Thermal Stimulation Method Research Articles

Oceanic methane hydrate utilization system design and reservoir scale numerical modeling

In recent years, the exploration and utilization of methane hydrate from permafrost and off-coast regions has become a very hot topic. Aiming at its potential as a vast energy resources in future use, the current study is focused on the low emission extraction and utilization system for oceanic methane hydrate and its economic and technological feasibility. In this study, recent real extraction tests in both China and Japan in 2017 are firstly discussed and compared, where the challenges in production period and production rate are explained. In those tests, depressurization method is used to dissociate the methane hydrate for gas production form the unconsolidated reservoir layers below the seabed. Based on the conditions of Japan, a systematic utilization system is proposed for oceanic methane hydratewhich is consisted of extraction system, power generation system, Carbon Capture and Sequestration (CCS) system and hot fluid injection system. Vertical well systems are adopted in this design and the strategic warming up process is used before depressurization extraction, so as to obtain optimal production rate of methane gas. The inclusion of CCS makes it possible to utilize methane hydrate with very low carbon emission. It is estimated that the electricity price obtained by this system can be comparable with the current civil electric price in Japan if the production rate is around 1.0×105 m3/d. Also in this study, the real extraction scenarios and possible strategies of Nankai Trough in Japan and Shenhu Area in China are re-visited and discussed by numerical models. It is found that the only depressurization or thermal stimulation method have the problems of wellbore deformation, pressure problem and energy efficiency problems, while combined strategic modes show the possibility of long-term continuous production. For the basic dissociation parameter analysis, the conditions near the equilibrium curve show higher production rate, which is due to higher initial reservoir temperature. Such results indicate the importance of heat supply inside the reservoir layer as the methane hydrate dissociation is endothermic. It is shown that with the increase of reservoir pressure, such temperature effects become more critical as the optimal initial temperature increases according to equilibrium curve. This results support the system design with strategic warming-up process in a sister well during methane hydrate extraction. In addition, the effects of reservoir layer thermal management, hot reservoir fluid flow effects on the dissociation behaviors are analyzed in this study. For vertical well analysis, the production rates under different permeability parameters show the same trend with real production, which in turn help confirm the in-lab permeability tests. Horizontal well simulation show the importance of overburden and under burden. If the confining layers are less permeable, the production rate becomes higher due to the limited intrusive water flow from the confining layers. In sum, this study summarized the advantages and disadvantages of oceanic methane hydrate extraction/utilization methods and put forward a low-carbon emission system. It is hoped that this study will contribute to the future development of both production prediction and real extraction tests.

Simulating the gas hydrate behavior at equilibrium dissociation: A study from Mahanadi basin of eastern offshore, India

Abstract A suitable model is developed by analyzing the effects of external stimuli such as changes in pressure and temperature on the gas production behavior for the multi-phase fluid system. Our ultimate goal was to assess and understand the response of the Mahanadi basin of eastern India, which is already identified as a prospective zone for gas hydrate exploration, while subjected to dissociation through numerical simulation. Further, a series of simulations were conducted to understand the spatial distributions of several key parameters including hydrate saturation, gas and aqueous phase velocities, heat flux, enthalpy changes, within the system. In this case, we have considered only the equilibrium reaction model to simulate the natural gas production during two vital hydrate dissociation mechanisms such as thermal stimulation and depressurization methods. Coupled equations of heat and mass balance were used in every subdomain of the simulation and the integral difference equations were used to model the hydrate-bearing geologic system of the studied region at different physicochemical environments. The simulation results suggest that the studied field has a great prospect for commercial gas production. Further, we identified that the speed of dissociation front varies with the boundary pressure, and depressurization is the suitable dissociation method to produce more gas from the studied field when compared to the thermal stimulation. The developed model can be used as an avenue for further detailed study of natural gas production from the hydrate reservoir of this region.

Influence of intrinsic permeability of reservoir rocks on gas recovery from hydrate deposits via a combined depressurization and thermal stimulation approach

Reservoir permeability is a crucial controlling factor for the successful exploitation of unconventional gas hydrate resources, which represent a vast natural gas reserve with substantial energy potential. Numerical simulations and analyses are essential tools for the prediction and evaluation of natural gas recovery from hydrate deposits. In this study, a two-dimensional axisymmetric model was developed and validated to investigate the effect of the intrinsic permeability of reservoir rocks on hydrate dissociation characteristics induced by a combined depressurization and thermal stimulation method. Simulation results indicate that the average gas production rate from hydrate deposits could be enhanced when thermal stimulation was additionally applied at the same production pressure, but the enhancement effect weakens as reservoir permeability increases. Pressure reduction propagates slowly from gas production wells into cores with low-permeability, and thermal stimulation dominates hydrate dissociation. However, depressurization can play a determining role for hydrate dissociation in high-permeability cores which benefit to the propagation of pressure reduction. Increased permeability promotes the characteristic shift from thermal-stimulation-governed radial hydrate dissociation to depressurization-determined uniform dissociation. To a certain extent, increased permeability enhances gas generation, but there is a threshold beyond which this effect is no longer felt as excessive consumption of sensible heat restricts further hydrate dissociation. Although there are many uncertainties in the hydrate dissociation process in porous media, numerical simulation can provide useful information for evaluating the feasibility of methodology for gas recovery from gas hydrate reservoirs.

Improving fluid flow in geothermal reservoirs by thermal and mechanical stimulation: The case of Krafla volcano, Iceland

The magmatic-hydrothermal system at Krafla Volcano, North-East Iceland, is an important source of fluids exploited for geothermal energy. Here, we employ laboratory measurements to constrain the porosity and permeability of the main lithologies forming the reservoir, and investigate the impacts of different thermal and mechanical stimulation practices to improve fluid flow.Six main rock types were identified and sampled: three basalts (a dense and a porous lava, and a surficial dyke); a hyaloclastite; an obsidian; an ignimbrite; a felsite; and a gabbro. Permeability measurements were made in a hydrostatic cell using the steady-state flow method at a range of confining pressures (1–100 MPa). The measurements show that permeability generally increases with porosity, but that permeability may vary significantly for a given porosity, depending on the presence of pore connectivity and micro-fractures. We note that an increase in effective pressure results in a decrease in permeability due to closure of pre-existing cracks, abundant in some rocks. When unloading, samples fail to recover pre-loading permeability, as cracks do not necessarily entirely reopen. To further examine the hysteresis imposed by crack closure, we cyclically loaded/unloaded a felsite sample ten times by varying pore pressure which resulted in a further nonlinear decreases in permeability with each pressurisation cycle; thus an understanding of the pressurisation path may be a requirement to constrain fluid flow variations in geothermal systems.To test the effects of thermal stimulation on fluid flow, samples of dense basalt and felsite were thermally stressed by heating to 450 °C and cooling at different rates (in air, in water and at a controlled rate of <5 °C·min−1). The results show that the permeability of originally highly fractured rocks is not affected by thermal stressing, but originally unfractured rocks show a nonlinear increase in permeability with each thermal stressing cycle, especially with the largest thermal shock imposed by quenching in water; thus thermal stimulation may not be expected to result in a similar magnitude of permeability creation along the length of a borehole.Finally, following the permeability measurements on intact rocks, the Brazilian tensile testing method was employed to impart one and two (orthogonal) macro-fractures, and permeability was measured after each step. The creation of one macro-fracture strongly enhanced the permeability of the rock (especially dense rocks), resulting in a narrower range of permeability (as a function of porosity) for the fractured rocks. Imparting a second fracture had trivial additional impact on the permeability of the rock. Yet, the presence of fine fragments and possible minor offset of fracture interfaces was found to obstruct fracture closure, which resulted in higher permeability irrespective of effective pressure; thus hydraulic fracturing may locally increase fluid flow, especially when employing proppants to obstruct fracture closure and ensure a stable permeable network in a reservoir.We discuss the implications of the findings for a first order constraint on the permeability of the reservoir rock and the potential of thermal and mechanical stimulation methods on energy production in geothermal systems nested in active volcanic fields.

Influence of core scale permeability on gas production from methane hydrate by thermal stimulation

The hydrate dissociation process involves heat transfer in the decomposing zone, multi-phase fluid flow during gas production, and the intrinsic kinetics of hydrate dissociation. The potential impact of laboratory-scale permeability on hydrate exploitation from hydrate-bearing sediments was predicted from a previously developed and verified two-dimensional axisymmetric model. We herein continue the previous work to investigate the influence of core-scale hydrate sediments’ permeability on gas production by the thermal stimulation method. The results show that the gas production in relatively low permeability reservoirs proceeded at a faster rate, requiring less time to complete the dissociation process, although an optimal permeability was associated with the fastest gas production. In addition, with the temperature continuously increased, the dissociation front displaced from the boundary wall to the core axis along the radial direction. In a lower permeability system, however, the hydrate dissociation process at the zone opposite the outlet valve was delayed. Due to the varying processes associated with hydrate dissociation, the overall thermal conductivity declined faster at an earlier stage in sediments of high permeability as compared with sediments of lower permeability. Furthermore, the effects of boundary heat transfer were more significant for low permeability systems.

Experimental investigation of quenching effect on mechanical, microstructural and flow characteristics of reservoir rocks: Thermal stimulation method for geothermal energy extraction

Thermal stimulation method can be effectively utilized to enhance the flow performance in the tight porous media with an induced thermal shock. This approach can be applied to many deep geo-engineering applications including geothermal energy extraction. In this regard, understanding the flow-mechanical behaviour of reservoir rock during and after thermal stimulation is necessary for safe and effective stimulation process. The aim of this study is, therefore to investigate the quenching effect on mechanical and flow behaviour of Australian Strathbogie granite by conducting a series of micro and macro-experiments. Experiments were conducted by profiling the micro-structure of rock with CT scanning, microscopic imaging and, quantifying the thermally induced damage due to quenching treatment. In order to understand mechanical response of rock due to quenching, strength tests were conducted under the unconfined condition, and the corresponding fracture propagation patterns were investigated using an acoustic emission (AE) system. In addition, flow performance of thermally treated rock was studied under a wide range of coupled high temperature and high-pressure conditions (temperatures 20–300 °C, confining pressure up to 45 MPa and injection pressure up to 40 MPa), simulating different geothermal temperatures and depths. According to the findings, thermal treatment resulted in around 70% of reduction of strength and elastic properties respectively due to the thermally induced damage caused by induction of both inter and intra-crystalline cracks. Further, increased porosity and crack density significantly enhanced the permeability of the rock compared to the intact rock (from approximately 1 × 10−19 m2 to 6 × 10−15 m2 under 10 MPa confining pressure). However with the increasing of normal stresses, permeability decreased non-linearly and further, increasing temperature resulted in significant reductions in permeability of granite (approximately 95% of reduction from room temperature to 300 °C) due to the thermally induced volumetric expansion which leads to enhancement of interlock effect.

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.

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.

Experimental Investigation into Methane Hydrate Dissociation by Thermal Stimulation with Dual Vertical Well

The gas production behavior of methane hydrate in porous media using the Thermal Stimulation method with dual vertical well was investigated in the Cubic Hydrate Simulator (CHS), a three-dimensional 5.8-L cubic pressure vessel with the internal length of 0.18 m. Three horizontal layers equally divide the CHS into four regions. 9-spot vertical wells and 25-spot thermometers are arranged on each layer, respectively. The production and injection wells are sited on the cross corners of the CHS. The thermal stimulation process is analyzed. The change characteristics of the injection temperature, pressure, and other related parameters during the injection are obtained. The result also verifies that the hydrate decomposition process is a moving boundary ablation process. The hydrate decomposition boundary moves from the injection well to the production well gradually during the thermal stimulation process. The higher injection rate leads to the lower energy efficiency.

An Application of the Results from the Large-Scale Thermal Stimulation Method of Methane Hydrate Dissociation to the Field Tests

Methane hydrate formation and the gas recovery from the hydrates using the thermal stimulation method was studied in a large-scale laboratory reactor. A large-scale laboratory reactor (59 L volume) was used in this study. The efficiencies of gas recovery and energy utilization were studied over two different values of initial hydrate saturation (30% and 50%) and three different values of heating rates (20, 50, and 100 W). Results obtained from the tests demonstrate that with the initial hydrate saturation (SH) remaining constant (50% SH); total recovery of methane increases from 40% to 52% to 73% with a heating rate increase from 20 W to 50 W to 100 W. The average thermal efficiency, however, decreases from 86% to 84% to 82% over the same heating rate range. Alternately at a constant heating rate (100 W), total recovery of methane increased from 67% to 74% when an initial hydrate saturation was increased from 30% to 50%. We show that maintaining a constant heating rate throughout the dissociation is not t...

Numerical analysis of core-scale methane hydrate dissociation dynamics and multiphase flow in porous media

Methane hydrate is one of the most promising future energy resources for humankind. In recent years, due to its vast existence in permafrost regions and deep ocean beds, increasing attention has been paid to the extraction, transportation and utilization of methane hydrate. The current study proposed core-scale numerical investigation models for the complex multiphase dissociation flows of methane hydrate inside porous media, which is a continuation and an extension of previous numerical investigations. The current numerical model focuses on the depressurization process and thermal boundary effects and discusses the parametric effects of the core-scale internal flows and controlling factors of the dissociation boundaries. The new findings with respect to the dissociation front movement and water–ice equilibrium effects during the dissociation process are also analyzed in this study. Ice formation and boundary heat conduction limitations are found to be critical for the smooth production of methane gas. Based on these results, trade off and production strategies for depressurization methods and thermal stimulation methods are also discussed in detail. It is hoped that this study will be useful for related core-scale analysis and possible engineering system designs.

Assessment of gas production from natural gas hydrate using depressurization, thermal stimulation and combined methods

The largest sources of hydrocarbons worldwide are distributed in the permafrost and submarine in the form of methane hydrates, but exploitation of these hydrocarbons is still years away from industrial production; thus, further research is needed.

Preliminary study on broodstock rearing, induced breeding and grow-out culture of the sea cucumberHolothuria scabrain Sri Lanka

Experiments were conducted to identify suitable methods for broodstock rearing, induced breeding and grow-out culture of Holothuria scabra in Sri Lanka. Two hundred and seventy-two brooders (500–600 g) collected from off Mannar were individually packed in oxygen-filled polythene bags with and without sea water and transported to a sea cucumber hatchery at Kalpitiya. Lagoon pens, sand-filled fibreglass tanks and bare tanks were used in triplicates to maintain brooders. Spawning was initiated using air dry, water jet and thermal-stimulation methods. Hatchery produced juveniles with an average weight of 11 ± 5 g were reared (2 individuals m−2) in lagoon pens, mud ponds and fibreglass tanks in triplicates. The significantly high evisceration rate was observed when brooders were transported without sea water (t-test, P < 0.05). Brooders maintained in bare tanks showed a significant weight reduction than the brooders in sand-filled tanks and lagoon pens (anova, P < 0.05, d.f. = 2). Thermal stimulation (ambient temperature ± 3–5°C) was found to be the most successful method of spawning initiation of H. scabra. The mean (±SD) percentage males and females participated for spawning per trial was 9.2 (±10) and 4.6 (±5.6) respectively. On an average, 1.16 millions of eggs (±1.03 SD, n = 5) were obtained per spawning trial. H. scabra juveniles reared in tanks showed significantly lower growth rate than the juveniles in pens and ponds (anova, P < 0.05). Lagoon pens and sand-filled tanks are suitable to maintain brooders and lagoon pens can be successfully used for mass rearing of juveniles.

Assessment of Gas Production Potential from Hydrate Reservoir in Qilian Mountain Permafrost Using Five-Spot Horizontal Well System

The main purpose of this study is to investigate the production behaviors of gas hydrate at site DK-2 in the Qilian Mountain permafrost using the novel five-spot well (5S) system by means of numerical simulation. The whole system is composed of several identical units, and each single unit consists of one injection well and four production wells. All the wells are placed horizontally in the hydrate deposit. The combination method of depressurization and thermal stimulation is employed for hydrate dissociation in the system. Simulation results show that favorable gas production and hydrate dissociation rates, gas-to-water ratio, and energy ratio can be acquired using this kind of multi-well system under suitable heat injection and depressurization driving forces, and the water production rate is manageable in the entire production process under current technology. In addition, another two kinds of two-spot well (2S) systems have also been employed for comparison. It is found that the 5S system will be more commercially profitable than the 2S configurations for gas production under the same operation conditions. Sensitivity analysis indicates that the gas production performance is dependent on the heat injection rate and the well spacing of the 5S system.

Experimental investigation on dissociation driving force of methane hydrate in porous media

As one kind of clean and widely distributed unconventional energy resource, natural gas hydrate (NGH) is attracting increasing attention these years. In this study, the methods of depressurization and thermal stimulation are adopted to dissociate the NGH experimentally, when the initial temperature is 1.0°C, the initial pressure is 3.1MPa, the porosity is 0.35 and initial NGH saturation is 32%. The results reveal that during the depressurization process, there exists a pressure difference threshold (ΔP=0.19MPa), and similarly a temperature difference threshold (ΔT=1.4°C) exists during the process of thermal stimulation. NGH will not dissociate until pressure difference or temperature difference outweighs the corresponding threshold value. In addition, the influencing factors of driving force on the gas production performance have been studied. When thermal stimulation is adopted, the driving force of the temperature difference will grow with the increasing hot-brine injection rate and the brine temperature, in which case the gas production rate will drive up. When depressurization is used, the pressure difference driving force will go up with the increasing depressurizing range and depressurizing rate. Driving force is a major factor during NGH dissociation by depressurization and thermal stimulation.

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.

가스하이드레이트 포화율 및 감압률에 따른 해리특성 분석

가스하이드레이트(GH: Gas Hydrate)는 전 세계적으로 약 10조 톤에 이르는 엄청난 양이 대부분 해양의 대륙사면에 부존되어 있으나(동토 지역 : 2 %, 해양 대륙사면 98 %), 현재까지 가스하이드레이트 저류층으로부터 상업화할 수 있을 만큼 가스를 회수하는 기술이 개발되어 있지 않은 실정이다. 일반적으로 회수하는 방법은 감압법, 열자극법, 억제재 주입법 및 치환법 등으로 크게 나누어 볼 수 있으며, 본 연구에서는 가스하이드레이트 포화율과 감압률에 따라서 가스하이드레이트 해리시간 및 가스생산이 어떻게 달라지는 지, 그에 대한 특성을 분석하고자 하였다. 연구분석 결과 감압률과 해리시간의 상관 관계식을 도출(<TEX>$Y=0.0004X^2-0.499X+176.86$</TEX>)할 수 있었고, 또한 감압률이 클수록 메탄생산량이 좋다는 것을 알 수 있었지만(감압률 40% 대비 50%에서 메탄가스생산량이 46.2% 향상), 감압률이 60%에서는 오히려 생산량이 줄어드는데, 이는 가스하이드레이트 재형성에 기인한 것으로 판단된다. The gas hydrate of 10 trillion tons are buried under continental slope in the world(permafrost : 2%, marine continental slope: 98%), but technology for the the commercial gas recovery has not developed yet. There are normally four representative recovery methods: depressurization method, thermal stimulation method, inhibition injection method, and displacement method. This study focuses on change of dissociation time and gas production according to gas hydrate saturation rate and depressurization rate. It was found that the correlation between depressrization rate and dissociation time was like as <TEX>$Y=0.0004X^2-0.499X+176.86$</TEX>. It was also found that the bigger depressurization rate is, the better production is(methane gas is produced over 46.2% at depressurization rate 50% compared with 40%). However, on the contrary to this, it is presumed that gas production is decreased at 60% due to gas hydrate reformation.

Evaluation of gas production from methane hydrates using depressurization, thermal stimulation and combined methods

Abstract To investigate the gas production from methane hydrate-bearing sediments, the gas production processes from methane hydrate in porous media using depressurization, two-cycle warm-water injection and a combination of the two methods were characterized in this study. The methane hydrates were formed in porous media with various initial hydrate saturation (Shi) in a pressure vessel. The percentage of gas production, rate of gas production, and energy efficiency were obtained and compared using the three methods. The driving force of the hydrate dissociation at different stages of depressurization was analyzed and ice formation during the gas production was observed. For the two-cycle warm-water-injection method, the percentage of gas production and the energy efficiency increased with increasing of Shi. However, due to the large amount of warm water needed to heat the porous media at the dissociation site, the percentage of gas production was lower than the other two methods under the same experimental conditions. The experimental results proved that the combined method had obvious advantages for hydrate exploitation over the depressurization and warm-water-injection method in terms of the energy efficiency, percentage of gas production and average rate of gas production, and with increasing of Shi, the advantages are enhanced. For the Shi of 51.61%, the percentage of gas production reaches 74.87%, which had increments of 18.63% and 31.19% compared with the depressurization and warm-water-injection methods. The energy efficiency for the combined method were 31.47, 49.93 and 68.13 for Shi of 31.90%, 41.31% and 51.61%, respectively.

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.

Heat Transfer during the Dissociation of Hydrate in Porous Media

The decomposition of gas hydrate accompanied with phase transformation is an endothermic process demanding approximately 52-60kJ of latent heat per mole of methane in hydrate. Energy is a key factor that restricts the hydrate exploitation. Hence the heat transfer plays a very significant role in the decomposition of hydrate. This paper describes a simple analytical model in cylindrical coordinate for simulating gas production from homogeneous hydrates in porous media by the thermal stimulation method. The results from the analytical solution can predict the temperature distribution in the decomposed and undecomposed hydrate zone, the decomposing interface location at any time, and the rate of the decomposition. This simple model can be used to perform the feasibility of commercial gas production from hydrates reservoirs in the South China Sea region using the warm-seawater at surface. It is of great significance to guide the practical production.