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Clayey-silty hydrate-bearing sediments are widely distributed worldwide, presenting significant potential for natural gas hydrate (NGH) extraction. However, fundamental thermodynamic and kinetic properties of CH 4 hydrate (MH) within these sediments remain subjects of debate, necessitating further investigation. This study aimed to investigate the kinetics of MH formation and dissociation in clayey-silty sediments suspensions, alongside morphological examinations. Results demonstrated that SH-W02-B sediments (SS) effectively shorted the induction time (t ind), with even a minute mass fraction of 0.1 wt% SS reducing t ind by 14 min compared to pure water (41 min). Lower mass fractions (<5.0 wt%) exhibited slight promotion of MH growth kinetics, whereas higher mass fractions (>10.0 wt%) markedly promoted MH growth kinetics, peaking at 144 v/v (20.0 wt%), correspondingly, and facilitating the conversion of H 2 O to MH to ∼70%. Morphological observation unveiled the formation of a thin clay layer at the gas-liquid interface, retarding MH formation and inducing the development of an anti-seepage hydrate layer, leading to the initial slow growth phase. Subsequent upward water migration facilitated the transport of fine clay particles, while the progressive encasement of clay particles by hydrate disrupted the fine clay layer, triggering the rapid hydrate formation phase and ensuing stratification between fine and coarse sediments particles. During hydrate dissociation, a heterogeneous distribution of fine and coarse sediments particles manifested in the reservoir. Moreover, the results carrying profound implications on understanding NGH occurrence pattern and developing recovery strategies from clayey-silty hydrate-bearing sediments. [Display omitted] • Clayey-silty core sediments were recovered near the China second field production test well. • X-ray diffraction analysis reveals 31 wt% clay with primary illite/smectite mixed layer in core sediments. • The presence of core sediments expedite CH 4 hydrate nucleation and enhance the growth kinetics. • Transport of fine clay particles leads to two-stage growth with stratified hydrate-sediments layers. [ABSTRACT FROM AUTHOR]

Methane hydrates are considered as the future energy due to its vast resource volume and high energy density. The fluid production and thermal response of two types hydrate-bearing sediments (i.e. , excess-gas and excess-water) under controlled depressurization are still unclear and warrant investigation. In this study, we devised two different hydrate-bearing sediments (HBS) synthesis methods and synthesized excess-water (S A = ∼27.2%) and excess-gas (S G = ∼26.5%) HBS with S H of 72.0%. The hydrate dissociation kinetics and fluid production behavior were examined under three bottom-hole pressures, i.e. , 3.0, 5.0, and 7.0 MPa. Gas production from the excess-gas HBS follows two-stage profile, while continuous gas production was observed after S G reaches 6.0% in the excess-water HBS. Water production from excess-gas HBS was significantly delayed compared with excess-water HBS and only started when S A reached above 22.5%. A logarithmic water production profile was observed in all cases. Rapid temperature drop due to hydrate dissociation is significantly delayed in excess-gas cases. Heat transfer from surroundings is relatively slow due to its low composite thermal conductivity. The findings on the contrast fluid production behavior between excess-gas and excess-water cases shed light on optimizing production strategies for future field production trials from these two different types reservoirs. • Excess-water (EW) and excess-gas (EG) HBS synthesized using novel method. • Controlled depressurization under BHP at 3.0 MPa, 5.0 MPa and 7.0 MPa. • Comparison of fluid production and heat transfer behavior of EW and EG HBS. • EG HBS is favored over EW HBS for production due to high R G , low R W and WGR. [ABSTRACT FROM AUTHOR]

A huge amount of natural gas hydrates remains untapped in permafrost and continental margin. While several short term field production tests have been carried out, the underlying challenges during hydrate dissociation in porous media, such as the interdependent production behavior of gas and water, is still not well understood. In this work, we employed depressurization technique to recover natural gas from a water saturated hydrate bearing sediment (40% S H , 50% S A and 10% S G ) at 281.5 K surrounding temperature. During depressurization, the bottom hole pressure (BHP) was maintained at constant pressures of 5.0, 4.0, 3.0 and 2.1 MPa respectively to evaluate its effect on gas and water production. As expected, a higher BHP (corresponding to a lower dissociation driving force) resulted in a slower gas and water production. At a BHP of 2.1 MPa, thermal buffering was observed below ice point (272.7 K), accompanied by enhanced gas production. By lowering BHP from 5.0 MPa to 2.1 MPa, the percentage methane produced increased from 45.5% to 83.0%; whereas the cumulative water production decreased from 217 mL to 157 mL. The difference in gas and water production was attributed to the preferential production of aqueous phase at higher BHPs (5.0 and 4.0 MPa) nearing the end of hydrate dissociation whereby less hydrates were present. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrate-bearing sediments under excess-water condition were synthesized systematically. • Fluid production induced by depressurization below the quadruple point were examined. • Effect of pressure drawdown rate and hydrate phase saturation were investigated. • Both gas production rate and recovery ratio increased under ultra-deep depressurization. Driven by the large resource volume, extraction of CH 4 from hydrate reservoirs has received worldwide attention. Depressurization is a feasible method and has been widely used in past field tests. However, fluid production behavior from water-saturated hydrate-bearing sediments below the quadruple point of CH 4 + H 2 O has not been well understood and warrants investigation. In this study, we designed a series of experiments using ultra-deep depressurization with bottom hole pressure (BHP) of 1.0 MPa, 1.5 MPa and 2.1 MPa in water-saturated hydrate-bearing sediments to investigate the fluid production behavior, particularly in the region with potential ice or secondary hydrate formation. The effect of hydrate saturations and pressure drawdown rates on fluid production below the quadruple point were systematically examined. Our experimental results show that ice formation was likely based on the temperature response, however, the self-preservation effect by ice formation prevents a further drop in temperature to its equilibrium temperature at the corresponding BHP. The gas and water recovery ratios increased by 6.5% and 49.1%, respectively for BHP below the quadruple point. No significant benefit of water trapping due to ice formation was observed. Faster pressure drawdown rate results in a more substantial temperature drop, which could trigger ice formation. Still, interestingly gas production was continuous and not affected in the phase diagram of ice + CH 4. Higher hydrate phase saturation improves gas production rate and final gas recovery ratio, yielding a lower water-gas ratio for more economic-viable CH 4 recovery. Our results indicate that ultra-deep depressurization could be one technical feasible option for increasing both gas production rate and final recovery ratio. The findings provide valuable information for designing an ultra-deep depressurization scheme based on S H of interest in future production tests. [ABSTRACT FROM AUTHOR]

• Hydrate dissociation induced by multi-stage depressurization (MDP) were conducted. • Fluid production behaviour and heat transfer characteristics were analysed. • MDP improves the energy efficiency and increases the minimum temperature approached. Natural gas hydrates (NGH) are globally abundant, high in energy density, and are a clean energy source with great potential. The efficient and safe production of CH 4 from NGH has attracted widespread attention in the scientific and industrial fields. Depressurization (DP) has been assessed to be a technically feasible method of producing CH 4 from hydrate-bearing sediments (HBS). However, technical challenges (e.g. sand production and flow assurance issues) remain when using direct fast single-stage DP. Multi-stage depressurization (MDP) has been proposed to be an effective solution but less understood. In this study, we designed a series of experiments employing MDP processes (2-stage, 3-stage, 6-stage, and up to 10-stage DP) to dissociate water-saturated HBS with high S H > 56 vol%. We examined the fluid production behaviour, the evolution of temperature, and evaluated the energy efficiency ratio of the MDP processes with a comparison with single-stage DP process. Increasing the number of DP stages with finer steps controlled the rate of hydrate dissociation and significantly reduced the cumulative fluid produced in each stage. However, the overall recovery was not altered and largely depends on the initial and final thermodynamic states. Fluid production was driven by different mechanisms: gas production driven by heat transfer yielded the highest recovery at final P = 3.0 MPa, whereas water production driven by pressure drawdown resulted in the highest recovery when P first dropped below P eq = 4.6 MPa. Moreover, increasing the number of DP stages leads to an increase in the minimum temperature by 2.2 °C and an overall energy efficiency ratio improvement by 36.6%. The findings of this study could be significant in tuning the water–gas production performance and in designing the optimal MDP strategies in future field production tests. [ABSTRACT FROM AUTHOR]

• Hydrate-bearing sediments with saturation above 0.40 was synthesized consistently. • Three drawdown rates were employed to induce hydrate dissociation and fluid production. • Two-stage water production and continuous gas production profiles were observed. • Cumulative water production rate increases linearly with drawdown rate. • Gas recovery of 0.83 and water recovery of 0.26 were attained at BHP = 3.0 MPa. Methane hydrates have been considered as the future source of energy due to its vast resource volume and high energy density. Depressurization has been demonstrated to be a technical feasible method in producing gas from hydrate reservoirs in the past field production tests. However, major technical challenges, i.e. low gas production rate, excessive water production, and uncontrollable sand production remain. Such undesirable production behaviour is directly linked to one key process variable, the rate of pressure drawdown. It controls the rate of hydrate dissociation and governs the multiphase fluid flow in sandy media. However, precise control of bottom hole pressure in large-scale production tests is difficult and it remains unclear how different drawdown rates affect the fluid production behaviour. Herein, we performed a series of hydrate formation experiments synthesizing aqueous-rich methane hydrate-bearing sediments with hydrate saturation above 40% and subsequently a series of depressurization experiments employing different controlled drawdown rates to dissociate the hydrate-bearing samples. Cumulative production of gas and water were monitored during the depressurization. The goals of the experiments were to examine the fluid production rate, the recovery ratio, and the temperature response in the hydrate-bearing system under different drawdown strategies. Cumulative water production rate increases linearly with drawdown rate, while cumulative gas production rate is directly linked to the hydrate dissociation rate, which is controlled by heat transfer at the early stage and dissociation kinetics toward the final stage. Water gas ratio can be regulated within a threshold by employing a slow drawdown rate. In addition, the final recovery of fluid is not controlled by the drawdown rates but is related to the final stable conditions of pressure and temperature. Our findings expand the understanding of fluid production behaviour from hydrate-bearing sediments and provide guidance in optimizing production strategies and in assessing the fluid production potential for future experiments and field production tests from hydrate reservoirs. [ABSTRACT FROM AUTHOR]

• Five different wellbore designs are employed in production from hydrate-bearing sediments. • Continuous gas production and step-wise water production are observed at all pressures. • Slotted-liner design offers better control of water production than bore-hole design. • Partial perforation and perforation location affect water production more than gas production. Methane hydrates have been considered as the future clean energy resource. To recover energy from hydrate-bearing sediments safely and effectively requires a fundamental understanding of the dynamic behaviour of hydrates in sandy media. Past field production tests have proved the technical feasibility of gas production from hydrate reservoirs. However, technical challenges (e.g. sand production and excessive water production) remain in realizing long-term economic-viable production. In this study, we investigate the effect of various wellbore designs (specifically the shape and the location of perforations) on the fluid production behaviour in aqueous-rich hydrate-bearing sediments. Slotted-liner design was first-time incorporated in depressurization under different bottom-hole pressures (2.5 MPa, 3.0 MPa and 4.0 MPa). Continuous gas production and step-wise water production during the early stage of depressurization were observed at all pressures because of the change of fluid flow path from reservoir to wellbore. Compared with traditional borehole design, slotted-liner design showed advantage in controlling water production under low BHP (2.5 MPa). Using wellbores with different perforation locations, it was observed that perforation location does not pose a significant impact on the behaviour of gas production; whereas, middle perforation away from the aqueous-rich and hydrate-rich region yielded the least water production. The experimental results from this study demonstrated the possibility of employing slotted-liner well and varying the perforation location in reducing water production from methane hydrate-bearing sediments. Our findings can pave way for the testing of novel wellbore designs to further enhance gas recovery and reduce water production from hydrate reservoirs. [ABSTRACT FROM AUTHOR]

Highlights • Vertical wellbore was incorporated within water saturated hydrate sediment. • Hydrate accumulation around wellbore reduced gas production and dissociation rate. • Cumulative gas production enhanced by up to 8% through vertical wellbore. • Simultaneously, water production was reduced by up to 43% at 4.5 MPa. Abstract Natural gas hydrates, an abundant source of natural gas in nature, have the potential of supplying energy for the future. However, due to the complexity of physical processes that occur simultaneously during the dissociation of hydrates within porous media, further understanding on the dynamic behavior of hydrates dissociation (e.g. heat transfer associated with phase transition, permeability changes, and simultaneous gas/water production) is required to elucidate the process of gas recovery from hydrates. In this work, we investigate the effect of a single vertical wellbore incorporation on the production behavior from hydrates formed in sandy sediments (0.1–0.5 mm). Depressurization approach (3.5, 4.0 and 4.5 MPa) was applied to dissociate the hydrate-bearing sand at a constant surrounding temperature of 281.5 K. Through the incorporation of a vertical wellbore, the gas production rate decreased with increasing bottom hole pressure, and continuous production of gas was observed at 4.5 MPa even after 10 h of dissociation. For water production, a majority of water was produced within the first 2 h, beyond which only a small amount of water was recovered. It was also observed that the incorporation of a vertical wellbore in the current design and apparatus impeded hydrate dissociation, which was attributed to a stronger flow resistance for fluids to exit through the vertical wellbore. Cumulatively, the incorporation of vertical wellbore enhanced gas production by up to 8% and reduced water production by up to 42%. In the future, innovative production schemes, such as heated wellbore and hydraulic fracturing can be applied to further optimize gas production from hydrate-bearing sediments. [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]

Natural gas hydrate is a potentially vast energy resource for the future. However, the fundamental behavior of hydrate dissociation during energy recovery is not fully understood due to the complex interplay of phase change and multiphase flow within porous media. In this study, the effect of horizontal wellbore incorporation on the simultaneous gas and water production during the dissociation of methane hydrates in sandy sediment (0.1–0.5 mm) was investigated. A horizontal perforated wellbore was incorporated within water saturated hydrate bearing sediments of 40% hydrate, 55% aqueous and 5% gaseous phase saturation to mimic marine hydrate sediments, and the gas and water production behavior from horizontal wellbore (HW) was compared with the base case (without well) at 3 bottom hole pressures (BHPs) of 3.5, 4.0, and 4.5 MPa under a constant surrounding temperature of 281.5 K. The evolution of temperature across 12 measured locations within the 1 L sediment demonstrated the significant effect of horizontal wellbore incorporation on heat transfer within sediment. Through the incorporation of horizontal wellbore, a continuous production of gas was observed for an extended period of time as compared to the base cases without a well. The resulting cumulative gas production was enhanced by 5.5–10% at various BHPs; whereas the cumulative water production was significantly reduced by 30.8–36.9% at different BHPs from the base cases. By estimating the percentage of hydrate dissociated, it was found that the incorporation of horizontal wellbore in the current apparatus caused a slower hydrate dissociation, as reflected by a longer time to dissociate 50% ( t 50, h ) and 90% ( t 90, h ) of the hydrates. This study demonstrates the potential of horizontal wellbore incorporation to simultaneously enhance gas production and reduce water production, unveiling a future direction in optimizing gas recovery from hydrate reservoirs through innovative wellbore design, configurations and well placement strategies. [ABSTRACT FROM AUTHOR]