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Hydrate reservoirs in the South China Sea belong to muddy silt‐type hydrate reservoirs with low permeability. Horizontal well fracturing is one of the main stimulation methods for low‐permeability oil and gas reservoirs. For muddy silt‐type hydrate reservoirs, the stability and effectiveness of hydraulic fractures may be a severe problem due to poor reservoir cementation, reservoir deformation, and sand production. In this paper, the time variability of fracture conductivity is considered for the first time. According to the geological data at offshore gas hydrate production test site in the South China Sea, a multilayer hydrate reservoir model was established, and the influence of the time variable fracture conductivity on the production behavior in the process of horizontal well fracturing was analyzed. The simulation results show that, compared with the case of constant fracture conductivity, the gas production rate with the case of time variable fracture conductivity is greatly reduced, and the 5‐year cumulative gas production is reduced by 49.9%. Sensitivity analysis shows that the larger the initial fracture conductivity, the higher the peak gas production rate; the larger the attenuation magnitude of the fracture conductivity, the lower the gas production rate in the late production period; the larger the decline rate coefficient of the fracture conductivity, the higher the gas production rate in the early production period. The analysis of the orthogonal experimental design shows that the influence of the attenuation magnitude of fracture conductivity, the initial conductivity of fracture, and the decline rate coefficient of fracture conductivity on the cumulative gas production decreases in order. The actual production process should try to avoid excessive attenuation magnitude of fracture conductivity, while improving the initial fracture conductivity as much as possible to ensure high cumulative gas production. [ABSTRACT FROM AUTHOR]

Sand production has become a common phenomenon in the exploitation of unconsolidated natural gas hydrate reservoirs, which will hinder the long-term production of natural gas hydrate reservoirs. However, there are few literatures reported on the influences in reservoir physical properties such as permeability and porosity, and production laws caused by sand production. This paper provides a numerical model, coupled with reservoir sand-gas-water multiphase flow processes, which is capable to simulate the process of sand production in natural gas hydrate reservoirs. The simulation results indicate that sand settlement is mainly concentrated near the wellbore due to the high concentration of migrated sand. The decrease in reservoir porosity and permeability caused by sand settlement has a significant impact on production. The impact of sand production on reservoir fluid fluidity shows that fluid flow is inhibited near the wellbore, while fluid flow performance increases far away from the wellbore. The numerical model and analysis presented here could provide useful insight into changes in reservoir physical properties and production laws caused by sand production in the natural gas hydrate-bearing marine sediments using depressurization method. [ABSTRACT FROM AUTHOR]

Offshore gas hydrates are being focused on as an unconventional energy source. The susceptibility of marine hydrate reservoirs to sand emergence makes them unsuitable for long-term exploitation. This research delves into the particle-transport law in hydrate reservoirs, revealing that sand blockage hinders long-term exploitation, and proposes a reservoir modification strategy to establish high-permeability channels, thereby enhancing production. After 110 days of mining, the range of porosity and permeability below the initial value was approximately 8.5 m. The study further optimizes reservoir modification parameters under sand blockage, suggesting that a balance between fracture and reservoir blockage can be achieved with a fracture half-length of 20 m, leading to increased production. It also highlights that the slow decomposition of blocked hydrate and the accumulation of decomposed methane gas in the upper part of the reservoir can be leveraged to increase production by providing a high permeability channel for free gas, concluding that maintaining long-term high permeability is crucial for hydrate reservoir fractures. • The highlights of the paper are: • Determined that sand blockage is the main reason why the hydrate reservoir cannot be exploited for a long time. • Proposed a method for reservoir modification to break through the blockage range to increase production. • The optimization of hydrate reservoir modification parameters under sand blockage is investigated. [ABSTRACT FROM AUTHOR]

Experimental simulation is an important approach to study the gas hydrate dissociation mechanism, and similarity theory is an effective tool for the design of experimental model. Based on hydrate kinetic reaction model in HydrateResSim, seventy-three similarity numbers are derived by means of inspectional analysis and dimensionless analysis. Gas production from the natural gas hydrate reservoir is controlled by two mechanisms, namely dissociation-controlled mechanism and flow-controlled mechanism. Two groups of schemes are put forward for the design of experimental model of natural gas hydrate according to these two mechanisms respectively: for flow-controlled hydrate reservoir the scale of injection rate of heat and water, and parameters associated with size is the same with that of length, the scale of time is the square of length scale; for dissociation-controlled hydrate reservoir, the scale of parameters associated with size is the same with that of length, the scale of absolute permeability is 4/3 times square of length scale, the scale of time is 2/3 times square of length scale, the scale of injection rate of heat and water is 7/3 times square of length scale. [ABSTRACT FROM AUTHOR]

Dynamic change of edge of sand detachment (ESD) is still unclear during the depressurization mining of gas hydrate reservoirs. Considering multi-physics such as the depose of NGH and its impact on the change of formation temperature and conductivity, a novel numerical method is provided which could be utilized to simulate and analyze the behavior of the edge of sand detachment (ESD). The simulation results show that the expansion of ESD has mainly consists of three phases, which has a tendency to be fast first, then slow, and finally tend to the ESD limit (ESDL). In our basic model referring to Shenhu field, the ESD rapidly reached around 44 m from day 0–360 days, then the increase rate slowed from 360 days to 720 days, and finally stabilized at 58 m (ESDL) after 720 days. In addition, the expansive rate of ESD and the ESD limit (ESDL) have great effect on the sand productions. The sand production rate increases rapidly and then reach peak in a rushed stage of ESD expansion, then decrease significantly and tend to be zero when ESD expands to the ESDL in NGH reservoirs. Based on the prioritization analysis of influencing factors, for a NGH reservoir with lower hydrate saturation, higher absolute permeability, higher initial reservoir temperature, lower bottom hole pressure, ESD expands much more fast and the ESDL is much larger. Our work could shed light on the dynamic behavior of ESD for the development of NGH reservoirs, and the finding of ESDL is helpful to determine the safe development area and facilitates adjustment on the sand controlling arrangements. [Display omitted] • There is an upper ESD limit (ESDL) during depressurization mining of NGH reservoirs. • The expansive rate of ESD and ESDL have great effect on the sand productions. • The ESDL could be reduced by increasing bottom hole pressure in low-saturated NGH reservoirs. [ABSTRACT FROM AUTHOR]

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

Gas productivity of current hydrate reservoir tests is too lower to meet the demand of the commercial level. The main reasons include the low permeability of hydrate-bearing layer (HBL) and the existence of permeable burdens in natural gas hydrate reservoir, which act as the major barriers for pressure drop propagation during depressurization. However, previous studies are focusing on either permeable burdens or hydraulic fracturing. This paper proposed a novel modified method of depressurization by hydraulic fracturing assisted with sealing burdens. The effects of the radial length and permeability of fractured domain, and the sealing location and length of overburden and underburden layers on hydrate dissociation and gas production were analyzed through numerical simulation. The results indicated:1) The larger permeability and longer radial length of the fracturing domain would promote propagation of pressure drop significantly, and the percentage of gas hydrate dissociation would be increased from 3.99% to 29.86% by hydraulic fracturing. 2) The sealing length of burdens should be larger than the fracturing radial length. The cumulative gas production could be enhanced by 93.25% while the cumulative water production could be decreased by 62.99% compared with no sealing burdens. Base on the novel method, the goals of enhancing gas production and reducing water production would be achieved simultaneously, providing important implications for hydrate commercial exploitation in the future. Low permeability of marine hydrate-bearing sediment and the existence of permeable burdens restrict the gas production of gas hydrate. Hydraulic fracturing is an efficient method to improve the permeability and enhance gas productivity. Sealing the permeable over/underburden weakens the invaded water and expands hydrate dissociation zone along the radial hydrate layer. The novel method of depressurization by hydraulic fracturing assisted with sealing burdens is introduced to achieve the goals of enhancing gas production and reducing water production simultaneously. [Display omitted] • A novel method of hydraulic fracturing assisted with sealing burdens was proposed. • Hydraulic fracturing was benefited to improve permeability and deepen the pressure drop. • Sealing burdens could effectively prevent water from burdens invading into HBL. • The goals of enhancing gas production and reducing water production could be achieved simultaneously. [ABSTRACT FROM AUTHOR]