This study assesses the feasibility of using offshore freshwater for improved oil recovery in passive-margin marine environments. Low-salinity waterflooding (<5‰) has recently been shown, on average, to improve oil recovery by 14%. Hydrogeologists estimate that up to 3 × 105 km3 (1.89 × 1011 bbl) of fresh (<1‰) water are sequestered in shallow (<500 m [1640 ft] depth), permeable, poorly lithified sand, sandstone, and carbonate aquifers along passive margins, within 100 km (60 mi) of the present-day coastline in ocean-water depths up to approximately 50 m (164 ft). The offshore distribution of fresh–brackish water is analyzed for five vertical cross sections from offshore Australia, Suriname, Indonesia, and the United States. The distribution of offshore freshwater is compared with offshore platform locations in three oil-producing marine basins, including the North Sea, the Gulf of Mexico, and the Niger Delta. The continental-shelf cross sections host between 0.8 and 8.6 km3 of fresh–brackish water (<5‰) per kilometer of shoreline (8 × 109 to 8.7 × 1010 bbl/mi of shoreline), most within 20 to 100 km (12 to 60 mi) from the coast in water depths of 10 to 50 m (33 to 164 ft). Because the majority of the offshore oil platforms are located less than 100 km (60 mi) from the shore, these resources could be used for low-salinity recovery. Cross sectional aquifer models of offshore fresh–brackish production indicate that a single, 1000-m (3280-ft)-long horizontal well could produce 19,200 m3/day (120,764 bbl/day) from a relatively permeable aquifer (10−11 to 10−12 m2 [10,000 to 1000 md]) overlain by a moderately tight (∼10−15 m2 [1 md]), confining unit for the typical life span of a well (30 yr).

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