Search

Abstract Gas hydrate dissociation, resulted from temperature and pressure changes in reservoirs, and natural submarine landslides demonstrate spatial consistency. Currently, large-scale production of oceanic gas hydrates is subject to extensive exploration. However, only few studies focus how large-scale hydrate mining affects near-future stability of the submarine slope and landslides. Thus, we used actual exploration data of the hydrate sediment in the Shenhu area in the South China Sea to establish a substantial three-dimensional and multi-field coupled numerical model of the hydrate submarine slope, which take into account dynamic strength changes of the hydrate layer during the hydrate decomposition process. Our numerical study indicated that severe reduction of hydrate sediments strength is the ultimate cause of the seabed settlement and slope instability. When horizontal wells are utilized to produce hydrates on the seabed slope, horizontal wells should be oriented in the vertical slope direction to ensure maximum stability of the slope as well as higher gas yield. Our results also revealed that seafloor subsidence caused by the increased effective stress corresponds to ∼20% of the total subsidence. Excessive production pressure drop (>9 MPa) and heat injection temperature (>80 °C) as well as large production scale (>150 m) will cause sharp decrease in slope stability and trigger submarine landslides. Our study also discussed methods of hydrate recovery and demonstrated benefits of simultaneous implementation of depressurization and the heat injection methods. Highlights • The horizontal well should be oriented vertically to slope to mine oceanic hydrate. • The decrease in strength after hydrate decomposition declines the slope stability. • Seafloor subsidence occurs in the whole process of hydrate decomposition. • High productivity is likely to induce large-scale submarine slope landslides. [ABSTRACT FROM AUTHOR]

Abstract Due to the complicated reservoir conditions in the oceanic methane hydrate (MH) reservoir in the Nankai Trough, Japan, the gas production rate for the economical extraction had not been achieved during the current field production tests in 2013 and 2017 using vertical wells. Therefore, this study aimed at the application of the horizontal wells to the oceanic MH production in the Nankai Trough. Since there existed three sub-hydrate-bearing layers with different physical properties (i.e., initial hydrate saturation, porosity, and intrinsic permeability) in the reservoir, some possible well configurations including single horizontal well patterns and dual horizontal well patterns were designed based on a multilayered geological model simulating the real oceanic MH reservoir in the Nankai Trough. Then, the effectiveness of these two kinds of well designs was verified via long-term simulations of the oceanic MH production by simple depressurization, and the optimal well configuration for each design was recommended. Furthermore, a combined method of depressurization and hot water injection was also tested based on the dual horizontal well pattern, and the sensitivity analyses indicated that a favorable gas production rate of 8.64 × 105 m3/day could be obtained within the first year, even under a relatively low injection temperature of 40 °C and a relatively small injection rate of 2 kg/s/m of well. Highlights • Horizontal well designs were tested in methane hydrate production in Nankai Trough. • Single horizontal well should be placed in upper high-permeability layers. • Dual horizontal wells should be placed in upper and lower high-permeability layers. • Hot water injection + depressurization promote gas production due to heat supply. • Large production rate is obtained in 1st year by low injection temperature and rate. [ABSTRACT FROM AUTHOR]

The effects of depressurization and the combination method with wellbore heating on vertical center well and vertical side well on hydrate exploitation were investigated and compared in this study. Six combinations of experimental groups were conducted using different exploitation methods (i.e., depressurization and the combination method) and different types of wells (i.e., vertical center well and vertical double-side wells). The results show that the lower the production pressure is, the better it can generate hydrate productions. Meanwhile, it is further confirmed that the heating process can greatly increase the gas production rate of hydrates. In addition, when using depressurization within a certain exploitation range, the location of the production well has little impact on the effects of hydrate exploitation. However, with double-side wells using the combination method, the effect of driving force in hydrate decomposition is weakened due to the larger heat loss. Therefore, when the heating well is located close to the boundary of the hydrate deposit, the use of double-side wells is less productive than that of a center well. As a result, when using the depressurization method, the location of the production well within the exploitation zone should be as close to the coastal line as possible with a stable hydrate deposit layer. On the other hand, when wellbore heating is applied, it is suggested that the heating well should be arranged near the center of the hydrate deposit. Furthermore, it is found that there is a right-skewed bell curve shape relationship between the exploitation time and the energy generation level using the combination method. [ABSTRACT FROM AUTHOR]

Gas hydrate reservoirs are considered as future global energy resource, however, minimal studies have been conducted on one of the most important historical seas to investigate the potential - the Mediterranean Sea. In this study, it is aimed to investigate this potential. According to the analyses in this study, there are many indicators of gas hydrates in the Mediterranean Sea. These indicators are mainly appropriate pressure-temperature conditions of gas hydrate formation, source gas potential, and coarse sands potential. In this study, by using the literature marine survey data and drilling data of the Mediterranean Sea such as pressure, temperature, salinity, porosity, geothermal gradient, sand content of sediments, etc., it was estimated that up to 98.2 (median) standard trillion cubic meters of methane may be available in the potential producible gas hydrates of the Mediterranean Sea. Due to the paucity of bottom simulating reflectance in the Mediterranean Sea, three hypothetical Class 3 gas hydrate reservoirs in the Mediterranean Sea conditions were simulated by using depressurization production method with/without wellbore heating with HydrateResSim numerical simulator. When the depressurization pressure is lower, much more gas is produced but until certain value. The main reason of gas production stop in these three reservoirs is hydrate reformation along the wellbore. Wellbore heating at 50 °C extended gas production for a while but could not avoid hydrate reformation along the wellbore. Ice formation due to the endothermic gas hydrate dissociation is not the cause of gas production stop because the temperature of the Mediterranean Sea sediments is high. [ABSTRACT FROM AUTHOR]

The in situ dissociation of methane hydrate is necessary for the commercial recovery of methane gas from sediments. Thermal stimulation and depressurization are both effective dissociation methods. To simulate methane gas production from a methane hydrate (MH) layer, we used laboratory experiments to investigate the use of depressurization with electrical heating on MH sediment. To clarify the effect of temperature around the production well of the MH layer, depressurization experiments were examined at three initial temperatures: 0 °C, 3 °C, and 10 °C. A MH sediment core saturated with NaCl electrolyte solution (3.5 wt.%) was used. After depressurization, the core temperature decreased to between 10 °C and 3 °C below the initial temperature because of the endothermic MH dissociation reaction. When electrical heating was applied during depressurization at the initial temperatures of 3 °C and 10 °C, this decrease in core temperature was suppressed, and the core temperature increased to between 1 °C and 9 °C above the initial temperature. Electrical heating at a current density of 10 A/m 2 , which corresponds to an electrical power of 1.6–0.8 W/kg, during depressurization was found to dissociate the hydrate effectively. Overall, the findings indicate that depressurization combined with the electrical heating of hydrated sediment saturated with electrolyte solution enables higher gas production compared to depressurization with lower electrical power. [ABSTRACT FROM AUTHOR]

Techniques have been developed to determine dissociation front (i.e., the boundary where hydrate saturation is decreased to 0) for hydrate development by combining depressurization and hot brine stimulation. Experimentally, hydrate dissociation is determined with a one-dimensional (1D) model by gradually injecting hot brine to examine gas and water production. Theoretically, simulation techniques are employed to determine the decay rate and relative permeability by fitting the experimental measurements. Subsequently, the numerical techniques are well matched with field test data and then extended to field applications by applying two different development methods (i.e., depressurization and combining it with hot brine injection). It is found that the combination method not only greatly improves gas recovery by approximately 35.00%, but also results in higher water production rate and lower gas water ratio compared with those of depressurization alone. The orthogonal design method is then used to perform sensitivity analysis and optimize operational parameters by maximizing energy efficiency as the objective function. The most sensitive parameters are found to be the brine temperature, producer bottomhole pressure, brine injection rate, and injection time. Two dissociation fronts are formed separately near the producer and injector, while the dissociation front of the producer is found to move slower than that of the injector due to the different driving mechanisms for the movement of two dissociation fronts. [ABSTRACT FROM AUTHOR]

Natural Gas Hydrate (NGH) reserves are large and regarded as an important alternative future energy source. Safely and efficiently exploiting natural gas hydrates has recently become a hot issue around the world. The decomposition of natural gas hydrate requires a great deal of absorbed heat. The decomposition process is a complicated heat and mass transfer process with phase changes, such that heat and mass transfer will have a great impact on the decomposition results. Focusing on the heat transfer effect, a two dimensional mathematical model of natural gas hydrate decomposition was established. Using a numerical simulation method, the thermal conductivities of different porous mediums were selected and the bottom hole pressure (BHP) was changed under different overburden and underburden temperatures to simulate the depressurization exploitation of natural gas hydrates. The reservoir temperature distribution, hydrate decomposition front, gas production rate, and cumulative gas production under different cases were obtained. The results show that the incoming heat directly influenced the reservoir temperature distribution, hydrate decomposition rate, and gas production and the temperature at any point on the decomposition front is constant under certain BHP. When there was no external heat introduced into the hydrate-bearing layers, the impact of thermal conductivity on decomposition was greater, but decreased with increasing temperature of overburden and underburden. When no external heat was introduced into the hydrate-bearing layers, the BHP became the main controlling factor of the cumulative gas production, and the effect of the BHP on the cumulative gas production was gradually weakened with increasing temperatures of the overburden and underburden. [ABSTRACT FROM AUTHOR]

Gas hydrates are considered as a promising energy source and the Black Sea has a high potential of gas hydrates. The Danube Delta of the Black Sea is the most well-known prospect in the Black Sea after many geological and geophysical studies such as bottom-simulation reflectors (BSR) and electromagnetic surveys. In this study, gas production simulations from two gas hydrate layers (6 m thick hydrate layer at 60 mbsf and 30 m-thick hydrate layer at 140 mbsf above BSR at 350 mbsf) at the same locations with approximately 50% hydrate saturation in the Danube Fan of the Black Sea were run with depressurization method separately at 2 MPa, 3 MPa, 4 MPa, 5 MPa, and 6 MPa by using HydrateResSim numerical simulators. Moreover, different production tests strategies were suggested in this region. [ABSTRACT FROM AUTHOR]

The exploitation of natural gas hydrate from the unconfined marine sediments, which is overlain by permeable layer, is challenging, as the traditional depressurization method cannot induce significant pressure drop in the hydrate reservoir to release the gas. To overcome this problem, we here numerically investigate the performance of joint depressurization and thermal stimulation to extract the gas in Shenhu area, South China Sea. The influences of warm water injection and the horizontal well placements on the gas production are mainly discussed. The results show that gas recovery can be improved significantly by the injection of warm water, attributed to the increase of the temperature in the hydrate-bearing sediment. A higher gas production can be obtained by locating vertically the injection well in the middle of the hydrate-bearing sediment, which can reduce the water recharge from the layers overlying and underlying the hydrate-bearing sediment. Moreover, the well spacing affects the methane production significantly when the thermal stimulation starts. The numerical experiments may be useful for future design and optimization of marine gas hydrate exploitation under similar conditions. [ABSTRACT FROM AUTHOR]

According to the available geological data of hydrate reservoir at the 1st offshore production site in the Eastern Nankai Trough in 2013, a simulation model matching the production test scheme was established to investigate a relatively long-term production performance and the effect of perforated interval on gas production by comparing the short simulation results and the field test data (6 days). The verification results indicate that the simulated gas production rate is consistent with the measured field value but a certain deviation of corresponding water production rates occurs. The relatively long-term simulation performed reveal that both gas released rate in the sediment and gas production rate in the well increase before complete dissociation of hydrates between bottom hole and underburden. After that, they convert to decreasing and the water production rate increases sharply. Meanwhile, the slight secondary hydrates are observed in the dissociation front. Additionally, the sensitivity analysis of completion interval indicates that the perforated interval design for candidate hydrate reservoir like this site should avoid the low-permeability hydrate layer and take the lower hydrate layers as the water blocking zones when using depressurization method, and that the optimum completion interval should be determined by together considering the production period and the expected production rate. [ABSTRACT FROM AUTHOR]

Marine gas hydrate always occurs in clayey silt sediments partially or fully saturated with water. Pore water is involved in gas hydrate formation and decomposition, which plays an important role in gas recovery from hydrate during depressurization. In this work, the depressurization of methane hydrate in the clayey silt sediments was experimentally investigated in a one-dimensional reactor. The variation of gas production and pressure gradient under the gas-rich and water-rich condition were studied. It was found that the gas production in water-rich environment was much lower than that in gas-rich environment as the movement of hydrate dissociating front is slower in water-rich environment. The permeability of the clayey silt sediments was measured before hydrate formation and after hydrate decomposition, which indicated that there was a large depression in the permeability of the clayey silt sediments after hydrate decomposition, which could be attributed to the hydration swelling of clay minerals with the expansion of diffuse double layer caused by the release of water with lower salinity from gas hydrate dissociation. And the permeability depression of the clayey silt sediments caused by hydrate decomposition was more obvious in water-rich environment, reaching up to ∼50%, and increased with the content of clay minerals. In addition, inorganic salt-based clay stabilizer solution was pre-injected into the clayey silt gas hydrate-bearing sediments before depressurization to mitigate the formation damage during gas hydrate production. The results showed the permeability of the sediments pre-saturated with NH 4 Cl was maintained at ∼75% of the initial values after gas hydrate decomposition as the exchange of Na+ with NH 4 + in sediments was proved to decrease the bound water in montmorillonite interlayers by NMR analysis and inhibit the hydration swelling of clay particles. • The depressurization of clayey silt gas hydrate was studied in a one-dimensional reactor. • Gas hydrate production in water-rich environment is slower than that in gas-rich environment. • There is a depression in permeability of clayey sediments after hydrate decomposition. • The formation damage associated with clay bound water can be mitigated by NH 4 Cl. [ABSTRACT FROM AUTHOR]

In 2013, gas hydrate accumulations were confirmed in the Dongsha Area of the South China Sea by the scientific drilling expedition GMGS2. The drilling sites of GMGS2-01, -04, -05, -07, -08, -09, -11, -12, and −16 were verified with the existence of hydrate bearing layer. The gas production behavior was evaluated at GMGS2-16 by numerical simulation in this work. The scenarios of single depressurization with single horizontal well and dual horizontal wells and the depressurization in conjunction with warm brine stimulation with dual horizontal wells were carried out in this work. Simulation results indicated that gas production rate for the single depressurization with dual horizontal wells was higher than that for the single horizontal well. In addition, the gas production performance was more favorable when using the depressurization in conjunction with brine stimulation. The average gas production rate with such scenario can obtain the same order of magnitude with the minimum production level of the commercial viability of the hydrate exploitation in the Gulf of Mexico. Furthermore, the sensitivity analysis indicates that gas production rate increases with the increase of injection temperature. However, the energy loss increases with the rise of injection temperature. Increasing the injection salinity or the injection rate can increase the gas production rate as well. However, the energy ratio is the highest for the case with the middle-higher injection rate. [ABSTRACT FROM AUTHOR]

Naturally occurring reservoirs of methane gas hydrate have been found in arctic and marine environments around the world. They are an attractive source of energy as natural gas can be produced from these hydrate deposits by depressurization or thermal stimulation. Reservoir simulations are used to find the optimum production strategies for methane gas hydrate reservoirs. Most of these simulation are carried out for homogeneous hydrate reservoirs in absence of substantial well data. These gas hydrate reservoirs are inherently heterogeneous because of the geological settings of the hydrate bearing sediments. Majority of the heterogeneity comes from the depositional layering at different geological time scales. Examples are Mount Elbert, block 818 in Gulf of Mexico, Walker Ridge 313 Site. The effects of geological layering on gas production and hydrate dissociation fronts in the reservoir are still unknown. In the present work, a 3-dimensional, multiphase, multi-component, thermal simulator is used to study layered gas hydrate reservoir, underlain by an aquifer, with cross-flow between the layers. Gas production from the reservoir and gas hydrate dissociation fronts in the reservoir are found for different aquifer permeability, different layering and well completions. In layered confined reservoirs, recovery is found to be dependent on (a) the total volume of the hydrate present in the reservoir which governs the enthalpy requirement for dissociation, (b) depressurization potential of the reservoir which depends on the effective permeability and (c) the enthalpy available for dissociation which depends on the reservoir enthalpy and heat transfer from over and under-burden. The layering suggests the positions and progress of the dissociation fronts and affects the gas recovery depending on the aquifer permeability. This effect is seen because of the pressure variation in different layers and enthalpy available for hydrate dissociation. In unconfined reservoirs the effect of layering on dissociation front positions is found to be insignificant as the pressure variation in the layers is insignificant because of the unconfined aquifer. A weakly un-confined aquifer also renders the depressurization ineffective. [ABSTRACT FROM AUTHOR]

This paper analyses the effectiveness of the depressurization technique for methane gas production from an unconfined gas hydrate setting in the Krishna Godavari Basin of India. TOUGH + HYDRATE reservoir modeling and production simulation software is used for identifying the depressurization range for sustained dissociation. For the identified depressurization range, a borehole based pumping technique is modeled using MATLAB software, to identify the effectiveness of the technique in dissociating the reservoir with post-dissociation permeability (PDP) ranging between 1 and 500 mD, the likely scenario for the KG basin. The results indicate that the depressurization technique shall be effective for reservoirs with higher PDP, while the reach of the bore well depends on the capacity of the pump and hydrate zone well perforation area. The simulation results indicate that a bore well with 50 m 2 of hydrate zone surface area could be effective in dissociating the reservoir up to a distance of 1000 m, with a pump of 128 kW electric capacities in formations with PDP of 500 mD. The results could serve as a basis for the economic planning of production wells in gas hydrate reservoirs. [ABSTRACT FROM AUTHOR]

Natural Gas Hydrate (NGH) is widely discovered in silty sediments with a high compressibility in the South China Sea. The permeability reduction in these reservoirs during depressurization is inevitable. Here, we innovatively take the effect of permeability reduction into account to improve the accuracy of production forecast and suggest the optimal production pressures for two different typical reservoirs (i.e., Class I: Site W19 and Class III: Site SH2) in this area. In this study, two typical reservoir-scale production models of NGH reservoir are built, and the effects of bottomhole pressure and pore compressibility and permeability correlation coefficients on gas and water extraction are quantitatively evaluated. The simulation results indicate that for silty NGH reservoirs, the effect of permeability reduction on gas and water recovery during depressurization cannot be overlooked. Pore compressibility and permeability correlation coefficients will significantly affect production, and with an increase in both parameters, the cumulative gas and water recovery will decrease. However, the total gas production will firstly increase and then decrease with a reduction in bottomhole pressure. In addition, the optimal production pressures of sites W19 and SH2 are 1.5 MPa and 5.5 MPa respectively. These findings can give some valuable suggestions for future field tests in this region. • Effect of permeability reduction on gas recovery of NGH reservoirs was studied. • A related sensitive analysis affecting the permeability reduction was conducted. • Optimal production pressures of site W19 and site SH2 were suggested respectively. [ABSTRACT FROM AUTHOR]

Recently, the hydrate-bearing sand reservoirs were founded with high saturation in the offshore India by the National Gas Hydrate Program Expedition 02 (NGHP-02). Based on the detailed geological data at the NGHP-02-16 site, a more realistic reservoir model is constructed to analyze the unique multiphase flow and hydrate phase change behaviors from the geologically descriptive hydrate reservoirs. The main goals of this work are to optimize the adjustable production parameters and to predict the long-term gas production performance for guiding the upcoming field hydrate production test in India. The results indicate that gas production rates increase, but water production rates decrease slowly with time from the heterogeneous hydrate-reservoir. This is completely different from the previous prediction of multiphase flow behavior based on homogeneous hydrate-reservoir models. A non-uniform hydrate dissociation front is formed in the hydrate-reservoir due to the reservoir heterogeneity promote the horizontal dissociation of solid-hydrate within the high permeability sand layers. The maximum distance of free gas zone expands to outside more than 500 m from the production well. This indicates that the field fault system should be carefully concerned during the long-term production. Sensitivity analysis results suggest that the complete perforation in the hydrate-bearing layer and the underlying water saturated layer is beneficial for improving the hydrate production performance. Over the 5 years depressurization, a total of 2.08 × 107 ST m3 methane gas can be produced using the recommended well placement. The average gas-to-water ratio and energy ratio reach 10.22 ST m3 CH 4 /m3 H 2 O and 30.14, respectively, showing a high gas production potential from this hydrate site. • A more realistic heterogeneous hydrate-reservoir model is constructed. • The time-dependent increase of methane gas production is evident in our model. • The best well perforation is determined for the specified hydrate site. • The unique multiphase flow and hydrate phase change behaviors are analyzed. • The effects of reservoir heterogeneity on hydrate production are clarified. [ABSTRACT FROM AUTHOR]

The depressurization method and its modified scheme may be the best way to efficiently recover marine natural gas hydrate (NGH) from the reservoir. However, the capacity of existing engineering tests still poses considerable challenges to the industrial development of NGH. The complex structure wells can effectively promote the NGH contact area to enhance the exploitation efficiency, but the influence of branch parameters on gas production performance is not well sorted out for reference when performing well type optimization. Therefore, a geological model of an ideal hydrate reservoir and 65 wellbore models were established based on the geological data from station AT1 in Nankai Trough to analyze the influence of different branch parameters on the production capacity for various well types. Results indicate that the open hole section length is heavily constrained by the size of the reservoir for a single well. Compared to the corresponding single well, the multi-branch vertical wells can increase productivity by about 2.12%–51.18%, the multi-branch horizontal wells by about 0.99%–29.75%, and the cluster horizontal wells by about 54.36%–250.66% well. Moreover, the correlation coefficients of various branch parameters were analyzed as a reference when optimizing the well type and show that the cluster horizontal well has the highest sensitivity to branch parameters, which can significantly promote gas production efficiency, and is expected to break the threshold of industrial development. • Complex structure wells were proposed to enhance gas recovery from natural gas hydrate. • A longitudinal heterogeneous reservoir model of 1010m × 1010m × 120m was established based on Japan's Nankai Trough. • Various well type cases were designed to study the influence of branch parameters on gas production. • The gas production characteristics of various well types were counted and compared. • The correlation of different branch parameters on gas production indicators was explored by Pearson's Law. [ABSTRACT FROM AUTHOR]

The Mallik gas hydrate deposit was found to consist of 3 distinct, highly concentrated, high quality zones of structure I hydrate with partial occupancy of 5.75–6.2. Earlier simulation studies focused on history matching the 6 days production test of the lower zone, assuming 100% hydrate occupancy. The focus of the current study is on a simulation comparison of the expected response of the three hydrate bearing zones (lower, middle and upper) of the Mallik well 2L-38 to a single vertical well depressurization test. Additionally, a revised history match of the bottom zone field test considering partial gas occupancy of the hydrate, and a further assessment of the kinetic dissociation model are studied. This study extends the previously developed model found to successfully represent the physical and thermodynamic mechanisms involved with hydrate dissociation in the Mallik field test. The simulation results indicate that hydrate production from the middle hydrate zone is feasible and attractive, while production from the upper zone is not, due to its low pressure/temperature condition. Generally speaking, all three zones showed a similar role of the bottom aquifer in determining the water and gas flows, and all three zones showed an upward gas migration block by different existing hydrate layers. This effect is contrary to gas production from conventional gas reservoirs and indicates that the use of horizontal wells in such reservoirs may not be attractive. This study has also explored the role of partial gas cavity occupancy in the lower zone production characteristics and a re-history match of the 6 days Mallik production test showed an improved match and some indication about partial occupancy close to 6.88. Finally an in-depth examination of a previously reported laboratory scale study of methane hydrate decomposition and some observations from even smaller scale molecular dynamics study gave exciting clues of how to further interpret and improve our kinetic gas hydrate model, to be used at multiple time and length scales. Such an improved model has been proposed and awaits further testing and matching of appropriate data. [ABSTRACT FROM AUTHOR]

The delineation of the Mallik gas hydrate field has utilized extensive well logging and substantial 3D seismic testing and interpretation. This study explores the use of seismic data to quantify the areally heterogeneous gas hydrate distribution. The available Mallik 3D seismic data was compiled and compared/contrasted with available well log data from two adjacent wells. Based on the seismic information, two areally variable (i.e. non-homogeneous) scenarios for gas hydrate distributions are considered: Scenario I having the same initial total hydrate amount as our earlier model areally uniform (homogeneous) distribution, and Scenario II with significantly less overall total hydrate, but honouring the same relative distribution. The scenarios of variable gas hydrate distributions are used in dynamic simulations of the lower Mallik zone. Simulations of each were conducted with and without the role of geomechanics.In Scenario I, we observed multiple gas production peaks (which quite similar to 6 days production behaviour) with higher localized pressure pulses occurred due to strong gas hydrate heterogeneity. In Scenario II, this drastic change in gas production rate was not observed (due to faster pressure evolution in the reservoir). In both Scenarios, the overall reservoir gas production peak is delayed compared to the homogeneous case. This is further delayed by the role of geomechanics. More interestingly, all simulation cases show a very similar overall production trend. This is probably a unique for the Mallik gas hydrate production using single vertical well, including a gas production peak but terminating in a stabilized period of lower but significant gas production.With geomechanics, gas production in general and the gas production peak is shifted and delayed. The geomechanics effect is not purely compaction drive (as in conventional reservoirs, gas production increases with geomechanics). The simulations utilized two set of geomechanical parameters obtained from logs (dynamic parameters) and rocks testing (static parameters). Geomechanical responses based on dynamic parameters were essentially equivalent to simulations ignoring geomechanical effects. The geomechanics simulations indicate an essentially elastic reservoir response (i.e. no plastic failure) assuming a cased vertical well. The Mallik upper zone A and middle zone B are closer to the permafrost and nearer to plasticity limits should be explored. Rapid rise and drop followed by asymptotic rise in gas production is proven to be unique in Class II GH dissociation. Shifted and delayed gas production in general and gas production peak with geomechanics. Occurred multiple gas production peaks with higher localized pressure pules due to non-uniform hydrate distribution. Indicated an essentially elastic reservoir response (i.e., no plastic failure) assuming a cased vertical well. [ABSTRACT FROM AUTHOR]

The commercial exploitation of natural gas hydrate reservoirs requires scientific policies to reduce the exploitation cost and improve production efficiency. A multineural network composite model for NGH reservoir exploitation is designed and coupled with the secondary-developed T + H code in the PYTHON environment. Under the preconditions of realizing multiobjective optimization in terms of efficiency, safety, and economy, the model achieves the optimal dynamic production policy and quantitatively presents the advantages and disadvantages of different exploitation policies in each exploitation state. In the initial exploitation period, high-temperature and increasingly low-rate exploitation policies achieve the strongest production performance and energy efficiency. The huff and puff heat injection method can effectively reduce the permeability drop caused by secondary hydrate formation. In the middle exploitation period, the injection density of thermal energy controls the hydrate exploitation efficiency, the huff and puff method is not applicable, and more intensive heat injection results in higher exploitation efficiency and less water production. In the posterior exploitation period, the methane recovery rate does not depend on heat injection, and more energy supply can only achieve a lower energy efficiency return. The proposed methodology provides a feasible means of NGH reservoir exploitation policy optimization and analysis. • Automatically and autonomously explore the dynamic optimal exploitation policy. • LSTM based reinforcement learning realizes policy long-term impact evaluation. • Maximize energy efficiency while achieving multi-objective optimization. • Quantify the value of adopting different policies in each exploitation state. • Optimization mechanism is analyzed from the mass and heat transfer perspective. [ABSTRACT FROM AUTHOR]

Natural gas hydrates (NGHs) buried in the Shenhu area are characterized by fine-grained sediments with interbedded hydrate-free layers. However, few is known about the influence of hydrate-free interlayers (HFIs) on gas production, which may control efficient and safe gas recovery from NGH reservoirs in long-term production. In this study, a field-data-based heterogeneous reservoir model is established to evaluate gas production behavior of Shenhu NGH reservoir with HFIs during depressurization. We compare the gas production from the hydrate reservoir with different HFIs characteristics. In a vertical well configuration, NGHs adjacent to HFIs dissociate preferentially over time, thereby significantly increasing average methane production and gas-to-water ratio. Notably, highly permeable HFIs (especially those near the permeable underburden), which can be regarded as high-permeability channels, increase pressure diffusion and then fluid flow, heat transfer and long-term gas production efficiency. The interlayers provide a better pathway for gas migration and secondary hydrates form inhomogeneously in the dissociation front. In contrast, low-permeability HFIs can negatively affect gas production and decrease average gas production amount by nearly 32.4% at most. The HFIs with low permeability are unfavorable for fluid migration and convective heat transfer from the burdens. Under the same initial conditions, the alternative of single horizontal well promotes the gas extraction volume limitedly. This study suggests that the vertical well configuration may provide more promising gas production efficiency than that of the horizontal one for the heterogeneous NGH reservoirs with HFIs in vertical direction. Our finding can provide a useful guide for gas production strategy in NGH reservoirs with HFIs. • Effects of HFIs with different characteristics on gas production are systematically evaluated. • The lower HFIs adjacent to underburden exhibit relatively better gas production. • Intrinsic permeability of HFIs can greatly affect the role of HFIs in gas production. • Single vertical wells are more appropriate for heterogeneous gas hydrate reservoirs. [ABSTRACT FROM AUTHOR]

Brine stimulation is a common method for assisting depressurization to produce hydrate, which significantly improves the efficiency of hydrate production. In this work, to study the influence of brine concentration on the natural gas hydrate production process, a novel cylindrical hydrate production simulation experimental device was established, and investigations of hydrate dissociation experiments induced by depressurization in conjunction with brine injection were carried out. The effects of different brine concentrations on the gas production rate and heat transfer during production were studied. The distribution, flow, and concentration evolution of brine during hydrate production were analyzed. Brine injection can significantly enhance cumulative gas production during the depressurization stage, increase the sensible heat for hydrate dissociation, enhance heat transfer between the hydrate reservoir and the environment during the recovery process, and significantly improve the gas production rate of hydrate. The increase in the gas production rate caused by brine injection mainly occurs at the end of the depressurization stage and the beginning of the constant pressure stage. The injected brine is mainly located in the middle and upper areas of the reactor. In the bottom area of the reactor, the hydrate has higher saturation and longer dissociation time, which is slightly affected by the injected brine. A higher hydrate saturation is not beneficial to brine injection and does not promote hydrate dissociation. Additionally, in the experiments with brine injection, the resistance decreases more obviously during the depressurization stage due to more hydrate dissociation. The resistance also shows that in the bottom area of the reactor, there is a higher hydrate saturation and a lower brine effect on hydrate. • Depressurization in conjunction with brine injection is applied for hydrate dissociation. • The distribution, flow and concentration evolution of the injected brine are analyzed. • Effects of injected brine on gas production and heat transfer are analyzed. • Promoting of hydrate dissociation by brine mainly occurs at the end of depressurization. [ABSTRACT FROM AUTHOR]

Abstract: Natural gas production from hydrate reservoirs is a physical and chemical seepage process of multiphase and multicomponent in nonisothermal condition. A gas–water two-phase hydro-mechanically coupled model is established to simulate the complex performance of depressurization-induced gas production from hydrate reservoirs. The model considers the heat conduction and convection, the variation of physical and mechanical properties as a result of hydrate dissociation, and the interaction between fluid seepage in porous and rock skeleton deformation. With the finite element approximation technique, a coupled simulator is developed. Case studies of gas production from hydrate reservoirs by depressurization are presented, and the fluid–solid coupling effect is mainly discussed. The results show that the overall fluid–solid coupling effect reduces the depressurization-induced gas production from hydrate reservoirs. The rock skeleton deformation can shrink reservoir porosity and increase the elastic drive energy, which favors to enhance gas production to some extent. In contrast, the permeability and porosity reduction due to sandstone deformation can lead to the decrease of gas production significantly. The physical properties variation in the fluid–solid coupling effect has a major impact on gas production of hydrate reservoirs. The fluid–solid coupling effect is a key factor worthy of close attention in the exploitation of hydrate reservoirs. [Copyright &y& Elsevier]

Natural gas hydrate has become a hotspot for researchers because of its considerable economic benefits and potential strategic significance. In the hydrate exploitation process, the hydrate-bearing sediments get damaged, and the coupling effect of multi-physical fields is involved. In this study, a damage constitutive model of hydrate-bearing sediments is established by considering decomposition damage, loading damage, and the influence of residual strength. Based on this damage constitutive model, a Thermo-Hydro-Chemo-Mechanical (THCM) multi-field coupling model considering the sediment damage is constructed on the COMSOL simulation platform. Compared with the experimental data, the THCM coupling model is demonstrated and employed to study the depressurization process of horizontal wells in permafrost regions. During the hydrate exploitation model, changes in displacement, saturation, average pore pressure, and temperature at different mining positions are recorded and analyzed. The results indicate that the damaging of sediments has a significant effect on soil deformation and temperature. After the sediment damaging effect is considered, the minimum temperature of the extraction points is reduced by 22.5% at most. The maximum deformation of the extraction points is about 11.8% greater than that without considering the sediment damage effect. Moreover, the model reveals the self-preservation effect of gas hydrate. The temperature of the surrounding soil at the exploitation site decreases due to hydrate decomposition, which would inversely promote hydrate formation. These numerical models in this study can be applied to estimate the strength of sediments and predict the reservoir settlement in the process of hydrate depressurization. • A damage constitutive model of hydrate-bearing sediments considering the sediment damage effect is established. • A Thermo-Hydro-Chemo-Mechanical (THCM) multi-field coupling model considering the sediment damage effect is constructed. • During depressurization, the effect of sediment damage on hydrate reservoir stability is analyzed by using a TCHM model. [ABSTRACT FROM AUTHOR]

Over 90% of the global hydrate resource is accumulated in the fine-grained sediments with very low permeability. Gas production from the low-permeability hydrate reservoirs is significantly restricted by the effective transport of fluids into the production well. To improve gas production from the low-permeability hydrate reservoirs, the high-pressure jet grouting (HPJG) technology is first proposed that involves construction of an artificial jet grouting column (JGC) with relatively high-permeability around the production well using the cellular concrete. In this work, the main target is to investigate the feasibility of this proposed method for enhancing gas production by numerical simulation, aiming at the clayey-silt hydrate reservoirs in the Shenhu area of South China Sea. It is demonstrated that the hydrate dissociation efficiency and the gas productivity greatly improved by about 157% and 110%, respectively, by constructing an artificial JGC with radius of 0.5 m around the production well. The results suggest the proposed method is a promising strategy for enhancing gas production from the low-permeability hydrate reservoirs. This is mainly because the artificial JGC significantly promotes the advance of depressurization and the transport of gas and water within the reservoir. In addition, gas production performance of the proposed strategy mainly depends on JGC radius and JGC height, while the JGC porosity has less effect. For the specified clayey-silt hydrate reservoirs in the Shenhu area, the artificial JGC radius, height, and permeability are recommended as 0.5 m, 30 m, and 1.0 × 10 −12 m2, respectively. [Display omitted] • The jet grouting method is first proposed to enhance hydrate production. • The feasibility of the novel hydrate production strategy is investigated. • The uncertain and adjustable parameters are evaluated in detail. • The optimal parameters are designed for the clayey-silt hydrate reservoirs. [ABSTRACT FROM AUTHOR]

Due to the ever-increasing energy demand, the study of gas production from marine methane hydrate attracts intensive interest around the world. Vertical wells were adopted in most of the previous production tests, although horizontal well might be with significant advantage over vertical well in production performance. To evaluate the gas production performance from a high irreducible water saturated hydrate reservoir with free gas underlain using horizontal well, a multilayered hydrate reservoir model is established, of which all the sediment layers are with high saturation of irreducible water and a free gas layer occurs beneath the hydrate zone. Gas production performances on single horizontal well and two paralleled horizontal wells are investigated. The simulation results obtained show that gas production with horizontal well can yield a high gas production rate, especially at the early stage of production with the contribution of free gas. A horizontal well located at the bottom of the hydrate layer is recommended, because it can not only extract free gas in gas production but also induce more dissociation of hydrate. When two paralleled horizontal well-configuration is applied, the middle and bottom well pattern can generate a synergistic effect that accelerates hydrate dissociation and reduces water production. The sensitivity analyses of free gas saturation and irreducible water saturation show that either of the two factors can result in a high gas-to-water ratio. • A multilayered hydrate reservoir model with free gas layer is established for numerical simulation of production performance. • Single horizontal and two paralleled horizontal well-configurations are investigated. • Effects of free gas saturation and irreducible water saturation on gas production are studied. • Gas-to-water ratio is high in the high irreducible water saturation methane hydrate reservoir contained free gas. [ABSTRACT FROM AUTHOR]

An innovative idea of "reservoir reformation" was proposed earlier and verified to be feasible for enhancing methane gas recovery efficiency and CO 2 sequestration by constructing an artificial impermeable CO 2 hydrate cap between the methane hydrate reservoir and overburden. However, the effects of different factors on the sealing performance and mechanical stability of the CO 2 hydrate cap need to be further clarified for the optimization of this new technology. In this study, experiments were conducted to simulate methane hydrate gas production by depressurization and analyze the influence of CO 2 emulsion injection temperature, injection rate and overlying pressure by controlling these variables. The results indicated that the desired CO 2 hydrate cap was formed by decreasing the injection temperature and rate. A cyclic process of "hydrate formation-fracture-regeneration" was proposed at a controlled injection rate of 3 mL/min. The degeneration rate of the CO 2 hydrate layer was accelerated with an increase in the overlying pressure. The mechanical stability of CO 2 hydrate cap was determined by the combined effects of methane hydrate decomposition and an increase of axial load. An impermeable CO 2 hydrate cap between the methane hydrate reservoir and overburden was constructed for enhancing methane gas recovery efficiency and CO 2 sequestration. [Display omitted] • Hydrate cap was formed by injecting CO 2 emulsion to enhance methane hydrate exploitation. • The decrease of injection temperature was conductive for forming CO 2 hydrate cap. • Low CO 2 emulsion injection rate promotes the formation of hydrate layer. • A circulation of formation-fracture-regeneration process was proposed for forming the cap. • Erosion rate of CO 2 hydrate layer interior increases under high overlying pressure. [ABSTRACT FROM AUTHOR]

The lithological features of sediments control the heterogeneity in permeability. This study numerically investigates the effect of lithological rhythm of a confined hydrate reservoir on gas production for warm water injection scenarios. The investigated lithological rhythm structures range from simple rhythms to complex composite rhythms. The depressurization in multiple wells causes significant gas production. However, the decrease in pressure gradient constrained the production of gas and water. The warm water injection enhances the production of gas and water with various lag times, though it results in a low energy efficiency. The positive and reverse rhythms have similar energy efficiencies. The gas and water production rate slightly decrease with the increase in the permeability contrast. The maximum gas production rate increases with the increase in number of composite rhythmic layers. However, the water production changes only slightly. The rhythm structures significantly affect the gas and water production rates. A high permeability region in the bottom part of the reservoir is conducive to warm water breakthrough. However, it is unfavorable for the dissociation of remnant hydrate in low permeability regions. Moreover, the results show that the rhythm structures also affect the gas productivity of each rhythmic layer. The results presented in this study help us understand the effect of complex and real rhythm structures on the gas production performance and the dynamic evolution of hydrate dissociation zone. • Depressurization in a limited domain tends to cause similar trend of gas rate. • High permeability region within rhythms is conducive to warm water breakthrough. • The remanent hydrate at low permeability region dissociated constrainedly. • A higher number of rhythmic layers facilitated a higher peak gas production rate. • Rhythmic structure affected the free gas distribution of each rhythmic layer. [ABSTRACT FROM AUTHOR]

Depressurization is an effective method to exploit natural gas hydrate reservoirs. However, ice might be formed due to the endothermic effect of hydrate dissociation, which will have a significant influence on gas production. In this work, a numerical model is established to investigate the hydrate dissociation performance by depressurization near the quadruple point. The impact of production pressure and intrinsic permeability on gas production and ice formation are also analyzed. It is revealed that the ice tends to be formed around the perforated interval due to the lower production pressure. A decrease in the effective porosity and permeability due to ice formation has been observed. But in fact, the formed ice has played a positive role in enhancing gas production owing to the released latent heat during ice formation. A large amount of ice is formed which results in a higher gas production rate when the production pressure is lower. The gas production rate and ice formation are greatly enhanced in the early production stage of a hydrate reservoir with a relatively high intrinsic permeability. For a hydrate reservoir with low permeability, ice formation is beneficial for gas production in the long term. [Display omitted] • A mathematical model considering the effect of ice on hydrate depressurizing dissociation is established and verified. • Ice formation plays a positive role in promoting hydrate decomposition. • The production pressure and intrinsic permeability are important parameters that affect the formation of ice. [ABSTRACT FROM AUTHOR]

The accuracy control of the production pressure difference is of great importance to the efficient development of marine gas hydrates through depressurization. But from the field trial production, the existing control method of bottom hole pressure is difficult to achieve the expected depressurization target. To this end, a transient gas-liquid two-phase flow model in marine gas hydrate production system was established, which considers the coupling interactions of the wellbore, the formation and the downhole electric submersible pump (ESP). Besides, an automatic control method of production pressure differential was proposed through real-time optimization of the pump displacement. Using the model, a series of numerical simulations were performed to study the multiphase flow rules in the production well. The results indicated the depressurization process could be divided into four stages based on the characteristics of gas-liquid flow in different pipelines. During the depressurization, the liquid level in the well exhibited a trend of first falling and then rising, and may eventually reach the wellhead for continuous drainage. In addition, sensitivity analysis showed that the smaller the pipe diameter, the higher the gas-liquid separation efficiency, and the greater the possibility of continuous drainage from the gas production pipeline. As the result, it was verified that the proposed model and method can achieve the expected depressurization well, which will provide useful references for the production design during the development of marine natural gas hydrate resources. • Transient gas-liquid two-phase flow model in gas hydrate production system was established. • An automatic control system and method of production pressure differential was proposed. • It was verified that the model and method can implement the depressurization target well. • The evolution rules of multiphase flow parameters during the depressurization were studied. • The effect of pipe diameter and gas-liquid separation efficiency was thoroughly analyzed. [ABSTRACT FROM AUTHOR]

In 2017, China successfully conducted its first offshore gas hydrate production test by using vertical well in the Shenhu area, achieving a total gas production of 3.09 × 105 m3. However, there is still a long way to commercial production. In this study, to evaluate and improve the gas production potential of China's first hydrate production site, depressurization, and the combination of depressurization and hot water injection, were applied to natural gas hydrate (NGH) reservoir in China's first offshore hydrate production site to analyze 10 years' hydrate production features. The results indicated that the decomposition front of hydrate when using depressurization in two horizontal production wells had obvious non-uniform characteristics, and there were rapid decomposition regions and hydrate reformation regions. Among them, the hydrate reformation region may have adverse effects on hydrate decomposition. The combination of depressurization and hot water injection could improve total gas production and alleviate hydrate reformation. However, when the temperature of hot water injected increased from 35 °C to 80 °C, the energy efficiency decreased from 10.22 to 1.53, as a lot of energy was consumed to heat the sediments in the reservoir. When the hot water injection time was reduced from 3650 days to 1825 days, the energy efficiency could be effectively increased from 10.22 to 21.78 without significant reduction of the gas production. This work contributed to the present understanding of production behavior of depressurization and the combination of depressurization and hot water injection, and helped to evaluate the influence of hot water injected on gas production from NGH reservoir. • Rapid decomposition region and hydrate reformation region coexist near the decomposition front. • Increasing hot water injection temperature 35 °C–80 °C will reduce energy efficiency from 10.22 to 1.53 • The reduction of hot water injection time from 10 years to 5 years will not significantly reduce gas production. [ABSTRACT FROM AUTHOR]

The stratification split grouting foam mortar method (SSGFM) was first proposed to stimulate the low-permeability gas hydrate reservoir through constructing the fast flow channel for enhancing gas production. Based on the low-permeability hydrate-bearing sediments (HBS) at SH2 site in the Shenhu area of South China Sea, the foam mortar layer (FML) reservoir model was constructed. The numerical simulation that coupled thermal-hydraulic-mechanical processes was employed to evaluate the efficiency and feasibility of this method. Results show that the FML could effectively promote the expansion of the low-pressure zone into the hydrate reservoir, and enhance gas hydrate dissociation rate, cumulative gas production and energy efficiency. The foam mortar layers (FMLs) have significant influence on the spatial distribution and the evolution characteristics of reservoir thermophysical and geomechanical parameters during gas production. The sensitivity analysis of FML shows the number of FMLs, thickness and permeability of FML exist the critical values for meeting the demand of promoting hydrate dissociation and gas production. Although the cumulative gas production and gas production rate will greatly increase with enlarging the radius, the gas-to-water ratio increase slightly. In view of the hydrate reservoir at SH2 site, the recommended number of FMLs is 4–6, and the distance between the FML and overburden/underburden should be more than 7.5 m. In addition, the optimal values of thickness, radius and permeability of FML are 5 cm, 40 m and 1 × 10−10 m2, respectively. The geomechanical response indicates that the FMLs are beneficial for reservoir stability while improving gas production. • The stratification split grouting foam mortar is a novel method of gas production from NGH. • The method could improve the fracture width and maintain the fracture during gas production. • The gas production and energy efficiency are effectively improved by SSGFM method. • The FMLs will help to maintain reservoir stability while improving gas production. • The optimizing parameters of FLMs in view of the hydrate reservoir at SH2 site were proposed. [ABSTRACT FROM AUTHOR]

Gas hydrates are considered as a strategic unconventional clean hydrocarbon resource. This paper analyses the techno-economic viability of the methane gas production from the fracture-dominated gas hydrate reservoir in the Krishna-Godavari basin, east coast of India based on multi-well configurations. The analysis is done using the gas hydrate field depressurization-based production numerical simulator NIOT-IndHyd 1.1 and the gas hydrate field development return-of-investment cost model with reservoir petro-physical properties and India's current natural gas import price as inputs. The results indicate that reservoirs with post-dissociation permeability (PoDP) of >200mD with hydrate saturations >75% or higher PoDP are economic production targets for the present Indian natural gas import price of US$ 4.66/MMBtu. The payback period for multi-well configurations with a floating production unit for various reservoir post dissociation permeabilities are also presented. • Methane gas production from gas hydrate reservoirs in the KG basin analysed. • Analysed using depressurization-based production numerical simulator IndHyd 1.1. • Reservoirs with PoDP >200mD with hydrate saturations >75% are economic targets. [ABSTRACT FROM AUTHOR]