Natural Gas Hydrate Production Research Articles

Mineral effects on methane hydrate formation and distribution in sand sediments

Methane hydrate is a promising energy resource widely occurring in the world, but the formation and distribution of methane hydrate in marine sediments with complex minerals remains unclear. Due to the small pore space and fine-grained size of clay minerals, the contact relationship between methane hydrate and clay mineral surface under excess water has not been characterized fully, which is required to investigate the mineral effect on methane hydrate formation. In this study, cryo-SEM was applied to observe the distribution of methane hydrate synthesized in pure quartz sands and the quartz sands mixed with montmorillonite or illite separately. According to the results of the research, methane hydrate dispersedly forms in the pore space away from and on the surface of quartz under a local excess water condition. In addition, methane hydrate occurs away from montmorillonite in pores with excess water and contacts the edge of montmorillonite under a local excess-gas condition. Moreover, methane hydrate was only observed to form away from illite aggregates in the pore with residual water. Montmorillonite and illite influence the preferable distribution of methane hydrate to occur away from mineral surface in excess water condition. In the sediments composed of different minerals, methane hydrate forms stochastically in the pore space and grows up individually. As a result, the findings of this study significantly expand the understanding of the formation and distribution of methane hydrate in marine sediments and the prediction of physical properties of hydrate-bearing sediments, and provide insights into natural gas hydrate exploration and production.

Study on the Multiphase Flow Behavior in the Jet Pump Lifted Natural Gas Hydrate Wells with Combined Depressurization and Heat Injection Method

Abstract Natural gas hydrate production tests have been carried out successfully with different exploitation methods and gas recovery drainage technologies. The purpose of this study is to elucidate the fluid flow behavior in the jet pump lifted natural gas hydrate wells and optimize hot water injection parameters for the jet pump drainage gas recovery technology. The mechanism of jet pump drainage recovery of natural gas hydrate wells was analyzed and its applicability to natural gas hydrate exploitation with combined depressurization and heat injection method was demonstrated. In addition, multiphase flow models of tubing and annulus were established respectively for the phenomenon of countercurrent flow of heat exchange in the jet pump drainage gas recovery process, and the corresponding multiphase flow laws were derived. Based on the study of multiphase flow, sensitivity analysis and optimization of heat injection parameters were performed. It has been demonstrated that jet pump drainage gas recovery technology is suitable for the exploitation of onshore natural gas hydrate with combined depressurization and heat injection method. The multihphase flow behavior in the tubing and annulus of the jet pump lifted natural gas hydrate wells was disclosed in this study. Numerical simulation results show that the temperature and pressure distribution in the wellbore of the jet pump lifted natural gas hydrate wells during the recovery process are affected by the injection conditions. This study provides theoretical guidance for the production optimization and hydrate prevention and control in the wellbore of the jet pump lifted natural gas hydrate wells.

Natural gas hydrate dissolution kinetics in sandy sediments: Implications for massive water production in field tests

The tremendous natural gas hydrate (NGH) reserve in globe makes it a promising future energy. The field test production indicates that massive water production may occur in the depressurization of sandy NGH reservoirs. The dissolution of NGH may play a significant role in this process. Therefore, quantifying the NGH dissolution kinetics and clarifying the relations between NGH dissolution and water production is crucial. In this work, a modified excess water method was developed to prepare quasi-homogenous water-saturated methane hydrate (MH) bearing sediments (HBS). The MH dissolution kinetics were investigated by flowing methane-free water through the HBS. The MH dissolution process could be divided into the rapid stage and the decayed stage. Preferential flow channels may form by the inhomogeneous MH dissolution. Water flux has a significant impact on the MH dissolution kinetics via affecting the mass transfer and the hydrate-water exposure in flow channels. When increasing the water fluxes from 2.0 mL·cm−2·min−1 to 6.1 mL·cm−2·min−1, the MH dissolution rate initially ramped up from 15.4 cm·d−1 to 36.0 cm·d−1 (mass transfer dominates), and then decreased to 28.7 cm·d−1 (flow channels dominate). MH saturation has an influence on dissolution kinetics. The MH dissolution rate increased from 27.1 cm·d−1 to 44.2 cm·d−1 when the MH saturations increased from 11.8 % to 60.1 %. Experimental results indicate that the preferential flow channels by inhomogeneous NGH dissolution could contribute to the massive water production in the depressurization of NGH reservoir. Large pressure gradient in NGH production may accelerate the NGH dissolution and the following massive water production. Homogeneous high-saturation HBS could more facilitate the hydrate-water exposure for dissolution, which could retard the onset of massive water production. This work may ignite some new thoughts on the water control and production optimization strategies in sandy NGH exploitation.

Study on the Multiphase Flow Behavior in Jet Pump Drainage and Natural Gas Hydrate Production Wells with Combined Depressurization and Thermal Stimulation Method

Natural gas hydrate (NGH) trials have been performed successfully with different development methods and gas recovery drainage technologies. Multiphase flow in a wellbore and the drainage of natural gas hydrate are two important parts for its whole extraction process. Additionally, the choice of the drainage method is linked to the development method, making the drainage of NGH more complex. Jet pump drainage is usable for NGH production wells with the combined depressurization and thermal stimulation method. The objective of this study is to shed more light on the multiphase flow behavior in jet pump drainage and NGH production wells and put forward suggestions for adjusting heat injection parameters. The mechanism of jet pump drainage recovery technology for NGH wells was analyzed and its applicability to NGH development by the combined depressurization and thermal stimulation method was demonstrated. In addition, multiphase flow models of tubing and annulus were established, respectively, for the phenomenon of the countercurrent flow of heat exchange in the process of jet pump drainage and gas production, and the corresponding multiphase flow laws were derived. On the basis of these studies, sensitivity analysis and the optimization of thermal stimulation parameters were conducted. It is demonstrated that jet pump drainage gas recovery technology is feasible for the development of onshore NGH with the combined depressurization and thermal stimulation method. The laws of multiphase flow in the tubing and annulus of jet pump drainage and NGH production wells were disclosed in this study. Numerical simulation results show that the temperature and pressure profiles along the wellbore of jet pump drainage and NGH production wells during the drainage recovery process are affected by injection conditions. Increasing injection rate and injection temperature can both improve the effect of heat injection and reduce the hydrate reformation risk in the bottom of the annulus. This study offers a theoretical basis and technical support for production optimization and hydrate prevention and control in the wellbore of jet pump drainage and NGH production wells.

Numerical Simulation of Gas Production Behavior Using Radial Lateral Well and Horizontal Snake Well Depressurization Mining of Hydrate Reservoir in the Shenhu Sea Area of the South China Sea

Improving the production capacity of natural gas hydrates (NGHs) is crucial for their commercial development. Based on the data of the first on-site testing production of NGHs in the Shenhu Sea area, numerical methods were used to analyze the production behavior of radial lateral well (RLW) and horizontal snake well (HSW) with different completion lengths when they deployed at different layers of the Class-1 type hydrate reservoir (with a fixed pressure difference of 6 MPa and continuous production for 360 days). The results indicate that compared with the single vertical well production, RLW and HSW can effectively increase production capacity by enlarging drainage area and the productivity is directly proportional to the total completion length. The RLW and HSW deployed at the three-phase layer (TPL) have optimal mining performance within a 360-day production period. Different to the previous research findings, during a short-term production period of 360 days, regardless of the deployment layer, the overall production capacity of HSW is better than RLW’s. The total gas production of HSW-2 circles well type is about four times that of a single vertical well, reaching 1.554 × 107 ST m3. Moreover, the HSW-1 lateral well type stands out with an average Qg of 3.63 × 104 ST m3/d and a specific production index J of 16.93; it has the highest J-index among all well types, which means the best mining efficiency. It is recommended to choose the HSW-1 circle well type, if the coiled tubing drilling technique is used for on-site testing production of NGHs in the future. The research results provide insights into the potential applications of RLW and HSW in this sea area.

Monitoring the developmental trend and competitive landscape of natural gas hydrate through patent analysis.

Natural gas hydrate (NGH) is a significant alternative energy resource in achieving carbon neutrality. The developmental trend and competitive landscape of NGH exploitation and production are crucial for policymakers in government, managers of enterprises, and researchers. This study introduces a novel framework for conducting an in-depth analysis of NGH, integrating patentometrics, technology evolution, and correlation relationships to monitor developmental trends and competitive landscape through patent analysis. The results indicate that China, the USA, and Japan have distinct technology advantages. Current technological developments in the NGH field focus primarily on extraction technologies, equipment, and processing systems. The co-opetition analysis among countries reveals that the most extensive international cooperation network is primarily in Europe and the USA, with national partnerships in Asia concentrated in China and Japan. Institutional cooperation remains limited, primarily within universities in China, while both the USA and Japan foster collaboration between enterprises. The competitive landscapes of key NGH-related technologies among countries and institutions are also examined. This study contributes not only to monitoring the developmental trend and competitive landscape in NGH but also to providing policy recommendations for government and enterprises regarding strategic management and collaborative innovation.

Time-dependent deformation of marine gas hydrate-bearing strata conditioned to a wellbore: Experiments and implications

Natural gas hydrate (NGH) is a promising alternative energy resource, whereas marine NGH exploitation suffers from various potential geo-environmental concerns. The existence of a production wellbore not only provides channel for gas extraction, but also causes specific boundary conditions for the occurrence of geo-environmental risks. Almost all kinds of geo-environmental risks during NGH production initiate from time-dependent deformation of the near-wellbore strata. In this paper, the pressuremeter test (PMT) method was firstly introduced to investigate the lateral creep behaviors of the NGH-bearing sediment. The PMT probe was used to apply outwards lateral stress onto the hydrate-bearing sediment to simulate the horizontal stress state of the near-wellbore reservoir during drilling, completion, and/or hydraulic fracturing, during which the pressure inside the wellbore is higher than the reservoirs static pressure. The results indicated that the decelerated creep accounts for more than 70% of the lateral strain before the effective stress reaches the long-term strength. The hardening modulus of the sediment was proven to be increased from the magnitude of ∼102 MPa–∼103 MPa, when the hydrate saturation increases from 2.2% to 50.0%. The PMT creep test results can be used for interpretation of the in-situ horizontal stress and geomechanical parameters such as pro-plastic pressure and ultimate pressure. Based on the fact that PMT test results complies with the previous results from other techniques, a field-applicable PMT technique for NGH reservoir test is suggested to be developed in future study, and a systematic hypothesis and workflow for NGH reservoir modelling is also proposed in this study.

A bibliometric analysis of two-decade research efforts in turning natural gas hydrates into energy

As a promising energy source, natural gas hydrate production has ignited scientific research in multidisciplinary fields globally, generating a vast amount of literature. A review in multidisciplinary and international perspectives will contribute to a comprehensive understanding about this field. This paper maps the two-decade spatiotemporal landscapes of scientific research on natural gas hydrate (NGH) production with a bibliometric analysis. The landscapes suggested that the study on gas hydrate production covers multiple research fields and international collaboration. Research topics on NGH production experienced three stages involving resource exploration, productivity enhancement, and environmental issues. An analysis of the research with the most activity showed that Energy & Fuel, Chemical Engineering, and Multidisciplinary Geoscience were the most active, with Thermodynamics, Geological Engineering, Ocean Engineering, and Civil Engineering experiencing fast growth. In terms of the landscape of international collaboration, the United States and European countries were the main contributors in the early stage, followed by China after 2008. The breakthrough of the commercial production threshold relies on the international collaboration and the multidisciplinary research among Thermodynamics, Geological Engineering, Ocean Engineering and Civil Engineering to characterize the energy consumption, gas production rate, and reservoir stability.

Experimental investigation on dynamic mechanical characteristics of casing during hydrate decomposition using depressurization

During marine hydrate production, some potentially undesirable consequences of casing strength failure are prone to occur. However, conventional methods used in casing mechanical analysis is unable to handle casing failure accurately, as the macro constitutive model employed in current simulation could not reflect the real stress-strain relationship of hydrate reservoir. This paper aimed at proposing an experimental method to investigate dynamic mechanical responses of casing during hydrate decomposition based on the self-developed experiment system, in which overlying stress was simulated to construct a similar mechanical environment of casing. By conducting laboratory-scale tests, the effect of overlying stress on casing mechanical behaviors could be revealed, and comparative analysis of casing axial stress obtained from experiment and simulation was carried out. Experimental results indicated that axial stress and extrusion pressure of casing increased 20.91 % and 7.18 % with the increase of overlying stress, respectively. Casing axial stress increased first and then gradually reached stable in different depressurization stage, which agreed well with the results calculated by numerical simulation. The maximum error between experimental data and simulation results was 11.7 %, 11.1 % and 10.2 % in different depressurization stage, respectively. The proposed approach could provide reference for casing safety design of natural gas hydrate production.

Pore-scale study of the dynamic evolution of multi-phase seepage parameters during hydrate dissociation in clayey silt hydrate-bearing sediments

Unlike conventional oil and gas sources, natural gas hydrate (NGH) is stored in solid form within the pores of loose submarine sediments. Production by any method poses a complex problem involving phase transition and multi-phase flow of gas and water. The evolution of hydrate dissociation from the solid phase to gas and liquid phases in a hydrate reservoir is a dynamic process incorporating pore structure and multi-phase seepage. The solid–gas-liquid phase dissociation of NGH affects reservoir seepage properties by altering the micropore structure of the hydrate-bearing sediments. This study, based on the lattice Boltzmann method of modeling fluids flow, clearly demonstrates the micropore-scale changes in pressure and heat transfer that take place during hydrate dissociation. Pore-scale heterogeneity is found to influence the complex development of hydrates in a porous clayey silt medium during dissociation. The effect of hydrate, gas and water saturation on the evolution of relative permeability is identified, and the relationship between relative permeability and hydrate saturation during hydrate dissociation is quantified. These findings provide theoretical support for the implementation of marine natural gas hydrate production testing and future commercial production in China.

Investigation of creep and transport mechanisms of CO2 fracturing within natural gas hydrates

Natural gas hydrates (NGH) are considered as a potential energy resource in the future because of its dramatic energy reserves. NGH formation owns the characteristic of viscoelasticity, to some extent behaves both like liquid and solid. Ignoring the viscoelastic properties of NGH can lead to inaccurate prediction of related characteristics. In 2020, horizontal-well drilling technology was adopted to improve the stimulation performance of NGH reservoirs in the Shenhu area of China. This indicates that it is possible to apply hydraulic fracturing or CO2 fracturing in the reservoir reconstruction of NGH. CO2 fracturing can generate more fracture networks and connect micro fractures. Additionally, CO2 can be stored geologically and integrated with injection, production, and storage. This study provides a relatively detailed demonstration of the formation and basic mechanical properties of NGH. Then, a comprehensive overview of potential NGH production enhancement methods is conducted. Finally, an evaluation method for dual conductivity considering creep effect is proposed, and the results show that the creep effect of NGH can reduce the conductivity of fractures and matrices. Thus, it is of great importance to investigate the viscoelastic properties of NGH formation, for the long-term prediction of the geo-mechanical response to gas extraction.

Exploration of production capacity-geomechanical evaluation and CO2 reinjection repair strategy in natural gas hydrate production by multilateral horizontal wells

Multilateral horizontal wells (MHW) can enhance gas production by expanding the natural gas hydrate (NGH) decomposition area, but inevitably leading to larger changes in reservoir pressure and rock stresses, causing subsidence or disasters. Therefore, this study established a THMC model that evaluated the production effect and geomechanical response of MHW in Class-I reservoirs with different permeability and proposed a CO2 reinjection repair strategy to achieve subsidence recovery and carbon storage. Results indicate that MHW with vertical branch arrangements performs best in short-term production for low-permeability reservoirs, while the temperature and pressure evolution in high-permeability reservoirs is more drastic than in low-permeability reservoirs, producing many unfavorable factors. Higher capacity also results in larger strata subsidence and stress changes, affecting the reservoir stability and wellbore safety: (i) subsidence occurs mainly in the NGH sediment and the upper zone of the wellbore; (ii) normal stresses around the wellbore increase most significantly in the vertical direction, while shear stresses generally decrease. The repair indicators of subsidence recovery rate, energy recovery rate, and hydrate conversion rate suggest that reinjecting CO2 into depleted NGH reservoirs can improve the mechanical properties of the strata and achieve carbon storage via the hydrate method as well.

Numerical Simulation of Hydrate Dissociation Behaviors in Hydrate Reservoir with Different Properties during Horizontal Well Drilling

The effectiveness of horizontal well drilling in improving the gas recovery efficiency of hydrate production makes it a promising technology for commercial exploitation. However, during horizontal well drilling in hydrate reservoirs, it is crucial to control hydrate dissociation to ensure the reservoir stability and drilling safety. In this work, a two-dimensional model using polar coordinates was built to study the influences of hydrate reservoir characteristics and drilling fluid salinity on gas production. The simulation applies to the hydrate reservoir of the second natural gas hydrate (NGH) production test in the Shenhu area of the South China Sea. The characteristics of hydrate dissociation and secondary formation and the drilling invasion behavior in the NGH layer and the mixing layer (free gas + hydrate) during horizontal well drilling were analyzed and compared. The simulation results indicated that the pressure and temperature transmission rates in the mixing layer (free gas + hydrate) are higher than those in the NGH layer. The invasion amount of drilling fluid in the mixing layer is 18.8 times more than that in the NGH layer. Under the high invasion of the drilling fluid, the hydrate dissociation amount in the mixing layer is similar to that of the NGH layer even though the initial hydrate saturation of the NGH layer was 2.65 times that of the mixing layer. The area of the hydrate dissociation in the mixing layer is much larger than that in the NGH layer, which may lead to the increase in risk of wellbore instability. The secondary hydrate formation is only observed in the NGH layer, which inhibits the drilling fluid invasion. The salinity of the drilling fluid has a more significant impact on the hydrate dissociation near the wellbore in the mixing layer compared to the NGH layer. With the increase in salinity from 3.05 wt% to 20 wt%, the hydrate dissociation range in the mixing layer increases from 0.16 m to 0.23 m, while the hydrate dissociation range in the NGH layer does not significantly change.

Pore-scale visualization of hydrate dissociation and mass transfer during depressurization using microfluidic experiments

Natural gas hydrate is a potential low-carbon energy resource. Extracting this resource efficiently remains a challenge, due to the intricacies involved in multiphase reactive transport during hydrate dissociation. This study employs microfluidic experimental techniques to observe pore-scale interactions during depressurization-induced hydrate dissociation, utilizing high-resolution spatial and temporal imagery. Supervised machine learning algorithms segmented these microscopic images, enabling the analysis of phase saturation changes and hydrate dissociation rates. The study identifies two distinct hydrate forms in microfluidic chips: hydrate films and crystals, each exhibiting unique dissociation characteristics. Hydrate films decomposed rapidly in response to pressure reductions below equilibrium with the dissociation rate of O(10−1 %/s) under stationary gas–water conditions. When considering gas–water migration, the gas encapsulated in the hydrate film will be displaced by water, leading to a reduced dissociation rate. The hydrate crystal dissociation is much slower with the dissociation rate of O(10−3 %/s) under stationary gas–water conditions compared to the hydrate film, hindered by mass transfer limitations. Gas-water migration can enhance crystal dissociation. The formation and expansion of gas microbubbles under depressurization accelerated hydrate dissociation within a confined temporal and spatial range, which was known as the self-promotion mechanisms. Significantly, depressurization-induced gas slug flow increases the dissociation rate by more than one order of magnitude, compared to that in stationary gas–water conditions. The depressurization process played a significant role, not only by facilitating thermodynamic feasibility for hydrate dissociation but also by inducing the critical gas slug flow. These results of the present study provide theoretical guidance for improving natural gas hydrate production efficiency.

An environmental-benign method to enhance hydrate production in clayey silty reservoir via splitting grouting

During the pivotal period of energy transition between carbon neutrality and carbon peaking, it is crucial to develop the natural gas hydrate (NGH) due to the advantages of high energy density and non-pollution. However, the NGH production also faces several significant environmental challenges that need to be addressed such as seawater intrusion, methane leakage, and submarine landslides. Therefore, an environmental-benign method to enhance hydrate production in clayey silty reservoir via splitting grouting was introduced. As a novel method for achieving the two goals of both reservoir skeleton reinforcement and permeability enhancement, is proposed to stimulate NGH reservoir. Herein, the formulation based on foamed ultrafine cement-based grout was established. The results showed that the grout has good fluidity with initial viscosity ∼500 mPa·s and consolidation time in the range of 28–50 min. Meanwhile, the consolidation performance evaluation was studied, and it showed that the consolidation is with porosity in the range of 9.41%–46.59%, permeability in the range of 51.23 mD-789.07 mD, and compressive strength in the range of 3.31 MPa–3.88 MPa. The internal pore channels are uniformly distributed with a large number of stomatal strings, which can be used as a multistage seepage channel for gas and water production from hydrate decomposition. To further examine the stimulating behavior, the effect of hydrate saturation, grouting well diameter, grouting flow rate, and in-situ stress on splitting pressure and grouting volume were analyzed, and it was found that the grout has good ability to split fracturing and grout diffusion under the conditions of axial pressure of 5 MPa, confining pressure of 4 MPa, and hydrate saturation not more than 0.30. In addition, an environmental-benign high-efficient reservoir stimulation method on a laboratory scale was proposed. The findings could provide fundamental insights and recommendations for the implementation of splitting grouting in NGH reservoirs.

Cementation breaking and grit separation characteristics of weakly cemented natural gas hydrate by a new structure hydrocyclone

Solid fluidization exploitation technology is one of the most promising methods for the safe exploitation of shallow natural gas hydrates (NGH) in the deep sea. However, after primary jet breaking in the solid fluidization exploitation, the hydrate and argillaceous silt particles are in a state of weak cementation and agglomeration. When the argillaceous silt particles enter the offshore drilling platform with the drilling fluid, it causes substantial sand production, high energy consumption, and pipeline blockage, considerably affecting the development process of the NGH test. Therefore, in-situ desanding has become one of the key technologies of solid fluidization exploitation. In this study, a coupling method of hydrocyclone cementation breaking and hydrocyclone desanding was proposed, and a new structure of cementation breaking and grit separation hydrocyclone was designed. In a lab-designed cementation breaking and grit separation hydrocyclone system, the cementation breaking and grit separation characteristics of weakly cemented NGH similar materials in the swirl flow field were studied. The experimental results under the operating conditions studied show that the swirl flow field exhibits good cementation breaking and grit separation performance for weakly cemented NGH similar materials. For weakly cemented NGH similar materials with a compressive strength of 0.06–0.42 MPa, the cementation breaking efficiency of the cementation breaking and grit separation hydrocyclone is in the range of 27.17–99.61%. After the cementation breaking, the mixture of hydrate similar materials (hydrophilic polyethylene powder) and quartz sand was separated in the hydrocyclone. The separation efficiency of hydrate similar materials is 27.59–99.64%, and the desanding efficiency is 87.66–99.98%. The cementation breaking and grit separation of weakly cemented NGH similar materials were successfully realized. Based on the experimental data, a multiple regression model for the cementation strength of similar materials and the swirl flow field strength was established using the least squares method. This method is expected to provide new technical support for in-situ desanding and safe exploitation of the deep-sea shallow NGH production process.

Numerical analysis of the production behaviors and geomechanical responses during natural gas hydrate production by vertical wells fracturing

Trial extraction of natural gas hydrates has made great progress, but the available capacity is still orders of magnitude away from commercial development. Reservoir modification measures are a potential approach to achieve commercial recovery, and previous studies have demonstrated the feasibility of fracturing modification in hydrate reservoirs. The laboratory experimental research on hydrate production by vertical well fracturing combined with depressurization method has been carried out to prove the effectiveness of improving capacity. The presence of fractures enhances conductivity near the wellbore, thereby facilitating the propagation of pressure drops and augmenting thermal energy transfer. Whereas hydrate decomposition causes a reduction in reservoir strength and stiffness, potentially resulting in formation subsidence, wellbore failure, and other catastrophic events. Therefore, it is important to understand the geomechanical response for designing fracturing modifications in hydrate reservoirs. As a continuation of previous research, we constructed a coupled model of thermal-hydro-mechanical to investigate the production characteristics and geomechanical response patterns during vertical well fracturing combined with depressurization method to extract hydrates. The results show that fracturing modification helps to shorten the duration of “self-locking effect” and improve the gas-water ratio of gas hydrate extraction. Fractures induces the stress concentration near the wellbore, exacerbating both the subsidence and uplift of caprocks. Under the condition of controlled geological risk, priority should be given to extracting reservoirs with high saturation and high original permeability.

On the Influence of Fine Particle Migration and Deposition on Gas Hydrate Production: Insights from the First Trial Production in the South China Sea

Summary As a promising alternative energy source with vast reserves, the primary challenge in exploiting natural gas hydrates (NGHs) lies in achieving long-term safety and efficiency. Several production tests conducted globally have demonstrated that the production of solid particles is one of the main factors contributing to this issue. However, most studies on assessing NGH production capacity have overlooked this phenomenon and its impact on production performance, potentially compromising the accuracy of simulation results. Therefore, this study aims to quantitatively analyze the impact of fine particle migration and deposition on gas production by numerically examining the first NGH production test conducted in the Shenhu area of the South China Sea (SCS). Through comparison of simulation results with the reconstructed gas production curve, the model was calibrated and enabled an explanation of unique gas production performance during field testing. It is found that the deposition of solid particles reduced the gas production rate by nearly an order of magnitude during the initial stage of exploitation. Long-term simulation results indicate that only the three-phase layer (TPL) remains partially unobstructed and serves as the primary source of gas production in later stages. Sensitivity analysis reveals that both depressurization and particle control strategies exert significant effects on particle migration, which in turn affects gas production. Specifically, the increase in depressurization amplitude does not necessarily lead to improved gas production behavior beyond a certain threshold (approximately 6 MPa). This study illustrates the inherent trade-off between preventing particle production and increasing gas production, highlighting the need for a safe and efficient production scheme that strikes a balance between these two objectives. Based on the current study, it is preliminarily recommended to maintain a depressurization amplitude of approximately 6 MPa with a maximum depressurization rate of 3 MPa/d and ensure a fine particle filtration ratio of no less than 95%.

Influence of depressurization mode on natural gas hydrate production characteristics: One-dimensional experimental study

The latest research suggests that using depressurization methods and their enhancements could be the most effective way to produce natural gas hydrates efficiently in marine areas. Therefore, it's crucial to investigate the gas production behavior and the changes in temperature and pressure within natural gas hydrate reservoirs under different depressurization methods. In this paper, the self-developed one-dimensional natural gas hydrate production simulation test system was utilized to conduct one-dimensional depressurization production tests during two depressurization modes of significant depressurization and slow depressurization. The findings indicated that the minimum temperature increment-depressurization rate curves in significant depressurization tests exhibited irregular patterns, primarily due to the frequency and intensity of hydrate reformation, while the smaller the depressurization rate, the larger the minimum temperature increment and the smaller the maximum dissociation rate under slow depressurization. In addition, when maintaining a fixed hydrate saturation level, the peak gas production rate showed a decline with decreasing depressurization rate. However, the exact correlation between the peak gas production rate and the depressurization rate remained uncertain due to the unpredictable nature of the “armoring” effect.

Fine Particle Migration in a Gas Hydrate Sand: Single- and Two-Phase Fluid Using a Device for Observation at the Pore Scale

The production of natural gas hydrates will change the cementation strength, porosity, and effective stress in the stratum, which may lead to engineering and geological disasters. Sand production is a phenomenon where sand particles are carried out of the reservoir along with fluids during gas extraction, posing challenges to safe and sustainable production. This study explored the mechanism of fine particle migration in multiphase flow by a microscopic visualization test device. The device can inject a gas–liquid–solid phase at the same time and allow real-time observation. Experimental tests on fine particle migration of single- and two-phase fluid flow were carried out considering different conditions, i.e., fine particle concentration, fine particle size, fluid flow rate, and gas–liquid ratio. The results show that in single-phase fluid flow, the original gas will gradually dissolve in the liquid phase, and finally stay in the test device as bubbles, which can change the pore structures, resulting in the accumulation of fine particles at the gas–liquid interface. In two-phase fluid flow with mixed gas–water fluids, there are two flow modes of gas–liquid flow: mixed flow and separated flow. The interfacial tension at the gas–liquid interface can effectively migrate fine particles when the gas–liquid flows alternately and the sand production rate further increases as the gas–liquid ratio increases. In addition, changes in the concentration of fine particles, particle size, fluid flow rate, and the gas–liquid ratio will affect the migration of fine particles, leading to differences in the final sand production.