An oil stop valve assembly for allowing a flow of water and blocking a flow of oil has a body having an inlet and an outlet, a seat formed adjacent to the outlet of the body, and a float positioned in the body. The float has a specific gravity of between 0.90 and 0.95 so as to be buoyant in water and to sink in oil. The float is away from the seat when a level of water within the body is above the seat. The float is seated in the seat when the level of water in the body is adjacent to the seat. The body is positioned in a container having a fluid inlet and a fluid outlet. The fluid outlet of the container is connected to the outlet of the body.

An oil stop valve assembly for allowing a flow of water and blocking a flow of oil has a body having an inlet and an outlet, a seat formed adjacent to the outlet of the body, and a float positioned in the body. The float has a specific gravity of between 0.90 and 0.95 so as to be buoyant in water and to sink in oil. The float is away from the seat when a level of water within the body is above the seat. The float is seated in the seat when the level of water in the body is adjacent to the seat. The body is positioned in a container having a fluid inlet and a fluid outlet. The fluid outlet of the container is connected to the outlet of the body.

A fluid density valve housing includes a pilot fluid inlet and outlet and a primary fluid inlet and outlet. A shuttle is disposed within the housing. The shuttle comprises a portion having a reference density and a portion defining a cavity fluidically connected to the pilot fluid inlet, the shuttle permitting or denying primary fluid flow depending upon a density of a pilot fluid flowing through the cavity versus the reference density. A method for controlling flow, including comparing density of a pilot flow with a reference density, moving a valve between fully open, fully closed, and choked depending upon the differential density between the pilot fluid and the reference density. A borehole system, including a borehole in a subsurface formation, a string in the borehole, and a fluid density valve disposed within or as a part of the string.