Experimental Studies on the Geomechanical Behavior and Heterogeneity in Laboratory Synthesized Hydrate-bearing Sands
Date
2023-09-21
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Abstract
Hydrate-bearing sands (HBS) store large volumes of methane gas, and along with their potential to be commercially exploited using current oil and gas production techniques, make them suitable as a future energy resource. When hydrates form in the pore space of a sediment, they significantly increase the strength and stiffness of the host sediment. The enhancement in mechanical properties is influenced by various factors, such as hydrate saturation, in-situ stress conditions, sediment type (fine-grained or coarse grained). Our current understanding of HBS behavior is based on studies of laboratory synthesized hydrate-bearing specimens, however significant variation in reported values exist that has been suggested to arise from differences in hydrate formation methods, test apparatus, test conditions. Results from testing natural HBS samples, obtained from offshore India, suggest that the particle size impacts geomechanical properties. However, laboratory studies on synthesized specimens are typically conducted on sand specimens with narrow particle size distributions (PSD) and exclude larger particles. As such, there is a lack of understanding of what may lead to variations in mechanical properties of HBS. This thesis reports on a comprehensive experimental program conducted on laboratory synthesized hydrate-bearing sands for two different PSDs, whose particle size was chosen to better represent the coarse fraction observed in natural cores. The laboratory study included detailed analyses of the shear modulus evolution during the formation of hydrates in the pore space of sands. The research also explored the effect that the initial water saturation had on the pressure and temperature conditions when hydrate formation was initiated, along with how it impacted the stress-strain response after hydrate formation. The results from the extensive testing highlighted an inherent heterogeneity in geomechanical behavior of HBS specimens formed and subjected to similar conditions. Conceptual models were developed to help visualize the experimental observations and gain better insights into the factors that led to differences in formation characteristics and resulting hydrate morphology that developed within different sands. The results of this study highlight that the variations in the geomechanical behavior of HBS reported in literature may arise due to the heterogenous distribution of hydrate within laboratory synthesized HBS studies, which is also likely to exist in natural hydrate bearing cores. Observations from the testing will help researchers better understand the behavior of HBS over a wide range of sediment types and formation conditions, like under permafrost, or within offshore seabed sediments.
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Keywords
Methane gas hydrate, Geomechanical properties, Particle size distribution, Stiffness, Strength, Sands, Heterogeneity, Hydrate distribution
Citation
Pandey, M. R. (2023). Experimental studies on the geomechanical behavior and heterogeneity in laboratory synthesized hydrate-bearing sands (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.