Geomechanical behavior of methane hydrate bearing sediments : from laboratory to field scale

Abstract

A number of constitutive models have been proposed to capture the geomechanical behavior of methane hydrate bearing soils (MHBS). However, the generality of these models is not assessed as they are only validated against a limited number of experiments. Meanwhile, the physical meanings of these model parameters are not always well understood, making obtaining these parameters through laboratory tests a substantial challenge. Moreover, although a few short-term field scale gas production tests from gas hydrate-bearing sediments have been conducted, a probabilistic framework that could adaptively predict the methane production and the wellbore deformation as operation progresses is still absent. Aiming to understand the behavior of MHBS and its influence over gas production, this thesis is composed of two parts. The first part tries to understand the laboratory-scale tests, conducted by the author and collected from literatures. The second part proposes a framework that links the laboratory-scale tests to field-scale productions. The first part focuses on the behavior of MHBS in laboratory. A series of triaxial tests conducted on homogeneous gas/water saturated hydrate-bearing sediments with different hydrate saturation degrees were reported. To model these experiments, modifications have then been made on the methane hydrate critical state model, which was used to study experimental observations, conducted by the author and reported in literatures. The physical significance of parameters in the modified model has been thoroughly studied with global sensitivity analysis. The evolvement of the role of each parameter at different loading stage was depicted quantitatively. To facilitate practitioners to use the modified model, design charts and practical recommendations to determine the values of parameters of the modified model have also been provided. The second part integrates the modified model to an adaptive framework to capture the behavior of MHBS during the field scale production. The framework is driven by sensitivity analysis and is capable of making adaptive predictions of wellbore responses as production progresses. The framework is consisted of three parts -identification of dominant parameters, calibration of dominant parameters through inverse analyses, and estimation of the wellbore response uncertainty at subsequent stages. The complex wellbore response during gas production is shown to be reasonably encapsulated using a small number of influential parameters such as the dilation enhancement parameters, which characterize the geomechanical enhancement from hydrate, and permeability of the underlying layer. The contribution of the proposed framework is that it allows researchers to focus on the experimental determination or numerical calibration of the most dominant parameters, which may be refined through stepwise inverse analyses, and then adopted for forward predictions either in a deterministic or probabilistic manner. The application of the modified model and the framework to the field trial production conducted in Nankai Trough, Japan was conducted and presented as a case study, which sheds light on future gas productions that could take place in other parts of the world.