Deep-sea Gas Hydrate Research Articles

Investigation on gas–liquid–solid three-phase flow model and flow characteristics in mining riser for deep-sea gas hydrate exploitation

In response to the problem of gas–liquid–solid three-phase flow in deep-sea hydrate extraction pipelines, a gas–liquid–solid three-phase flow model considering the dynamic decomposition of hydrates is established using continuity equations, momentum equations, and energy equations. The numerical solution of the theoretical model is achieved using the finite difference method. Comparing the theoretical model with the experimental results, the results showed that the average error of gas holdup, liquid holdup, solid phase content, gas phase velocity, liquid phase velocity, and solid phase velocity obtained from the theory and experiment are 8.24%, 0.41%, 1.88%, 5.80%, 2.81%, and 2.22%, respectively, which verified the correctness of the theoretical model. On this basis, the influences of hydrate abundance, liquid phase displacement, and wellhead backpressure on the gas–liquid–solid three-phase flow characteristics in the pipeline were investigated, and it was found that the gas holdup rate will increase with the increase in hydrate abundance, liquid phase displacement, and wellhead backpressure, with the influence of hydrate abundance being more sensitive. The liquid holdup rate increases with the increase in hydrate abundance and liquid phase displacement, but decreases first and then increases toward the wellhead position with the increase in wellhead backpressure. The solid phase content decreases with the increase in hydrate abundance, and first increases and then decreases toward the wellhead position as the liquid phase displacement and wellhead backpressure increase. The influence of gas phase velocity on the abundance of hydrates is relatively small, but it increases with the increase in liquid phase displacement. When the wellhead backpressure increases, the instantaneous increase then tends to flatten out. The influence of hydrate abundance on the liquid phase velocity is also relatively small, but it increases with the increase in liquid phase displacement and decreases with the increase in wellhead backpressure. The solid phase velocity will increase with the increase in hydrate abundance and liquid phase displacement, but it will not show significant changes with the change of wellhead backpressure. The research results can provide a theoretical basis for the safety of hydrate mining.

Assembly process analysis and system design for deep in-situ fidelity corer

The deep in-situ corer is an essential tool for exploring the physical and mechanical behaviours of rocks at varying depths. Corers are multi-part products, encompassing extensive redundant assembly information stemming from the intricate assembly processes and numerous components. Thus, designing assembly systems for corers poses complexity challenges. This paper proposes an automated assembly system design framework, comprising assembly process analysis and system development. The proposed framework offers comprehensive design processes for multi-part product assembly systems, emphasizing managing information amount and complexity during the design phase. Specially, design principles and methods for minimizing information amount are analysed. And the directed graph is utilized to express the assembly information. Building upon this foundation, automated assemblability assessment and assembly process modular clustering is proposed to amalgamate and streamline assembly information, thus reducing design complexity. Leveraging pre-processed assembly information, the sub-functional solutions for assembly system are solved and optimized with AD (Axiomatic Design) and TRIZ (the Theory of Inventive Problem Solving). A case study of the deep-sea gas hydrate corer validated the effectiveness of the proposed methods in reducing information amount and yielding sub-functional solutions, thereby facilitating overall system design realization. Assembly experiments verified the designed system accomplished functional requirements. This study serves as a valuable reference for automated assembly analysis and system development for multi-part products, offering technical insights into developing deep in-situ cores from an assembly perspective.

Assembly sequence planning and evaluating for deep oil and gas corer based on graph theory

Fidelity corers play a crucial role in exploring and evaluating deep oil and gas resources. Due to the complexity of the corer parts, an improper assembly sequence can lead to inefficiencies and reduced reliability, impacting the success of obtaining oil and gas samples. This paper presents a novel assembly sequence planning method based on graph theory and multi-objective evaluation. The graph-theoretic cut set algorithm is utilized to generate all possible assembly sequences based on a directed graph which represents corer component assembly information. To address deep oil and gas corer requirements, the interpretative structural modeling method (ISM) is employed to construct a comprehensive stability-reliability-accuracy assurance-easiness-cost economy (SRAEC) indicator system. Through indicators weight assignment and multi-objective evaluation, the optimal solution of assembly sequence is identified from a global perspective. The proposed method is applied to a deep-sea gas hydrate corer, resulting in the identification of the most suitable assembly sequence for both field drilling and laboratory testing. This optimized sequence improves the assembly reliability and efficiency, contributing to the overall success and efficiency of the corer in obtaining oil and gas samples. The study provides valuable insights for designing and practically applying deep oil and gas coring tools.

Application of unlabelled big data and deep semi-supervised learning to significantly improve the logging interpretation accuracy for deep-sea gas hydrate-bearing sediment reservoirs

Due to the extremely complex reservoirs and strong heterogeneity, deep-sea gas hydrate logging porosity calculations still have problems, which further leads to insufficient resource calculation accuracy. Logging reservoir evaluation methods based on intelligence may be able to provide more reliable prediction results, especially the logging evaluation model based on deep learning with great potential. This paper proposes a new method to form unlabelled logging big data, and based on this, establishes a semi-supervised deep learning method suitable for deep-sea gas hydrate-bearing sediments porosity calculation, forming a porosity evaluation model. The method of forming logging big data expands 380 original data samples into 2280 labelled samples and 60050 unlabelled samples, which reduces the sampling requirements for deep-sea sediment formations with high sampling costs. The evaluation results show that the model not only obtains better results than other methods in the inspection wells corresponding to the areas where the training wells are located, but also obtains very good results in other wells that are not involved in the modelling. Compared with traditional prediction methods, the average relative error of porosity prediction is less than 4%. As far as we know, this is the first time that deep learning has been successfully applied to deep-sea hydrate sediment reservoir logging evaluation. It provides a new idea for intelligent logging evaluation of deep-sea hydrate sediment reservoirs.

Artificial Fracture Stimulation of Rock Subjected to Large Isotropic Confining Stresses in Saline Environments: Application in Deep-Sea Gas Hydrate Recovery

The low permeability of gas hydrate deposits leads to poor extraction rates. Artificial fracture stimulation could significantly improve the recovery rate of an estimated 300 trillion m3 of this untapped future energy source, which form in seabed sedimentary deposits. Because conventional methods of rock fragmentation are inapplicable in such deposits due to sudden release of energy that may impose the risk of methane release to the atmosphere, alternative rock fragmentation technologies are necessary for artificial fracture stimulation of gas hydrate deposits. We checked the effectiveness of a new hydrophobic non-explosive demolition agent as a rock fracturing technique, which could potentially be used as a third-generation disruptive technology for mining (3G-DTM). Laboratory experiments performed by mimicking deep-sea environments suggest that the density of the gradually generated rock mass fractures increases with confining pressure and pore fluid salinity. Importantly, due to the fracturing nature of 3G-DTM, the fracture density can be significantly improved (by 116%) with increasing the confining pressure (from 70 kPa to 20 MPa). Increased salinity of the rock pore fluid also improved the fracture density by 38% at 20 MPa confining pressure when the salinity increased from 0% to 20%. Furthermore, the rock is subjected to a gradual fracturing process in the 3G-DTM fracturing (10–15 h in the laboratory experiments) allowing for a safer, more controlled fracture propagation, making 3G-DTM a substitute for conventional rock fragmentation in marine environments.

Pressure-core-based reservoir characterization for geomechanics: Insights from gas hydrate drilling during 2012–2013 at the eastern Nankai Trough

Pressure coring and analysis technology has progressed remarkably over the last decade. This technology allows for methane-hydrate-bearing sands, silts, and/or clays to be recovered from the deep-sea gas hydrate reservoir and then maintained in the laboratory within the hydrate stability phase boundary for evaluation and characterization. In this study, a series of pressure-core-based undrained/drained confined compression tests, unconfined (uniaxial) compression tests, isotropic loading and unloading tests, and permeability tests are conducted to investigate and characterize the intact strength, compressibility, and permeability of the gas hydrate reservoir. The consolidation tests and fluid flow tests showed that clayey silt sediments have permeability values of approximately tens of microdarcies, and sandy sediments with and without hydrates have permeability values of tens of millidarcies. The sediment from the bottom of the reservoir, which is sandy in nature with fines, also has permeability values of tens of microdarcies. From previous reports, the undrained shear strength of clayey silt sediments with low gas hydrate saturation is relatively low. Results from the drained shear tests and uniaxial compression tests show the first Mohr−Coulomb failure criteria for natural gas hydrate-bearing sediments, in which both the effective cohesion c' and the friction angle ϕ’ increase with increasing hydrate saturation. The results indicate in situ existence of pore filling and patchy gas hydrate. Finally, a new failure envelope for hydrate-bearing sediment is proposed that includes both hydrate saturation and confining pressure; it shows a good relation with the experimental results.

Recovery of Methane from Gas Hydrates Intercalated within Natural Sediments Using CO2 and a CO2/N2 Gas Mixture

The direct recovery of methane from massive methane hydrates (MHs), artificial MH-bearing clays, and natural MH-bearing sediments is demonstrated, using either CO(2) or a CO(2)/N(2) gas mixture (20 mol % of CO(2) and 80 mol % of N(2), reproducing flue gas from a power plant) for methane replacement in complex marine systems. Natural gas hydrates (NGHs) can be converted into CO(2) hydrate by a swapping mechanism. The overall process serves a dual purpose: it is a means of sustainable energy-source exploitation and greenhouse-gas sequestration. In particular, scant attention has been paid to the natural sediment clay portion in deep-sea gas hydrates, which is capable of storing a tremendous amount of NGH. The clay interlayer provides a unique chemical-physical environment for gas hydrates. Herein, for the first time, we pull out methane from intercalated methane hydrates in a clay interlayer using CO(2) and a CO(2)/N(2) gas mixture. The results of this study are expected to provide an essential physicochemical background required for large-scale NGH production under the seabed.

Methane under-saturated fluids in deep-sea sediments: Implications for gas hydrate stability and rates of dissolution

Deep-sea sediments contain Earth's largest reservoir of methane (CH 4, 3000 GTons C) trapped within ice-like crystals known as gas hydrates. Understanding the controls on gas hydrate stability is critical because methane released from hydrate destabilization is hypothesized to be a powerful agent of past and potentially future climate change. Hydrates are stable under high pressure, low temperature, moderate salinity, and saturated gas conditions. Yet, the degree of gas saturation is rarely known in nature because in situ dissolved pore-water CH 4 concentrations are rarely measured. Here, we report measurements of these concentrations in sediments immediately surrounding deep-sea gas hydrate deposits and show that pore-fluids are greatly under-saturated with respect to expected values for equilibrium with methane gas hydrate. This indicates that the hydrates are dissolving, even though they are found within the appropriate pressure and temperature stability field. However, dissolution rates calculated from the in situ CH 4 data are significantly less than dissolution rates predicted for methane-under-saturated pore-water in direct contact with pure methane gas hydrate if equilibrium CH 4 concentrations exist immediately adjacent to the hydrate surface. Diffusion-retarding factors found naturally in ocean sediments, such as oil coatings or biofilms, appear to enhance stability in outcropping hydrate deposits. The in situ seafloor evidence provided herein leads us to hypothesize that the stability of the worldwide hydrate deposits may be much greater than predicted from diffusion kinetics because biological (microbial excretion of slime or surfactants) and/or physical processes (oil coatings) effectively armor and stabilize exposed hydrate surfaces, substantially retarding their dissolution.

Correction of Hydrate Dissociation in Shale Media via Correlation of Kim's Model

Deep-sea gas hydrates are ice-like crystalline substances in which hydrocarbon and non-hydrocarbon gases of specific molecular diameters are held by hydrogen bonding within rigid cages of water molecules. Sudden release of 1100-2100 Gt of carbon from dissociation of up to 10% of the global hydrate reservoir has been suggested as the cause of both the C-isotope excursion and coeval global atmospheric warming through an enhanced greenhouse effect. Kim's model assumed that, at a constant temperature, a two-step process could describe gas hydrate decomposition, including destruction of the clathrate host lattice at the surface of a particle and desorption of the guest molecule from the surface. In this paper, first, the thermodynamics and the kinetics of gas hydrate decomposition via Kim's model in the seafloors was reviewed; then, by a stainless steel test cell reactor and Omega Data Acquisition software, their correlation to sandy environments was formulated via numerical fitting of experimental data. The correlation of the conventional Kim's model is that the configuration of gas hydrate formation as a type of surface coating around sand grains was assumed. In the first stage of new mechanism dissociation, when the rate-controlling factor is dissociation heat, the formulation of the correlated dissociation rate was expressed. In the second stage of the mechanism, when the rate-controlling factor is mass transfer, the modified rate constant factor instead of Kim's constant factor was determined. The coefficient of correlation between the results of the estimation and experiment was more than 0.98 for both 85 and 216-μm sands.

Source and receiver geometry corrections for deep towed multichannel seismic data

High resolution seismic data were collected using a Deep‐Tow Acoustics/Geophysics System (DTAGS) near ODP site 889/890, offshore Vancouver Island for deep sea gas hydrate study. The near bottom configuration and wide high‐frequency bandwidth allow much better resolution of the structures in the upper ∼400 m. However, the short 2ndash;6 m wavelengths imply accurate positioning of the source and receivers to properly determine moveout velocities and to stack the data. Consequently, a method of estimating the depths of the source and hydrophones using sea‐surface reflection times was developed. Travel time corrections based on these sea surface measurements were applied to the data by marching a time window along the seismic trace to account for the angular dependency of the reflected raypath. The need for these corrections is clearly illustrated by the results, which show non‐linear depth variations of up to 50–100 m for both the source and the hydrophone array.

Is the extent of glaciation limited by marine gas‐hydrates?

Methane may have been released to the atmosphere during the Quaternary from Arctic shelf gas‐hydrates as a result of thermal decomposition caused by climatic warming and rising sea‐level; this release of methane (a greenhouse gas) may represent a positive feedback on global warming [Revelle, 1983; Kvenvolden, 1988a; Nisbet, 1990]. We consider the response to sea‐level changes by the immense amount of gas‐hydrate that exists in continental rise sediments, and suggest that the reverse situation may apply—that release of methane trapped in the deep‐sea sediments as gas‐hydrates may provide a negative feedback to advancing glaciation. Methane is likely to be released from deep‐sea gas‐hydrates as sea‐level falls because methane gas‐hydrates decompose with pressure decrease. Methane would be released to sediment pore space at shallow sub‐bottom depths (100's of meters beneath the seafloor, commonly at water depths of 500 to 4,000 m) producing zones of markedly decreased sediment strength, leading to slumping [Carpenter, 1981; Kayen, 1988] and abrupt release of the gas. Methane is likely to be released to the atmosphere in spikes that become larger and more frequent as glaciation progresses. Because addition of methane to the atmosphere warms the planet, this process provides a negative feedback to glaciation, and could trigger deglaciation.

Methane Hydrate in Slope Sediments on West Coast of Central America: ABSTRACT

Offshore Mexico and Guatemala slope sediments are classic sites of deep-sea gas hydrate occurrence. Gassy, frozen sediment was recovered in cores from this region on Legs 66, 67, and 84 of the Deep Sea Drilling Project. In addition, a massive 3 to 4-m thick layer of nearly pure methane hydrate at a depth of 250 m was cored on Leg 84 and preserved for study. The gas from the hydrate is 99+% methane with a few tenths percent carbon dioxide and traces (10-4v/v) of ethane. Most of the sites with gas hydrate in the sediments have methane with ^dgr13C of -70 to -60 ^pmil, indicating origin from methane-generating bacteria. The massive gas hydrate contained methane with ^dgr13C of -40 ^pmil, and the surrounding sediment had bicarbonate in the por water with ^dgr13C of +35 ^pmil. The 75 ^pmil separation in ^dgr13C between coexisting methane and bicarbonate is consistent with kinetic fractionation during bacterial reduction of carbon dioxide to methane, with continuous replenishment of carbon dioxide by fermentation processes. The areal extent of the massive gas hydrate is not known, but the single point yields a gas-in-place estimate of 5.2 × 108m3/km2 or 48 bcf/mi2. End_of_Article - Last_Page 245------------