Intrinsic Properties of Sediment from the Bpxa-Doe Mount Elbert Gas Hydrate Research Well
Reservoir characteristics significantly control regional and pore-scale distribution of gas hydrates (GH). Sediment physical properties provide important parameters for long-term production test and completion program designs, stratigraphic interpretation, reservoir flow effects, pore water geochemistry, and microbial interactions. This study characterizes sediment recovered from the February 2007 BPXA-DOE Mount Elbert gas hydrate research well on Alaska’s North Slope.
Gas hydrate occurs in two unconsolidated very fine to fine-grained sand-rich intervals. The upper interval, Unit D, lies between 614.5 m (2016 ft) and 627.6 m (2059 ft) and the lower interval, Unit C, lies between 649.8 m (2132 ft) and 660.8 m (2168 ft). These layers are bounded by an upper seal of very fine-grained clay- and silty-clay intervals. A deeper interval (Unit B) is similar lithologically to Units C and D, however it is water-saturated and contains no hydrate.
With a few exceptions, median grain sizes (MGS) for the hydrate-bearing layers typically fall within the coarse silt to very fine sand (31 to 125 microns) ranges. The bounding clay-rich beds typically have MGS values in the very fine to fine silt (4 to 16 microns) ranges. Consistent with grain-size results, XRD analyses report higher quartz (82% by weight) in Units C and D, compared to 55% in surrounding sediment. Clay minerals comprise an average of 10% of Units C and D compared to an average of 31% in finer-grained sediment. Units C and D contain about equal amounts of chlorite and illite, and lesser amounts of kaolinite and only trace amounts of carbonate. Pyrite content increases near the top of Unit D.
High sediment permeabilities are important to the formation of concentrated hydrate deposits. Measured permeability values on plugged samples agree with those calculated from grain size and porosity. Within GH-bearing intervals, permeability to air is approximately 600 to 2000 mD (after GH dissociation), whereas permeabilities of finer-grained surrounding sediments range from 0.8 to 10 mD.
Preliminary results indicate that water contents range from 18 to 22% based on solid mass in the GH-bearing intervals. Grain density varies from 2.66 to 2.74 g/cm3, however, two high values (3.19 and 3.21 g/cm3) are associated with hard carbonate-cemented layers. Porosity and bulk density values calculated from discrete sediment samples agree with wireline log measurements.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009