Acoustic characterization for creep behaviors of marine sandy hydrate-bearing sediment.

Marine natural gas hydrate (NGH) is a promising substitutive low-carbon energy resource, whereas NGH-production induced geoengineering concerns remain challenging. Advanced forecast of possible geoengineering risks is the fundamental for eco-friendly NGH exploitation. Reservoir creep deformation is an early symptom of the geoengineering risks. However, whether the creep deformation behaviors of the NGH-bearing strata is predictable remains controversial. In this study, a series of multi-step loading creep test are conducted for sandy gas hydrate bearing sediment (GHBS) samples, during which the ultrasonic responses are recorded simultaneously. The acoustic velocity, compression-to-shear velocity ratio, Poission's ratio, main frequency, and main frequency amplitude are used to characterize creep failures of the GHBS for the first time. Combining analyses of the creep behaviors and acoustic responses yield the following conclusions. Firstly, the long-term strength derived from creeping test is 0.45-0.60 times of the shear strength derived from triaxial shearing. Ignoring the creep effect might underestimate the scale and intensity of possible geoengineering risks during long-term NGH exploitation. Secondly, the acoustic velocity increases gently and then decreases continuously during creeping. Once the accelerated creep appears, the acoustic velocity plummets significantly, together with a sudden decrease in the compression-to-shear velocity ratio, and fluctuations in the main frequency and its amplitude. Furthermore, the main frequency and its amplitude shall fluctuate abruptly prior to the emergence of the accelerated creep. Therefore, we anticipate that the combination of abnormal fluctuations of main frequency and its amplitude can be used as early-warning indicators for possible creep failure of the GHBS. The results might have great significance for in-situ detection and prediction of possible reservoir failure during long-term NGH exploitation.

Integrating test device and method for creep failure and ultrasonic response of methane hydrate-bearing sediments.

Clarifying the creep behaviors of hydrate-bearing sediment (HBS) under long-term loading is crucial for evaluating reservoir stability during hydrate exploitation. Figuring out a way of characterizing deformation behaviors and their geophysical responses to HBS is the basis for modeling creep behaviors. In this study, we propose a novel device to test time-dependent deformation and the ultrasonic response of HBS under high-pressure and low-temperature. The experimental device consists of a high-pressure chamber, an axial-load control system, a confining pressure system, a pore pressure system, a back-pressure system, and a data collection system. This testing assembly allows temperature regulation and independent control of four pressures, e.g., confining pressure, pore pressure, back pressure, and axial loading. Columned artificial HBS samples, with a diameter of 39 mm and a height of 120 mm, can be synthesized in this device. Afterward, in situ creep experiments can be achieved by applying stable confining pressure and axial load, together with geophysical signals acquisition. During loading, the stress-strain relationships and ultrasonic data can be obtained simultaneously. Through analyzing the stress-strain relationship and ultrasonic data, the macroscopical failure and microcosmical creep deformation law of the samples can be figured out. Preliminary experiments verified the applicability of the device. The method provides some significance for field observation of reservoir failure via geophysical techniques during hydrate exploitation.