Electrical anisotropy of gas hydrate-bearing sand reservoirs in the Gulf of Mexico

Abstract

a b s t r a c tWe present new results and interpretations of the electrical anisotropy and reservoir architecture in gas hydrate-bearing sands using logging data collected during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II. We focus specifically on sand reservoirs in Hole Alaminos Canyon 21 A (AC21-A), Hole Green Canyon 955 H (GC955-H) and Hole Walker Ridge 313 H (WR313-H). Using a new logging-whiledrilling directional resistivity tool and a one-dimensional inversion developed by Schlumberger, we resolve the resistivity of the current flowing parallel to the bedding, R k and the resistivity of the current flowing perpendicular to the bedding, R t . We find the sand reservoir in Hole AC21-A to be relatively isotropic, with R k and R t values close to 2 U m. In contrast, the gas hydrate-bearing sand reservoirs in Holes GC955-H and WR313-H are highly anisotropic. In these reservoirs, R k is between 2 and 30 U m, and R t is generally an order of magnitude higher.Using Schlumberger's WebMI models, we were able to replicate multiple resistivity measurements and determine the formation resistivity the gas hydrate-bearing sand reservoir in Hole WR313-H. The results showed that gas hydrate saturations within a single reservoir unit are highly variable. For example, the sand units in Hole WR313-H contain thin layers (on the order of 10e100 cm) with varying gas hydrate saturations between 15 and 95%. Our combined modeling results clearly indicate that the gas hydratebearing sand reservoirs in Holes GC955-H and WR313-H are highly anisotropic due to varying saturations of gas hydrate forming in thin layers within larger sand units.

Gulf of Mexico Gas Hydrate Joint Industry Project Leg II logging-while-drilling data acquisition and analysis

“…But it is acknowledged that complex sand-shale interbedded reservoirs can be strongly anisotropic in nature, which can lead to inaccurate estimation of hydrocarbon saturations, particularly when the interbed thickness approaches the vertical resolution limits of the log data. The anisotropic nature of complex sand-shale interbedded gas hydrate reservoirs are discussed in Cook et al (2012).…”

“…These measurements can be used to determine electrical anisotropy of the reservoir as well as formation dip and azimuth (Li et al, 2005;Sun et al, 2010). GOM JIP Leg II scientists have used the directional resistivity measurements to analyze electrical anisotropy associated with gas hydrate-bearing reservoir sands in Cook et al (2012).…”

Architecture of gas-hydrate-bearing sands from Walker Ridge 313, Green Canyon 955, and Alaminos Canyon 21: Northern deepwater Gulf of Mexico

“…Overlay of the structure map with the pattern of strong peak amplitudes suggests that the gas hydrate occurrence is strongly influenced by fault compartmentalization. In addition, these faults are likely conduits for further vertical gas migration, leading to shallower, fault-related gas hydrate occurrences within fractures formed in fine-grained sediments Cook et al, 2012). At both the H-and Q-well locations, gas hydrate appears in sands in structurally high fault blocks (labeled Fault blocks H and Q on Fig.…”

“…This internal structure is consistent with high-resolution log images where the unit is hydrocarbon bearing, creating strong resistivity contrasts (further amplified by the occurrence of gas hydrate) between reservoir quality, gas-hydrate-bearing sands and intervening units. These interbeds are likely at or below the resolution of the LWD gamma-ray tool, and may represent gas-hydrate free sand layers (see Cook et al, 2012); although it most likely that the interbeds are finer-grained units, with the reduced reservoir quality being the primary cause for the substantial reduction or lack of gas hydrate.…”

“…Sh within these units appears to be restricted to the coarsest-grained layers, and is likely to be very high (exceeding 80% in most units), although the thinbedded nature of the unit creates significant reservoir anisotropy Cook et al, 2012). The interpretation of the detailed nature of the bounding sands is complicated by poor LWD data quality due to enlarged bore-holes.…”