Abstract
Over recent years, there has been a growing interest in producing methane gas from hydrate-bearing sands (MHBS) located below the permafrost in arctic regions and offshore within continental margins. Geotechnical stability of production wellbores is one of the significant challenges during the gas extraction process. The vast majority of geotechnical investigations of MHBS have been conducted on laboratory-formed samples due to the complex procedure of undisturbed sample extraction. One of the most commonly used hydrate laboratory-formation methods is the excess-gas method. This work investigates fundamental aspects in the excess-gas formation of MHBS that are affecting the geotechnical interpretation and modeling. The work finds that (1) the measured temperature in the experimental system may be quite different from the in-sample temperature, and can reach 4 ^circC difference during thermodynamic processes. This potential difference must be considered in investigation of hydrate formation or dissociation, (2) various calculation approaches may yield different hydrate saturation values of up to tens of percentages difference in high hydrate saturations. The calculation formulas are specified together with the fundamental difference between them, (3) the water mixture method during the sample assembling is critical for homogeneous MHBS laboratory formation, in which a maximum initial water content threshold of 9.1 to 1.3 % are obtained for a minimal fraction size of 0.01 to 0.8 mm, respectively, (4) the hydrate formation duration may influence the MHBS properties, and should be rigorously estimated according to the real-time gas consumption convergence. The outcomes of this work may contribute to the integration of data sets derived from various experiments for the study of MHBS mechanical behavior.
Highlights
Over recent years, there has been a growing interest in producing methane gas from hydrate-bearing sands (MHBS) located below the permafrost in arctic regions and offshore within continental margins
This paper presents an experimental study of aspects that influence the geotechnical investigation of laboratory formed MHBS using the excess-gas method
We propose a set of experimental recommendations for geotechnical testing based on the excess-gas formation method, that will serve as a basis for integrating various data-sets:
Summary
Methane gas is consumed from the second day onwards. At the initial formation stage, the hydrate is formed rapidly at the water-gas interfaces and the formation rate decays as the hydrate separates between the free gas and the remaining water[40]. Where mw and mm are the molar masses of water (18.02 g/mol) and methane (16.04 g/mol) This calculation ignores the fact that the initial water content may not be entirely converted into hydrate in the excess-gas method. We show an example of internal water content distribution, ω , in the vertical and horizontal directions, of a soil sample which initially mixed with a global amount of ω = 6%. While the top measuring point of the ω = 4 and 5% samples increases in accordance to the applied global water content, the top ω values of both the 6 and 8% samples are similar (∼ 5.3% ) This ω capillary threshold may be considered as a soil property, associated with the soil skeleton specific surface area.
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