Non-Embedded Ultrasonic Detection for Pressure Cores of Natural Methane Hydrate-Bearing Sediments

Abstract

An apparatus for the analysis of pressure cores containing gas hydrates at in situ pressures was designed, and a series of experiments to determine the compressional wave response of hydrate-bearing sands were performed systematically in the laboratory. Considering the difficulties encountered in performing valid laboratory tests and in recovering intact hydrate bearing sediment samples, the laboratory approach enabled closer study than the marine environment due to sample recovery problems. The apparatus was designed to achieve in situ hydrate formation in bearing sediments and synchronous ultrasonic detection. The P-wave velocity measurements enabled quick and successive ultrasonic analysis of pressure cores. The factors influencing P-wave velocity (Vp), including hydrate saturation and formation methodology, were investigated. By controlling the initial water saturation and gas pressure, we conducted separate experiments for different hydrate saturation values ranging from 2% to 60%. The measured P-wave velocity varied from less than 1700 m/s to more than 3100 m/s in this saturation range. The hydrate saturation can be successfully predicted by a linear fitting of the attenuation (Q−1) to the hydrate saturation. This approach provided a new method for acoustic measurement of the hydrate saturation when the arrival time of the first wave cannot be directly distinguished. Our results demonstrated that the specially designed non-embedded ultrasonic detection apparatus could determine the hydrate saturation and occurrence patterns in pressure cores, which could assist further hydrate resource exploration and detailed core analyses.

Insights into the climate-driven evolution of gas hydrate-bearing permafrost sediments: implications for prediction of environmental impacts and security of energy in cold regions

“…The elastic wave velocities were measured using the ultrasonic transmission method which requires accurate measurements of the travel time ( t ) of the elastic waves together with the propagation distance through the specimen ( L ). 25 Having the inherent travel time ( t 0 ) of both P- and S-waves, the velocity ( v ) could be calculated according to eqn (1) : where i stands for P and S.…”

“… 2 Given the conventional exploration techniques primarily rely on the evolution of geophysical response of gas hydrate-bearing sediments, considerable work has been conducted in laboratory to investigate the pore-scale processes accompanying hydrate formation in porous media in order to understand how hydrates alter the skeletal properties of the host sediment and influence the magnitude of the compressional (P) and shear (S) wave velocities. 25 Various empirical and physics-based methods have been also developed to establish a relationship between the elastic wave velocities and hydrate saturation, and thereby predict the physical properties of gas hydrate-bearing sediments. 26 However, there is still lack of fundamental knowledge regarding the pore-scale phenomena influencing the physical properties of permafrost, particularly gas hydrate-bearing permafrost sediments compared to merely frozen or hydrate-bearing unfrozen sediments.…”

Formation, Exploration, and Development of Natural Gas Hydrates

“…Although the phase equilibrium [7], structural properties [8], mass transfer mechanism [9], heat transfer analysis [10], growth kinetics [11,12], and phase change parameters [13] of NGH have been well-studied in the past, a need for a fundamental and thorough understanding of NGH formation and occurrence still remains. In particular, developing high-precision, multi-dimensional, and comprehensive exploration technologies are the future direction of deep-sea NGH exploration [14]. The advancement in this field will be conducive to ascertaining the geological condition, revealing the reservoir-forming mechanism, proving the NGH reserve, and disclosing the NGH distribution.…”