Water phase which is injected from surface to injected to the subsurface reservoir via water injectors to support reservoir pressure and improve oil displacement
It can be sourced form any part of Earth's hydrosphere: subsurface aquifer, ocean, sea, lake , river or it can be processed formation water after it is lift up on surface alongside with hydrocarbons.
See also
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Natural Science / Physics / Chemistry / Chemical Substance / Water
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petroleum Geology / Petroleum reservoir fluids