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Methane hydrates (MHs) have been considered a promising future energy source due to their vast resource volume and high energy density. Understanding the behavior of MH formation and dissociation at the pore-scale and the effect of MH distribution on the gas–liquid two phase flow is of critical importance for designing effective production strategies from natural gas hydrate (NGH) reservoirs. In this study, we devised a novel high-pressure microfluidic chip apparatus that is capable of direct observation of MH formation and dissociation behavior at the pore-scale. MH nucleation and growth behavior at 10.0 MPa and dissociation via thermal stimulation with gas bubble generation and evolution were examined. Our experimental results reveal that two different MH formation mechanisms co-exist in pores: (a) porous-type MH with a rough surface formed from CH4 gas bubbles at the gas–liquid interface and (b) crystal-type MH formed from dissolved CH4 gas. The growth and movement of crystal-type MH can trigger the sudden nucleation of porous-type MH. Spatially, MHs preferentially grow along the gas–liquid interface in pores. MH dissociation under thermal stimulation practically generates gas bubbles with diameters of 20.0–200.0 μm. Based on a custom-designed image analysis technique, three distinct stages of gas bubble evolution were identified during MH dissociation via thermal stimulation: (a) single gas bubble growth with an expanding water layer at an initial slow dissociation rate, (b) rapid generation of clusters of gas bubbles at a fast dissociation rate, and (c) gas bubble coalescence with uniform distribution in the pore space. The novel apparatus designed and the image analysis technique developed in this study allow us to directly capture the dynamic evolution of the gas–liquid interface during MH formation and dissociation at the pore-scale. The results provide direct first-hand visual evidence of the growth of MHs in pores and valuable insights into gas–liquid two-phase flow behavior during fluid production from NGHs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Reveals the inhibition effect of representative clays from South China Sea on CH 4 hydrate experimentally. • Thermodynamic inhibition is more significant for Na-montmorillonite than kaolinite and illite. • Develops a thermodynamic model accounting for the reduced water activity on clay surface. • Implications provided on the stability and fluid production from South China Sea clayey hydrate-bearing sediments. Clay minerals are widely distributed in hydrate-bearing sediments worldwide, especially in the clayey-silty sediments in the South China Sea. The debate on the influence of clay on the thermodynamics of natural gas hydrate (NGH) remains and impedes the fundamental understanding of hydrate-clay interactions and the design of effective production strategies for energy recovery. Specifically, the phase equilibria of MH in the presence of clay and how different types of water associated with clay particles affect NGH thermodynamics and stability are not fully elucidated and warrant investigation. In this study, the phase equilibria of methane hydrate (MH) were measured in the presence of typical clays from the South China Sea, i.e., montmorillonite, kaolinite, and illite. The shift of MH phase equilibria curve to a more stringent pressure was identified with Na-montmorillonite showing the most significant inhibition. The leftward shift (inhibition) of phase equilibrium temperature (T eq) is up to 3.9 K at a pressure of 10.40 MPa for Na-montmorillonite compared with pure water. Furthermore, we develop a thermodynamic model based on the classical Chen-Guo model first accounting for the change in water activity (a w) due to cation exchange of Na-montmorillonite. The developed model exhibits good predictability with the measured phase equilibira data. The inhibition on the phase equilibria of MH is linked to the reduction of a w due to the clay-bound water for clays with a swelling nature. The findings of this study offer direct evidence and explanations on how clay minerals thermodynamically inhibit CH 4 hydrate. It provides valuable insights into the resource estimation of clayey-silty NGH in nature and facilities the design of suitable production strategies targeting clay-rich hydrate-bearing sediments. [ABSTRACT FROM AUTHOR]

[Display omitted] • The phase equilibria of 72CH 4 /28CO 2 hydrate with DIOX no more than 5.56 mol% were measured. • Kinetics and morphology were acquired for CH 4 /CO 2 + DIOX hydrate varying 1.00–5.56 mol% • DIOX below 5.56 mol% induces CH 4 /CO 2 + DIOX hydrate formation with heterogeneous compositions. • CH 4 /CO 2 + DIOX hydrate growth for DIOX below 5.56 mol% yields DIOX concentrated in hydrate phase with a lean DIOX solution. • Co-storage of CH 4 /CO 2 in hydrate was suggested based on GC measurement and Raman analysis. Hydrate technology could provide a novel approach to deliver convenient CO 2 -rich natural gas. The identification of a suitable and environmental-friendly promoter for enhancing CO 2 /CH 4 hydrate kinetics is critical. In this study, we evaluated the thermodynamics, kinetics, and morphology of CH 4 /CO 2 + DIOX mixed hydrates in the presence of a thermodynamic promoter 1,3-dioxolane (DIOX) at and below its stoichiometric concentration (SC, C DIOX = 5.56 mol%). Due to the thermodynamic preference of DIOX enclathration in mixed hydrates formation, a decrease in C DIOX in the unconverted solution was observed post nucleation below SC and the hydrates formed exhibit significant heterogeneity in cage occupancy. As C DIOX in the unconverted solution decreases, the DIOX concentration in hydrate (R DIOX) and the thermodynamic stability of the newly formed hydrates decreases. Based on the step-wise heating design, the thermodynamic stability of the hydrates formed below SC with high- R DIOX is close to that of hydrates formed at SC, while hydrates with low -R DIOX undergo dissociation at their P-T trajectories above the measured phase equilibria. Raman measurement indicates that the lower- R DIOX hydrates possess a higher CO 2 composition. This study provides new insights into the tuning effect of DIOX on mixed hydrates formation and the basis for using low thermodynamic promoter concentration in hydrate-based technologies. [ABSTRACT FROM AUTHOR]

Oceanic hydrate-based CO 2 sequestration (HBCS) holds great promise for achieving carbon neutrality. However, the presence of inorganic salts, particularly MgCl 2 besides NaCl, in seawater can significantly impact the formation rate and the stability of CO 2 hydrate. In this study, experimental investigations were conducted to examine the thermodynamics, kinetics, and the resulting morphological features of CO 2 hydrate in the presence of MgCl 2 , covering mass fractions ranging from 0 to 5.0 wt%. The experimental findings reveal that MgCl 2 exerts a thermodynamic inhibitory effect with its inhibitory capacity increasing with higher mass fractions. The solubility model of CO 2 in MgCl 2 solution was modified, demonstrating a gradual weakening of CO 2 solubility as MgCl 2 mass fraction increases. Additionally, the growth kinetics of CO 2 hydrate decreases with increasing MgCl 2 mass fraction. Regarding CO 2 hydrate morphology, it was observed that at low mass fractions of MgCl 2 (<1.0 wt%), a dense hydrate film rapidly formed at the gas-liquid interface after CO 2 hydrate nucleation, hindering the further conversion of CO 2 into hydrate. Conversely, at higher mass fractions (>3.0 wt%), CO 2 hydrate exhibits a more porous and slurry-like structure, facilitating more gas-liquid contact and mass transfer, thereby enhancing the conversion of CO 2 into hydrate. During hydrate dissociation, a salt-removal effect associated with CO 2 hydrate formation was observed, leading to the accumulation of concentrated electrolyte (MgCl 2) and facilitating CO 2 hydrate dissociation. These findings have implications for understanding the CO 2 hydrate formation and dissociation in the presence of MgCl 2 relevant in the subsea environment and can contribute to the development of effective hydrate-based CO 2 sequestration strategies. [Display omitted] • Measured phase equilibria of CO 2 hydrate in the presence of 0–5.0 wt% MgCl 2. • Increasing MgCl 2 reduced CO 2 solubility in H 2 O by 22 % for 5.0 wt% MgCl 2. • Increasing MgCl 2 yielded sluggish CO 2 formation kinetics but enhancement observed at 1.0 wt% MgCl 2. • CO 2 hydrate morphology transits from surface film growth to slurry with increasing MgCl 2. • CO 2 hydrate dissociation triggered by concentrated MgCl 2 after CO 2 hydrate formation. [ABSTRACT FROM AUTHOR]

Implementing efficient measures to increase production is crucial for the commercial development of hydrate. Currently, several potential stimulation methods, including well type optimization, depressurization amendment and reservoir reconstruction, have been proposed. However, most studies tend to focus on a single measure, and there are few reports on the stimulation performance under multiple means. Therefore, our study concentrated on constructing production models for simplified layered hydrate reservoirs in the Nankai Trough using four different horizontal well systems. The production enhancement resulting from aggressive depressurization and various well patterns were explored. Additionally, the production performances under conditions of burden sealing and reservoir permeability enhancement were further evaluated respectively. Lastly, the common impact of employing both burden sealing and reservoir permeability enhancement was assessed. The prediction results show that: (i) Decreasing the bottomhole pressure effectively enhances gas recovery and can be combined with other techniques for better expectation; (ii) There exists an optimal burden sealing ratio that maximizes gas production while increasing the permeability enhancement ratio consistently stimulates gas recovery; (iii) Both single and dual well systems, assisted by aggressive depressurization, are expected to achieve commercialization whereas the triple well pattern is unfavorable for achieving a significant increase in yield. • Stimulation performance of multiple means for NGH was systematically evaluated. • The optimal sealing ratios under different situations were firstly obtained. • Effects of well type, production pressure and permeability increase were studied. • Some promising schemes reaching commercial standard were suggested. [ABSTRACT FROM AUTHOR]

Kinetic hydrate inhibitor laboratory testing before field application is one of the key priorities in the oil and gas industry. The common induction-time-based technique is often used to evaluate and screen for kinetic hydrate inhibitors (KHIs). However, the main challenge relates to the stochastic nature of hydrate nucleation observed in fresh systems, which often results in scattered data on hydrate formation with unacceptable uncertainties. A much more precise KHI evaluation method, called crystal growth inhibition (CGI), provides comprehensive insights into the inhibitory behavior of a kinetic hydrate inhibitor, including both hydrate formation and decomposition. Given that industry does not require this much information, it is not feasible to expend either much time or cash on this strategy. This study aims to provide a cost-effective technique that presents maximum data accuracy and precision with relatively little time and cost expenditure. Hence, the impact of water-hydrate memory on improving the accuracy and repeatability of the results of the induction-time-based technique (IT method) was examined. First, the concept of water-hydrate memory, which contains information about how it is created, was reviewed, and then, the factors influencing it were identified and experimentally investigated, like the heating rate of hydrate dissociation and the water-hydrate memory target temperature during heating. Finally, a procedure was developed based on the background information in the earlier sections to compare the consistency of the results, originating from the conjunction of water-hydrate memory with the IT technique. The results of replications at KHI evaluation target temperatures of 12.3-12.4°C and 11.5-11.7°C showed that more repeatable data were obtained by applying water-hydrate memory, and a more conclusive decision was made in evaluating KHI performance than with an IT method. It seems that combining the IT method with water-hydrate memory, introduced as the "HME method", can lead to more definitive evaluations of KHIs. This approach is expected to gain in popularity, even surpassing the accurate but complex and time-consuming CGI method. [ABSTRACT FROM AUTHOR]

[Display omitted] • The amount of dissociation of natural gas hydrates below the quadruple point is non-linearly correlated with the depressurization rate. • The existence of a metastable state could be beneficial for increasing the gas production rate of methane hydrates and for improving the recovery rate of reservoir temperatures. • When hydrates disengagement the metastable region, the gas production rate of hydrates is improved. • The time that the hydrate is in the metastable region needs to be kept at the right time interval. The depressurization method has been proven to be an energy-efficient and relatively feasible production technique for natural gas hydrates. However, during the depressurization process, especially when the hydrates reservoir approaches the quadruple point, it is inevitable to experience phenomena such as freezing and hydrates secondary formation. Therefore, most researchers think that the P-T near the quadruple point may not be suitable for the dissociation of hydrates and a series of depressurization control methods were adopted to avoid this. However, according to other researchers, the effect of depressurization near the quadruple point on the dissociation behavior of hydrates is not inhibitory, but even a promoting effect to a certain extent. In addition, A special region called metastable state appears with the decrease of pressure near the quadruple point, which makes the dissociation behavior of hydrate below the quadruple point different from that above it. Therefore, in this paper, to reveal more clearly the dissociation behavior of natural gas hydrates near the quadruple point and the influence of the existence of metastable state on the depressurization dissociation of hydrate, the dissociation experiment of natural gas hydrates in the porous medium peri -freezing point is investigated in different depressurization rates and amplitudes, peri -freezing point and pressure stabilization times quadruple point. Experimental studies have shown that the amount of dissociation of natural gas hydrates below the quadruple point is non-linearly correlated with the depressurization rate. This correlation is synergistically regulated by the depressurization magnitude per unit of time and the environmental heating efficiency. Compared with the dissociation of hydrates above the freezing point, the metastable region does not show an obvious inhibitory effect on the dissociation of hydrates, and it even effectively shortens the dissociation time of hydrates and improves the heat transfer efficiency of the reservoir. Under the dissociation pressure of 2.5 MPa (metastable state), the reservoir temperature rose from 0 °C to 1 °C in 2.9 h, 3.6 h less than that under 2.6 MPa (above the quadruple point), and 0.7 h more than that under 1.0 MPa (below the metastable state). In addition, the gas production rate of hydrates is improved when hydrates disengagement the metastable region, increasing by 65.12 % compared with those above the quadruple point. However, pressure stabilization times when hydrates are in a metastable state need to be kept within a certain interval, too long or short pressure stabilization time will inhibit the dissociation of hydrates, and reduce the promotion effect on the dissociation of hydrates. [ABSTRACT FROM AUTHOR]

The transition to a low-carbon economy requires the implementation of effective carbon capture and sequestration (CCS) strategies. One of the potential CCS strategies is to capture industrial CO 2 emissions and inject them into the oceanic sediments to be stored as CO 2 hydrates. However, the success of this technique depends on a few key factors such as the type of sediments where CO 2 is injected, the kinetics of CO 2 hydrate formation and dissociation, the accuracy of the models for prediction of formation kinetics, and the CO 2 hydrates morphology. So, in this first-gen work, a highly complex set of experiments was carried out to examine the CO 2 hydrate formation and dissociation processes by injecting CO 2 via injection tube into different size wet sediments, i.e., coarse (diameter: 0.5–1.5 mm), granules (diameter:1.5–3.0 mm) and dual-layered sand (coarse + granules), embedded inside high-pressure reactor as the artificial seabed. The experiments were carried out at 3.5 MPa at T = 1.5–2.0 °C with 500 ppm of the eco-friendly hydrate promotor (l -tryptophan). The images of morphological changes during hydrate formation/dissociation, the Scanning Electron Microscope analysis of the sediments, and the estimated water-to-hydrate conversations have been reported in this work. A novel mathematical four-parameter-based CO 2 hydrate kinetics model was also developed. A set of 32,843 experimental data points was used to train a supervised machine learning algorithm using two parameters with the other two taken from published literature. The water-to-hydrate conversion was estimated and follows the order of dual-layered sand [88.26 (±4.62) %] > coarse [77.77 (±5.72) %] > granules [65.36 (±2.3) %]. The proposed ML-based model predicted the water-to-hydrate conversion with an Average Absolute Relative Deviation [%AARD] of 4.23–13.29 %. This work serves as a step forward in developing a sustainable hydrate-based oceanic carbon storage technology. [Display omitted] • CO 2 hydrate kinetics and morphology in dual-layered sediments. • Effect of injecting CO 2 inside the sediments directly via vertical injection tube. • Developed and trained CO 2 hydrate formation kinetics model in sediments via a supervised machine learning (ML) algorithm. • ML model can predict CO 2 hydrate formation kinetics in sediments with accuracy in terms of AARD of 4.23–13.29 %. [ABSTRACT FROM AUTHOR]