numerical investigations, we conclude that depressurization-induced gas production can be accelerated by ice formation during hydrate dissociation at a pressure below the quadruple point.

Different Modes of Gas Hydrate Dissociation to Ice Observed by Microfocus X-ray Computed Tomography

To investigate the kinetics of gas hydrate dissociation to ice, we conducted in situ observations of argon and krypton hydrates decomposed at atmospheric pressure and 245 K using microfocus X-ray computed tomography (MFXCT). The MFXCT observations for Ar samples revealed encapsulation of shrinking pure hydrate cores by ice products, confirming the previously proposed model of hydrate dissociation to ice. In stark contrast, CT images taken for Kr samples revealed entirely different distributions of the two phases: preserved hydrates homogeneously trapped in the ice matrix. Results obtained in this work indicate that the mode of hydrate dissociation to ice depends on the hydrate systems, probably because of differences in interaction between gas and water molecules.

Numerical Analysis of the Dissociation Experiment of Naturally Occurring Gas Hydrate in Sediment Cores Obtained at the Eastern Nankai Trough, Japan

Oceanic gas hydrate deposits at high saturations have been found within continuous thick sands in areas such as the Eastern Nankai Trough and the Gulf of Mexico. The recent discovery of these deposits has stimulated research and development programs exploring the use of gas hydrates as energy resources. Because the permeability of hydrate-bearing sediments is a crucial factor for successful gas production from oceanic hydrate reservoirs, the permeability of these sediments and the dissociation process of hydrates should be investigated using hydrate cores obtained at these oceanic hydrate reservoirs. In this study, to investigate the permeability of actual hydrate-bearing sediments and the dissociation process of hydrates by a depressurization method, a numerical simulation was conducted using a state-of-the-art hydrate reservoir simulator. A dissociation experiment of hydrate-bearing sandy cores obtained from turbidite sediments at the Eastern Nankai Trough was analyzed. By choosing appropriate model parameters, the simulator precisely reproduces the dissociation behavior such as cumulative gas production, cumulative water production, and pressure change. The model parameters associated with permeability indicate a pore-filling tendency rather than a coating tendency of the hydrate in the pore space. Although the permeability of the hydrate-bearing cores obtained at hydrate reservoirs in nature was relatively low, the effective water permeability obtained in this study seems promising for achieving depressurization-induced hydrate dissociation. It has been found that the pressure reduction propagates deeply into the hydrate-bearing zone and the hydrate is spatially dissociated. Also, the permeability is beyond the lower limit of threshold permeability, which is absolutely necessary for successful gas production by depressurization. This study confirms the advantage of employing depressurization as a gas production method, using the hydrate in sandy turbidite sediments at the Eastern Nankai Trough as our test sample. The numerical analysis method used is effective to analyze the dissociation behavior of hydrate-bearing cores obtained at natural hydrate reservoirs, and it enables evaluation of gas productivity in those reservoirs.

A natural hydrate dissolution experiment on complex multi-component hydrates on the sea floor

Dissolution of natural hydrate cores was measured using time-lapse photography on the seafloor at Barkley Canyon (850 m depth and 4.17 °C). Two types of hydrate fabrics in close contact with one another were studied: a “yellow” hydrate stained with condensate oil and a “white” hydrate. From thermogenic origins, both fabrics contained methane as well as heavier hydrocarbons. These multi-component hydrates were calculated to be well within p– T stability conditions (<200 m water depth needed at 4.17 °C). While stable in pressure and temperature, the hydrates were bathed in under-saturated seawater, which promoted dissolution. The flux of gas from the shrinking yellow hydrate core was 0.15 ± 0.01 mmol gas/m 2 s, while the white hydrate dissolved faster at 0.25 ± 0.02 mmol gas/m 2 s. To determine the controlling mechanism for the observed changes in the hydrate cores, experimental results were compared with an engineering correlation for convective mass transfer. Using water velocity as a fitting parameter, the correlation agreed well with results from a previous dissolution experiment on well-characterized synthetic hydrates. Even with a number of other unknowns, when applied to the natural hydrate, the mass transfer correlation predicted the dissolution rate within 20%. This seafloor-based experiment, along with visual observations of seafloor hydrate dissolution over a 3-day period, were used to further understand the fate of natural seafloor hydrates exposed on the seafloor. By showing that mass transfer is the rate-controlling mechanism for dissolution of these natural hydrate outcrops, proper hydrodynamic calculations can be employed to give a refined estimate on hydrate dissolution rates.

メタンハイドレートコアの減圧法実験におけるガス生産律速因子の解析

Depressurization method is regarded as a promising gas production technique from methane hydrate reservoirs. There are three major factors that determine gas production rate by depressurization : kinetics of hydrate dissociation, gas flow through the reservoir, and heat transfer to the dissociating zone. As the gas productivity of methane hydrate wells significantly changes depending on the factors that determine the rate, it is important to find the most governing factor. To find such rate-determining factors, we developed a new method by introducing formulations of three potential methane fluxes generated by kinetics of hydrate dissociation, gas flow and heat transfer.The proposed method was applied to the analysis of hydrate dissociation and gas production behaviors in laboratory scale depressurization experiments on artificial methane hydrate cores. From the calculation and comparison of those rate-determining factors, we concluded that gas productions in those experiment cases were mainly limited by heat transfer. Hydrate dissociation rates during the experiments were well matched with the methane fluxes calculated by heat transfer, which increased in proportion to the heat flux into the core. According to the core experiment results, gas production rate from depressurization of high-permeability hydrate cores is probably limited by heat transfer. Future application of this method to the analysis for hydrate reservoir performance should give us a clue to finding a best production method.

Methane hydrates: problems in unlocking their potential

Methane hydrates have been recovered or postulated for virtually all continental margins around the world and a few areas onshore. Volumes of about 2 × 10 14 m 3 have been estimated for this potential resource. However, only a few sites have been suggested offshore northwest Europe, despite extensive hydrocarbon exploration and academic studies of the margin. Reasons for this anomaly are unclear. To aid the search a new hydrate stability zone map for the UK is presented. As well as identifying a resource, hydrate studies are also important in assessing geohazards to deep-water exploration and development. Stability, processes and distribution information contribute to the wider climate change debate as methane hydrates are estimated to hold a significant part of the global organic carbon budget. To quantify reserve potential and to identify suitable methods of methane extraction, a full understanding of how hydrates are held within sediments is required. Although modelling (physical and theoretical) can contribute to an understanding, it is important to evaluate in situ conditions to ‘ground truth’ acoustic data and imagery. How hydrate is held and its control of dynamic geotechnical behaviour within the sedimentary system is still very poorly understood. Parameters such as pore size, fluid saturations, sediment mineralogy and cementation will affect hydrate morphology, distribution, behaviour (during dissociation) and potential recovery from porous media. Assessing physical parameters and processes under in situ conditions provides the next step along the route to exploiting methane hydrates as a resource. The requirement to recover samples under in situ pressures and temperature conditions provides a significant technological challenge that has been attempted over the last few years with some success. Currently, the European HYACINTH project is developing systems to recover, analyse and manipulate hydrate-bearing sediments under in situ pressures and temperatures. On Leg 204 of ODP this equipment was used for the first time to recover hydrate cores at in situ pressure, transfer them without loss of pressure into laboratory chambers and to log them geophysically. As the database of in situ properties grows, integrated laboratory studies of synthetic sediment-hosted hydrates can be developed to provide important benchmarking, which is crucial for the study of rare natural core samples.