The effect of morphology on the overall physical properties of hydrate-bearing sediments

Introduction With the world’s energy demand increasing daily in the midst of climate change concerns, alternative cleaner energy sources are required. Methane gas hydrates are an untapped resource of natural gas. Due to their wide geographical distribution and the estimated staggering magnitude of this resource, methane gas hydrates have stimulated significant international interest due to their potential as a future energy resource, but furthermore as a geotechnical hazard for offshore operations related to hydrocarbon recovery. Methane gas hydrates are icelike crystalline solids composed of water and natural methane gas that can only exist under specific thermobaric conditions of low temperature and high pressure restrictions. Due to these conditions, gas hydrates naturally occur in submarine sediments or in on-shore sediments in permafrost regions (Kvenvolden, 1993). The occurrence of gas hydrate within sediments significantly alters the physical properties of the host sediments. If the pressuretemperature conditions are changed, for instance by climate change, hydrates are destabilised, dissociating into water and methane gas. This would cause seafloor instabilities and foundation failures of offshore works. To contain this potential geotechnical hazard as well as to tap into this energy resource, the ability to detect and quantify the presence and concentration of hydrate in submarine sediments and understand the effect it has on host sediments has become increasingly important. Due to the metastable nature of gas hydrate, the identification of their insitu presence via the recovery of actual samples has been problematic. Hence the detection and quantification of gas hydrate has been inferred via exploratory seismic surveys, which measure indirectly the bulk dynamic properties of sizeable volumes of sediment insitu. Seismic data are then interpreted using an effective medium model, which employs theoretical assumptions to relate wave velocities to gas hydrate content of the sediment. Wave velocity can then be used to infer hydrate concentration levels. Methane gas hydrates occur in a variety of sediments ranging from fine-grained clays to coarse-grained sands, each hosting a variety of morphologies, which occur as two basic types: pore-filling and graindisplacing (Holland et al, 2008). A number of published effective medium models for gas hydratebearing sediments are available in the literature. The models employ different techniques and of these models it is unclear which to use to interpret seismically measured wave velocities as the models predict quite different hydrate saturation estimates for the same dataset and site (Chand et al, 2004; Westbrook et al, 2008; Minshull & Chand, 2009; Ghosh et al, 2010). The existing models are predominantly limited to pore-filling hydrate morphologies and there is a lack of modelling techniques that consider grain-displacing morphologies. Thus the effect of hydrate morphology on submarine sediments is poorly understood. This justifies the need for the development of a new modelling approach that can conceptualise and model more complex morphologies such as graindisplacing morphologies by taking geometry explicitly into account, thus complementing the existing models in the field by providing a means to investigate a larger range of hydrate morphologies. An alternative modelling approach for gas hydrate-bearing sediments based on first-order computational homogenisation is formulated for specific graindisplacing morphologies. This approach is novel as it applies geotechnical engineering principles along with modelling techniques from the material science field, which have not been implemented in the hydrate-research industry to date. The aim is to investigate the effect of morphology and hydrate content on the overall stiffness and seismic wave velocity of gas hydrate-bearing sediments.