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The effects of geologic conditions and production methods on gas production from hydrate-bearing sediments (HBS) have been widely investigated. The reservoir was usually treated as horizontal distribution, whereas the sloping reservoir was not considered. In fact, most strata have gradients because of the effects of geological structure and diagenesis. In this study, based on currently available geological data from field measurements in Shenhu area of the South China Sea, the effects of formation dip on gas production were investigated through depressurization using a horizontal well. The modeling results indicate that the strategy of horizontal well is an effective production method from the unconfined Class 2 HBS. The predicted cumulative volume of methane produced at the 1000 m horizontal well was 4.51 × 107 ST m3 over 5-year period. The hydrate dissociation behavior of sloping formation is sensitive to changes in the reservoir pressure. As in unconfined marine hydrate reservoir, the sloping formation is not conducive to free methane gas recovery, which results in more dissolved methane produced at the horizontal well. The obvious issue for this challenging target is relatively low exploitation efficiency of methane because of the recovery of very large volumes of water. Consequently, the development of the favorable well completion method to prevent water production is significantly important for realizing large scale hydrate exploitation in the future. [ABSTRACT FROM AUTHOR]

Dinoflagellate cysts (dinocysts), biogenic elements and metals were analyzed from sediment cores collected from Zhelin Bay of the South China Sea in December 2008 to understand the environmental changes over the past 50 years. Dinocyst concentrations ranged from 0 cysts/g to 770 cysts/g, and they were dominated by heterotrophic taxa. There was a clear increase trend upcore for biogenic elements, except for biogenic silica. Metals originated from both the lithogenic source and human activities, and significantly increased after 1985–1995. Environmental changes in the past 50 years can be divided into three stages: (1) before 1985, during which biogenic elements, cyst flux and metals were low; (2) from 1985 to the early 2000s, characterized by an obvious increase of dinocysts, TOC, TN and most metals, while TOC/TN and BSi decreased; and (3) after 2000, the period of rapid increase of dinocysts, TOC and TN but decrease of certain contamination metals. [ABSTRACT FROM AUTHOR]

The recovery of natural gas hydrate (NGH) from marine sediments faces challenging due to not only the gas productivity itself but also the possible geohazards, such as seafloor subsidence, submarine landslide, wellbore instability and possible sand production. The coupled thermal-hydrodynamic-mechanical (THM) processes during NGH recovery are generally complex, and numerical simulation tools are needed to assess related geomechanical responses. We have extended the Biot consolidation model in our previous simulator TOUGH2Biot, and incorporated into the existing TOUGH + hydrate code, resulting in a THM simulator for the NGH recovery. The THM simulator is used to assess the geomechanical responses to gas recovery from an unconfined hydrate-bearing sediment (HBS) in the Shenhu area, South China Sea. We investigated depressurization using constant bottom-hole pressure through a horizontal well. Results show that methane production quickly reaches a stabilized state and the water rate increases linearly. The drawdown of pore pressure around the well controls the increase in effective stress. Subsidence becomes significant after depressurization due to the quickly propagation of pore pressure. Depressurization in early stage could contribute to more than half of the total subsidence. The decreasing production pressure leads to an increase in the methane production rate but deterioration in subsidence. A decrease in intrinsic permeability of overlying and underlying layer is undesirable due to its decrease in methane production rate and the worse of subsidence. A balance between gas productivity and related geomechanical response must be achieved. The methods and preliminary results presented in this study could help us to understand the geomechanical behaviors during NGH recovery and to design trial production schemes under similar conditions. [ABSTRACT FROM AUTHOR]