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This study reports an investigation into both isotropic and anisotropic permeability effects on gas production behavior during depressurization-induced natural gas hydrate dissociation at site NGHP-01-10D in the Krishna-Godavari basin. Numerical simulations were performed on a reservoir-scale model incorporating a single vertical well, examining different scenarios of permeability ratios (rrz). The investigation assessed gas and water production rates, cumulative production volumes, the gas-to-water ratio, and the spatial distribution of reservoir parameters throughout a production duration of 3 years. The findings indicate that permeability anisotropy has a substantial impact on hydrate dissociation and gas recovery. For rrz > 1, horizontal pressure propagation was promoted and gas production increased. For example, at t = 1100 days, the total gas production improved from 7.88 × 105 ST m3 for rrz = 1 to 55.9 × 105 ST m3 for rrz = 10. For rrz < 1, vertical pressure propagation resulted in higher water production with concomitantly lower rates of gas production rates. Spatial distribution analysis revealed that higher rrz values led to more extensive radial propagation of pressure drop, temperature decrease, gas saturation increase, and hydrate dissociation. The study concludes that higher horizontal permeability enhances depressurization effects, resulting in higher gas production rates and more favorable gas-to-water ratios. [ABSTRACT FROM AUTHOR]

The sediment type in the "Shenhu" sea area of the South China Sea is mud sediment, with poor reservoir physical conditions, and the seabed is loose with a small pressure window. These conditions pose heightened safety challenges for the extraction of hydrates in this region. This study develops a multiphase flow model that accounts for the endothermic decomposition of hydrates and employs the finite difference method for its solution. Utilizing this model, the multiphase flow characteristics during depressurization extraction at a specific well in the "Shenhu" area are investigated. Building on this, the study analyses the impact of varying pump rates and geothermal gradients on multiphase flow properties, hydrate yield, and engineering safety. Based on the analytical findings, recommendations are proposed to balance hydrate production with engineering safety, effectively mitigating potential engineering accidents during depressurization extraction. The outcomes of this research offer technical guidance for the commercial exploitation of hydrates in the "Shenhu" area of the South China Sea, laying a foundation for future related studies. [ABSTRACT FROM AUTHOR]

This paper is an investigation of the gas mixing and depressurization effects of containment spray on hydrogen risk during a typical severe accident in a light water reactor (LWR). Two spray tests (ST3_0 and ST3_1) were simulated using the OECD/SETH-2 project frame; the tests were performed with different preconditions in two interconnected vessels of the PANDA facility by focusing on the breaking-up of the stratified helium-rich layer and helium transport between the interconnected vessels with and without heat and mass transfer. The computational fluid dynamics simulations were performed using an Eulerian-Lagrangian approach, in which the dispersed droplets were tracked with a Lagrangian framework and the heat and mass transfer model between the droplets and gas were developed through user-defined functions. The simulation results are in reasonable agreement with the test data and reproduce the main phenomena of the spray tests. Although the proposed approach is adequate for addressing similar problems, some discrepancies still exist in the simulations. Based on these discrepancies, some recommendations are suggested to improve the accuracy of the proposed approach. [ABSTRACT FROM AUTHOR]

Geomechanical and heat transfer characteristics of gas hydrate-bearing sediment (GHBS) are significantly affected by hydrate dissociation during gas production from reservoirs, which is typically tens of meters in thickness. This paper presents the development of an innovative in-flight apparatus that is capable of modeling hydrate dissociation in GHBS on a geotechnical centrifuge, by which a series of model tests are conducted under normal gravity (1g) and hypergravity (100g and 80g). The effects of the hypergravity field on the development of pore pressure, soil deformation, as well as particle migration and gas production during hydrate dissociation are explored. Results show that gas released from hydrate dissociation increases excess pore pressure and changes the soil pore structure. During hydrate dissociation, the accumulation of excess pore pressure leads to the development of gas-driven fractures and the subsequent formation of a dominant seepage channel. The critical excess pore pressure of fracture formation in the 100g test is higher than that in the 1g test. The dominant seepage channel promoting fluid seepage and fine particle migration is more likely to be formed into obvious fracture structures under 100g compared with slender pipe structures under 1g. Two peaks are witnessed in gas production in the 100g test, corresponding to the stage at maximum pressure difference and at the complete formation of dominant seepage channels, which is consistent with that in the field trails. These results indicate that the formation of fracture during hydrate dissociation is beneficial to efficient gas production, while the problem of particle migration should be carefully paid attention to. [ABSTRACT FROM AUTHOR]

In this paper, a one-dimensional model of gas–water two-phase productivity for hydrate depressurization is established, which takes into account permeability variation and gas–water two-phase flow. By solving the coupled algebraic equations of dissociation front position, equilibrium temperature, and pressure in an iterative scheme, the movement law of the hydrate dissociation front and the evolution process of temperature and pressure near the well were obtained, and the effects of bottom hole pressure, reservoir temperature, and hydrate saturation on productivity were analyzed. The results show that the hydrate reservoir is divided into a decomposed zone and an undecomposed zone by the dissociation front, and the temperature and pressure gradients of the former are greater than those of the latter. Reducing bottom hole pressure, increasing reservoir temperature, and increasing hydrate saturation all lead to an increase in temperature and pressure gradient in the decomposed zone. Methane gas production is a sensitive function of bottom hole pressure, reservoir temperature, and hydrate saturation. The lower the bottom hole pressure, the higher the reservoir temperature, the lower the hydrate saturation (within a certain range), and the higher the gas production rate. The trend of the water production curve is the same as that of gas, but the value is 3–4 orders of magnitude smaller, which may be due to the large difference in the viscosity of gas and water, and the gas seepage speed is much larger than that of water. [ABSTRACT FROM AUTHOR]

Natural gas hydrate is a kind of low-carbon and clean new energy, so research on its efficient extraction in terms of theory and technology is particularly important. Combined thermal injection and depressurization is an effective method for extracting natural gas hydrate. In this study, the classical local heat equilibrium model was modified, and a pore-scale fully coupled unsteady heat transfer model for hydrate reservoirs was set up by considering multiple forms of heat flow accompanying hydrate's decomposition and gas–liquid flow. Based on this model and the basic geological information of the X2 hydrate reservoir in the western Pacific Ocean, a numerical model of gas hydrate extraction using combined heat injection and depressurization was constructed to simulate the production performance of the hydrate reservoir. The results were fully compared with the results obtained by the depressurization method alone. The results indicated the hydrate extraction via a combined heat injection and depressurization would have a cumulative gas production of 31.609 million m3 and a cumulative water production of 1.5219 million m3, which are 72.57% higher and 31.75% lower than those obtained by depressurization alone, respectively. These study results can provide theoretical support for the industrial extraction of gas hydrate in seas. [ABSTRACT FROM AUTHOR]

In situ electric heating is an important method used to increase production capacity during the extraction of natural gas hydrates. This work numerically evaluated the sensitivity of different heating parameters on gas production behavior with a vertical well depressurization in the Shenhu Sea area hydrate reservoir, the production pressure difference of 4 MPa, and continuous depressurization for 1080 days. The results showed that the in situ electric heating method can effectively enhance production capability by promoting hydrate dissociation and eliminating secondary hydrates. Compared with scenarios without heating, implementing whole wellbore heating (100 W/m) increases cumulative gas production (Vg) by 118.56%. When intermittent heating is applied to the local wellbore (15 m) located in the three-phase layer (with an interval of 30 days) and stops heating in advance at 480 days, there is no significant difference in Vg compared to the whole wellbore heating case, and the cumulative heat input is only 4.76%. We recommend considering intermittent heating of the local wellbore and stopping heating in advance during vertical well depressurization as this approach significantly reduces heating energy consumption while simultaneously improving production capacity. [ABSTRACT FROM AUTHOR]

Multilayer commingled production is a widely used development method to improve the production capacity of gas reservoirs. However, there is currently limited research on the gas production characteristics of multilayer commingled production in hydrate reservoirs with underlying gas. The objective of this study was to analyze the characteristics of multilayer commingled production in order to determine suitable hydrate reservoirs for such a development method. Firstly, we employed analytical solutions to the equations of fluid flow in porous media to determine the factors affecting the production capacity. Then, by employing numerical simulation and depressurization methods, the rates of gas production and gas release from hydrate dissociation in a single production well were estimated. Additionally, the production capacity ratio of multilayer commingled production and separated-layer production was proposed. The influence of different reservoir characteristics on multilayer commingled production yield was determined and plotted. When there is an interlayer between hydrates and the underlying gas, the formation pressure ratio is the decisive factor affecting the multilayer commingled production yield. When there is no interlayer, the multilayer commingled production rate will increase with an increase in the permeability ratio, hydrate saturation, and underlying gas saturation. This study provides a theoretical foundation for predicting the production capacity of hydrate reservoirs, as well as assistance in selecting the hydrate reservoirs most suitable for multilayer commingled production. [ABSTRACT FROM AUTHOR]

Natural gas hydrate has huge reserves and is widely distributed in marine environment. Its commercial development is of great significance for alleviating the contradiction between energy supply and demand. As an efficient research method, numerical simulation can provide valuable insights for the design and optimization of hydrate development. However, most of the current production models simplify the reservoir as a two-dimensional (2D) horizontal layered model, often ignoring the impact of formation dip angle. To improve the accuracy of production prediction and provide theoretical support for the optimization of production well design, two three-dimensional (3D) geological models with different dip angles based on the geological data from two typical sites are constructed. The vertical well, horizontal well and multilateral wells are deployed in these reservoirs with different permeabilities to perform production trial, and the sensitivity analysis of dip angles is also carried out. The short-term production behaviors in high and low permeability reservoirs with different dip angles are exhibited. The simulation results show that 1) the gas and water production behaviors for different well types in the two typical reservoirs show obviously different variation laws when the short-term depressurization is conducted in the inclined formation; 2) the inclined formation will reduce the gas production and increase the water extraction, and the phenomena becomes pronounced as the dip angle increases, particularly in the low-permeability reservoirs; 3) and the impact of formation dip on hydrate recovery does not change significantly with the variation of well type. [ABSTRACT FROM AUTHOR]

This study reports an investigation into both isotropic and anisotropic permeability effects on gas production behavior during depressurization-induced natural gas hydrate dissociation at site NGHP-01-10D in the Krishna-Godavari basin. Numerical simulations were performed on a reservoir-scale model incorporating a single vertical well, examining different scenarios of permeability ratios (rrz). The investigation assessed gas and water production rates, cumulative production volumes, the gas-to-water ratio, and the spatial distribution of reservoir parameters throughout a production duration of 3 years. The findings indicate that permeability anisotropy has a substantial impact on hydrate dissociation and gas recovery. For rrz > 1, horizontal pressure propagation was promoted and gas production increased. For example, at t = 1100 days, the total gas production improved from 7.88 × 105 ST m3 for rrz = 1 to 55.9 × 105 ST m3 for rrz = 10. For rrz < 1, vertical pressure propagation resulted in higher water production with concomitantly lower rates of gas production rates. Spatial distribution analysis revealed that higher rrz values led to more extensive radial propagation of pressure drop, temperature decrease, gas saturation increase, and hydrate dissociation. The study concludes that higher horizontal permeability enhances depressurization effects, resulting in higher gas production rates and more favorable gas-to-water ratios.

Natural gas hydrate (NGH) is generally produced and accumulated together with the underlying conventional gas. Therefore, optimizing the production technology of these two gases should be seen as a relevant way to effectively reduce the exploitation cost of the gas hydrate. In this study, three types of models accounting for the coexistence of these gases are considered. Type A considers the upper hydrate-bearing layer (HBL) adjacent to the lower conventional gas layer (CGL); with the Type B a permeable interlayer exists between the upper HBL and the lower CGL; with the type C there is an impermeable interlayer between the upper HBL and the lower CGL. The production performances associated with the above three models are calculated under different conditions, including only a depressurized HBL (only HBL DP); only a depressurized CGL (only CGL DP); and both the HBL and the CGL being depressurized (HBL + CGL DP). The results show that for Type A and Type B coexistence accumulation models, when only HBL or CGL is depressurized, the gas from the other layer will flow into the production layer due to the pressure difference between the two layers. In the coexistence accumulation model of type C, the cumulative gas production is much lower than that of Type A and Type B, regardless of whether only HBL DP, only CGL DP, or HBL + CGL DP are considered. This indicates that the impermeable interlayer restricts the cross-flow of gas between HBL and CGL. For three different coexistence accumulation models, CGL DP has the largest gas-to-water ratio. [ABSTRACT FROM AUTHOR]

CO2 sequestration in sediments as solid hydrate is considered a potential way to capture and store anthropogenic CO2. When CO2 hydrate is formed in front of CO2 migration, the injection channel will be blocked, and the removal of hydrate blockage becomes the first problem that must be faced. This work proposed an N2 injection method to remove CO2 hydrate blockage. Based on numerical simulation, a study was conducted using TOUGH+MIXHYD v.1.0 to confirm the feasibility of N2 injection and compare it to depressurization. The spatial and temporal distribution characteristics of pressure, temperature, hydrate saturation, and gas saturation were investigated. Under the combined effects of temperature, pressure, and gas composition, secondary CO2-N2 hydrate can form far from the injection point, causing an increase in local temperature and hydrate saturation. The rate of CO2 hydrate dissociation using direct depressurization is significantly slower compared to N2 injection methods. As the pressure of N2 injection increases, the rate of CO2 hydrate dissociation notably accelerates, which does not show a significant increase with increasing injection temperature. This work introduced a novel approach to addressing the issue of CO2 hydrate blockage, which holds prominent significance for the advancement of hydrate-based CO2 geological sequestration. [ABSTRACT FROM AUTHOR]

In this work, the factors that influence indoor radon (Rn) level and most usual methods to prevent or mitigate Rn are presented and discussed, according to their way of action. The findings show that it is possible to select the most appropriate and effective solutions for each situation, even in cases where there is a high risk of Rn concentration, by combining these methods to increase overall efficiency. Finally, most relevant strategic recommendations are presented to guarantee the success of Rn protection measures, to reduce problems associated with Rn.

The residual oil saturation of the matrix near the well zone of a tight reservoir is high due to the tight reservoir’s complex conditions, such as the small pore throat radius and low permeability of the matrix and the development of microfractures, which can result in serious water channeling, even after long-term water injection development. The aim of this paper is to improve the effects of depressurization and augmented injection for tight reservoir waterflooding development by reducing the tight matrix’s residual oil saturation, increasing and maintaining its water phase permeability near the well zone using a nano-SiO2 microemulsion system with a small particle size and high interfacial activity. Therefore, four nano-microemulsion systems were evaluated and screened for their temperature resistance, salt resistance, interfacial tension, solubilization, and dilution resistance. A microemulsion system of 13% A + 4% B + 4% C + 4% n-butanol + 6% oil phase + 69% NaCl solution (10%) + 1% OP-5 + 0.5% anti-temperature agent + 0.3% nanosilica material was preferred. According to the core displacement experiment, the depressurization rate can reach 28~60% when the injection concentration of the system is 1~10% and the injection volume is 2~5 PV. The results of the on-site test show that the water injection pressure dropped to 17.5 MPa, which was lower than the reservoir fracture re-opening pressure. The pressure reduction rate was approximately 20%. The validity period of the depressurization and augmented injection has reached 23 months to date.

Natural gas hydrate is a kind of low-carbon and clean new energy, so research on its efficient extraction in terms of theory and technology is particularly important. Combined thermal injection and depressurization is an effective method for extracting natural gas hydrate. In this study, the classical local heat equilibrium model was modified, and a pore-scale fully coupled unsteady heat transfer model for hydrate reservoirs was set up by considering multiple forms of heat flow accompanying hydrate’s decomposition and gas–liquid flow. Based on this model and the basic geological information of the X2 hydrate reservoir in the western Pacific Ocean, a numerical model of gas hydrate extraction using combined heat injection and depressurization was constructed to simulate the production performance of the hydrate reservoir. The results were fully compared with the results obtained by the depressurization method alone. The results indicated the hydrate extraction via a combined heat injection and depressurization would have a cumulative gas production of 31.609 million m3 and a cumulative water production of 1.5219 million m3, which are 72.57% higher and 31.75% lower than those obtained by depressurization alone, respectively. These study results can provide theoretical support for the industrial extraction of gas hydrate in seas.

In this paper, a one-dimensional model of gas–water two-phase productivity for hydrate depressurization is established, which takes into account permeability variation and gas–water two-phase flow. By solving the coupled algebraic equations of dissociation front position, equilibrium temperature, and pressure in an iterative scheme, the movement law of the hydrate dissociation front and the evolution process of temperature and pressure near the well were obtained, and the effects of bottom hole pressure, reservoir temperature, and hydrate saturation on productivity were analyzed. The results show that the hydrate reservoir is divided into a decomposed zone and an undecomposed zone by the dissociation front, and the temperature and pressure gradients of the former are greater than those of the latter. Reducing bottom hole pressure, increasing reservoir temperature, and increasing hydrate saturation all lead to an increase in temperature and pressure gradient in the decomposed zone. Methane gas production is a sensitive function of bottom hole pressure, reservoir temperature, and hydrate saturation. The lower the bottom hole pressure, the higher the reservoir temperature, the lower the hydrate saturation (within a certain range), and the higher the gas production rate. The trend of the water production curve is the same as that of gas, but the value is 3–4 orders of magnitude smaller, which may be due to the large difference in the viscosity of gas and water, and the gas seepage speed is much larger than that of water.

In situ electric heating is an important method used to increase production capacity during the extraction of natural gas hydrates. This work numerically evaluated the sensitivity of different heating parameters on gas production behavior with a vertical well depressurization in the Shenhu Sea area hydrate reservoir, the production pressure difference of 4 MPa, and continuous depressurization for 1080 days. The results showed that the in situ electric heating method can effectively enhance production capability by promoting hydrate dissociation and eliminating secondary hydrates. Compared with scenarios without heating, implementing whole wellbore heating (100 W/m) increases cumulative gas production (Vg) by 118.56%. When intermittent heating is applied to the local wellbore (15 m) located in the three-phase layer (with an interval of 30 days) and stops heating in advance at 480 days, there is no significant difference in Vg compared to the whole wellbore heating case, and the cumulative heat input is only 4.76%. We recommend considering intermittent heating of the local wellbore and stopping heating in advance during vertical well depressurization as this approach significantly reduces heating energy consumption while simultaneously improving production capacity.

Horizontal drains (HDs) are commonly deployed at open pit mines to depressurize the pit walls and thereby improve pit wall stability. If successful, depressurization maintains current pit wall angles and improves stripping ratios, which directly influences mine economics. HDs are commonly designed using empirical approaches that have been developed based on experience at other mines and are often aided by 2D and 3D analysis using groundwater models at sites with complex geological and structural settings. Specialized techniques for simulating HDs in these models became available nearly 20 years ago with the introduction of discrete feature elements in FEFLOW, a proprietary groundwater modelling package. More recently, introduction of unstructured grids allowed direct representation of HDs and their sequencing in MODFLOW, a publicly available modelling package that is widely used at mine sites. This paper provides an example of how HD design can be aided by 3D numerical analysis that utilizes these new MODFLOW simulation capabilities. [ABSTRACT FROM AUTHOR]

The residual oil saturation of the matrix near the well zone of a tight reservoir is high due to the tight reservoir's complex conditions, such as the small pore throat radius and low permeability of the matrix and the development of microfractures, which can result in serious water channeling, even after long-term water injection development. The aim of this paper is to improve the effects of depressurization and augmented injection for tight reservoir waterflooding development by reducing the tight matrix's residual oil saturation, increasing and maintaining its water phase permeability near the well zone using a nano-SiO2 microemulsion system with a small particle size and high interfacial activity. Therefore, four nano-microemulsion systems were evaluated and screened for their temperature resistance, salt resistance, interfacial tension, solubilization, and dilution resistance. A microemulsion system of 13% A + 4% B + 4% C + 4% n-butanol + 6% oil phase + 69% NaCl solution (10%) + 1% OP-5 + 0.5% anti-temperature agent + 0.3% nanosilica material was preferred. According to the core displacement experiment, the depressurization rate can reach 28~60% when the injection concentration of the system is 1~10% and the injection volume is 2~5 PV. The results of the on-site test show that the water injection pressure dropped to 17.5 MPa, which was lower than the reservoir fracture re-opening pressure. The pressure reduction rate was approximately 20%. The validity period of the depressurization and augmented injection has reached 23 months to date. [ABSTRACT FROM AUTHOR]

A pressure relief valve (PSV) cannot depressurize a vessel or system; it can only limit its maximum pressure during upset or emergency conditions. A blowdown system is usually specified to accomplish the removal of the pressurized, volatile, hazardous, and flammable compounds such as hydrocarbon or ammonia vapor in vessels or systems. The pressure is removed from the vessel using a blowdown system; however, some hydrocarbon liquid will remain in the vessel. This paper explains different criteria for designing a blowdown system. Examples are presented to show how the blowdown time is affected by several factors. The Joule–Thomson effect and its effect on the vessel's inner wall temperature are illustrated using charts and tables. Several applications of the blowdown system are provided that relate to actual offshore and refining operations. Different mechanisms by which the blowdown system is activated are discussed, and a simplified design procedure to design a blowdown system is also provided. [ABSTRACT FROM AUTHOR]

Carbon Capture and Storage (CCS) involves transportation of liquefied CO 2 from capture sites to permanent storage locations, which is usually achieved by pipeline transportation or maritime shipping. Over long distances, maritime shipping at pressures around 16 bara has proved to be a more effective solution than transportation through pipelines. These conditions, although safe, offer a significant pressure margin with respect to the triple point of CO2 for more cost-effective ship design with larger transportation capacity. However, the reliability of this technology has not been fully demonstrated regarding depressurization operations commonly performed to minimize hazards associated to emergencies. For CO2, this operation entails risks such as dry ice formation. This paper focuses on two purposes. First, it reports detailed and extensive experimental tests of CO2 depressurization of a tanks at low pressure (< 20 bara). An extensive review of public domain literature showed that this data is scarce [20] This work focuses on contributing to close that gap. Finally, this work will present detailed simulations of the experiments using commercial flowsheeters to assess the reliability of standards models that are commonly used for engineering studies and design. All the experimental work was carried out using a bench scale tank of 2 m3 with ample instrumentation according to an experimental design. The results of the runs were modelled using the EO-Blowdown utility implemented in UniSim Design®. The results showed that modelling of CO2 systems with the EO-Blowdown utility is reliable and can reproduce accurately most key process variables, although it has some limitations such inability to model solid phases and metastable liquids [Display omitted] [ABSTRACT FROM AUTHOR]

A wide range of recent applications require high-quality sprays that are characterized by extremely small-sized droplets, a narrow droplet size distribution, and a short breakup length. Fuel injection systems in propulsion units, energy storage, medical implementations, printings, and coatings are just a few examples. Flash-boiling atomization is a unique method that was extensively developed during the past two to three decades and has been proven to generate high-quality demanded sprays. In flash-boiling atomization, the liquid is forced to reach a metastable superheated state by either rapid heating or rapid pressure drop, where vapor bubbles nucleate, become fast-growing, and subsequently break down the liquid into a fine spray in a very short time. This present article focuses on flash-boiling atomization via rapid depressurization, which is presently more relevant to energy systems. The field of flash-boiling atomization has seen rapid growth and popularity in the past two decades. The aim of this article is to quantitatively portray the landscape and evolutionary trajectory of flash-boiling atomization research and applications and to detect new research frontiers and emerging trends in the literature on flash-boiling atomization. We briefly review the basic theories of the flash-boiling atomization mechanism present a comprehensive overview of the field, from its birth in approximately the 1970s to the present, and provide a database comprising 386 articles published on the topic of flash-boiling atomization. We visualize the intellectual structure of flash-boiling atomization research and applications and track its evolvement over the past five decades, thus providing a global overview and a comprehensive understanding of the development of flash-boiling atomization research and emerging applications. [ABSTRACT FROM AUTHOR]

Leak detection based on volume changes is common in conventional liquid processing systems, but present challenges in gas systems. This paper estimates the gas leak volume to quantify gaseous fluid flow in processing plants using prior-calibration-relation techniques. The estimation of volume following this pressurebased model requires that the pressure indicator of the gas leak is normalized against the initial volume before the pressure drop. Normalization makes it possible for equivalent pressure and gas volume data to be presented in percentage or fraction, so they are comparable. (This is similar to the pressure reading or drop on the regulator gauge of a gas cylinder, which is proportional to the equivalent gas cylinder weight or volume difference due to the depressurisation) An example of leak estimation for Gas Plant JK - 52 real-time test case modelling is shown in this work. The change in pressure dP was plotted against the change in volume dV in a prior calibration to derive a relationship between dP and dV. For a Pressure drop of 5 bars, Leak Volume = 2.40 m3 or 84.7 scf of gas. Such estimation of actual gas volume is useful in tying gas leak to environmental impact, HSE and in costing of economic loss. [ABSTRACT FROM AUTHOR]

The sediments in the South China sea are mainly composed of clayey silt, characterized by weak cementation, low strength, and poor permeability. These characteristics lead to slow gas and water transport and low gas production efficiency in the production process, which is not conducive to reservoir stability. Therefore, this paper studied the influence of different factors on the displacement and permeability of hydrate-bearing sediments by using remolded cores from the South China Sea. It was found that, when the depressurization method was used for hydrate decomposition, the displacement change in sediments could be divided into three stages: depressurization stage, decomposition stage, and creep stage. During the decompression stage, sediment deformation was rapid and displacement was small. During the decomposition process of hydrates, sediment deformation was slow and displacement was maximum. The creep stage had the slowest deformation and the smallest displacement. The displacement increased with the increase in initial porosity, hydrate saturation, effective pressure, and depressurization amplitude. The permeability of the sediments was lower than that of the original sediments after hydrate decomposition. This permeability damage increased with the increase in the sediment porosity, hydrate saturation, depressurization range and effective pressure. Furthermore, the displacement of sediments was positively correlated with the permeability damage. [ABSTRACT FROM AUTHOR]

Most hydrate-bearing sediments worldwide exhibit mixed pore- and fracture-filling hydrates. Due to the high exploitation value, pore-filling hydrate production is the focus of current hydrate production research, and there is a lack of systematic research on the decomposition of fracture-filling hydrates and their effects on the evolution of temperature and pressure in hydrate-bearing sediments. If only the decomposition characteristics of pore-filling hydrates are studied while the fracture-filling hydrates decomposition and its effects on the hydrate-bearing sediments production process are ignored, the obtained research results would be inconsistent with the actual situation. Therefore, in this study, the effects of fracture-filling hydrates with different dipping angles on the hydrate production process were studied, and the necessity of considering the phenomenon of mixed pore- and fracture-filling hydrates in hydrate-bearing sediments was illustrated. On this basis, the simulation of a typical site (GMGS2-16) with mixed pore- and fracture-filling hydrates was constructed, and the production process was researched and optimized. The results indicated that: (a) fracture-filling hydrates formed in shallow fine-grained sediments and gradually approached the area of pore-filling hydrates, before a stable mixed zone was formed; (b) the occurrence of fracture-filling hydrates was conducive to the hydrate-bearing sediment depressurization production, and the promoting effect of the fracture-filling hydrate with smaller dipping angles was stronger; and (c) depressurization combined with heat injection could effectively compensate for the local low temperature and secondary hydrate caused by the mass decomposition of fracture-filled hydrates. [ABSTRACT FROM AUTHOR]

To further understand the characteristics of clay and sand production (hereafter collectively referred to as sand production) and to provide optimization designs of sand control schemes are critical for gas production from clayey silt natural gas hydrate reservoirs in the South China Sea. Thus, gas-water-sand production behavoirs and coupling reservoir subsidence characteristics before, during, and after hydrate dissociation of the clayey silt hydrate reservoirs with different clay contents (5%, 10%, 15%, 20%, 25%, and 30%) have been studied through a self-developed experimental system. The results show that with the increase of clay content, the total mass of sand production first increases and then decreases, and it reaches maximum when the clayey content is 20%. The sand production is the lowest before hydrate dissociation and increases significantly during hydrate dissociation, which mainly occurs in the high-speed gas and water production stage at the beginning of hydrate dissociation. After hydrate dissociation, the sand production decreases significantly. During the whole depressurization process, the clay and free sand particles generally move to the sand outlet due to the fluid driving force and overlying stress extrusion. However, for conditions of high clay contents, those particles fail to pass through the sand control screen and gradually accumulate and block the screen by forming a mud cake, which greatly reduce the permeability of the screen and limite sand production as well as gas and water production. Our research lays a foundation for sand production prediction and sand control scheme selection during gas recovery from clayey silty hydrate reservoirs that greatly need to consider a balance between sand control and gas productivity.

Natural gas hydrate (NGH) is a clean and efficient energy resource with extensive distribution in the permafrost regions and marine sediments. A few short-term production tests focusing on reservoir depressurization have been conducted in recent years. However, the long-term production performance and the transient evolution characteristics of reservoir properties are not well known. In this work, a more realistic hydrate-reservoir model that considers the heterogeneity of permeability, porosity and hydrate saturation is constructed, according to the available geological data at the Mallik site. The model is validated by reproducing the field depressurization test. The main purposes of this work are to evaluate the long-term gas production performance and to analyze the unique multiphase flow behaviors from the validated geologically descriptive hydrate-reservoir model. The results indicate that the long-term gas production through depressurization from hydrate reservoirs at the Mallik site is technically feasible, but the gas production efficiency is generally modest. The hydrate dissociation front in HBS is strongly affected by the reservoir heterogeneity and shows a unique dissociation front. The vertically heterogeneous HBS is beneficial for depressurization production compared to the massive hydrate reservoirs. Furthermore, the vertically heterogeneous hydrate-reservoir with low permeability of clay-layer can effectively block methane gas diffusion in the vertical direction. These emphasize that constructing realistic reservoir models is very important to accurately predict the hydrate production performance. At the end of 1-year depressurization, a total of 1.80 × 106 ST m3 of methane gas can be produced from the validated hydrate-reservoir, while which is far from the commercial value. In addition, reducing the production pressure in the wellbore is beneficial for increasing gas production volume, but is not conducive to improving the hydrate production efficiency at the Mallik site.

The hydrate deposits in offshore were mainly hosted in the clayey silt sediments, which had the characteristics of looseness and low permeability. Improving the permeability of hydrate reservoirs is the key to promote gas production efficiency. In this work, the enlarged permeable well wall method based on the local reconstruction around the production well was employed for the gas production. The enlarged permeable well wall was created by filling the hydrophobic porous materials to form a cylindrical high permeability area around production well. The gas production of hydrate deposits at the site W-17 in Shenhu area of the South China Sea was simulated by depressurization method. The simulation results showed that the enlarged permeable well wall method could promote the propagation of pressure drop and ensure the continuity of the high-permeability channel around the well. When the radius of enlarged permeable well wall were 1, 3 and 5 m, the gas production increased by 1.45, 2.08 and 2.66 times than that without the enlarged permeable well wall method for 10 years, respectively. Compared with the results of the first actual trial production in the South China Sea, the enlarged permeable well wall method has better production promotion effect on hydrate reservoirs. Moreover, the gas production was sensitive to the intrinsic permeability of the sediments, but not to the thermal conductivity.

This paper describes the method and results of an experimental study of unsteady burning rate of high-energy materials containing metal powder additives under depressurization at a rate of 140–160 MPa/s. The method based on stating and solving the inverse problem of internal ballistics is used to determine the unsteady burning rate. The studies are carried out for high-energy materials, including energy additives in the form of metal powders (ASD-4, ASD-6, and Alex aluminum powders), aluminum diboride, and dodecaboride. It is shown by analyzing the this study that the unsteady burning rate of high-energy materials under sharp depressurization is oscillatory and depends on the energy additive type. [ABSTRACT FROM AUTHOR]

In addition to the temperature and pressure conditions, the pore fluid composition and migration behavior are also crucial to control hydrate decomposition in the exploitation process. In this work, to investigate the effects of these factors, a series of depressurization experiments were carried out in a visible one-dimensional reactor, using hydrate reservoir samples with water saturations ranging from 20% to 65%. The results showed a linear relationship between gas production rates and gas saturations of the reservoir, suggesting that a larger gas-phase space was conducive to hydrate decomposition and gas outflow. Therefore, the rapid water production in the early stage of hydrate exploitation could release more gas-phase space in the water-rich reservoir, which in turn improved the gas production efficiency. Meanwhile, the spatiotemporal evolution of pore fluids could lead to partial accelerated decomposition or secondary formation of hydrates. In the unsealed reservoir, the peripheral water infiltration kept reservoir at a high water saturation, which hindered the overall production process and caused higher water production. Importantly, depressurization assisted with the N2 sweep could displace the pore water rapidly. According to the results, it is recommended that using the short-term N2 sweep as an auxiliary means in the early stage of depressurization to expand the gas-phase space in order to achieve the highest production efficiency. [ABSTRACT FROM AUTHOR]

Leak detection based on volume changes is common in conventional liquid processing systems, but present challenges in gas systems. This paper estimates the gas leak volume to quantify gaseous fluid flow in processing plants using prior-calibration-relation techniques. The estimation of volume following this pressure-based model requires that the pressure indicator of the gas leak is normalized against the initial volume before the pressure drop. Normalization makes it possible for equivalent pressure and gas volume data to be presented in percentage or fraction, so they are comparable. (This is similar to the pressure reading or drop on the regulator gauge of a gas cylinder, which is proportional to the equivalent gas cylinder weight or volume difference due to the depressurisation) An example of leak estimation for Gas Plant JK – 52 real-time test case modelling is shown in this work. The change in pressure dP was plotted against the change in volume dV in a prior calibration to derive a relationship between dP and dV. For a Pressure drop of 5 bars, Leak Volume = 2.40 m3 or 84.7 scf of gas. Such estimation of actual gas volume is useful in tying gas leak to environmental impact, HSE and in costing of economic loss.

To explain the dissociation law in the depressurization extraction of natural gas hydrate (NGH), a coupled thermal–hydrologic–chemical (THC) model was established. The coupled model was confirmed through the data of Masuda’s depressurization experimentation and the first NGH extraction in the Shenhu Area. After verifying the applicability of the numerical model, a long-term (1000 days) gas productivity of the NGH reservoir at the W17 test site was explored. This paper quantitatively analyzed the effect of reservoir permeability, temperature and porosity on NGH production capacity (cumulative methane production). The results revealed that the increase in reservoir permeability and temperature could significantly enhance methane production. The subsequent NGH extractions should raise the permeability and temperature of the target hydrate reservoir. Under the same NGH saturation, cumulative methane production did not always increase with increasing reservoir porosity. When the reservoir porosity increased to 0.40, the cumulative methane production decreased with the increase in porosity. The results showed that the combined extraction approach of depressurization and thermal flow injection is more efficient than only the depressurization method.

Widely employed in hydrate exploitation, the single well method is utilized to broaden the scope of hydrate decomposition. Optimizing the well structure and production strategy is necessary to enhance gas recovery efficiency. Complex wells represented by the multilateral wells have great application potential in hydrate mining. This study focused on the impact of multilateral well production methods on productivity, taking the Nankai Trough in Japan as the study area. The spatial distribution of physical parameters such as porosity, permeability, and hydrate saturation in the Nankai Trough has significant heterogeneity. For model accuracy, the Sklearn machine learning and Kriging interpolation methods were used to construct a three-dimensional heterogeneous geological model to describe the structure and physical property parameters in the study area of the hydrate reservoir. The numerical simulation model was solved using the TOUGH + Hydrate program and fitted with the measured data of the trial production project to verify its reliability. Finally, we set the multilateral wells for hydrate high saturation area to predict the gas and water production of hydrate reservoir with different exploitation schemes. The main conclusions are as follows: ① The Sklearn machine learning and Kriging interpolation methods can be used to construct a three-dimensional heterogeneous geological model for limited site data, and the fitting effect of the heterogeneous numerical simulation model is better than that of the homogeneous numerical simulation model. ② The multilateral well method can effectively increase the gas production rate from the hydrate reservoir compared with the traditional single well method by approximately 8000 m3/day on average (approximately 51.8%). ③ In the high saturation area, the number of branches of the multilateral well were set to 2, 3, and 4, and the gas production rate was increased by approximately 51.8%, 52.5%, and 53.5%. Considering economic consumption, the number of branching wells should be set at 2–3 in the same layer. [ABSTRACT FROM AUTHOR]

The results of the second trial production of the gas hydrate reservoir in the Shenhu area of the South China Sea show that the production of a gas hydrate reservoir by horizontal wells can greatly increase the daily gas production, but the current trial production is still far below the minimum production required for commercial development. Compared with a single horizontal well, a fishbone well has a larger reservoir contact area and is expected to achieve higher productivity in the depressurization development of gas hydrate reservoirs. However, there is still a lack of systematic research on the application of fishbone wells in Class I gas hydrate reservoirs. In this paper, a grid system for gas hydrate reservoirs containing fishbone wells is first established using the PEBI unstructured grid, and fine-grained simulation of reservoirs near the bottom of the wells is achieved by adaptive grid encryption while ensuring computational efficiency. On this basis, Tough + Hydrate software is adopted to simulate the productivity and physical field change of a fishbone well with different branching numbers. The results show that: the higher the number of branches in a fishbone well, the faster the free water production rate, reservoir depressurization, and free gas production rate in the initial stage of depressurization development, and the faster depressurization can effectively promote hydrate dissociation. Compared with a single horizontal well, the cumulative gas production of a six branch fishbone well can increase by 59.3%. Therefore, using multi-branch fishbone depressurization to develop Class I gas hydrate reservoirs can effectively improve productivity and the depressurization effect, but the hydrate dissociation will absorb a lot of heat and lead to a rapid decrease in reservoir temperature and hydrate dissociation rate. At the end of the simulation, the hydrate dissociation rate of all schemes was lower than 50%. In the later stage of depressurization development, the combined development method of heat injection and depressurization is expected to further provide sufficient thermal energy for hydrate dissociation and promote the dissociation of the hydrate. [ABSTRACT FROM AUTHOR]

Test exploitation equipment and technology have progressed considerably in marine natural gas hydrate (NGH) exploitation, but many critical technical issues still need to be resolved before commercial production. Previous studies have proposed a non-drilling exploitation device—a self-entry exploitation device (SEED)—but reaching the NGH commercial exploitation threshold in its initial state is difficult. Consequently, we verified and evaluated some production enhancement measures to improve the exploitation system of the SEED. In this study, based on the geological data from the SHSC-4 site in the Shenhu sea and the material characteristics of the SEED, we carried out four production enhancement measures by numerical simulation. The results indicate that: (i) open-hole position adjustment can expand the contact areas between the device and NGH reservoirs; (ii) the effect of inner wall heating is limited but sufficient to achieve the goal of preventing clogging; (iii) it is necessary to select a reasonable spacing according to a combination of expected production cycle time and pressure when carrying out clustered depressurization; and (vi) when performing depressurization combined with thermal stimulation exploitation, factors such as permeability and thermal conductivity play a decisive factor in gas production. [ABSTRACT FROM AUTHOR]

Natural gas hydrate (NGH) is regarded worldwide as the most promising clean energy. Research on the exploration and exploitation of NGH has come to a stage of breakthrough in China. However, the accumulation mechanism and occurrence regularity of NGH from muddy silt in the South China Sea remain unclear, and the hydrate decomposition and phase transformation mechanisms during exploitation as well as safe and efficient technologies require further study. This study reviewed the research and application status of NGH exploitation technologies including depressurization and solid fluidization. Moreover, corresponding suggestions were proposed considering the existing problems regarding current NGH exploitation technologies. Specifically, basic research and major engineering should be promoted for multi-gas exploitation of NGH and deep oil and gas; technology innovation and integration should be strengthened to promote synergy between theory and practice; a numerical simulation big science platform should be established for pilot scale tests and multi-field coupling; and relevant standards need to be established and improved. This can improve the research level and technical maturity of NGH exploitation in China, facilitate its large-scale commercial development, and ensure energy security of China.

The five-spot well is a promising technique for enhancing the inherent low gas production rate from silty natural gas hydrate (NGH) reservoirs with low permeability in the South China Sea (SCS). However, the production characteristics and the responses to key geological and engineering factors are still unclear and warrants investigation. Herein, we first establish a three-dimensional reservoir model based on the SH2 NGH deposits at SCS with the implementation of a five-spot well. We systematically examine the gas production enhancement effect and the unexpected well-to-well interaction induced by long-term depressurization. A sensitivity analysis on the effects of bottomhole pressure (P btm), permeability (k) and hydrate saturation (S H) is conducted to elucidate the key factors controlling the gas production enhancement. Our results reveal that the five-spot well significantly improves the gas productivity with cumulative gas production increased to 4.12 times that of a single well over 10 years. The well interference on gas production is not significant in the early stage but becomes more significant as depressurization sustains. Due to the relatively slow pressure propagation at the inner well, contribution to gas production from the inner well decreases over time while the outer wells increase. Based on the sensitivity analysis, gas production increases with decreasing P btm , increasing k , and decreasing S H with P btm and k as the key controlling factors. The findings provide valuable design basis for the adoption of a multi-well system in enhancing gas production for targeted NGH reservoirs in the next field production trial at SCS. [Display omitted] • A five-spot well is designed for enhancing gas production from silty natural gas hydrate deposits • A thermo-hydraulic-chemical model is set up to examine the gas production enhancement effect • A 33 sensitivity study of bottomhole pressure, permeability and hydrate saturation is conducted • Gas production increased to 4.12 times that of a single well but with well-to-well interference [ABSTRACT FROM AUTHOR]

Methane hydrates, as a potential energy resource for the future, remain a considerable interest in the field of geo-energy and geo-resources. However, there is still a challenge to accurately characterize the coupled multiphysics during the entire exploitation process, specifically when involving sand production and sand control. In this study, an innovative semi-analytical model is developed to fully consider the coupled interactions between sand migration and multiphysics (seepage, temperature, hydrate dissociation, and mechanical behaviors) around a vertical production well. Furthermore, the mud cake formed near the screen is also taken into consideration. Meanwhile, the influence of reservoir deformation, hydrate dissociation, and hydraulic drag on sand production are taken into account in the current coupled multiphysics framework. Additionally, the effect of solid particle detachment on the mechanical properties of the formation and the effect of solid mass variation on the permeability and porosity of the reservoir, etc. , are fully considered. As a step of validation, a good agreement is observed for gas production and sand production, between the presented solutions and field measurements. Based on the proposed solutions, depressurization-driven exploitation problems are analyzed for different cases, meanwhile, recommendations for engineering applications are presented from the perspectives of engineering safety, efficiency, and sustainability. [ABSTRACT FROM AUTHOR]