Patent ID: 7093662

Claim:
A system for controlling the density of a drilling fluid in a wellbore in well drilling operations, comprising: a first drill tube having a top end and a bottom end, the top end of said first drill tube being located adjacent the surface, the bottom end of said first drill tube being located in the wellbore, said first drill tube for delivering a drilling fluid having a predetermined density from the surface to the wellbore, said first drill tube having a predetermined outer diameter; a second drill tube having a top end and a bottom end, the top end of said second drill tube being located adjacent the surface and the bottom end of said second drill tube being located in the wellbore, said second drill tube having a predetermined inner diameter which is greater than the outer diameter of the first drill tube, said second drill tube being arranged such that the first drill tube is contained within the second drill tube to define an annular space between the outer diameter of the first drill tube and the inner diameter of the second drill tube, said second drill tube comprising at least one port therein for establishing communication between the annular space within the second drill tube and the wellbore, said second drill tube for delivering a base fluid having a predetermined density from the surface to the wellbore via the port to create a combination fluid, said base fluid having a density different than the predetermined density of the drilling fluid, said combination fluid having a predetermined density that is defined by a selected ratio of the drilling fluid and the base fluid, said combination fluid rising to the surface; a drilling device connected to the bottom end of the first drill tube; a drilling rig located at the surface to facilitate offshore drilling operations; and a riser having an upper end adjacent the drilling rig and a lower end in fluid communication with the wellbore, said riser for delivering the combination fluid from the wellbore to the drilling rig at the surface.