Comparative Formation Evaluation Methodologies for Gas Hydrate Evaluation in Ignik Sikumi #1, Alaska North Slope
Ignik Sikumi #1 was drilled in early 2011 on the Alaska North Slope for evaluation of CO2/CH4 exchange technology in methane-hydrate bearing sands. Hydrate saturation and water saturation are critical to exchange: excess water may react with injected CO2 to form CO2-hydrate, which may diminish reservoir permeability. Quantification and characterization of pore water is a primary focus of this petrophysical evaluation. High-porosity sandstones of the Tertiary Sagavanirktok Formation host North Slope gas hydrate deposits. Water-bearing sandstones are also present in Ignik Sikumi #1, well above the calculated base of the hydrate stability.
Ignik Sikumi #1 was drilled vertically through permafrost to 1750 feet (MD), below which hydrate-bearing sediments were penetrated with chilled, oil-based mud. Openhole log suite includes gamma-ray, neutron porosity, high-resolution bulk density, compressional and shear sonic scanner, resistivity scanner, micro-resistivity imaging, and magnetic resonance tools. The interplay of different responses from different tools optimizes detection and quantitative evaluation of hydrate-bearing reservoirs.
One method for evaluation of the gas-hydrate sands is a combination of density and magnetic resonance. Combining magnetic resonance with bulk density measurements allows determination of hydrate saturation without inference of connate water resistivity, one shortcoming of the Archie Equation. Sagavanirktok “Upper C” sandstone has average hydrate saturation of 75% and water saturation of 25%. The Archie Equation also successfully determined saturations, using density porosity calculated with fluid density equal to that of hydrates and Rw from nearby wet sands. A third, partially successful method was simultaneous solution using gamma-ray, neutron porosity, compressional transit time, and bulk density, to solve for hydrate volume. A fourth method utilizing compressional and shear wave velocities consistently underestimated hydrate saturation compared to the other methods. High-resolution density porosity and micro-resistivity were combined via the Archie Equation as a fifth water saturation calculation technique.
Characterization of pore water as clay-bound,
capillary-bound and free water is possible with magnetic resonance log
measurements. Evaluation of relaxation time distributions and optimization of
cutoff times indicates that of the water present in the pores (SWavg = 25%),
41% is mobile or “free” water.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California