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Natural gas hydrate is widely spread in marine environments around the world. It has great energy potential due to its high methane gas content. High-precision exploration and evaluation of marine gas hydrate still face great challenges as it is affected by the complex reservoir control mechanisms and distribution characteristics. Resistivity is widely used in geophysical logging and theoretical research on gas hydrate-bearing reservoirs by utilizing the high sensitivity electrical response. In this paper, based on the examination of the global marine gas hydrate occurrences, resistivity logging results are summarized. Then the key remaining gas hydrate resistivity experimental concerns are reviewed. In summary, resistivity properties are a reliable means to derive the gas hydrate reservoir characteristics, despite the effect induced by the anisotropic properties of hydrate reservoirs and drilling technology. The overall resistivity change associated with the occurrence of pore filling gas hydrate in reservoirs are relatively small, and the specific value is affected by sediment lithology and hydrate saturation. On the other hand, fracture filling hydrate reservoirs have strong anisotropy, and massive hydrate occurrences (i.e., layers of gas hydrate with no sediment) section shows very high resistivity variation. Clay minerals are an important factor restricting the accurate estimation of gas hydrate saturations from in situ resistivity measurements. Many experimental studies have proposed the correction of Archie empirical formula, but widely representative models have not yet been developed. It is worth noting that more complex resistivity measurements may be able to provide additional electrical response information on various gas hydrate systems. [ABSTRACT FROM AUTHOR]

Natural gas hydrate is critical for its tremendous potential to impact the energy supply field, accelerate global warming if methane reaches the atmosphere, and affect the safety of deep-sea oil and gas production. The estimated gas hydrate resource density in this area is typical, and the total gas hydrate stored is expected to be equivalent to around 74.4 billion tons of oil. Gas production is the third step after the assessment of gas hydrate resource potential and hydrate saturation detection with geophysical methods. [Extracted from the article]

Class 1 hydrate reservoirs are regarded as the most promising targets for gas production. Previous investigations in the northern South China Sea have proven that the low‐permeability hydrate reservoir at site GMGS3‐W19 is a Class 1 hydrate deposit. However, the dynamic production behaviors of this type of reservoir are not yet fully understood. Here, we report gas recovery from hydrate reservoirs containing underlying free gas layers with different initial gas saturations and investigate the effects of absolute permeability and depressurization pressure on gas production performance. The results show that high initial gas saturation of the underlying free gas layer and low bottom‐hole pressure are advantageous for enhancing the gas production performance. The effects of absolute anisotropic permeability and isotropic permeability on gas recovery from Class 1 hydrate reservoirs are different. For long‐term gas production from this hydrate reservoir with any initial gas saturation, a higher horizontal permeability (permeability anisotropy more than 10) in the hydrate layer is remarkably beneficial to stimulate gas extraction and decrease the amount of produced water replenished by inflows from permeable overlying and underlying sediments. Moreover, the gas production rate is highly consistent with the hydrate dissociation ratio, one of the main factors affecting the gas production performance, in Class 1 hydrate reservoirs with different permeabilities. However, for this type of hydrate reservoir with permeability anisotropy larger than 5, the larger the horizontal permeability in the hydrate layer can form more secondary hydrates. The increment in secondary hydrate formation negatively influences the hydrate dissociation ratio and gas recovery. Thus, it is also significant to weigh the permeability anisotropy of the hydrate layer and the secondary hydrate formation amount during the production progress. These findings can contribute to estimating the gas recovery efficiency in Class 1 hydrate reservoirs and optimizing the production process in similar areas. [ABSTRACT FROM AUTHOR]

The permeability anisotropy (ie, horizontal, kr, to vertical, kz, permeability ratio) of gas hydrate reservoirs may be a significant factor that affects hydrate dissociation and gas production. Here, site SH2, a candidate for field testing comprising a clayey silt gas hydrate reservoir in the Shenhu area in the South China Sea, was chosen to investigate the effect of permeability anisotropy on gas production behavior through numerical simulations. The spatial distribution of physical fields (ie, pressure, temperature, and saturation) and the evolution of gas and water recovery are comparatively analyzed. The simulation results show that permeability anisotropy has a significant impact on gas extraction, especially when the horizontal permeability is higher than the vertical one. The sensitivity analyses indicate that permeability anisotropy in both medium‐permeability (10‐100 mD) and low‐permeability (1‐10 mD) sediments are conducive to hydrate dissociation and that a higher horizontal permeability is more favorable for gas production. Comparatively, permeability anisotropy in low‐permeability sediments is not beneficial for improving production efficiency. In addition, our work also suggests that heterogeneous hydrate saturation in a reservoir‐scale model should be employed in future predictions because the gas production potential will be overestimated in a homogeneous reservoir under the same conditions. These findings contribute to a clear understanding of the influence of reservoir properties on gas hydrate dissociation processes and provide a reference for future field trials and commercial production. [ABSTRACT FROM AUTHOR]

The process of extracting natural gas from gas hydrate‐bearing sediments (GHBS) may yield significant sand influx due to the metastable nature of GHBS. Selecting appropriate sand control media is vital to addressing the challenges caused by excessive sand production. This study proposes a protocol called holding coarse expelling fine particles (HCEFP) for sand control design. The protocol aims to provide a new optimization method for screen mesh size selection for clayey silt hydrate reservoirs. Detailed optimizing procedures of proper candidate screen mesh sizes in hydrate exploitation well in clayey silt hydrate reservoirs are depicted based on the HCEFP. Then, the site W18, which is located in the Shenhu area of the northern South China Sea, is taken as an example to illustrate the optimization procedure for screen mesh size selection. The results reveal that complete solid retention via a standalone screen is rarely beneficial as high clay contents can adversely affect wellbore productivity due to excessive plugging. Screen aperture size selection for clayey silt hydrate wells should strike a balance between retaining coarser particles and avoiding screen blockage by the relatively fine particles. Furthermore, longitudinal heterogeneity of the PSDs also increases the difficulties associated with sand control design. Multistage sand control optimization is necessary in hydrate production wells. For Site W18, we recommend that the entire production interval can be divided into two subintervals for multistage sand control operations. [ABSTRACT FROM AUTHOR]

A spiral multilateral well network is a promising production method to enhance long-term gas recovery from prevalent fine-grained hydrate reservoirs. However, practical application is greatly restricted before the optimal well network parameters are determined and the mechanism behind a unique phenomenon in multilateral wells, namely inter-branch interference, is clear. In this study, we numerically optimized the well configuration and spacing when spiral multilateral wells were deployed in two typical fine-grained hydrate reservoirs, i.e., ultra-low permeability hydrate reservoirs (ULPHR, <1 mD) and low-permeability hydrate reservoirs (LPHR, >1 mD). The mechanism behind inter-branch interference was innovatively revealed. The results indicated that the number of spiral branches should be increased, and equidistant branches should be deployed uniformly in the lower ULPHR or throughout LPHR to enhance production efficiency. A wide spacing of spiral multilateral wells with long branches contributed to long-term productivity in fine-grained hydrate reservoirs with any permeability; however, narrow spacing was more favorable for short branches or short-term production. Our study found three inter-branch interference stages during gas production, namely, "no effect" stage, "positive" stage, and "negative" stage; all the three stages are controlled by reservoir permeability, production distance, and production time. Owing to the "positive" interference effect, longer equal-length branches resulted in superior long-term production enhancement in ULPHR, particularly for lengths greater than 30 m. Gas production from LPHR using only two optimal spiral multilateral wells exhibited high production performance similar to that of the sandy hydrate deposits in Japan, suggesting that the optimal spiral multilateral well network is promisingly suitable for commercial production in the future. • Long-term gas production researched using a novel spiral multilateral well network. • Well network parameters optimized in two typical fine-grained hydrate reservoirs. • The mechanism behind inter-branch interference effect was firstly revealed. • Inter-branch interference depends on reservoir permeability, production distance and time. • Due to "positive" interference effect, long equal-length branch was recommended. [ABSTRACT FROM AUTHOR]

Understanding dynamic mechanical properties of hydrate reservoirs is essential for ensuring safety and economic hydrate production. The modulus, damping ratio, and Poisson's ratio are critical parameters in interpreting seismic surveys and well logging data and the stability prediction of hydrate reservoirs during production or under earthquake conditions. In this paper, the shear and Young's moduli and damping ratios were evaluated by conducting resonant column tests on the synthetic hydrate-bearing specimens with respect to hydrate saturation, stress state, strain range, void ratio, pore pressure, and stress history. Regardless of the test conditions, a distinct increase in the damping ratio with an increase in the modulus of hydrate-bearing specimens could be used to identify the hydrate occurrence. The stress and hydrate improve the modulus of hydrate-bearing specimens; however, the exponent enhancement of stress on the modulus of hydrate-bearing specimens is suppressed by high hydrate saturation. In addition, a rapid nonlinear decrease in the normalized modulus of hydrate-bearing specimens with high hydrate saturation occurred under identical strain increments, and the corresponding damping ratio also increased rapidly. Along with the modulus data of other synthetic and natural hydrate-bearing specimens determined using various methods, a definite exponential relationship between E h = E 0 ∗ e n ∗ S h and different parameter settings was established to satisfy various application conditions. In contrast, the Poisson's ratio of hydrate-bearing sediments should be determined with caution because of its relationship with specimen deformation or damage. • The modulus degradation and damping ratio curves of hydrate-bearing specimens were governed by the hydrate. • An empirical elastic modulus equation was validated for hydrate-bearing specimens with different application scenarios. • The deformation-related Poisson's ratio of hydrate-bearing specimens were compared under various strain ranges. [ABSTRACT FROM AUTHOR]

It is of great importance to investigate the mechanical properties and constitutive models of hydrate-bearing sediments (HBS) for the safe extraction of hydrate. However, there are few experiments and constitutive models for fine-grained HBS to date. In this work, a range of triaxial tests were first performed to reveal the mechanical properties of hydrate-bearing clayey-silty sediments (HBCSS). Subsequently, the compression properties and shear behaviors of HBS were summarized in depth. Finally, a constitutive model that could describe the mechanical behaviors of clayey sediments and sandy sediments uniformly was established by proposing the compressibility parameter (reflecting compression characteristics), cementation factor (reflecting cementation effect), and dilatancy factor (reflecting filling effect). The results show that increasing saturation can significantly improve the stiffness and strength of HBCSS. While strain softening can be induced by high hydrate saturation. Strain hardening often produces volume contraction, and strain softening produces volume dilatation. The established constitutive model can not only effectively reflect strain softening and hardening, volume dilatation and contraction, but also describe the characteristics of isotropic compression and one-dimensional consolidation. Meanwhile, it has a good prediction on the effects of confining pressure and hydrate saturation. The model is suitable for both coarse-grained sandy HBS and fine-grained clayey HBS. • The mechanical behaviors of HBCSS are revealed. • A strength prediction formula considering hydrate saturation and confining pressure is proposed. • The compression characteristics of HBS can be reflected by a compressibility parameter. • A constitutive model suitable for clayey HBS and sandy HBS is established. [ABSTRACT FROM AUTHOR]

In this study, we propose a production enhancement strategy that combines multiplanar fractures with a multilateral well network, namely multi-fractured multilateral well network (MMWN), to improve the production efficiency of ultralow-permeability hydrate reservoirs (ULPHs). Considering that natural gas hydrate is generally disseminated in nonhorizontal strata, the influence of topography on the productivity of multilateral wells was first revealed, and then the stimulation effects of MMWN with different geometries and properties in advantageous structural positions of ULPHs on gas production were systematically analyzed. Results indicate that gas extraction from multilateral wells is good at the structural low of ULPHs. MMWN significantly increased the stimulated reservoir volume, leading to an increase in gas production by more than 23 times compared to using the depressurization method alone. Using the MMWN did not require extra energy and caused no ice formation. Improving the planar fracture number, area, width, and permeability effectively facilitated gas recovery. Enlarged fracture spacing is suitable for long-term production. Gas production is not influenced by porosity, overlarge width, or high permeability (over 10 D) of planar fractures. This novel study suggests that the combination of multilateral well networks and highly intensive reconstruction is an effective method to produce gas hydrates. • Multi-fractured multilateral well network (MMWN) is proposed. • Favorable deployment of structural position and fracturing strategy revealed. • Structural low of reservoir is most suitable for multilateral wells to extract gas. • Appropriate geometry and properties of multi-planar fractures are necessary. [ABSTRACT FROM AUTHOR]

Multi-well systems can increase the drainage area and hence enhance the productivity of natural gas hydrate (NGH) production. Fundamentally, the double-well system is the key and basis of understanding the multi-well system and formulating field strategies for gas extraction from hydrate-bearing sediments. However, production performance and multi-well production strategies for recovering NGH by using depressurization have not been treated in detail. Here we develop a three-dimensional model to describe double-well production characteristics from the low-permeability hydrate reservoir and analyze well-deployment strategies. The productivity of this double-well system is 1.5–2.5 higher than that of the single well under the same conditions, which is triggered by enlarging the drainage area. The well spacing alters flow characteristics and corresponding geophysical fields, which is the key variable for the double-well system in gas recovery. Its range can be determined based on the combination of productivity and hydrate dissociation front. Besides, due to the limited enhancement in productivity using the double-well system, the multi-well system assisted with reservoir reconstruction technologies and stimulation methods is of significant practical value in realizing commercial exploitation of NGH. • Propose a 3D thermal-hydro-chemical coupling model for production using double-well systems. • Gas extraction performance and reservoir responses are investigated. • Effects of well spacing on productivity and physical fields are analyzed. • Discuss production strategies of multi-well systems considering commercial hydrate production. [ABSTRACT FROM AUTHOR]

Buried depth, as an inherent occurrence feature of hydrate reservoir, plays a significant role in fluid production during hydrate dissociation. In this study, we experimentally investigate the production performance of hydrate reservoirs at various buried depths beneath the seafloor. The hydrate-bearing system is synthesized in quartz sand with grain size varying between 100 and 500 μm in a 0.98 L reactor. Similar hydrate saturation is obtained at different prevailing pressures between 5.6 and 8.8 MPa. Depressurization experiments are designed to investigate the effect of buried depths on fluid production behavior. The results show that gas and water production increase with elevated buried depths at the same production pressure. However, based on the gas to water ratio, a deep-buried reservoir has a higher production potential at the initial stage. In contrast, a shallow-buried reservoir is an ideal candidate for fluid production in the later stage. Depressurization to the equilibrium P - T condition could lead to potential hydrate dissociation, but the rate is less intensive with less than 46.0 vol% hydrates dissociated in a prolonged time. The experimental results also reveal that both the design of depressurization and the reservoir depths have a combined effect on the overall fluid production performance. • A series of hydrate-bearing sediments with different buried depths were synthesized. • An accurate calculation method quantifying multi-phase saturation was developed. • Stability of hydrate reservoir and fluid production performance were analyzed. • Different production pressures but with the same degree of pressure driving force were employed. [ABSTRACT FROM AUTHOR]