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

Abstract

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.

Numerical Simulation and Optimization of Well Parameters for Depressurization-Assisted CO2 Replacement Using a PSO Algorithm

“…There have been five offshore and six permafrost field tests of hydrate globally, yet none have managed to achieve efficient and stable production. − The depressurization method, due to its efficacy, convenience, and cost-effectiveness, , has been utilized in all offshore and four permafrost field tests. However, hydrate dissociation not only impacts the mechanical stability of the formation but also severely constrains the efficient production of hydrates in the later stages of depressurization-based production due to insufficient reservoir heat. − The CO 2 replacement method can simultaneously maintain reservoir mechanical properties, release heat to promote the dissociation of CH 4 hydrates, and facilitate the sequestration of CO 2 . − However, the problem of inefficiency still exists with the replacement method. These two methods demonstrate complementary advantages, rendering the combined method the focal point of current research endeavors.…”