Coral sand is widely distributed in offshore oil and gas basins, and beneath the foundations of offshore wind turbines that are exposed to persistent cyclic loads, including wind, ocean waves, and seismic activity. Under these cyclic loads, coral sand is prone to particle breakage, resulting in unexpected differential settlements that potentially threaten the security of offshore structures. A series of cyclic simple shear tests were conducted in this study to investigate the process of coral sand particle breakage during cyclic loading. The findings revealed that the shear stress and shear modulus of coral sand significantly increase after the first cycle, and then increase moderately with the number of loading cycles. Particle breakage becomes more severe as the cyclic shear strain amplitude, vertical stress, and total cycles increase. There is a critical shear rate that induces significant particle breakage, leading to a more regular particle shape in coral sand. The detachment interlocking and mechanical grinding models are proposed for analyzing the mechanism of the particle breakage in coral sand under cyclic loading. The level of input energy that controls the particle breakage in coral sand is set based on analysis of the energy dissipation.

The hydrate reservoirs exhibit significant inhomogeneity due to the presence of various hydrate types, potentially impacting the effectiveness of hydraulic fracturing. The investigation of the relationship between hydraulic fracture extension patterns and pure hydrate blocks for varying conditions is highly valuable. The extended finite element method (XFEM) is applied to reveal the influence mechanism of hydrate heterogeneity on fracture propagation law. First, compared with the results of the fracturing experiments in hydrate-bearing clayey silt sediments, the accuracy of the method is verified. Subsequently, a model for fracturing near-well in deep-sea inhomogeneous hydrate-bearing-sediments is established, containing sedimentary matrix and pure hydrate blocks. The effects of disseminated hydrate saturation, injection rate, horizontal stress difference and the distribution location of pure hydrate blocks on the fracture extension path are investigated. Due to the pure hydrate is much more brittle, most hydraulic fractures, once encountered with it, choose to bypass. If the fracture is unable to bypass the pure hydrate block, the fracture appears to arrest and penetrate. The seafloor sediment matrix is characterized by its loose nature, and a continuous increase in the injection rate may result in the competing development of the fractures in both length and width directions.

In the second depressurization test production of marine natural gas hydrates (NGHs) in China, gas was successfully produced from low-permeability clayey silt NGHs through a horizontal depressurization well, setting a new record for marine NGHs production. Although significant success has been achieved at the field scale, there is still limited research on its production behavior. In this paper, a layered heterogeneous horizontal well production numerical model was established based on logging data from this field production site. First, the cumulative gas production volume for 30 and 42 days was successfully fitted through numerical simulation, and the short-term production behavior under different irreducible water saturation (S-irw) was studied in detail. An increase in S-irw during short- and long-term production can increase the flow of free gas toward the production well, thereby increasing gas production. During the long-term production process, a barrier area with high hydrate saturation in the bottom of the hydrate-bearing layer (HBL) hinders the further upward expansion of the decomposition front. Meanwhile, due to the relatively large amount of movable water, the barrier area formed at low S-irw is relatively small. Through this study, it is helpful to reveal the hydrate production behavior of the horizontal well under high S-irw and a strong sealing effect.

Particle breakage at coral sands–structure interface is common in marine geological environments and is critical to the stability of geotechnical structures. However, due to the knowledge gap regarding the relationship between structure roughness and coral sand particle breakage, previous studies have not provided a clear understanding of this phenomenon. To address this gap, we conducted ring shear tests to investigate the evolution of the fractal dimension of particle breakage and its dependence on structure surface roughness and vertical stress based on fractal theory. The results show that both the roughness of the steel plate simulating the structure surface in the ring shear test and the particle size of coral sand have significant impacts on the evolutions of particle breakage and morphology at the contact interface. In detail, the particle size distributions (PSDs) of coral sands after shearing have obvious self-similarity and converge to the limit distribution state, especially when the sands contain more large particles. When the steel plate is smooth, the fractal dimension of the broken coral sand is relatively low, which indicates an early stage of fractal dimension development. Moreover, the relationship between the fractal dimension and vertical stress exerted on a rough steel plate can be approximately described using a second-order polynomial function. Moreover, there exists a critical vertical stress that corresponds to the maximum fractal dimension for each kind of coral sand in our tests. The particle breakage rates of coral sand samples on smooth steel plates are substantially lower than those on rough steel plates.

Carbon sequestration through CO2 injection into a formation is an effective strategy for reducing greenhouse gas emissions. In this study, a one-dimensional long reactor was constructed to simulate the CO2 injection process under various sediment temperatures, pressures, and flow rates. The formation of CO2 hydrate and the resulting blockages were investigated in detail through a series of indoor experiments. Due to the increasing driving force for CO2 hydrate formation, reducing sediment temperature and increasing sediment pressure can cause hydrate blockage to form near the injection end, leading to an increase in CO2 injection pressure and a reduction in the storage range. Furthermore, CO2 injection rate has a substantial impact on the pattern of hydrate blockage. A lower injection rate facilitates full contact between CO2 gas and pore water, which helps to increase the formation and blockage degree of CO2 hydrates, thereby decreasing the amount of CO2 injection. The experimental investigation presented in this paper examines the laws of CO2 injection and clogging under various sediment conditions and injection processes on a one-dimensional scale, which can provide valuable insights for the design of CO2 sequestration processes.

Improving the fundamental theory of hydraulic fracturing in deep-sea hydrate reservoirs is of great significance in enhancing the efficiency of gas hydrate mining. In this paper, a multi-cluster fracturing model for a horizontal well was established with a three-dimensional extended finite element method and the measured geo-mechanical data from hydrate reservoirs in the South China Sea. The performance, fracture initiation, and expansion mechanisms of multi-hydraulic fractures are investigated under different fracturing sequences, hydrate saturation and cluster deployment patterns. Some interesting results have been discovered: In the case of three-cluster, the fractures produced by sequential fracturing are notably more affected by pre-existing fractures. Furthermore, the effect of fracture interference between simultaneous and sequential fracturing is reflected in the middle fracture is compressed by the fractures on its left and right sides. The increase in hydrate saturation enhances the cementing ability of the sediments and results in an increased difficulty of fracture initiation for the five-cluster simultaneous fracturing. It also results in a decrease in the geo-stress field, where the interaction between fractures plays a dominant role in the process of fracture expansion, leading to non-uniform expansion.

Poor seepage capacity always occurs in the clayey silt hydrate reservoir, making it more difficult to achieve depressurization. Hydraulic fracturing is an important means of enhancing reservoir seepage and increasing gas production rates. In this work, a series of depressurization experiments were performed in a triaxial hydraulic fracturing apparatus. The samples were prepared according to the physical parameters of samples obtained from the South China Sea. The fracturing behavior and gas production characteristics of methane hydrate-bearing sediments were investigated. The effects of hydrate saturation, fracturing fluid properties, and in-situ stress on fracturing behavior were analyzed. It was found that it was feasible to be fractured for sediments with low hydrate saturation (S-h < 0.30), and the fracture initiation pressure was positively correlated with the hydrate saturation. The results showed that medium-viscosity fracturing fluid had excellent flow and filtration loss reduction, which were beneficial to the maintenance of fractures inside sediments. The in-situ stress difference determined the direction of fracture propagation, which was always along the vertical direction of the minimum principal stress. Besides, the gas production enhancement and permeability improvement by hydraulic fracturing were evaluated. The average gas production rate and peak rate in the fractured run were 3.01 times and 5.84 times of those in the unfractured run, respectively. To further enhance the seepage capacity, the inorganic salt-based clay stabilizer solution was added to the fracturing fluid. The results showed that the permeability of the sediments fracturing with NH4Cl was 3.07 times of that in the unfractured run due to the inhibition of the hydration swelling of clay particles. The results are valuable to provide a basic understanding and suggestions for the application of hydraulic fracturing in gas hydrate reservoirs.

The combination of depressurization and thermal fluid injection (D + T-i) is a very promising method to realize the efficient development of marine natural gas hydrate (NGH). However, the performance of D + T-i for hydrate production in the reservoir with impermeable boundaries has received little attention. In this work, the hydrate exploitation performances of D + T-i in NGH reservoir with impermeable overlying layer or impermeable underlying layer (O-imp + U-p, O-p + U-imp, O-imp + U-imp) were numerically investigated. The reservoir with permeable overlying layer and underlying layer (O-p + U-p) was used for comparison. Moreover, the effect of thermal fluid injection temperature (T-i) and injection pressure (P-i) on hydrate decomposition and gas production behaviors in the reservoir with O-imp + U-imp or O-p + U-p were studied in detail. The results indicate that the hydrate decomposition potential of the reservoir with O-p + U-p, O-imp + U-p or O-p + U-imp is similar, which is significantly lower than that of the reservoir with O-imp + U-imp. The presence of impermeable layer in reservoir can enhance gas production and reduce water production, especially when both overlying and underlying layers are all impermeable. The gas to water ratio and energy ratio of the reservoir with O-imp + U-imp are 5.0-7.4 times and 5.5-8.6 times that of the other three reservoirs, respectively. On the other hand, the improvement of Pi increase on hydrate decomposition and gas production is far superior to the improvement caused by T-i rise. Moreover, the increase of Pi and Ti has a more significant improvement on gas production in the reservoir with O-imp + U-imp than that in the reservoir with Op + Up. The maximum improvement of gas production in the reservoir with O-imp + U-imp is 69.6%. These results can provide theoretical support for the exploitation of marine NGH by D + T-i.

Horizontal well multistage fracturing technology has potential as a technical method to improve the permeability of clayey silt natural gas hydrate (NGH) reservoirs as it can increase the contact area between a horizontal well and an NGH reservoir. Although this technology has been successfully applied to the exploitation of shale gas and coalbed methane, the relevant technical experience cannot be directly applied to clayey silt NGH reservoirs. Based on the properties of the clayey silt NGH reservoir at the SH2 site in the Shenhu area of the South China Sea, herein, we established a three-dimensional hydraulic fracturing model based on cohesive elements to analyze the influence of the fracturing fluid injection rate on the initiation and propagation of single-cluster fractures and the influence of fracture spacing on simultaneous fracturing and sequential fracturing of multi-cluster fractures. We comprehensively analyzed the distribution characteristics of fracture morphology and the phenomenon of stress interference between fractures under different conditions. The results showed that without fracture interference, fractures tended to propagate to the middle and upper parts of the reservoir owing to the low fracture propagation resistance. Simultaneous fracturing and sequential fracturing produced different staggered fracture distribution patterns, which were caused by the geostress field changing owing to the generation of fractures. Our findings broaden the understanding of hydraulic fracturing in clayey silt NGH reservoirs.

Marine hydrates mainly occur in clayey silt sediments, and their occurrence environment has the characteristics of a high clay content and low permeability, which seriously inhibit an increase in the gas production rate. This study presents a new method, the high-pressure jet breaking and sand filling method, to transform the natural gas hydrate (NGH) reservoir near the production well, whereby a water jet is used to form an area of sediment with large particle diameters and high permeability. Based on indoor experiments, we report the characteristics of hydrate formation, decomposition, and phase equilibrium in the transition area formed by this method under different sand particle sizes and mass fractions. The addition of sand at a high mass fraction with a large particle diameter significantly shortened the time required for rapid hydrate formation, prolonged slow hydrate formation, and reduced the increase in temperature. With an increase in the mass fraction of large-diameter sand, the free water content of pores increased, which made it easier to form hydrates with large particles, thus delaying the hydrate decomposition rate. The phase equilibrium curves of hydrates in clayey silt sediments with sand added at mass fractions of 30% and 50% were close to that of bulk hydrate in pure water at lower temperatures. As the temperature increased, the phase equilibrium curves gradually approached those of hydrates in clayey silt sediments.

The gas production from clayey silt natural gas hydrate (NGH) reservoir in the South China Sea faces the problem of low connectivity between the reservoir and the production well, which seriously reduces the gas production rate. Multistage fractured horizontal well (MFHW) is regarded as an effective technical means to improve gas production for an unconventional reservoir with low permeability. In this paper, a three-dimensional numerical simulation model was built to study the promotion effects of MFHW technology on gas production from a clayey silt NGH reservoir. The temporal and spatial evolution characteristics of the NGH reservoir with and without multiple fractures were compared and analyzed in detail. In addition, the influences of the fracture number, permeability, and morphology on the stimulation effect on gas production through MFHW technology were discussed. The results indicated that the fractures with high conductivity provided a fast channel for gas and water flow and increased the contact area between the horizontal well and the NGH reservoir, which had a positive effect on increasing gas production from the clayey silt NGH reservoir. Increasing fracture number, fracture permeability, and the area of fracture morphology effectively improved the gas production rate and total gas production, but the stage of the high gas production rate only lasted for a short time. This study demonstrated the production behavior of MFHW technology in the clayey silt NGH reservoir, which was helpful for understanding this technology’s stimulation effect.

At present, reservoir reconstruction technology is a possible and practical way to solve the commercial exploitation problem of hydrates, such as low production, limited production range, and high production cost. The research mechanism of hydraulic fracturing in NGH (natural gas hydrate) reservoirs has not formed a theoretical system for the most common reservoir reconstruction technology. In this work, according to the measured geological conditions of site GMGS3-W19, a multi-cluster hydraulic fracturing model for horizontal wells in NGH reservoirs is established with the extended finite element method. The effects of fracture spacing, hydrate saturation, and horizontal in situ geostress difference on the final propagation path of multi-cluster fractures are analyzed. When the alternating fracturing and sequential fracturing are adopted, the subsequent fracture will have a certain extrusion force on the formed fractures, which makes the existing fractures continue to propagate and produce a second expansion. The total lengths of fractures generated by alternating and sequential fracturing modes for values of fracture spacing from 8 to 20 m are longer than that of simultaneous fracturing mode. The hydrate saturation increases from 20 to 60%, and the fracture propagation path hardly changes. However, the fracture propagation of HBS (hydrate-bearing-sediments) with high hydrate saturation is hindered, which leads to a small fracture length in alternating fracturing and simultaneous fracturing modes. When sequential fracturing is adopted, the interference between fractures of HBS with low hydrate saturation is apparent, and the bending degrees of hydraulic fractures are large. When the hydrate saturation reaches 60%, the last fracture will hardly bend and expand along the perforation direction. The difference of horizontal geo-stress increases, and the interference between fractures is more substantial, specifically reflected in the increase of the second fracture expansion lengths and further expansion along the direction of the maximum horizontal geo-stress.

There are critical problems in the field trials of natural gas hydrate (NGH) production, such as low production rate, limited recovery range and short period of stabilized production. Reservoir stimulation is a potential means to promote gas recovery from NGHs since previous studies have proved that gas production of other unconventional gas reservoir can be highly enhanced by hydraulic fracturing. However, few studies pay attention to the hydraulic fracturing behavior of clayey silt NGH reservoir. In this work, the effects of sediment properties and fracturing fluid parameters on fracture initiation, propagation and morphology were experimentally investigated by hydraulic fracturing of frozen clayey silt and methane hydrate-bearing clayey silt. The results indicated that the uneven distribution of ice in sediments led to non-uniform fracture propagation phenomenon, such as singlewing fracture and fracture deflection. The fracturing fluid with injection rate of 90 mL/min was conducive to the formation of complex fractures in sediments, which indicated that that increasing injection rate could greatly increase the effect of reservoir reconstruction. When the axial pressure (8 MPa) was 2 MPa higher than the confining pressure, the fracture propagated in the direction perpendicular to the minimum principal stress, which revealed the magnitude of minimum in-situ stress difference controlling the fracture propagation direction. In addition, this work showed the fracability of clayey silt hydrates even with low saturation, which revealed the feasibility of hydraulic fracturing of marine methane hydrate reservoir.

Natural gas hydrates in the Shenhu area of the South China Sea occur in clayey silt sediments, and its occurrence environment has low permeability characteristics, making it difficult to commercialise gas production from gas hydrates. A feasibility study of hydraulic fracturing and horizontal well applied to the hydrate reservoir in China's first offshore hydrate production site to increase gas production from hydrates was done. According to the well logging curve, a numerical model was built by Tough+Hydrate to study the influences of horizontal well location and horizontal fracture length on gas production. It was found that compared with the cases without fracturing, the total gas production of the cases with 5 m horizontal fracture length when the horizontal well was in the middle of the gas hydrate-bearing layer (GHBL), the middle of the three-phase layer (TPL), and the top of free gas layer (FGL) increased by 53.20%, 60.29%, and 16.68%, respectively. However, when the fracture length increased from 5 to 20 m, the gas production increased only by 5.41%, 7.77%, and 2.27%, respectively. This meant that the preferred fracture length in this study was 5 m, which could effectively increase the production and reduce the construction cost. Meanwhile, the promotion effects of horizontal fracture on the gas production of the horizontal well at the top of the FGL was smaller than that in the middle of the GHBL or TPL. However, higher gas production could be achieved when the horizontal well was at the top of the FGL with a fracture length of 5 m during 1 and 3 years of production, which was 2.61 and 1.61 times of that in the middle of GHBL, respectively; however, the total gas production of 10 years when the horizontal well was in the middle of GHBL was comparable to that at the top of FGL when horizontal fracture length was 5 m.

There are major problems in offshore hydrate production tests, such as low gas production, limited hydrate decomposition area, and short stable production duration. Hydraulic fracturing is regarded as an effective way to improve gas production from a natural gas hydrate (NGH) reservoir. However, the fracture initiation, propagation, and morphology of hydraulic fracturing in the NGH reservoir are rarely investigated. In this work, a 2D numerical model based on the cohesive element is built to study the effects of reservoir properties and fracturing execution parameters on hydraulic fracturing of the NGH reservoir. With the increase of gas hydrate saturation, the fracture initiation pressure increases obviously, and the fracture becomes longer and narrower, which can be attributed to the increase of the strength and elastic modulus of hydrate-bearing sediments. Fracture initiation pressure decreases with the increase of reservoir intrinsic permeability due to the filtration of fracturing fluid. The stress in the normal direction of the fracture surface has a more significant influence on the initiation, propagation, and size of the fracture. With the increase of in situ horizontal stress, the strength of hydrate-bearing sediments increases, leading to an obvious increase of fracture initiation pressure and the formation of wider and shorter fractures. In addition, a higher injection rate of fracturing fluid is conducive to the formation of wider and longer fractures. At high injection rates, the effects of fracturing fluid viscosity on fracture initiation pressure and fracture morphology are more obvious. The results obtained in this work will bring a better understanding of hydraulic fracturing in NGH reservoirs and help to construct potential reservoir stimulation strategies.

The stratification split grouting foam mortar method (SSGFM) was first proposed to stimulate the low-permeability gas hydrate reservoir through constructing the fast flow channel for enhancing gas production. Based on the low-permeability hydrate-bearing sediments (HBS) at SH2 site in the Shenhu area of South China Sea, the foam mortar layer (FML) reservoir model was constructed. The numerical simulation that coupled thermal-hydraulic-mechanical processes was employed to evaluate the efficiency and feasibility of this method. Results show that the FML could effectively promote the expansion of the low-pressure zone into the hydrate reservoir, and enhance gas hydrate dissociation rate, cumulative gas production and energy efficiency. The foam mortar layers (FMLs) have significant influence on the spatial distribution and the evolution characteristics of reservoir thermophysical and geomechanical parameters during gas production. The sensitivity analysis of FML shows the number of FMLs, thickness and permeability of FML exist the critical values for meeting the demand of promoting hydrate dissociation and gas production. Although the cumulative gas production and gas production rate will greatly increase with enlarging the radius, the gas-to-water ratio increase slightly. In view of the hydrate reservoir at SH2 site, the recommended number of FMLs is 4-6, and the distance between the FML and overburden/underburden should be more than 7.5 m. In addition, the optimal values of thickness, radius and permeability of FML are 5 cm, 40 m and 1 x 10(-10) m(2), respectively. The geomechanical response indicates that the FMLs are beneficial for reservoir stability while improving gas production.

In this study, a NC35 tool joint with double shoulder is considered as the research object and studied by finite element method (FEM) using nonlinear thermo-mechanical coupled model and implicit & explicit conversion method. Von mises stress field, temperature field, contact stress, and ultimate working torque of tool joint under different axial loads at high temperatures are calculated and compared with those at normal atmospheric temperature. High temperature doesn't change the stress and contact stress distribution under make-up torque condition but reduces the value of stress and contact stress due to material properties decline. Ultimate working torque decreases with the increase of temperature under the same axial load. Coupled influence of axial load and temperature on shoulder contact state is analyzed. Furthermore, failure modes of tool joint under different temperatures and axial loads are discussed. According to the results of calculation, a guiding diagram considering safety factor is presented which can provide a safe operating range at normal atmospheric temperature and high temperature. It can help engineers choose appropriate operating parameters under different conditions.

Gas hydrates in the Shenhu area are mainly hosted in clayey silt sediments, which have the relatively high irreducible fluid saturation and gas entry pressure. And then, they will have an impact on gas production from hydrate-bearing clayey silt sediments, which was evaluated by the numerical simulations of SH2 site in Shenhu area in this paper. The results showed that, with the increase in irreducible water saturation and irreducible gas saturation, the amount of water production and gas production was obviously reduced. When the irreducible water saturation increased from 0.10 to 0.50, the cumulative CH(4)production volume decreased from 1668799 m(3)to 1536262 m(3), and the cumulative water production volume dropped from 620304 m(3)to 564797 m(3), respectively. When the irreducible gas saturation increased from 0.01 to 0.05, the cumulative CH(4)production volume dropped from 1812522 m(3)to 1622121 m(3), and the cumulative water production volume dropped from 672088 m(3)to 600617 m(3), respectively. In addition, the capillary pressure increased obviously with the increase in gas entry pressure, but the effect on gas production was small and the effect on water production could be negligible. In conclusion, irreducible water and gas saturation had an important effect on the gas production from gas hydrate, whereas the effects of gas entry pressure could be ignored.

In 2017, China successfully conducted its first offshore gas hydrate production test by using vertical well in the Shenhu area, achieving a total gas production of 3.09 x 10(5) m(3). However, there is still a long way to commercial production. In this study, to evaluate and improve the gas production potential of China's first hydrate production site, depressurization, and the combination of depressurization and hot water injection, were applied to natural gas hydrate (NGH) reservoir in China's first offshore hydrate production site to analyze 10 years' hydrate production features. The results indicated that the decomposition front of hydrate when using depressurization in two horizontal production wells had obvious non-uniform characteristics, and there were rapid decomposition regions and hydrate reformation regions. Among them, the hydrate reformation region may have adverse effects on hydrate decomposition. The combination of depressurization and hot water injection could improve total gas production and alleviate hydrate reformation. However, when the temperature of hot water injected increased from 35 degrees C to 80 degrees C, the energy efficiency decreased from 10.22 to 1.53, as a lot of energy was consumed to heat the sediments in the reservoir. When the hot water injection time was reduced from 3650 days to 1825 days, the energy efficiency could be effectively increased from 10.22 to 21.78 without significant reduction of the gas production. This work contributed to the present understanding of production behavior of depressurization and the combination of depressurization and hot water injection, and helped to evaluate the influence of hot water injected on gas production from NGH reservoir.

Gas hydrate-bearing sediments at the first offshore gas hydrate production test site in the Shenhu area, South China Sea, have the characteristics of high irreducible water saturation and low permeability. The stratum structure of this production test site is complex, its gas hydrate-bearing layer (GHBL), three-phase layer (TPL) and free gas layer (FGL) all contain methane gas or gas hydrates, that make the production more complicated. According to the available geological data at the first offshore gas hydrate production test site in the Shenhu area, a 2D numerical simulation model is built to study the short-and the long-term production behavior of the gas hydrates in the Shenhu area. In the short-term, the total gas production from hydrates, when the TPL gas saturation is 0.078, is determined to be 2.75 x 10(5) m(3), which is close to the actual total gas production of 3.09 x 10(5) m(3). When the irreducible water saturation varies from 0.40 to 0.63, the total gas production changes from 1.41 x 10(5) to 2.75 x 10(5) m(3), and the total water production decreases from 1.04 x 10(4) to 8.7 x 10(3) m(3), which indicates that irreducible water is a favorable factor for gas production from hydrates in clayey silt sediments. In the long-term gas production from hydrates, the gas production rate first decreases, then increases, and finally decreases again. Gas hydrate reformation occurs at the interface between the TPL and the FGL, which will adversely affect the gas production from hydrates.