Pilot-Scale Hydrate Simulator Research Articles

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

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

Large-scale experimental study of heterogeneity in different types of hydrate reservoirs by horizontal well depressurization method

The heterogeneity of hydrate distribution and decomposition in sandy reservoirs is an important issue. Because large hydrate simulators are scarce and visualization difficult, heterogeneity in large-scale hydrate reservoirs has been consistently underestimated. In this study, a pilot-scale hydrate simulator with an effective volume of 117.8 L was used to simulate the horizontal well depressurization decomposition of two different types of hydrate reservoirs: gas-saturated and water-saturated. The production performance and heterogeneity of these two hydrate reservoirs were analyzed during the hydrate decomposition process. For the first time, hydrate decomposition heterogeneity was classified into planar heterogeneity and interlayer heterogeneity based on the heterogeneous characteristics in the horizontal and vertical directions. Based on the pressure difference and duration, a dimensionless factor was proposed to quantify the intensity of the heterogeneity at a specific location. Furthermore, this study related the hydrate decomposition heterogeneity intensity variation process to the variations in the two-dimensional geometric model for hydrate distribution in the pore space. The experimental results showed that horizontal well extraction using the depressurization method was highly efficient. During the hydrate decomposition process, planar heterogeneity was stronger in the water-saturated hydrate reservoir than in the gas-saturated hydrate reservoir. In gas-saturated hydrate reservoir, hydrate saturation and heterogeneity increased from top to bottom. In water-saturated hydrate reservoir, hydrate saturation and interlayer heterogeneity increased first and then decreased from top to bottom. The study of heterogeneity was critical for optimizing the hydrate extraction scheme, and emphasis should be placed on the placement of production enhancement facilities in regions with significant heterogeneity.

Pilot-Scale Experimental Investigation of Multifield Coupling and Heterogeneity during Hydrate Dissociation

Natural gas hydrates are considered as a potential energy resource for the future. In this study, for the first time, the pilot-scale hydrate simulator (PHS), with an effective volume of 117.8 L, w...

The consistency of the normalized hydrate dissociation rate in the hydrate simulator with different scales

Recently, in order to perform the gas production test from methane hydrate reservoir in laboratory, the size of the experimental simulator is developing towards a larger scale. The characteristics of the heat transfer and the mass transport in the hydrate-bearing sediments with different scales are varied, thereby resulting in the different hydrate dissociation behaviors. In this study, the SCHS (Small Cubic Hydrate Simulator, 0.729 L), the CHS (Cubic Hydrate Simulator, 5.832 L), and the PHS (Pilot-scale Hydrate Simulator, 117.8 L) in our laboratory were applied to study the hydrate dissociation by depressurization. The experimental results showed that, in the depressurization stage, the volume of the hydrate dissociation was determined by the sensible heat of the sediments and the heat transferred from the surroundings. During the constant pressure stage, the normalized hydrate dissociation rates (vnorm) was first proposed to define the average rate of the hydrate dissociation per temperature and per shape factor. The vnorm excluded the influence of the driving force of the heat transfer and the scale of the hydrate simulator on the hydrate dissociation. Therefore, for the hydrate simulator with different scales, the value of the vnorm for different production pressures were similar. The calculated value of the vnorm for different runs were not completely be same due to the experimental error. However, the average value of the vnorm for different runs could be used to predicate the average hydrate dissociation rate in the hydrate simulator with other scales. For example, during the gas production from a hydrate reservoir with the same shape and double length/radius of the PHS, the average hydrate dissociation rate can be calculated as 12.44 ml/min, and the total duration time of the hydrate dissociation can be calculated as 8.58 days.

Heterogeneity properties of methane hydrate formation in a pilot-scale hydrate simulator

The accumulation and distribution features of gas hydrates are the significant issues affecting the hydrate resource assessment, the hydrate dissociation behaviors, and the risk evaluation of hydrate exploitation. This work aims to investigate the methane hydrate formation process by numerical simulation based on the experimental data in a pilot-scale hydrate simulator. Methane hydrate is formed by multi-step water injection method. Simulation results of the evolutions of the temperature, the system pressure, the mass of formed hydrate, and the remaining mass of methane gas are in good agreement with those of the experiment. The spatial distributions of the gas, water, and hydrate are all found to be very heterogeneous in the vessel during the whole simulation period. The free methane gas tends to accumulate in the pores near the upper boundary because of buoyancy and diffusion effects, while the liquid water shows an opposite trend under the gravity and capillary effects. Finally, the formed hydrates are found to be mainly accumulated in the upper area of the reactor due to the more favorable gas-water contact conditions, and the hydrate saturation decreases from the roof to the bottom layer by layer. In addition, the inhomogeneity of methane hydrate is more pronounced along the vertical direction than the horizontal one during the formation process. Generally, the heterogeneous formation of gas hydrate is dominated by the migration processes of gas and water in porous media. Moreover, the selected kinetic model is also important for the description of the heterogeneous hydrate formation behavior.

Influence of heat conduction and heat convection on hydrate dissociation by depressurization in a pilot-scale hydrate simulator

Natural gas hydrate, as an unconventional energy resource, has generated considerable research interest. It is generally accepted that depressurization method is the most practical and economically promising way to produce gas from gas hydrate sediments. Rates of hydrate dissociation by depressurization depend on heat transfer rate, and the heat transfer during hydrate dissociation mainly includes heat conduction and heat convection. In this paper the Pilot-Scale Hydrate Simulator (PHS), with an inner volume of 117.8 L, was applied to investigate the influence of heat conduction and heat convection on hydrate dissociation. Different thermal boundary conditions and different flow directions during gas recovery from hydrate reservoir by depressurization were performed in the PHS. In addition, the method of studying the effect of different directions of heat convection by changing well locations was firstly proposed in this paper. It was obtained from experimental results that the hydrate dissociation rate with an isothermal boundary is faster than that with a semi-adiabatic boundary, and heat conduction is the dominant factor in hydrate dissociation by depressurization in the constant pressure stage. The influence of heat convection on hydrate dissociation in the constant pressure stage may not be obvious, but during the depressurizing stage, the opposite direction of fluid flow and heat transfer can promote hydrate reformation, and has effect on fluid flow characteristics inside the reservoir. These findings can provide theoretical references for field tests of exploiting natural gas hydrate.

Large scale experimental investigation of influence of heat conduction and heat convection on hydrate dissociation by depressurization in sandy sediment

Natural gas hydrate can be regarded as alternative energy source in future due to huge reserves of methane gas trapped in hydrate bearing formations. According to the laboratory studies and field programs, depressurization method has been considered as the most cost-effective and practical way to dissociate gas hydrates. Rates of hydrate dissociation by depressurization mainly depend on heat transfer rate. The heat transfer during hydrate dissociation mainly includes heat conduction and heat convection. In this work, the Pilot-Scale Hydrate Simulator (PHS), with an inner volume of 117.8 L, was applied to investigate the influence of heat conduction and heat convection on hydrate dissociation. Different thermal boundary conditions and different flow directions during gas recovery from hydrate reservoir by depressurization were performed in the PHS. The experimental results indicate that hydrate dissociation rate with isothermal boundary is fast than that with semi-adiabatic boundary. However, the influence of heat convection direction on heat dissociation in the CP stage may not be obviously. The heat transfer rate in the CP stage of the depressurization mainly depends on the heat conduction rate.

Pilot-scale experimental test on gas production from methane hydrate decomposition using depressurization assisted with heat stimulation below quadruple point

Natural gas hydrate can be regarded as alternative energy source in future. Therefore, developing approaches for enhancing gas recovery from hydrate reservoir is attracting extensive attention. A Pilot-Scale Hydrate Simulator (PHS) with the effective volume of 117.8 L was applied for investigating gas recovery from hydrate dissociation below quadruple point in porous media, where hydrate exists with ice, water, and methane gas. Depressurization and depressurization assisted with heat stimulation below quadruple point were selected as the hydrate decomposition method. The influence of heat stimulation on hydrate decomposition below quadruple point was evaluated. The experimental results indicate that the hydrate decomposition rate can be greatly enhanced by decreasing the pressure below quadruple point, because ice can be generated during hydrate decomposition below quadruple point. Heat released by ice formation can immediately supply to hydrate decomposition. During hydrate decomposition experiment by depressurization assisted with heat stimulation, the influence of heat stimulation on hydrate recovery below quadruple point is not obviously, because injected heat is used for ice melting rather than hydrate dissociation. Therefore, heat stimulation may not enhance hydrate dissociation below quadruple point.

Pilot-scale experimental evaluation of gas recovery from methane hydrate using cycling-depressurization scheme

Methane hydrate is considered as a potential source of methane for energy supply. Therefore, developing approaches for enhancing gas recovery from hydrate reservoir is attracting extensive attention. The Pilot-Scale Hydrate Simulator (PHS), with an inner volume of 117.8 L, was applied to investigate gas recovery approach from hydrate reservoir. A novel cycling depressurization was carried out to improve the production efficiency of depressurization method. Three different schemes for gas recovery from hydrate reservoir were performed in the PHS, which were the Regular Depressurization (RD), the Semi-Cycling Depressurization (Semi-CD), and the Cycling Depressurization (CD), respectively. The production behaviors and heat transfer characteristics during hydrate dissociation in sandy sediments by different methods were compared and investigated. The advantages of the novel cycling depressurization were analyzed. The experimental results indicate that the effective average gas production rate in the experiments by CD is 17 times larger than that by RD. The energy cost per volume of gas production by the CD scheme can be significantly reduced by comparing with the RD scheme. Therefore, the production efficiency can be strongly enhanced by using cycling depressurization method. If the hydrate is dissociated by RD, the heat transfer is strongly coupled with the hydrate dissociation. However, if the hydrate is dissociated by Semi-CD or CD, the coupling of heat transfer and hydrate dissociation may be changed. During the well closing stage in the Semi-CD or CD scheme, the lower fluids flow rate in pores leads to a lower heat transfer rate, which leads to a lower hydrate dissociation rate in well closing stage.

Large Scale Experimental Evaluation to Methane Hydrate Dissociation below Quadruple Point by Depressurization Assisted with Heat Stimulation

The Pilot-Scale Hydrate Simulator (PHS), a three-dimensional 117.8 L pressure vessel, is applied to study the methane hydrate dissociation below the quadruple point in the sandy sediment in this work. The hydrate dissociation behaviors below and above the quadruple point by depressurization method and depressurization assisted with heat stimulation method are compared. The results indicate that methane hydrate dissociation below the quadruple point causes ice formation, which can strongly enhance the dissociation rate of the hydrate. The water generated from hydrate dissociation below the quadruple point may immediately form ice. Meanwhile, the hydrate dissociation below the quadruple point consumes the latent heat released by ice formation. In addition, it is found by depressurization assisted with heat stimulation that the heat injection has little influence on hydrate dissociation, because the injected heat is used for ice melting rather than hydrate dissociation.

Fluid flow mechanisms and heat transfer characteristics of gas recovery from gas-saturated and water-saturated hydrate reservoirs

Due to the huge reserves, natural gas hydrate is considered as a potential energy resource in future. Therefore, developing methods of gas recovery from hydrate reservoirs for commercial production are attracting extensive attention. In this work, hydrate dissociation and gas recovery from the gas-saturated and water-saturated hydrate accumulations are investigated in a pilot-scale hydrate simulator. Depressurization, thermal stimulation, and depressurization assisted thermal stimulation method are adopted in this work. Furthermore, the mechanisms of fluid flow and the heat transfer during hydrate dissociation in different hydrate accumulations are elucidated by large-scale experimental results. The experimental results indicate that the fluid flow mechanisms and the heat transfer characteristics during the gas recovery from hydrate reservoirs are greatly influenced by the initial water saturation. The Optimum gas production method is also different for different hydrate accumulations. The depressurization is optimized method for hydrate dissociation in the gas-saturated reservoir considered from the aspect of gas-water ratio. Thermal stimulation results in the lowest gas-water ratio and the lowest hydrate dissociation ratio, and is not effective for both the gas-saturated and water-saturated hydrate reservoir. The depressurization assisted thermal stimulation is the optimum method for the hydrate dissociation in the water-saturated sample.

Entropy generation analysis of hydrate dissociation by depressurization with horizontal well in different scales of hydrate reservoirs

Based on the hydrate conditions of the South China Sea, hydrate samples were synthesized in the Cubic Hydrate Simulator (CHS) and the Pilot-Scale Hydrate Simulator (PHS), and hydrate dissociation experiments by depressurization with single horizontal well were carried out. In order to illuminate the characteristic of the irreversible energy loss during the gas production process in a large-scale hydrate simulator and a smaller hydrate simulator, the entropy generation model was established. Results show that the evolutions of the pressure, temperature, gas production, and water production during hydrate dissociation process with different scales of hydrate reservoir are similar. Moreover, entropy generation in the mixed gas release stage is the largest. In addition, in the dissociated gas release stage, the ratio of entropy generation decreases remarkably with the increase of the hydrate reservoir scale, and constant-pressure depressurization method is favorable for hydrate dissociation in a larger scale hydrate reservoir.

Experimental and modeling analyses of scaling criteria for methane hydrate dissociation in sediment by depressurization

Three high pressure reactors with different inner volumes, which are named as the Pilot-scale Hydrate Simulator (PHS), the Cubic Hydrate Simulator (CHS), and the Small Cubic Hydrate Simulator (SCHS), are applied for investigating hydrate dissociation by depressurization method. The volume of the PHS, the CHS, and the SCHS are 117.80L, 5.80L, and 0.73L, respectively. Meanwhile, the model of scaling criterion for hydrate dissociation by the depressurization method is developed as well. The scaling criteria are verified and modified by the hydrate dissociation experiments with different scales. Finally, the gas production from a field scale hydrate reservoir (FSHR) is predicted by scaling the experimental results using the modified scaling criteria. The results indicate that the ratios of gas production in the depressurizing (DP) stage are similar to the ratios of inner volume, which verify the scaling criteria in the DP stage. However, the scaling criteria for the experiments in the constant-pressure (CP) stages need to be modified by the experimental results. The correction factor is 0.89. By using the modified scaling criteria, the gas production behavior, the hydrate dissociation process, and the heat transfer process in a larger scale hydrate reservoir can be predicted. The maximum deviations between the calculated value and experimental result are less than 16%, which can be accepted. In the FSHR with the diameter of 50m and the length of 60m, the predicted results indicate that 3.74×106m3 of gas are produced in 120h (5days) during the DP stage, and 1.94×106m3 of gas are produced in 1.86×105h (7750days) during the CP stage.

A pilot-scale study of gas production from hydrate deposits with two-spot horizontal well system

The hydrate dissociation and gas production behaviors are investigated in a pilot-scale hydrate simulator (PHS) using a novel two-spot horizontal well system through experimental and numerical simulations. There are two horizontal wells located in the same symmetrical plane of the cylindrical reactor. Gas and water are produced from the lower well under depressurization, while hot water with the temperature of 80.0°C is injected into the PHS from the upper one at a constant rate of 50mL/min. Both the experimental and numerical results show that nearly all the produced gas is originated from hydrate dissociation, and the initial free gas in the vessel contributes little to the final gas production. The injected hot water is more likely to flow downwards under the gravity effect and be produced from the lower well, and hydrate dissociation is mainly stimulated by the heat conduction in the reactor. Two irregular dissociation interfaces are observed to surround the hydrate undissociated areas. One interface expands from the upper well, while the other one shrinks from the boundaries. The results of the gas-to-water ratio and the obtained net energy indicate that it is commercially feasible for hydrate exploitation using this kind of well design. In addition, it is found that the production efficiency of the system is strongly dependent on the initial hydrate saturation and the hot water injection rate.

Large Scale Experimental Investigation on Influences of Reservoir Temperature and Production Pressure on Gas Production from Methane Hydrate in Sandy Sediment

The Pilot-Scale Hydrate Simulator (PHS), a three-dimensional 117.8 L pressure vessel, was applied to study the methane hydrate dissociation with different reservoir temperatures and different production pressures in the sandy sediment. The volume of the vessel is big enough to simulate the field-scale gas production from hydrate reservoir. The depressurization method and the depressurization assisted with heat stimulation method were performed as the hydrate dissociation methods. Three different temperatures, which are 4.7 °C, 8.8 °C, and 13.0 °C, were selected as the reservoir temperatures. The range of temperature in this work is the most common temperatures of hydrate reservoir in the ocean sediment. The experimental results indicate that, for the depressurization method, the temperature drop in the reservoir during hydrate dissociation is the key factor for the amount of hydrate dissociation in the depressurization (DP) stage and the rates of hydrate dissociation in the constant-pressure (CP) stage, w...

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.

Large scale experimental evaluation to methane hydrate dissociation below quadruple point in sandy sediment

The Pilot-Scale Hydrate Simulator (PHS), a three-dimensional 117.8L pressure vessel, is applied to study the methane hydrate dissociation below the quadruple point in the sandy sediment in this work. The hydrate dissociation behaviors below and above the quadruple point are compared. The influences of the production pressure, the initial reservoir temperature, and the water saturation on the hydrate dissociation below the quadruple point by depressurization are investigated. The results indicate that methane hydrate dissociation below the quadruple point causes ice formation, which can strongly enhance the dissociation rate of the hydrate. The water generated from hydrate dissociation below the quadruple point may immediately form ice and the pore water in the reservoir turns into ice at the same time. Meanwhile, the hydrate dissociation below the quadruple point consumes the latent heat released by ice formation. The lower production pressure causes the higher driving force for hydrate dissociation and ice formation, which results in the higher dissociation rate of the hydrate. In addition, when the production pressure is lower than the quadruple point, a lower initial reservoir temperature is favorable for ice formation, which leads to the higher hydrate dissociation rate. The experimental results from hydrate dissociation in the ‘water-saturated’ reservoir and ‘gas-saturated’ reservoir indicate that the rate of ice formation is slower in the ‘water-saturated’ reservoir.

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.

Effect of horizontal and vertical well patterns on methane hydrate dissociation behaviors in pilot-scale hydrate simulator

Exploitation of natural gas hydrate is expecting to be an important strategic way to solve the problem of energy depletion. Understanding the effectiveness of the well configuration plays a pivotal role in gas production from the hydrate reservoir. This study evaluates the methane hydrate dissociation behaviors using both vertical well and horizontal well experimentally. Methane hydrate in porous media has been synthesized in a 117.8 L pilot-scale hydrate simulator (PHS), which is equipped with 9 (3 x 3) vertical wells and 9 (3 x 3) horizontal wells. The condition of hydrate formation is corresponding to the ocean depth of 1200 m and it is similar to the hydrate characteristics of the South China Sea. Hydrate is dissociated under depressurization and thermal stimulation. The results indicate that, for the depressurization and thermal stimulation methods, the gas production rate, the heat transfer rate, and the accumulative dissociation ratio with the horizontal well pattern are higher than those with the vertical well pattern. Meanwhile, the evaluations of the energy ratio and the thermal efficiency indicate that the horizontal well pattern has the advantage of higher production efficiency by the thermal stimulation. Thus, it is determined that the production performance is better using the horizontal well pattern. (C) 2015 Elsevier Ltd. All rights reserved.

Depressurization induced gas production from hydrate deposits with low gas saturation in a pilot-scale hydrate simulator

The kinetic behaviors of methane hydrate dissociation under depressurization in porous media are investigated through experimental and numerical simulations. Hydrate samples with low gas saturations (SG⩽0.10) are synthesized in the pilot-scale hydrate simulator (PHS), a novel three-dimensional pressure vessel with effective inner volume of 117.8L. Three experimental runs with different production pressure at the central vertical well have been carried out. The intrinsic dissociation rate constant k0 is fitted to be approximately 4578kg/(m2Pa s) using the experimental data of run 1, and it is used for the kinetic simulation in all the three runs. The whole production process can be divided into two stages: the free gas and mixed gas production (stage I) and the gas production from hydrate dissociation (stage II). Both the experimental and numerical simulation results show that the gas production rate increases with the decrease of the production pressure, while the water extraction rate will rise much higher if the wellbore pressure is dropped extremely low. The free gas saturation is found to be a key factor that affects the overall production behaviors of marine hydrate deposits. In addition, the comparisons of the kinetic and equilibrium models indicate that the kinetic limitations are very small in the PHS. The hydrate dissociation under depressurization in the PHS is mainly controlled by the mass and heat transfer processes.