Site Summary – Walker Ridge Block 313
The drill sites at Walker Ridge 313 lies in ~6,500 ft of water within the western part of the “Terrebonne” mini-basin in the northern Gulf of Mexico. The primary target of drilling were a series of strong seismic anomaly that lay approximately 3,000 fbsf (feet below the seafloor). These anomalies exhibit strong “positive” amplitude response, indicating a horizon in the subsurface across which the speed of sound waves significantly increases. In addition, these same horizons, when traced deeper to the west, are observed to switch “polarity” to a strong negative response. Pre-drill interpretations determined that this collection of seismic responses was indicative of free gas accumulations (the negative anomalies) being trapped within porous and permeable sand horizons by significant accumulations of overlying gas hydrate within the sediment pore space. The primary goal of JIP drilling at this site was to test the validity of this interpretation through drilling and logging of wells at this site.
The Q4000 initially arrived at the WR313-G drill site around noon on April 17 and immediately began preparing to drill while onloading supplies from the supply ship MV Mia. At 5:30 PM on April 18, the drill bit tagged the sea-floor and drilling commenced. While drilling the primarily muddy sediments above the target, the logging tools showed the first signs of hydrocarbon at a depth of 650 ft below the sea-floor. This zone continued for approximately 500’, and initial interpretation is that this zone represents a significant accumulation of fracture-filling gas hydrate. As drilling proceeding, the lack of use of heavy drilling fluids and slow penetration rates (both designed intentionally to maximize the quality of the data recorded by the logging tools) made it difficult to remove cuttings or well-bore cavings from around the drill string. At a depth of about 9,600 ft., drill mud was added and penetration rate was increased, and the bit reached the primary target at 2,796 fbsf at mid-day, April 20. As predicted from the available regional seismic data, log data indicated the occurrence of approximately 50 ft of gas hydrate within a sand reservoir. Geophysical data developed prior to the expedition indicates that this gas hydrate deposit covers at least 500 acres, with several other prospective horizons in the area as well.
On April 29, the Q4000 returned to Walker Ridge 313 to test a second prospect in a position about 1 mile east of the “G” well. The WR313-H well had two primary targets. The first was the up-dip extension of the sand that had been logged as saturated with gas hydrate in the G-well. Geophysical data indicated that this sand should be of minimal thickness and/or gas hydrate saturation at this well location. The second was a deeper sand expected to lay just above the inferred base of gas hydrate stability that exhibited strong geophysical evidence of gas hydrate pore fill. In addition, the well also offered an opportunity to further evaluation the character and extent of the thick fracture-filling gas hydrate occurrence observed in the shallow section of the “G” well. The fracture-filling occurrence was again observed, in this location at a thickness of ~315 ft. The underlying sediments contained interbedded muds and thin sands, including four gas hydrate bearing sands ranging from 4 to 8 ft thick. The top of the equivalent horizon to the “pay” sand from the G-well was reached at 2,305 ft below the sea-floor and was clearly seen to have graded into a mud-rich sandy interval with minimal porosity and only limited occurrence of gas hydrate. The main target sand was logged at 2,646 ft and consisted of ~40 ft of clean sand in two lobes fully saturated with gas hydrate.
The results of the WR 313 drilling confirms the geological/geophysical model that links phase reversals of strong amplitude and appropriate polarity to substantial accumulations of gas hydrate in deeply buried sand reservoirs. The presence of significant gas hydrate accumulations in both pore-filling mode in sands and expected fracture-filling mode in shallow muds, should make Walker Ridge 313 a prime location for future research into the implications of marine gas hydrates.
Derrick of the Q4000 at WR313
Map of the geophysical response of the target horizon. Red areas have seismic response consistent with that confirmed to indicate gas hydrate in the WR313-G well.
Schlumberger’s David Myerson and Chevron’s Greg Pine inspect the WR313 drill string after completion of the WR313-G well
Dan McConnell of the JIP Leg II science team and AOA Geophysics aboard the Q4000