Marine Hydrate Reservoirs Research Articles

Assessing hydraulic fracturing feasibility in marine hydrate reservoirs: Impact of closure pressure and hydrate decomposition on fracture conductivity

Hydraulic fracturing is a potentially promising technology for stimulating gas production and improving energy efficiency in low-permeability hydrate reservoirs. However, marine hydrate reservoirs, with weak cementation, unconsolidated nature, low mechanical strength, and the potential for decreased strength and fines migration resulting from hydrate decomposition, pose significant challenges for effective fracture propping and maintaining high conductivity. To assess hydraulic fracturing feasibility in marine hydrate reservoirs, we conducted experiments to investigate the impact of closure pressure, hydrate saturation, and hydrate decomposition mode on fracture conductivity, with a particular focus on understanding the propping mechanism and identifying potential sources of conductivity damage in artificial fractures. The results demonstrated that the conductivity of propped fractures within hydrate-bearing clayey silt sediments linearly decreased with increasing closure pressure, primarily due to proppant compaction and embedding. The conductivity disparity under different hydrate saturation levels decreasesd and became similar at 16 MPa closure pressure. The decomposition mode of hydrates had a significant impact on fracture conductivity damage, with higher degree of damage observed with increasing depressurization range (up to 90.3% at a 3.0 MPa range), whereas slow decomposition induced by heating only resulted in minor decreases in fracture conductivity (damage degree: 6.6%), primarily attributed to the migration of fine particles during gas-water production. Proppant embedding and fines migration were the main causes of fracture conductivity damage in hydrate reservoirs. Proppant embedding damage occurred at higher closure pressures or slow hydrate decomposition, while fines migration damage occurred with rapid hydrate decomposition. In proppant-filled propped fractures, fines migration posed a greater risk to fracture conductivity than proppant embedding, as the migration of fine particles can block the pore spaces within the proppant-filled fractures, leading to a significant reduction in conductivity. Therefore, it was recommended to induce hydrate decomposition by slowly reducing pressure during production to minimize fines migration damage to fracture conductivity. These research findings hold significant importance in the application of hydraulic fracturing during field tests conducted on hydrate reservoirs, as they assist in optimizing the propping mode of fractures and ensuring the effectiveness and prolonged success of fracturing stimulation.

Enhancing gas production and CO2 sequestration from marine hydrate reservoirs through optimized CO2 hydrate cap

The invasion of formation water during marine hydrate production gravely affects gas productivity. CO2 hydrate caps effectively inhibit water invasion while sequestering CO2, which is considered an excellent prospective application. However, forming field-scale artificial CO2 hydrate caps remains a global challenge. This study proposed an innovative method for forming an artificial CO2 hydrate cap. The fracture length, injection time, production pressure, and production well distance, which are all factors that affect sealing capacity and production enhancement, were optimized. Results indicated that the sealing capacity exhibited a trend of initially increasing and subsequently decreasing with fracture length. The injection time was positively correlated with the sealing capacity and was sufficient to seal both vertically and horizontally once it reached 4 years. In addition to sequestering CO2, the CO2 hydrate cap enhanced gas production by 146.56 % and reduced water production by 37.47 % compared to direct depressurization. CO2 invasion occurred when the well spacing was too close, while being distant was not favorable for pressure propagation and heat utilization by the CO2 hydrate cap. This method simultaneously achieves multiple goals for energy and environmental, providing essential implications for the future commercial application of marine hydrates.

Numerical Simulation on Production Trials by Using Depressurization for Typical Marine Hydrate Reservoirs: Well Type and Formation Dip

Numerical Simulation on Production Trials by Using Depressurization for Typical Marine Hydrate Reservoirs: Well Type and Formation Dip

Sand control during gas production from marine hydrate reservoirs by using microbial-induced carbonate precipitation technology: A feasibility study

Sand production is one of the bottleneck problems restricting the safe and efficient production of marine hydrate reservoirs. Current research on this issue mainly focuses on the active sand control methods such as screen mesh and gravel packing, however, passive methods are rarely reported. In this study, a novel sand control strategy which uses microbial-induced carbonate precipitation (MICP) technology to reinforce hydrate reservoirs around the well is proposed. Based on the geological data of hydrate reservoirs in Japan, a reservoir-scale microbial reaction-migration model is established. The feasibility of MICP reinforcement to prevent sand production is demonstrated by the combinations of indoor triaxial testing, productivity prediction and microscopic sand production prediction. The results show that the MICP technology can effectively enhance reservoir strength, and has a negligible impact on the gas recovery. Although the calcium carbonate generated will slightly decrease the porosity and permeability, the cumulative gas production does not significantly decrease and the water extraction is also prevented. Most importantly, the amount of sand production can sharply reduce because of the cementation of calcium carbonate among sand particles. Additionally, the effectiveness and applicability can be improved by selecting appropriate bacteria fixation methods and grouting pressure at different stages.

Influence of gravity on methane hydrate dissociation characteristics by depressurization in marine hydrate reservoirs

Since the thickness of hydrate-bearing sediments is usually tens to hundreds of meters, the influence of gravity on hydrate dissociation characteristics can no longer be ignored. In this study, experimental simulations were designed to investigate the influences of gravity on hydrate dissociation characteristics by depressurization. The experimental results showed that for sealed water-rich marine hydrate reservoir, gravity could cause more water to migrate downward. The free pore space in sediments was thus rapidly vacated, resulting in faster hydrate dissociation and gas production. The average gas production rate increased from 0.018 mol/min to 0.038 mol/min. In addition, the free pore space at the top of the sediments was vacated first. Hydrate dissociation there was thus faster. For unsealed water-rich marine hydrate reservoir, gravity could not only cause more and faster migration of water, but also more easily cause the formation of dominant channels for water migration, leading to an effective reduction in reservoir pressure. The cumulative water yield increased from 1380g to 8950g. Hydrate thus dissociated. Hydrate dissociated faster at positions close to the mining well where the pressure was lower. These findings can provide deep insights into the spatial evolution of multiphase flows during the exploitation of natural gas hydrates.

The pore-structure characteristics of foraminiferal shells and their relations with natural gas hydrate formation in the marine sediments

Many marine hydrate reservoirs exhibit significant foraminifera abundance. These foraminifera alter the original properties of the sediments, potentially affecting the formation and accumulation of hydrates. This present study analyzed the pore-structure characteristics of foraminifera-containing sediments from the South China Sea and their influences on the hydrate formation process by using microscopic imaging techniques. Scanning electron microscopy (SEM) and X-ray computed tomography (X-CT) reveal the unique dual-porosity characteristics of these sediments, characterized by matrix pores and foraminiferal pores. These two types of pores diverge significantly in their size distribution, morphological attributes, connectivity, and other aspects. Further experiments show the formation and occurrences of tetrahydrofuran (THF) hydrate under different sedimentary conditions and solution concentrations. Foraminiferal shells are found to promote the formation of hydrate by increasing the volume fraction of hydrate-favoring pores, which is more pronounced under conditions of low hydrate saturation. These findings obtained may contribute to global hydrate resource assessments and enhance understanding of hydrate reservoirs.

Low-temperature gel breaking fracturing fluid for marine hydrate reservoir fracturing and its effect on phase equilibrium of methane hydrates

Hydraulic fracturing has been proven useful for improving the gas production efficiency of low-permeability hydrate reservoirs. However, no specific fracturing fluid system exists for marine hydrate reservoirs. Applying conventional water-based fracturing fluids in low-temperature (<293.2 K) marine hydrate reservoirs can lead to difficulties in gel-breaking and hydrate instability caused by external additives. In this study, we developed two water-based fracturing fluid systems that exhibited good gel-breaking performance at low temperatures (278.2–288.2 K), one comprising ammonium persulfate (APS), ferrous sulfate (FeSO4), and citric acid (CA), the other APS and triethanolamine (TEA). In addition, we investigated the influence of crosslinking and gel-breaking reactions of the fracturing fluids on the methane hydrate phase equilibrium. The breaking time of the APS-FeSO4-CA system at 278.2K, 283.2K, and 288.2K was 30 min, 75 min, and 120 min respectively. On the other hand, the APS-TEA system accomplished the same task in 3 h, 6 h, and 10 h respectively. Furthermore, methane hydrate phase-equilibrium data showed that the fracturing fluid's three states, base fluid (BF), fracturing fluid gel (FFG), and gel-breaking liquid (GBL), all exerted an inhibitory effect on the phase equilibrium, with the hydrate suppression temperature (ΔT) ranging from 0.4 to 1.0 K. The differences in their inhibitory effect were primarily related to the micromorphology of the hydroxypropyl guar gum (HPG) molecules and the number of active hydroxyl (-OH) groups in the solution. Cryo-scanning electron microscopy (SEM) measurements revealed that the micromorphology of HPG in BF, FFG, and GBL transformed from regular ribbon-like structures to 3-D network structures and then to hole-containing, short ribbon-like structures. FFG formed a regular 3-D network structure after adding crosslinking agents to the BF. The ΔT values reversed from FFG < BF to FFG > BF when the HPG concentration increased from 0.3 wt% to 0.5 wt%, possibly due to the difference in the HPG molecules' water adsorption capacity before and after crosslinking. The broken HPG molecules with OH− groups in the solution were transformed into residues after gel-breaking, decreasing the GBL's inhibitory effect. Therefore, our results indicated that micromorphology variations could cause differences in the HPG molecules' water adsorption capacity, ultimately leading to ΔT differences. Based on our findings, the gel-breaking fracturing fluid systems we developed demonstrated potential for optimizing hydraulic fracturing in marine hydrate reservoirs.

Effect of sand production on physical properties and fracturing development of gas hydrate reservoir

Offshore gas hydrates are being focused on as an unconventional energy source. The susceptibility of marine hydrate reservoirs to sand emergence makes them unsuitable for long-term exploitation. This research delves into the particle-transport law in hydrate reservoirs, revealing that sand blockage hinders long-term exploitation, and proposes a reservoir modification strategy to establish high-permeability channels, thereby enhancing production. After 110 days of mining, the range of porosity and permeability below the initial value was approximately 8.5 m. The study further optimizes reservoir modification parameters under sand blockage, suggesting that a balance between fracture and reservoir blockage can be achieved with a fracture half-length of 20 m, leading to increased production. It also highlights that the slow decomposition of blocked hydrate and the accumulation of decomposed methane gas in the upper part of the reservoir can be leveraged to increase production by providing a high permeability channel for free gas, concluding that maintaining long-term high permeability is crucial for hydrate reservoir fractures.

Sustained production of gas hydrate through hybrid depressurization scheme with enhanced energy efficiency and mitigated ice blockage

Several trial productions have been conducted on marine natural gas hydrate reservoirs, demonstrating its enormous potential as an alternative energy source. Nonetheless, sustained and efficient production still remains challenging, with crucial issues on low gas production efficiency and blockages. The large amount of hydrate decomposition and resulting heat absorption can lead to prolonged subzero conditions close to the well; this can readily induce ice formation and blockage. This study aims to investigate the issue of ice blockages caused by insufficient energy supply during the recovery process. A systematic analysis of the energy consumption and ice blockage quantification at each stage of the traditional multi-stage direct depressurization and constant speed depressurization methods is conducted. A hybrid optimized scheme is proposed, which enhances energy utilization efficiency by appropriately utilizing reservoir sensible heat during the initial stage of production. Additionally, a multi-stage constant speed depressurization method is introduced in the later stages to mitigate production fluctuations and reduce blockage duration. Results showed a 69.5 % increase in energy efficiency and an average 67.1 % decline in gas blockage duration with comparable gas productivity. The proposed method could be a viable solution to mitigate challenges of icing blockages and fluctuating production during trial tests of marine hydrate reservoirs, achieving improved energy efficiency and sustainable production.

Unraveling the complex pressure propagation of two-time depressurization production in Shenhu hydrate reservoir: Insights for long-term exploitation

Depressurization serves as the primary method for exploiting marine hydrate reservoirs, and understanding the long-term pressure propagation efficiency and identifying the potential limiting factors are crucial for enhancing and optimizing this approach. In this study, we employed numerical modeling to simulate two-time depressurization exploitation in the Shenhu hydrate reservoir, aiming to predict the long-term gas production and pressure propagation efficiency under various depressurization schemes. Our simulation results indicate: 1) The pressure propagates rapidly near the wellhead and gradually decelerates with distance due to the impeding influence of the reservoir's low intrinsic permeability and groundwater seepage in the covering strata. 2) Pressure propagation in single-well systems exhibits substantial variation among sublayers, resulting in limited areas of hydrate dissociation. Long-term gas production depends on the hydrate dissociation. Consequently, the total 5-year gas production for single-vertical-well and single-horizontal-well systems are only 3.59 × 106 m3 and 5.04 × 106 m3, respectively. 3) Multi-horizontal-well systems can significantly extend pressure propagation, but the maximum increase in gas production is only 47.0%. This is due to the reservoir's low permeability (2.3 mD), which severely hampers hydrate dissociation pressure, limiting it to 8 MPa. Therefore, optimizing depressurization requires reservoir modifications to enhance reservoir permeability.

Sustainability of CO2 replacement processes in marine hydrate reservoirs: Factors causing changes on mechanical properties of Gas-Hydrate after CO2/CH4 exchange

The energy content of Gas Hydrated in form of natural gas is approximately twice with respect to the total energy still producible from all the known conventional energy sources. To reduce the risk induced by exploitation of hydrate reservoirs a good knowledge of the mechanical properties is crucial. Here we simulate CH4 replacement with CO2 and then we analyze the mechanical properties of replaced hydrates through porosity, Vp and Vs measurements. Replacement tests with CO2 have been performed by depressurization and thermal stimulation. Both procedures result in a final CH4/CO2 concentration of ∼30/70%. We then measured the petrophysical properties of pure CO2 and CH4/CO2 hydrates hosted in synthetic (Glass beads -GB) and natural sand (Iceland Sand-IS). Porosity and velocity within the same hosting sediments, show a clear inverse relation for the sub-angular and poorly sorted IS being this less evident for the spherical and highly sorted GB sands. CO2 replacement increases Vp and dynamic Young’s modulus for GB samples whilst IS are less influenced by the presence of CO2. These results demonstrate that reservoir sediments severely affect the mechanical properties of hydrates and thus their characteristics must be taken in account for safety well and reservoir management.

Enhanced gas production from low-permeability hydrate reservoirs based on embedded discrete fracture models: Influence of branch parameters

The low permeability of marine hydrate reservoirs makes it difficult for trail results to meet the demand for exploitation. Hydraulic fracturing and multi-branch wells are the primary methods for enhancing productivity. This study proposed a new production enhancement method of multi-branch wells combined with multi-stage fracturing (MWMF) based on the embedded discrete fracture model (EDFM). By simulation of the Shenhu area, the multi-physical field evolution of the MWMF production process was first revealed, followed by the systematic analysis of the MWMF production enhancement effect with branch parameters. Results indicated that the cumulative gas production of MWMF increased by 327% compared with only horizontal wells. The production of four-branch wells exhibited a 7% reduction compared to horizontal wells using the MWMF method, suggesting increased branch numbers inhibit production. Increasing branch length was positively correlated with enhanced production. Notably, the influence of branch length on gas production became more pronounced as the number of branches increased. The interface of the three-phase layer (TPL) and hydrate-bearing layer (HBL) was the most suitable location for branch well placement. This novel study provides valuable insights into new applications of the MWMF method in developing low-permeability hydrate reservoirs.

MRI insight on multiphase flow in hydrate-bearing sediment and development mechanism of hydrate seal

Gas and water migration through the hydrate-bearing sediment are characteristic features in marine gas hydrate reservoirs worldwide. However, there are few experimental investigations on the effect of water-gas flow on the gas hydrate reservoir. In this study, gas-water migration in gas hydrate stability zone (GHSZ) was investigated visually employing a high-resolution magnetic resonance imaging (MRI) apparatus, and the formation of hydrate seal was experimentally investigated. Results revealed that normal flow of gas-water at the low flow rate of 1–0.25 mL/min will induce the hydrate reformation. Conversely, higher gas-water flow rates (at 2–0.5 and 4–1 mL/min) need higher reservoir pressure to induce the hydrate reformation. In addition, the hydrate reformation during the gas-water flow process produced the hydrate seal, which can withstand an over 9.0 MPa overpressure. This high overpressure provides the development condition for the underlying gas and/or water reservoir. A composite MRI image of the whole hydrate seal was obtained through the MRI. The pore difference between hydrate zone and coexistence zone produces a capillary sealing effect for hydrate seal. The hydrate saturation of hydrate seal was more than 51.6%, and the water saturation was more than 19.3%. However, the hydrate seal can be broken through when the overpressure exceeded the capillary pressure of the hydrate seal, which induced the sudden drop of reservoir pressure. This study provides a scientific explanation for the existence of high-pressure underlying gas below the hydrate layer and is significant for the safe exploitation of these common typical marine hydrate reservoirs.

Gas recovery from marine hydrate reservoir: Experimental investigation on gas flow patterns considering pressure effect

Gas recovery from marine hydrate reservoir: Experimental investigation on gas flow patterns considering pressure effect

Analysis of Methane Hydrate Dissociation Experiment in a Pilot-Scale Hydrate Simulator by a Full Implicit Simulator of Hydrate

Behaviors of gas and water production from a gas hydrate reservoir are based on fully coupled Thermal–Hydraulic–Chemical (THC) processes in porous media. Studies using experiments and mathematical models could be improved in hydrate-bearing sediments in the laboratory. The experiment of depressurization-induced methane hydrate dissociation via a single vertical well was carried out in quartz sands in a pilot-scale hydrate simulator (PHS). The multi-stage hydrate dissociation process, including the free liquid release, well shutdown, depressurization, and constant pressure, was achieved by complex manual operation of the production well in the experiment. A full implicit simulator of hydrate (FISH), a four-component, three-phase mathematical model, was developed and employed to reproduce the hydrate dissociation experiment. The kinetics of the hydrate reaction, heat/mass transfer in porous media, and mass conservation in the pressure vessel were validated by the experimental results. The key finding of this study is the acquisition of a group of unified parameters in a suitable mathematical model that could clearly elucidate the hydrate dissociation process in all stages. The mean absolute percentage error (MAPE) values of the gas and aqueous production are, respectively, as low as 2.338 and 9.630%. The strong confidence of the accuracy of the numerical simulator has been built up after the analysis of the experiment and the numerical simulation in this study, which could be used to evaluate gas recovery from marine hydrate reservoirs.

Numerical analysis of hydrate exploitation performances of depressurization combined with thermal fluid injection in marine hydrate reservoir with impermeable boundary

The combination of depressurization and thermal fluid injection (D + Ti) is a very promising method to realize the efficient development of marine natural gas hydrate (NGH). However, the performance of D + Ti for hydrate production in the reservoir with impermeable boundaries has received little attention. In this work, the hydrate exploitation performances of D + Ti in NGH reservoir with impermeable overlying layer or impermeable underlying layer (Oimp + Up, Op + Uimp, Oimp + Uimp) were numerically investigated. The reservoir with permeable overlying layer and underlying layer (Op + Up) was used for comparison. Moreover, the effect of thermal fluid injection temperature (Ti) and injection pressure (Pi) on hydrate decomposition and gas production behaviors in the reservoir with Oimp + Uimp or Op + Up were studied in detail. The results indicate that the hydrate decomposition potential of the reservoir with Op + Up, Oimp + Up or Op + Uimp is similar, which is significantly lower than that of the reservoir with Oimp + Uimp. The presence of impermeable layer in reservoir can enhance gas production and reduce water production, especially when both overlying and underlying layers are all impermeable. The gas to water ratio and energy ratio of the reservoir with Oimp + Uimp are 5.0–7.4 times and 5.5–8.6 times that of the other three reservoirs, respectively. On the other hand, the improvement of Pi increase on hydrate decomposition and gas production is far superior to the improvement caused by Ti rise. Moreover, the increase of Pi and Ti has a more significant improvement on gas production in the reservoir with Oimp + Uimp than that in the reservoir with Op + Up. The maximum improvement of gas production in the reservoir with Oimp + Uimp is 69.6%. These results can provide theoretical support for the exploitation of marine NGH by D + Ti.

Dynamic Dissociation Behaviors of sII Hydrates in Liquid Water by Heating: A Molecular Dynamics Simulation Approach.

An understanding of the dynamic behavior of subtle hydrate dissociation in the liquid water phase is fundamental for gas production from marine hydrate reservoirs. Molecular dynamics simulations are performed in this study to investigate the dissociation kinetics of pure propane and binary propane + methane sII hydrates in a liquid water environment. The results show that faster hydrate dissociation rates are observed at higher initial temperatures. The hydrate phase dissociates from the cluster surface to the inside in a layer-by-layer manner under the simulation temperature conditions, which is similar to the behavior of sI hydrates and is independent of the hydrate crystal type. Compared to the binary sII hydrate, the pure sII hydrate dissociates more easily under the same initial temperature conditions, which can be attributed to the stabilizing effect of guest molecules in the hydrate cages. The empty cages collapse in one step, in contrast to the two-step pathway induced by the guest-host interaction. In addition, a hydrocarbon phase forms in the binary hydrate dissociation system instead of nanobubbles. These results can provide molecular-level insights into the dynamic mechanism of hydrate dissociation and theoretical guidance for gas recovery by thermal injection from marine hydrate reservoirs.

A developed transient gas–liquid–solid flow model with hydrate phase transition for solid fluidization exploitation of marine natural gas hydrate reservoirs

The multiphase flow characteristic is one of the most concerning problems during solid fluidization exploitation of marine natural gas hydrate reservoirs. In this research, a new transient gas–liquid–solid multiphase flow model with hydrate phase transition was developed. Meanwhile, this model considered the coupling relationship among convective heat transfer, hydrate dynamic decomposition, and multiphase flow. The model can simulate the change of flow pattern from solid−liquid to gas–liquid–solid flow, and describe the distribution character of volume fraction of phase, wellbore temperature and pressure, and hydrate decomposition rate during transportation. The simulation results indicate that the hydrate decomposition region in the wellbore gradually expands, but the hydrate decomposition rate gradually decreases during the solid fluidization exploitation of hydrate. When mining time lasts for 4 h, and the bottom hole pressure decreases by about 0.4 MPa. Increasing NaCl concentration in seawater helps expand hydrate decomposition regions and improves the wellbore hydrate decomposition rate. When the NaCl mass fraction in seawater reaches 15%, it will raise the hydrate decomposition regions to the whole wellbore. In addition, the higher the wellhead backpressure, the lower the decomposition area and decomposition rate of hydrate in the wellbore. When wellhead backpressure reaches 2 MPa, the volume fraction of gas near the wellhead will reduce to about 12%. This work is expected to provide a theoretical basis for the development of marine hydrate reservoirs.

Numerical Study of the Effect of Thermal Stimulation on Gas Production from Marine Hydrate Reservoirs

The temperature of hydrate reservoirs will decrease in a gas production process because gas hydrate dissociation is endothermic, leading to a decrease in the gas production rate as sensible heat is exhausted. Heat supply should be an effective way to alleviate the effect of temperature decrease on the production rate, by thermal stimulation, such as hot water injection into a hydrate reservoir. However, the feasibility of thermal stimulation combined with depressurization is controversial. Hence, to investigate the effect of this combined strategy, gas production performance is studied by numerical simulation based on a 2D Cartesian hydrate reservoir model with a two parallel horizontal well configuration. One of the two horizontal wells, at the top of the hydrate-bearing layers, is for gas production, and the other at the bottom of the hydrate-bearing layers is for hot water injection. The influence of the injection rate, temperature of hot water, and permeability of hydrate-bearing layers on gas production is investigated. When only the hot water injection is applied, the released gas, which originates from the dissociation of hydrates around the injection well, tends to be trapped and forms hydrates again. When the production strategy of hot water injection combined with depressurization is engaged, the hydrate dissociation shows a depressurization-dominant feature, and the production is at a relatively high rate, but it seems the hot water injection is still a negative factor in gas production. Because the hot water injection will replenish reservoir pressure and increase the water production rate, the feasibility of depressurization will be compromised. Sensitivity analyses reveal that the increased permeability of hydrate-bearing layers can enhance gas production almost proportionally. It is also found that increasing the temperature of the injected hot water only has a very limited effect on gas production.

Long-term numerical simulation of a joint production of gas hydrate and underlying shallow gas through dual horizontal wells in the South China Sea

Recent field tests to recover natural gas from marine gas hydrate reservoirs in Japan and China have exhibited worldwide attention to this strategic energy form; significant challenges still remain in improving the accumulative gas yield for a better economic efficiency. According to the geological survey in marine hydrate reservoirs, there exists a concomitant free gas layer underlying the gas hydrate reservoir. Consequently, here we propose a new scheme to jointly produce the gases from hydrate layer and its underlying shallow gas layer. It was found that a dual horizontal well deployment respectively in the corresponding two layers could contribute to a 4.1 times cumulative gas yield comparing that of a single vertical well scenario. This implies a remarkable enhancement of gas productivity via making full use of the gases in the shallow gas layer. Notably, a potential risk of interlayer failure could occur upon the great interbedded pressure difference (maximum 10.7 MPa) arising from the varying behavior of pressure propagation in the layers. This can be effectively alleviated by separately controlling the depressurization scheme in different layers; a mild step-wise depressurization was suggested in the more permeable shallow gas layer while the hydrate layer with a lower permeability should experience a sharper pressure drop. This was found beneficial reducing the interbedded pressure difference by about 70% without intervening the gas production. An energy return on investment (EROI) analysis showed a promising positive energy harvest of our optimized layer-dependent pressure scheme. It could be therefore a potential method to be applied in the field tests in the South China Sea to improve the economic efficiency without disturbing the reservoir stability.