Depressurization In Conjunction Research Articles

Heat utilization efficiency analysis of gas production from hydrate reservoir by depressurization in conjunction with heat stimulation

The heat utilization efficiency of heat injection is a key factor for the available heat stimulation method in natural gas hydrate, and the heat transfer characteristics during natural gas production from hydrate reservoir require further analysis. In this work, based on the laboratory tests by the Pilot Hydrate Stimulator (PHS), the heat transfer process of hydrate decomposition in the PHS was investigated, and the real-time decomposition rate and the energy utilization efficiency of the hydrate decomposition were quantitatively studied. The experimental results showed that the energy consumption of hydrate decomposition was mainly obtained from the sensible heat hydrate bearing sediments and the heat transfer from the environment in the depressurization process. In the initial stage of heat injection, the energy consumption of hydrate decomposition mainly came from the heat contained in the injected hot water and the heat transfer through the boundaries. In the later stage of heat injection, all of the energy consumption for hydrate decomposition was obtained from heat injection. In this work, the heat transfer coefficient between the reservoir and the environment was first proposed to quantify the heat transfer through the boundary. The results showed that the heat transfer coefficient presented large fluctuations during the initial stage of the depressurization and the period when the reservoir temperature reached the environment temperature in the heat injection process. Meanwhile, the coefficient remained stable at the later stage of heat injection. These results indicated that the change in gas-water hydrate inside the reservoir and the change in reservoir temperature affected the heat transfer coefficient between the reservoir and the environment. Furthermore, the energy utilization efficiency and gas production rate were optimized for hydrate production when the reservoir temperature was controlled within the range close to the environmental temperature. These results may be of great significance for heat injection control during field production from hydrate reservoirs.

Experimental study on the influence of brine concentration on the dissociation characteristics of methane hydrate

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.

Heat Utilization Efficiency Analysis of Gas Production from Hydrate Reservoir by Depressurization in Conjunction with Heat Stimulation

Heat Utilization Efficiency Analysis of Gas Production from Hydrate Reservoir by Depressurization in Conjunction with Heat Stimulation

Evaluation of thermal stimulation on gas production from depressurized methane hydrate deposits☆

Natural gas hydrates have gained worldwide attention asan important potential non-conventional fossil fuel resource. Understanding the gas production behavior from hydrate deposits is critical to the utilization of the gas hydrate resource. In this study, the hydrate dissociation reaction was induced by depressurization in conjunction with thermal stimulation. Profiles of temperature, pressure, gas production rate, and cumulative gas production during the gas production processes were analyzed. The results show that the gas production process upon ice generation can be divided into five main stages: (1) a free gas release, (2) hydrate dissociation along the equilibrium curve driven by the reservoir sensible heat, (3) hydrate dissociation driven by the exothermic ice generation reaction, (4) ice melting and hydrate dissociation under ambient heat transfer, and (5) hydrate dissociation under ambient heat transfer. During the gas production process, two thermal stimulation methods—ambient heat transfer and warm water injection—were employed to supply heat for hydrate dissociation. The larger the heat flux supplied by ambient heat transfer, the greater the gas production. During the warm water injection process, the gas production time decreased as the temperature of the injected water increased. These two methods can effectively promote gas production from gas hydrate deposits. The findings of this study can provide some insight for designing and implementing optimal production techniques for use of hydrate resources.

Gas Production from Methane Hydrate Deposits Induced by Depressurization in Conjunction with Thermal Stimulation

To investigate the gas production behavior, the gas production processes including the profile of temperature, pressure and gas production rate from methane hydrate in porous media induced by depressurization in conjunction with thermal stimulation were characterized in this study. The process of hydrate dissociation was analyzed. A two-stage gas production process was observed: rapid gas production stage (free gas and dissociated gas from hydrate production), and slow gas production stage (single dissociated gas production). Five groups of experiments were performed to investigate the effect of thermal heat flow on the hydrate dissociation behavior. The promote effect of rising air bath temperature on gas production was obtained.

Hydrate dissociation induced by depressurization in conjunction with warm brine stimulation in cubic hydrate simulator with silica sand

To study the effect of salt concentration of brine injection on hydrate dissociation, hydrate dissociation experiments induced by depressurization in conjunction with warm brine stimulation have been carried out in a Cubic Hydrate Simulator (CHS). The dual horizontal wells were set as the well configuration. The results indicate that the salinity in the reservoir decreases continuously during the depressurizing stage under the mixture of fresh water from hydrate dissociation. However, the salinity increases overtime during the constant-pressure stage (the injection stage) by the mass transfer with the injected brine. The gas production rate and heat-transfer rate for pure water injection are lower than those for brine injection. In addition, raising the injected salinity can enhance the rates of heat transfer and gas production when the salinity is lower than 10.0%. However, the promotion effect of brine injection on hydrate dissociation is limited when the injected salinity is beyond 10.0%. This is because the specific heat of the brine declines with the increase of the salinity, which causes the decrease of heat injection rate. The water production rate equals to the water injection rate in the process of brine injection. The energy analysis and the evaluation of energy ratio indicate that the optimal injected salinity in this work is 10.0%.

Energy and entropy analyses of hydrate dissociation in different scales of hydrate simulator

To investigate the effect of the reservoir scale on hydrate dissociation by depressurization in conjunction with warm water stimulation with dual horizontal wells, experiments of hydrate dissociation by such method have been carried out in CHS (Cubic Hydrate Simulator) and PHS (Pilot-scale Hydrate Simulator). The results show that there is little difference of temperature variation during the depressurizing stage with different scales of hydrate simulator. However, during the constant-pressure stage (the injection stage), the difference is obvious, and the heat transfer rate in the PHS is faster than that in the CHS. Additionally, the system entropy production during the injection stage is the largest, implying that the injection stage is the main source of energy consumption. Moreover, both the ratio of the amount of the dissociated gas in the PHS to that in the CHS and the ratio of the entropy production for hydrate dissociation with the PHS to that with the CHS approximately equal to the volume ratio of the PHS to the CHS.

Influence of Hydrate Saturation on Methane Hydrate Dissociation by Depressurization in Conjunction with Warm Water Stimulation in the Silica Sand Reservoir

In this study, the different saturations of hydrate samples were formed in a cubic hydrate simulator (CHS) filled with silica sand. Subsequently, the hydrate was dissociated by depressurization in conjunction with warm water stimulation using dual horizontal wells. The hydrate dissociation process includes the depressurizing period and the injection period (the constant-pressure period). Hydrate was dissociated simultaneously in the whole reservoir during the depressurizing period. Meanwhile, gas production in the depressurizing period is mainly determined by the depressurizing rate, and it has little relation to the hydrate saturation (when the hydrate saturation ranges from 15.5% to 39.1%). During the injection period, more gas can be produced for the reservoir with the higher hydrate saturation, whereas the highest average gas production rate can be obtained for the reservoir with the middle-higher hydrate saturation. With respect to the gas production in the depressurizing period, gas production in th...

Production performance of gas hydrate accumulation at the GMGS2-Site 16 of the Pearl River Mouth Basin in the South China Sea

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.

Three dimensional experimental and numerical investigations into hydrate dissociation in sandy reservoir with dual horizontal wells

In this study, both three dimensional experimental and numerical investigations of hydrate dissociation have been carried out by depressurization in conjunction with warm water stimulation using dual horizontal wells. Hydrate has been synthesized in the porous sediment in a CHS (Cubic Hydrate Simulator). The results of gas and water production, hydrate dissociation, and temperature distribution by numerical simulation are in good agreement with the experimental results. The results show that hydrate dissociation evolves by means of ablation with double-moving boundary. One dissociation interface moves from the injection well to the neighboring regions, and the other dissociation interface spreads from the boundary to the central reservoir. The assessment of the energy ratio shows that the depressurization in conjunction with warm water stimulation using dual horizontal wells is a promising method for hydrate dissociation. Sensitivity analyses show that raising the injection temperature causes a sharply decline of energy ratio, although it increases the gas production rate at a certain time. Additionally, reducing the intrinsic permeability of the reservoir can decrease the energy ratio and increase the time for the completion of hydrate dissociation.

Analytic modeling and large-scale experimental study of mass and heat transfer during hydrate dissociation in sediment with different dissociation methods

An analytic model of the mass and heat transfers during hydrate dissociation in porous media by depressurization, thermal stimulation, and depressurization in conjunction with thermal stimulation without any empirical correlation is established in this work. Meanwhile, the PHS (Pilot-Scale Hydrate Simulator), a three-dimensional 117.8 L pressure vessel, is used for the investigation into the characteristics of the heat transfers and gas production behaviors during hydrate dissociation with the above dissociation methods. The model is solved analytically, and the predicted results are in good agreement with the experimental data. The results indicate that the sensible heat in porous media is firstly consumed for the hydrate dissociation, and then the heat transferred from the boundaries is employed for dissociating the hydrate in the reservoir by depressurization. The maximum deviations of the predicted gas production and the predicted temperatures with the model are 7.4% and 0.36%, respectively. With the thermal stimulation method, the maximum deviation of the predicted moles of the dissociated hydrate is 7.6%. There are two moving boundaries of the hydrate dissociation in the hydrate reservoir by depressurization in conjunction with heat stimulation. A synergistic effect of depressurization and heat stimulation on hydrate dissociation enhances the hydrate dissociation rates.

Investigation into optimization condition of thermal stimulation for hydrate dissociation in the sandy reservoir

Abstract Investigation of the optimal injection temperature for the hydrate dissociation plays a significant role in the gas hydrate exploitation in the practical field. In this work, the experiments of hydrate dissociation by depressurization in conjunction with thermal stimulation (DT) with the different injection temperatures are carried out in a Cubic Hydrate Simulator (CHS). Evaluation of the entropy production minimization (EPM), the energy ratio and the thermal efficiency are employed to investigate into the optimized injection temperature for hydrate dissociation. The thermal efficiency decreases with the increase of the injection temperature. The optimal injection temperatures for the hydrate dissociation from the points of the maximization of the energy ratio and the minimization of the entropy production, which are equivalent to maximizing the energy production and minimizing the energy consumption, respectively, are 38.8 °C and 37.9 °C. The results of evaluations from the two aspects are in a quite good agreement. Thus, the warm water injection of approximately 38–39 °C is suitable for hydrate dissociation with the DT method, and the hot water injection beyond 39 °C is uneconomical for hydrate dissociation.

Production behaviors and heat transfer characteristics of methane hydrate dissociation by depressurization in conjunction with warm water stimulation with dual horizontal wells

To investigate into the synergistic effect of depressurization and heat stimulation on hydrate dissociation and the three-dimensional heat transfer characteristics during hydrate dissociation in the porous media, a series of the hydrate dissociation experimental runs by the depressurization in conjunction with warm water injection with DWDH (dual horizontal wells) and single depressurization have been carried out in a three-dimensional CHS (cubic hydrate simulator). The results indicate that the gas production process can be divided into the free gas release stage, the mixed gas release stage, and the dissociated gas release stage. In the first two stages, the gas production is mainly controlled by the depressurizing rate. In the third stage, the duration of the hydrate dissociation with the DWDH method (water injection temperature equals to environmental temperature) is much shorter than that by the single depressurization. It is due to the fact that water injection enhances the heat convection and further increases the rate of the hydrate dissociation. The analysis of three-dimensional heat transfer shows that the heat transfer rate along the injection well is the fastest. Energy analysis indicates that the sensible heat of the hydrate reservoir is insufficient for the hydrate dissociation, and the heat for the hydrate dissociation mainly originates from the boundaries.

Preparation of Warm Brine in Situ Seafloor Based on the Hydrate Process for Marine Gas Hydrate Thermal Stimulation

An energy-efficient hydrate-based warm brine preparation method in situ seafloor was first proposed for marine natural gas hydrates (NGH) exploitation. The detailed preparation process and key technologies are discussed. The optimal hydrate-former, cyclopentane + CH4, is viable for preparing warm brine under various seawater depths. The heating coefficient of the warm brine preparation reaches 3.0. The NGH production performance by depressurization in conjunction with the prepared warm brine stimulation was studied by numerical simulation. The warm brine stimulation accelerates gas production. The gas production behavior performs better with the higher salinity and temperature. However, these positive effects are limited by the direct seepage of the brine from the injection well to the production well. The massive water production from the overburden and underburden layers causes low RGW and energy efficiency. Compared to the conventional hot brine injection, the good performance of the warm brine injecti...