SEDIMENT CONTROL ON HYDRATE OCCURRENCE IN NATURAL SEDIMENTS – FROM EXPERIMENTAL RESULTS

Hailong Lu¹, Fred Wright¹, Toshiharu Okui², Scott Dallimore¹, James Zheng¹, John A. Ripmeester^3
1 Terrain Sciences Division, Geological Survey of Canada
² Technology Research Center, Japan National Oil Corporation
³ Steacie Institute for Molecular Sciences, National Research Council of Canada

Sediment controls on the occurrences of natural gas hydrate have been documented in the studies conducted in Alaska, Blake Ridge, Nankai Trough and Mallik. In fine-coarse intercalative sediment sequences, gas hydrate typically occurs preferentially in the coarse sand layers. Because gas hydrate is stable only at relatively low temperatures and high pressures, it generally underwent significant dissociation during the core recovery process. As a result it is difficult to relate core observations to the actual characteristics of gas hydrate as it existed in original reservoir. Therefore, we must rely on the acquisition of reliable data from controlled laboratory experiments to provide baseline information for the calibration of geophysical and geochemical methods for assessing gas hydrate in natural geologic settings.

Methane hydrates were synthesized over various time periods in three distinct sediments types: sand, silty sand, and sandy clay silt. Gas hydrate saturation levels in each test specimen were subsequently estimated from measurements of gas yield upon dissociation of the entrained hydrate. In order to minimize gas hydrate dissociation during handling, specimens were frozen overnight prior to their extraction from the hermetically sealed reaction chamber. All subsequent sample handling was conducted in liquid nitrogen bath. Results indicate that methane hydrate saturation levels varied according to sediment type. For a reaction period of about one month, the hydrate saturations ranged from 79% to about 100% of pore space in sand, from 15-40% in silty sand, and from 2-6% in sandy clay silt. After only 3 days reaction time, methane hydrate saturation in sand had reached about 30%, implying a kinetic influence on hydrate formation. However, in a sandy clay silt specimen, hydrate saturation was only about 2% after a reaction period of approximately 3 months. The hydrate saturation levels achieved in these experiments are consistent with the observations on natural gas hydrate samples from Blake Ridge, Nankai Trough, and Mallik, where evidence indicates that gas hydrate saturation levels in sands are as high as about 90%, while in silts saturation levels were typically in the range of 2-5%. Because gas hydrates in these settings were almost certainly formed over geological time, we can assume that kinetic factors may be ignored in a comparative assessment of gas hydrate saturation levels. As a result, we can conclude that the features of sediment, especially the grain size of sediment particles, exert a significant influence on gas hydrate saturation levels in natural sediments.

Previous experimental studies have found that the 3-phase equilibrium threshold for methane hydrate stability in sediment shifts towards lower temperatures and higher pressures as compared to the situation in free water. A synthesis of the results from present and previous studies supports the conclusion that sediment can exert considerable influence on gas hydrate stability and pore saturation levels.