In order to know well the range and degree of gas hydrate dissociation during the large-scale exploitation of marine natural gas hydrate, the finite element software COMSOL is used to simulate the electromagnetic monitoring of the natural gas hydrate exploitation through the downhole controlled-source electromagnetic (CSEM) method. By constructing the 3D formation model containing the high-resistivity hydrate reservoir layer, the comparison between the downhole vertical source and the conventional towed horizontal source indicates that the former has advantages on the electric field responses for small source-receiver offsets required for monitoring. Then, the influence of vertical source monitoring system parameters, such as metal casing, vertical source depth, emission frequency, observation errors and noises, on the seafloor electric field responses are analyzed. The simulation results show that the metal casing, vertical source depth and emission frequency have significant influences on the seafloor electric field responses, and especially for the existence of metal casing, the vertical source should be placed below the hydrate reservoir and the low frequency such as 1 Hz of emission electrical current should be considered. The observation errors and noises of the monitoring system would not significantly affect the performance of the monitoring system with metal casing under the condition of small offsets. When the production range of the hydrate reservoir increases, the downhole vertical source monitoring system ensures that seafloor receivers can detect the electric field changes caused by hydrate exploitation and effectively identify the lateral boundary of production area. Hence, it is feasible to use the downhole CSEM method to monitor the dynamic reservoir process during marine hydrate exploitation. 

The RAYINVR software, which is widely used in academia for 2-D velocity structure model from fourcomponent OBS (ocean bottom seismometer) data, is unable to invert for the converted S-wave automatically, resulting in the low-efficiency of modeling process. Using MATLAB’s genetic algorithm, the RAYINVR software is improved and able to automatically and synchronously invert for Poisson’s ratios of each layer with all subblocks for the S-wave velocity structure model, and thus can provide Young’s modulus, Poisson’s ratio and other important mechanical information for gas hydrate survey. This method is applied to process the OBS data collected at the Slipstream submarine slide, and a fine P- and S-wave velocity structure model is obtained, which is comparable to the logging data of nearby borehole U1326. Therefore, the validity of the auto-synchronous inversion method is verified for the S-wave velocity structure modeling. The optimal velocity model reveals two structural interfaces with large Poisson’s ratio contrast. One is BSR (bottom simulating reflector) at 230±10 mbsf (meter beneath sea floor), which represents the bottom boundary of the gas hydrate stability zone, and the other is the basal boundary of a shallow abnormal high-speed body (possibly a sand body enriched with high saturation gas hydrate) at 75?100 mbsf. The latter agrees roughly with the glide plane of Slipstream submarine slide, indicating that the hydrate is related to the formation of submarine landslide.

In order to control the geological and environmental risks during the exploitation of marine gas hydrate, 4-component ocean bottom cable (4C-OBC) is considered to perform time-lapse seismic monitoring on hydrate reservoirs, which can collect P- and S-wave simultaneously, and satisfy the requirement of real-time and long-term monitoring. This paper uses ray tracing method to carry out forward simulation of 4C-OBC time-lapse seismic system for the horizontal well environment in the future commercial gas hydrate exploitation. Based on the seismic illumination of the formation model, the optimal OBC layout parameters is obtained to ensure that the acquired seismic data has good imaging effect. Then, the travel time and amplitude of the time-lapse seismic data in different exploitation stages is analyzed. The results show that both differential travel time and amplitude could reflect the exploitation degree of gas hydrate reservoir, especially significant for converted S-wave. The error analysis results of the observation system show that seismic source vessel’s positioning error would not significantly affect the time-lapse monitoring system. In sum, it is effective to monitor dynamic process of marine gas hydrate reservoir using 4C-OBC time-lapse seismic system.

It is the scientific basis for seismic exploration and resource evaluation of natural gas hydrate to study the acoustic properties of gas hydrate-bearing unconsolidated sediments in the laboratory. For this purpose, an instrument was designed to experimentally study the change in acoustic velocity of water saturated unconsolidated sediments during gas hydrate formation. The improved ultrasonic transducer was employed to measure the acoustic velocity. The saturation of methane hydrate was continuously monitored as referring to the gas pressure change in the reactor, which was then related to the corresponding acoustic measurement. With the artificial sample simulating the sediments of core HY-3 from SH-7 borehole in Shenhu area of South China Sea, several round experiments of methane hydrate formation and dissociation were conducted. The results showed that the measured compressional and shear wave velocities were located between the curves predicted as to the rock physics models with and without hydrate cementing sediment grains in pore space. Although methane hydrate preferred to suspend in pore space or precipitate on sediment grains in dissociation and formation processes, there was always a very small amount of hydrate cementing the grains, and it significantly increased the acoustic velocities of sediments.

We present and establish a seafloor deformation model for gas hydrate exploitation in the sea area based on Okada’s linear elasticity theory. The seafloor deformation field is simulated by using high precision tiltmeters, and the model parameters are inverted by simulated annealing method. The results show that the accurate information of dip, azimuth and volume of hydrate dissociated zone can be obtained by tiltmeters. The test results at different noise levels show that the model parameter inversion method has good anti-noise performance. In addition, the feasibility of tiltmeter monitoring in practical application is analyzed according to the hydrate exploitation test in Shenhu area of South China Sea in 2017, and the results show that the type of decline in production has advantages in seabed stability.