Online Detection Technology And Physical Properties Analysis Of Natural Gas Hydrate-bearing Sediment Cores

With the development of natural gas hydrate sampling techniques, detection and analysis of gas hydrate pressure cores become the bridge which connects hydrate exploration and exploitation. Natural gas hydrate pressure cores reflect the reservoir characteristics and hydrate accumulation. This information is important for the study of hydrate exploitation. Online detection of natural gas hydrate pressure cores provides the data for the transfer of natural gas hydrate pressure cores to conduct further analysis in laboratory.In this work, a system for natural gas hydrate-bearing sediment cores online detection with in-situ pressure was designed and developed for the first time, and a series of experiments for acoustic properties of hydrate-bearing sediment cores were performed systematically in laboratory and field. The influence factors, including hydrate saturation, particle size, porosity, and density of porous sediments to P-wave velocity (VP) and amplitude of hydrate-bearing sediment cores were investigated. The results showed VP increased with the rise of hydrate saturation. Particularly, VP increased more dramatically when the hydrate saturation exceeded 50%, which could be the criterion for the existence of natural gas hydrate. In the same hydrate saturation, VP kept constant when the particle size of the porous media changed in the range of particle sizes. Furthermore, VP decreased with the increasing initial porosity of the porous media. However, VP was determined mainly by the density of the porous media when the hydrate saturation reached 100%. Taking the effective media theory into account, the results showed that the hydrate occurred in the pore space first, which structure seemed to be pore-filling, and with the increasing hydrate saturation, the pore space within the porous media was connected by hydrate, which resulted in load-bearing structure. Using ice to replace hydrate, field scale experiments concentrated on different hydrate occurrences were conducted. The results showed that the system can achieve massive and lamellar hydrate discrimination, but powdered hydrate certain difficulties.Laboratory based studies for the physical properties of sediment matrix hosting gas hydrate were carried out on the non-pressured hydrate-bearing sediment samples from GMGS2 drilling expedition. Measurement of index properties, surface characters, thermal properties, acoustic properties, seepage properties and mechanical properties of the sediments were performed on ten sediment cores. The mean grain sizes mostly ranged from 7?m to 11?m throughout all intervals. Based on the images of X-Ray CT and SEM, bioclast has not been found yet, although the flaky clay may be conducive to hydrate formation in the pore. The thermal conductivity measured by plate heat source method were relatively low compared to other miners, changing from 1.0 to 1.45 W/(mK), which suggests that thermal stimulation may not be a good option for gas hydrate production, also the depressurization could exacerbate the gas hydrate reformation or ice formation. Using the artificial hydrate-bearing sediment cores from sediment samples, the experiments of stress-strain and failure strength for hydrate-bearing sediment cores were performed under different confining pressures. Results demonstrated the failure strength decreased quickly during the dissociation process of hydrate. Therefore, it was vital to drain the decomposition water during gas production from reservoir. Furthermore, the non-dimensional number D15/D85 and ? were employed to evaluate the sediment transport properties and the rule of the hydrate formation in pores. The results indicated the sediments were self-filtering, which helped to prevent particle migrations during gas production. Moreover, hydrate formation was inclined to be in pore space within sediments, but not act as the cement to contact the particles.