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Recently, drilling wells have encountered rich gas hydrates in fine-grained sediments in the northern South China Sea. Gas hydrate in fine-grained sediments is very heterogeneous, and its physical properties are different from those of oil and gas reservoirs. The reliability of the classical logging saturation evaluation models established for diagenetic reservoirs is questionable. This study used four wells in GMGS3 and GMGS4 to evaluate the effects of the application of three typical methods for evaluating saturation with different principles in the unconsolidated fine-grained sediments: nuclear magnetic logging, sigma logging, and the Archie formula. It was found that the value of the lithologic capture cross-section in sigma logging and the rock's electrical parameters in the Archie formula affect the accuracy of the model. Therefore, to obtain a reliable saturation value for fine-grained sediments, an innovative method for the calculation of resistivity and acoustic time is proposed to estimate gas hydrate saturation based on logging data, which is most consistent with the results of core analysis. The overall relative error of the verification well was 5.87%, whereas that of the density NMR logging method was 56%, showing that the accuracy of the newly proposed resistivity DT logging method's saturation formula was significantly improved. Finally, a new model-based cross chart was developed, which can rapidly differentiate gas saturation during drilling. [ABSTRACT FROM AUTHOR]

Recent studies have confirmed the coexistence of gas hydrate (GH) and free gas (FG) in gas hydrate stability zone (GHSZ). However, the acoustic properties of the coexisting sediment, GH-bearing sediment, and FG-bearing sediment are needed to be specified, for discriminating these three coexisting phases using sonic logs. In this study, the compressional- (P-) and shear (S-) wave attenuations of the sediment with coexisting GH and FG are calculated from the sonic logs in the South China Sea (SCS). The obtained results reveal that the P-wave and S-wave velocities of the sediment with coexisting GH and FG are decreased by 14% and 11%, P-wave attenuation enhanced by 104%, and S-wave attenuation decreased by 44%, respectively, as compared with the sediment only containing GH at well SHSC-4J1. A similar result can be obtained for well GMGS6-SH02. The highest P-wave attenuation appears in the sediment with coexisting GH and FG, ∼0.66 at well SHSC-4J1 and ∼0.60 at well GMGS6-SH02, respectively. Moreover, cross plots of the measured P- and S-wave velocities and attenuations suggest that attenuation is more sensitive to the coexistence of GH and FG than velocity. The saturation of coexisting GH and FG in sediment can be calculated from the sonic velocities using rock physics modeling, a maximum value of ∼26% (av. 13%) at well SHSC-4J1 and a maximum value of ∼23% (av. 7%) at well GMGS6-SH02. • The average velocity and attenuation of the sediment with coexisting GH and FG are estimated. • The highest P-wave attenuation appears in the sediment with coexisting GH and FG. • Attenuation is more sensitive to the coexistence of GH and FG than velocity. • The obtained results indicate the possible presence of multiple hydrate morphologies in the Shenhu Area. [ABSTRACT FROM AUTHOR]

In 2018, Guangzhou Marine Geological Survey conducted the fifth gas hydrate drilling expedition (GMGS5) in the northwestern continental slope of South China Sea. We use pore water geochemistry, pressure core CT scanning, degassing quantification, seismic data and in situ temperature test at four drill sites (W01, W07, W08 and W09) to investigate the gas hydrate characteristics and discuss the dynamics of the gas hydrate system. Based on the CT scanning and pressure core degassing, we demonstrate the coexistence of pore-filling and vein-like gas hydrates. Gas hydrate saturation estimated based on the chloride concentration (0–92%) is higher than that based on the degassing quantification (0–52.9%). The saturation discrepancy is mainly resulted from the sampling discrimination. Based on the average gas hydrate saturation estimated using chloride concentration and degassing, the methane gas volume contained in the gas hydrate of one cubic meter sediment at drill sites W07, W08 and W09 are calculated to be 13.7 m3, 26.82 m3 and 22.24 m3 and 3.03 m3, 14.30 m3 and 16.25 m3. The geothermal gradients estimated based on the in situ temperature measurements at sites W07, W08 and W09 (102–111 °C/km) are more than 50% higher than that at the background site W01 (65 °C/km). It is inferred that the fractures and faults in the gas chimney at sites W07, W08 and W09 are the major contributors for the efficient fluid pathways and high geothermal gradients. The pore water sulfate profiles and inferred sulfate-reduction methane-oxidation interface (SMI) depths indicate that methane flux is lowest at site W01 and highest at site W08. The SMI depths and top of gas hydrate occurrence is linearly correlated which provides an experienced function for understanding the local gas hydrate distribution. Kick-type sulfate profiles at sites W07 and W09 indicate an ongoing increase of methane flux and corresponding shoaling SMI depths. In addition, chloride enrichment in the shallow sediment at site W08 indicates a recent fast gas hydrate formation process. The kick-type sulfate profiles, positive chloride anomalies and authigenic carbonates at multiple depths suggest that the system experienced dynamic changes in fluid flux and multi-stage gas hydrate evolution on time scale from months to thousands of years. Understanding the controlling mechanism and periodicity of the composite pressure system is important for evaluating the gas hydrate resource and carbon cycling. • Gas hydrates were drilled for the first time in the Qiongdongnan Basin. • Pore-filling and vein-like gas hydrates co-exist in the cores. • Kick type sulfate profiles indicate the transit state of methane flux. • Positive pore water chloride anomalies indicate the recent fast gas hydrate formation. • Dynamic fluid flux and multi-stage gas hydrate evolution happened on time scale from months to thousands of years. [ABSTRACT FROM AUTHOR]

The Shenhu area is the key target area for marine gas hydrate industrialization in China. Significant progress has been made with respect to gas hydrate exploration and production in this area in recent years. However, a comprehensive understanding of the reservoir characteristics and hydrate accumulation mechanism are still lacking; in particular, the hydrate enrichment factors are not, as yet, completely clear. In this study, we systemically characterize the geological controls on gas hydrate accumulation in the Shenhu area, including the sedimentary environment, gas sources, fluid and gas pathways, and reservoir-caprock system. We subsequently provide a quantitative evaluation of some critical factors influencing hydrate enrichment. The depositional environment in the study area was characterized by low energy (sedimentary hydrodynamic conditions), and anoxic environment, and the gas hydrate accumulations were closely associated with turbidites. The methane captured in the gas hydrates in the GMGS1 area and at the SHSC-4 and SHSC2-6 production test sites was mainly derived from the microbial reduction of CO 2 , while thermogenic gas was a more important source of hydrocarbon gases in the GMGS3 and GMGS4 areas. In addition, we used a combination of 8-slice spiral X-ray computed tomography (CT) and micro-CT techniques for the first time for the in-situ investigations of the microfractures and foraminifera in the hydrate cores. The results indicate that microfractures and foraminiferal shells are crucial to the occurrence and accumulation of gas hydrates in fine-grained sediments of the Shenhu area. Interestingly, microfractures were found to be more developed in the hydrate layers where most sediment particles were relatively small in size. Hydrate saturation increased with i) a decrease in the specific surface area of sediment grain, ii) an increase in the median pore size, and iii) increased foraminifera abundance in the reservoir. Our findings could help locate high-saturation gas hydrate layers ('sweet spot') in fine-grained marine sediments of the Shenhu area, South China Sea. • Microscopic factors influencing the hydrate enrichment are comprehensively discussed. • A conceptional model for hydrate accumulation was proposed in the Shenhu area. • Microfractures prefer to develop in the hydrate layers where the sediment particles have relatively small grain size. • Thermogenic gas makes an important contribution to hydrate accumulation in turbidites connecting deep faults. [ABSTRACT FROM AUTHOR]

Gas hydrates and deep-sea cold seeps are of interest to industry and academia for several reasons, including their implications for offshore geohazards and unconventional resources, and for global climate change and carbon cycling. Guangzhou Marine Geological Survey (GMGS) implemented its fifth gas hydrate drilling expedition in the QiongDongNan Sea Area (QDN-SA) of the northern South China Sea (SCS) from June to September 2018. Multidisciplinary investigations, including Logging While Drilling (LWD), pressure and non-pressure coring, gas composition analyses, in situ temperature and permeability measurements, pore water geochemical analyses, and a regional seismic survey, reveal a complex distribution of gas hydrate within a vertical chimney structure. LWD profiles record high resistivity anomalies and high P-wave velocity variations at 9–174 mbsf, suggesting the coexistence of free gas and gas hydrate in the sediment column throughout the investigated gas chimney. Hydrate-bound and void gas is an admixture of methane (70.1%–98.3%) and heavier hydrocarbons (C 2 H 6 : 0.16%–17.59%, C 3 H 8 : 0.05%–9.88%, i-C 4 H 10 : <0.001%–2.06%, n-C 4 H 10 : <0.001%–0.85%, i-C 5 H 12 : <0.001%–0.12%, n-C 5 H 12 : <0.001%–0.02%), indicating a thermogenic origin for the gas. Based on the high content of C 2 + hydrocarbons in the gas composition, it is suggested that sII/sH gas hydrate exists in the study area. The gas chimney is interpreted to provide efficient pathways for fluid migration from an intermediate gas reservoir below the BSR to the gas hydrate stability zone (GHSZ). Gas hydrate crystallization decreases the permeability of the sediment, impeding fluid flowing into the upper sediment layers. Bivalve fragments embedded within carbonate rocks were recovered at the seafloor and multiple levels at two main sediment levels of 3mbsf and ∼54mbsf, indicating recurrent seepage activity. • First results from the 5th gas hydrate drilling expedition in the QiongDongNan Area of the South China Sea in 2018. • Gas hydrates co-exist with free gas between 9 mbsf and 174 mbsf at the drilling site. • High C 2 + (>10%) in the hydrate-bound gas indicate the presence of sII/sH gas hydrates and a thermogenic gas source. • Authigenic carbonates with cold seep bivalve fragments at multiple depths indicate recurrent seepage in the past 0.54 Ma. [ABSTRACT FROM AUTHOR]

Abstract Because of the metastable character of natural gas hydrate-bearing sediments (GHBS), they can be easily affected by drilling operations, thereby inducing the wellbore instability risk. This issue is studied in this paper by means of numerical simulations based on the first exploration well performed in the Shenhu area of the South China Sea in 2007. The corresponding simulated results were compared against the field caliper logging curve. Besides this analysis, the effects of both drilling mud properties (i.e., temperature, density, and salinity) and initial reservoir conditions (i.e., permeability, and saturation) on the wellbore stability were further investigated under the condition of drilling mud invasion into the GHBS. It is anticipated that the coupled multiphysics phenomena triggered under operation conditions can lead to wellbore instability and may also trigger the formation of fractures. The simulation results show that the predicted yield region around the borehole of well SH2 conforms to the borehole enlargement observed in the field. The sensitivity analyses indicate that a local increase in the pore pressure is likely to be generated in the wellbore vicinity, triggered by the large amount of free gas to be released during hydrate dissociation, which is in turn induced by heating (i.e., coming from the drilling-friction effects) and the high salinity of the drilling muds typically used. This study shows that by engineering the salinity of the drilling mud, the amount of free gas to be produced from marine GHBS can be controlled, preventing in this way further wellbore instability issues. This research also shows that an increase in the drilling-mud density not only impact on the development of pore pressures near the borehole, but it also affects the extent of drilling-mud invasion. This effect will extend the hydrate dissociation to a wider area and will also reduce the resistance of the formation, leading to further deformation and potential failure. The analyses also show that the increase in the initial hydrate saturation is associated with a stronger sediment and a more stable wellbore. The model also predicts that GHBS with higher absolute permeability will dissipate faster the excess of pore pressure in the surrounding sediment, contributing to wellbore stability; however, the extent of the hydrate dissociation zone will expand and thereby will increase the yield region. Highlights • A coupling method was proposed for analyzing wellbore stability drilling in GHBS. • The latest function between mechanical properties and S H was used in our model. • The model was demonstrated by the field caliper logging results. • The effect of drilling mud properties on wellbore stability was reported. • Wellbore stability affected by hydrate reservoir conditions was analyzed. [ABSTRACT FROM AUTHOR]

According to the preliminary geological data of gas hydrate bearing-sediments (GHBS) at site GMGS3-W19 in the third Chinese expedition to drill gas hydrates in 2015, a production model using three different recovery pressures was established to assess the production feasibility from both production potential and geomechanical response. The simulation results show that for this special Class 1 deposit, it is a little hard for gas production rate to reach the commercial extraction rate because the degree of hydrate dissociation is limited due to the low reservoir permeability and the permeable burdens. However, the free gas accumulating in the lower part of the GHBS can significantly increase gas-to-water ratio. It also generates many secondary hydrates in the GHBS at the same time. Decreasing the well pressure can be beneficial to gas recovery, but the recovery increase is not obvious. In term of geomechanical response of the reservoir during the gas recovery, the permeable burdens are conducive to reduction of the sediment deformation, though they don't facilitate the gas recovery rate. In addition, significant stress concentration is observed in the upper and lower edges of GHBS around the borehole during depressurization because of high pressure gradient, and the greater the well pressure drop, the more obvious the phenomenon. Yield failures and sand production easily take place in the edges. Therefore, in order to achieve the purpose of safe, efficient and long-term gas production, a balance between the production pressure and reservoir stability should be reached at the hydrate site. The production pressure difference and sand production must be carefully controlled and the high stress concentration zones need strengthening or sand control treatment during gas production. Besides, the sensitivity analyses show that the hydrate saturation heterogeneity can affect the production potential and geomechanical response to some extent, especially the water extraction rate and the effective stress distribution and evolution. Increasing GHBS and its underlying free gas formation permeabilities can enhance the gas production potential, but it probably introduces geomechanical risks to gas recovery operations. [ABSTRACT FROM AUTHOR]

Gas chimney and the associated fluid migrations are important for understanding the accumulation of gas hydrates worldwide. Two adjacent gas chimneys (GC1 and GC2) both originate from the Songnan low uplift in the Qiongdongnan Basin, and many fluid seepage pathways with various intensities were identified inside the shallow massive transport deposits (MTDs) on the top of the gas chimneys. Seismic attributes indicate that GC1 covers wide areas, and the seepage pathways are sporadically and irregularly distributed over GC1. GC2 is another type of gas chimney which is associated with a fault that communicated deep uplift to the MTDs, and more seepage pathways with larger diameters above GC2 were present along the fault. From the X-CT images of pressure cores with fracture-filling hydrates and elastic-electrical anisotropy analysis of well logs, hydrates are mainly present in vertical fractures in the seepage pathways above GC1, while many horizontal fractures in those pathways near the fault above GC2 were identified. Because the gas pressure needed to induce vertical fractures is smaller than horizontal fractures, the minimal heights of the gas column in GC1 and GC2 were at least 90.9m and 163.26m, respectively, based on the fracturing criterion. It also indicates that GC2 owns a higher migration efficiency. Meanwhile, rock physic inversion reveals that high-intensity seepages above GC2 create more fracture-filling hydrates, and increase the primary porosity of the sediments as well. This study reveals these new and quantitative core-well logging-seismic evidences of two types of chimneys with completely different fluid migration mechanisms, offering insights into studying fluid migration related to gas chimneys worldwide. • The gas chimney with a fault is a high-efficiency supply pathway. • The distribution of seepages and the fracture dips are related to the migration efficiency. • Adequate gas source produces more fracture porosities in shallow sediments. [ABSTRACT FROM AUTHOR]

The second gas hydrate expedition of the Guangzhou Marine Geological Survey drilled 13 sites and recovered a large amount of gas hydrate in the east of the Pearl River Mouth basin of the South China Sea. In this study, we examine three of these sites (sites 05, 08, and 16) using multichannel seismic data, LWD logging data, and multi-beam echo sounder data. The geophysical data suggest and core samples confirm that gas hydrates occur near the BSR at all three sites, whereas near-surface gas hydrates exist at sites 08 and 16 with high fluid flux. We present a conceptual model for the formation and accumulation of gas hydrates in our study area, controlled by the activity of dissolved and free gas-rich fluid. Interpreted abundant gas sources below the BSR contribute to the generation of overpressure, resulting in the movement of gas-rich fluid. Dissolved gas-rich fluid forms pore filling hydrates near the BSR, and the relatively deep hydrate layer thickens over time by making its way through the growing hydrate layer and adding to the top, if the fluid is supplied sufficiently. Free gas-rich fluid is more favorable to generate near-surface nodules or massive hydrates, which could be stable in natural conditions. As sediments deposit, the relatively shallow hydrates are buried deeper and keep their morphologies. Furthermore, gas hydrate formation and stability imply that gas hydrates may be increasingly considered as a potential energy resource. [ABSTRACT FROM AUTHOR]

Porosity, saturation, thickness of gas hydrate-bearing layers and their spatial distributions are key factors for production tests. The gas hydrate-bearing layers ranging from a few to tens of meters thickness are confirmed by Logging While Drilling (LWD) data and core samples at Sites W11 and SC-03, in the Shenhu area, South China Sea. Gas hydrate-bearing layers of two wells (about 200 m) show that not all layers exhibit good lateral continuity. Several thin layers (~4 m) with low gas hydrate saturations (15–20% of the pore space) or non-gas hydrate are laterally discontinuous. The interbedding of gas hydrate-bearing and non-hydrate layers with different saturations, thickness and random distribution cause the heterogeneities in the reservoir. The gas hydrate saturation and distribution inverted from deterministic and traditional geostatistical inversion have high uncertainties due to the limitations of the empirical relationship and probability density functions (PDFs). A better understanding of spatial distribution and saturation for each gas hydrate layer is useful to build a realistic net hydrate pay and resource estimation. In this study, we first divide gas hydrate saturations, porosity, acoustic impedance into low and high values based on the estimated LWD results, which is call as lithofacies. Then, we propose to create multivariate PDFs by lithofacies constraints. The acoustic impedance, saturation, thickness, porosity of gas hydrate-bearing layers and non-gas hydrate layers are inverted by displaying geostatistical inversion with lithofacies constraint. We found the spatial variations and heterogeneous distributions with a thickness ranging from 4 to 35 m and the laminated gas hydrate layers with saturations ranging from 10% to 40% are clearly shown from seismic data, which are useful for the horizontal well development method of a hydrate production test. Image 1 • Geostatistical inversion by lithofacies constraint can clearly distinguish thin layer. • Hydrate saturation is used as litholofacies to improve the inverted accuracy of clayey-silt sediments. • Boundaries between different saturations and thin layers clearly display the heterogeneity. [ABSTRACT FROM AUTHOR]