Method for determining residual oil content of a formation using thermal neutron decay measurements

Claim 2

The process of claim 1 where sufficient aqueous liquid is injected to remove the displaced aqueous liquid from within a radius surrounding the borehole that exceeds the radius of investigation of said measuring device.

Claim 3

A process for determining the fractional water saturation of an earth formation by means of measurements of thermal neutron capture rates, said process comprising: flowing an aqueous liquid through a zone of oil-containing earth formation around a borehole until its oil content is reduced to not more than a waterflood residual oil saturation within the radius of investigation of a thermal neutron capture rate measuring unit; emitting a pulse of neutrons into the borehole of a well and said zone of earth formations and measuring the rate at which the thermal neutrons decay; adjusting the salt concentration of an aqueous liquid to a measured difference from the salt concentration of the aqueous liquid that was flowed through said zone of earth formations; injecting enough of the so-adjusted aqueous liquid to displace the first aqueous liquid in the pores of the earth formation throughout the region of investigation of the above measuring step; measuring the rate at which the thermal neutrons decay after the injection of said liquid having an adjusted salt concentration; measuring the difference between the two measured thermal neutron decay rates; measuring the difference in the neutron capture cross sections of the two aqueous liquids having different salt concentrations; and combining said two difference measurements with other properties of said zone of oil-containing earth formation to obtain a determination of the fractional water saturation of the formation surrounding the borehole.