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The prediction of gas productivity and reservoir stability of natural gas hydrate (NGH) reservoirs plays a vital role in the exploitation of NGH. In this study, we developed a THMC (thermal-hydrodynamic-mechanical-chemical) numerical model for the simulation of gas production behavior and the reservoir response. The model can describe the phase change, multiphase flow in porous media, heat transfer, and deformation behavior during the exploitation of NGH reservoirs. Two different production scenarios were employed for the simulation: depressurization and depressurization coupled with CO2 exchange. The simulation results suggested that the injection of CO2 promotes the dissociation of NGH between the injection well and the production well compared with depressurization only. The cumulative production of gas and water increased by 27.88% and 2.90%, respectively, based on 2000 days of production simulation. In addition, the subsidence of the NGH reservoir was lower in the CO2 exchange case compared with the single depressurization case for the same amount of cumulative gas production. The simulation results suggested that CO2 exchange in NGH reservoirs alleviates the issue of reservoir subsidence during production and maintains good reservoir stability. The results of this study can be used to provide guidance on field production from marine NGH reservoirs. [ABSTRACT FROM AUTHOR]

Highlights • Numerical analysis of MH dissociation and fluids production by depressurization. • A function describing the change in MH surface area is incorporated in the T+H code. • Flow, thermal, kinetic parameters are optimized using a history-matching technique. • Simulation validated by experiment elucidates the issue of spatial heterogeneity. Abstract Methane Hydrates (MHs) are a promising energy source abundantly available in nature. Understanding the complex processes of MH formation and dissociation is critical for the development of safe and efficient technologies for energy recovery. Many laboratory and numerical studies have investigated these processes using synthesized MH-bearing sediments. A near-universal issue encountered in these studies is the spatial heterogeneous hydrate distribution in the testing apparatus. In the absence of direct observations (e.g. using X-ray computed tomography) coupled with real time production data, the common assumption made in almost all numerical studies is a homogeneous distribution of the various phases. In an earlier study (Yin et al., 2018) that involved the numerical description of a set of experiments on MH-formation in sandy medium using the excess water method, we showed that spatially heterogeneous phase distribution is inevitable and significant. In the present study, we use as a starting point the results and observations at the end of the MH formation and seek to numerically reproduce the laboratory experiments of depressurization-induced dissociation of the spatially-heterogeneous MH distribution. This numerical study faithfully reproduces the geometry of the laboratory apparatus, the initial and boundary conditions of the system, and the parameters of the dissociation stimulus, capturing accurately all stages of the experimental process. Using inverse modelling (history-matching) that minimized deviations between the experimental observations and numerical predictions, we determined the values of all the important flow, thermal, and kinetic parameters that control the system behaviour, which yielded simulation results that were in excellent agreement with the measurements of key monitored variables, i.e. pressure, temperature, cumulative production of gas and water over time. We determined that at the onset of depressurization (when the pressure drop – the driving force of dissociation – is at its maximum), the rate of MH dissociation approaches that of an equilibrium reaction and is limited by the heat transfer from the system surroundings. As the effect of depressurization declines over time, the dissociation reaction becomes kinetically limited despite significant heat inflows from the boundaries, which lead to localized temperature increases in the reactor. [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] • Depressurization followed CO 2 -N 2 injection for CH 4 recovery and CO 2 sequestration. • Effect of MH dissociation ratio on subsequently mixed hydrate formation. • CH 4 hydrate re-formation and dissociation occurred depending on CH 4 hydrate left. • CO 2 hydrate formation depended on the CH 4 left in both gas phase and hydrate phase. • Increasing CH 4 production decreased the CO 2 sequestration and hydrate restoration. Natural gas hydrates (NGHs) buried below the seafloor are globally abundant, energy-dense, and essential to the world's future energy mix. The combination of depressurization and CO 2 -N 2 injection processes for NGHs recovery has been identified as an environmental method to harvest energy, CO 2 sequestration, and to maintain the mechanical stability of sediment. In this regime, partial CH 4 was produced by depressurization followed by CO 2 -N 2 injection to form CH 4 -CO 2 -N 2 mixed hydrate (Mix-H). In this study, a series experiments were designed to examine the key factors in the depressurization process (i.e. bottom hole pressure BHP and CH 4 production ratio) on the subsequent processes of Mix-H formation, CO 2 sequestration and hydrate restoration. It was observed that Mix-H formation occurred in all cases, but the final amount of Mix-H were all lower than the initial amount of MH before depressurization. Particularly, the formation of CO 2 /N 2 hydrate in Mix-H is positively correlated with the amount of remaining CH 4 in the combined phases of gas and hydrate after depressurization. The formation of CH 4 hydrate in Mix-H during the reformation process occurred in five cases when MH dissociation ratio (D MH) reaches above 45.1 % during depressurization. MH was only observed to dissociate continuously after CO 2 /N 2 injection when D MH is low at 30.8 %, which indicates possible "CH 4 -CO 2 replacement reaction". Overall, increasing CH 4 production at the same BHP decreases the CO 2 sequestration and hydrate restoration. We first verified the feasibility of the combination method for synergistic CH 4 recovery and CO 2 sequestration in marine conditions. The experimental results provide possible guidance on the optimal design of the coupled processes and shed light on the relationship between CH 4 recovery and CO 2 sequestration that need to be wholistically balanced. [ABSTRACT FROM AUTHOR]

A large quantity of natural gas is sequestered in clathrate hydrate form beneath permafrost and continental margin. However, the simultaneous production behavior of gas and water from clathrate hydrate resources in porous media is not well understood. In this study, the gas and water production behavior from hydrate bearing sediment (HBS) using depressurization method is presented. The HBSs of 40% hydrate saturation, 50% water saturation and 10% gas saturation was produced in an excess water environment, and each formation cycle takes around 14 days, achieving a final methane conversion of ∼81%. Upon the formation of quantitatively similar hydrates, the sediments were brought to 281.5 K and exposed to a production pressure of 4.5 MPa. It was found that the localized temperature in several measurement points within the sediment dropped to the equilibrium temperature at the production pressure. By analyzing both gas and water production behavior simultaneously, it was found that hydrate dissociation occurred continuously even after the measured temperatures were stabilized outside the hydrate stability zone. The production profiles were also compared with thermal stimulation results to highlight the distinctive difference in production profile between the two approaches. [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]

[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]

• Depressurization followed by CO 2 -N 2 injection for CH 4 recovery and CO 2 sequestration. • A pilot-scale investigation on the combined processes at South China Sea NGH condition. • Spatio-temporal evolution of mixed CH 4 /CO 2 /N 2 hydrate formation was investigated. • Hydrate saturation reverts back to 50%-70% of its original after mixed hydrate formation. • Employment of horizontal well enhanced the storage of CO 2 after CO 2 /N 2 injection. To ensure sustainable energy supply while mitigating climate change, combining CH 4 recovery with CO 2 storage in marine natural gas hydrate (NGH) reservoirs is a promising carbon–neutral technology. In this study, a pilot-scale reactor (V = 22 L) was used to study the combined processes of CH 4 recovery from the methane hydrate-bearing sediment by using depressurization followed by in-situ CO 2 /N 2 injection to form mixed CH 4 /CO 2 /N 2 hydrate (Mix-H) at marine NGH reservoir conditions (T = 283.8 K and P = 11.0 MPa). The bottom hole pressure (BHP = 3.2 MPa, 5.3 MPa, and 6.8 MPa) during depressurization and the setting of horizontal wellbore (HW), were systematically investigated to optimize the synergistic CH 4 recovery and CO 2 storage. The CH 4 recovery ratio (55.4–80.6%) and the CH 4 production rate (5.15–134.98 mol/d) increased as BHP decreased. After CO 2 /N 2 injection, Mix-H formed first in the CH 4 -rich region in the upper section of the reactor and propagated downward to the lower. The Mix-H formation kinetics were significantly enhanced with an increase in BHP, leading to a decrease in CO 2 composition in the Mix-H. The use of a horizontal wellbore (HW) facilitates a more even distribution of the CO 2 /N 2 within sediments, resulting in a 13.5% increase in CO 2 composition of Mix-H compared with no HW setting. The CO 2 conversion ratio and hydrate restoration ratio reaches 52.8% and 69.4% at BHP = 5.3 MPa with HW, respectively. The technical feasibility of CO 2 storage in exploited marine NGH reservoirs was first proved at pilot-scale. The results provided insights into the synergistic method of NGH exploitation and CO 2 storage at the marine NGH 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 five-spot well is a promising technique for enhancing the inherent low gas production rate from silty natural gas hydrate (NGH) reservoirs with low permeability in the South China Sea (SCS). However, the production characteristics and the responses to key geological and engineering factors are still unclear and warrants investigation. Herein, we first establish a three-dimensional reservoir model based on the SH2 NGH deposits at SCS with the implementation of a five-spot well. We systematically examine the gas production enhancement effect and the unexpected well-to-well interaction induced by long-term depressurization. A sensitivity analysis on the effects of bottomhole pressure (P btm), permeability (k) and hydrate saturation (S H) is conducted to elucidate the key factors controlling the gas production enhancement. Our results reveal that the five-spot well significantly improves the gas productivity with cumulative gas production increased to 4.12 times that of a single well over 10 years. The well interference on gas production is not significant in the early stage but becomes more significant as depressurization sustains. Due to the relatively slow pressure propagation at the inner well, contribution to gas production from the inner well decreases over time while the outer wells increase. Based on the sensitivity analysis, gas production increases with decreasing P btm , increasing k , and decreasing S H with P btm and k as the key controlling factors. The findings provide valuable design basis for the adoption of a multi-well system in enhancing gas production for targeted NGH reservoirs in the next field production trial at SCS. [Display omitted] • A five-spot well is designed for enhancing gas production from silty natural gas hydrate deposits • A thermo-hydraulic-chemical model is set up to examine the gas production enhancement effect • A 33 sensitivity study of bottomhole pressure, permeability and hydrate saturation is conducted • Gas production increased to 4.12 times that of a single well but with well-to-well interference [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]