FROZEN HEAT | Volume 2

2.5 CASE STUDIES OF GAS HYDRATE OCCURRENCES

2.5.1 PORE-FILLING GAS HYDRATES IN SAND RESERVOIRS

kumi #1 research well confirmed similar occurrences of gas hydrates in four separate sand reservoirs (Schoderbek and Boswell 2011) in the western Prudhoe Bay unit. The geology of the shallow sediments of the Alaska North Slope, and of many other Arctic regions in which gas hydrates occur, is dominated by sediments deposited in shallow-water marine, coastal, and terrestrial environments. These conti- nental deposits generally include significantly greater pro- portions of sand-sized sediments than are typically found in deep-water settings. Virtually all known gas hydrate occur- rences in the Arctic are associated with sands. Prudhoe Bay gas hydrates are charged primarily by the upward migra- tion (aided by many faults) of gas leaking from the deeper Prudhoe Bay oil and gas fields. It appears likely that gaseous methane began to charge sand reservoirs prior to the evolu- tion of gas hydrate stability conditions roughly 1.6 million years ago (Collett 1993). Conversion to gas hydrates occurred after the climate cooled dramatically during glacial times, ag- grading thick occurrences of terrestrial permafrost. Collett (1995) assessed Alaska North Slope in-place gas re- sources from gas hydrates at 16.7 trillion cubic metres. Sub- sequently, using information from the 2007 Mount Elbert well and recent advances in numerical modelling (Anderson et al. 2010), Collett et al. (2008) provided the first assessment of technically recoverable resources from gas hydrates, indi- cating a mean of 2.4 trillion cubic metres from Alaska North Slope sand reservoirs using existing technologies. Compared to the data available about permafrost gas hydrates on the Alaska North Slope and the Mackenzie Delta, very lit- tle is known about gas hydrates in the vast deep-water ba- sins of the world. Perhaps the best-characterized occurrences in sand reservoirs are those located in the eastern Nankai Trough, off the southeastern coast of Japan (Figure 2.7). The

This most promising form of deposit, in terms of production potential, has been observed widely across the globe. Gas- hydrate-bearing sands have been discovered offshore Korea, where they are currently under evaluation as future produc- tion test sites (Lee, S-R. et al. 2011; Moridis et al. , 2013), and have been reported as well from the Cascadia margin (Riedel et al. 2006), the permafrost of Siberia (Makogon 1981), and elsewhere. The best-studied occurrences are permafrost-as- sociated sands on the Alaska North Slope (Collett et al. 2008) and the Mackenzie Delta of Arctic Canada (Dallimore et al. 1998; Dallimore and Collett 2005), in the extensive deep- water turbidites of the Nankai Trough, and the deeply buried sands of the northern Gulf of Mexico. The Alaska North Slope has a long history of oil and gas ex- ploration. Gas hydrates were first inferred in 1972 during initial exploration of the Prudhoe Bay oil field. Drilling data from more than 1 000 wells in the area indicate that gas hy- drates likely occur throughout the Alaska North Slope. They have been confirmed within a thick sequence of sand reser- voirs below the base of permafrost throughout a broad area known as the Eileen Trend (Collett 1993). A second trend, the Tarn Trend, was discovered in the early 1990s overly- ing the Kuparuk River oil field (Collett 2002). In this case, gas-hydrate-bearing sands are present largely within the low- ermost permafrost-bearing section. Inks et al. (2009) used standard industry seismic data to interpret more than a doz- en specific gas hydrate prospects within the Milne Point unit at the northern end of the Eileen Trend. In February 2007, the most promising of these, the Mount Elbert Prospect, was drilled, logged, and cored (Hunter et al. 2011), confirming the occurrence of a sand reservoir with gas hydrate saturations ranging from 50 to nearly 80 per cent. In 2011, the Ignik Si-

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