between the drillstring and the wellbore from a selected depth in the wellbore to the earth 39 s surface introduction of drilling fluid to the inside of the drillstring is stopped and fluid flow in the annular space from a point below the selected depth to a point above the selected depth is selectively controlled to cause a substantially constant fluid pressure at a predetermined depth in the wellbore application_number US 7917002 A description exemplary embodiments of the invention will be described with reference to the accompanying drawings like items in the drawings are shown with the same reference numbers the present invention provides a solution to certain problems in deepwater drilling more specifically extended reach or long horizontal well drilling in general dual gradient drilling dgd allows drilling in deep water with fewer casing strings than possible using conventional drilling techniques this enables drilling wells in a shorter time however in u201c open hole u201d horizontal wells full circulating bottom hole pressure reaches the drilling limit relatively early this limit defines either the point at which an additional string of casing must be set or the maximum reach for this well when casing is set additional drilling may not be possible especially in highly inclined and horizontal wells in dgd during normal circulation of the drilling mud there is a hydrostatic imbalance between the mud column in the drill string 2 in fig1 and the mud column in the wellbore 24 28 in fig1 and drilling riser.