a possibility of the fluid in annulus 22 plugging or otherwise damaging the operation of valve 20 valve 20 can be filled with hydraulic fluid and a bellows 25 or a piston unit not shown is attached to inlet 21 to physically separate the hydraulic fluid from the fluid in annulus 22 in operation well 10 is cased and production tubing 15 including valve 20 is run into place valve 20 will be positioned at a depth below the recognized danger zone in which intersection by directionally drilled well 30 is most likely to occur in most known instances this danger zone will lie above 2 500 feet which is within the operating range of commercially available spring closing safety valves where the danger zone extends below 2 500 feet it may be necessary to use a commercially available dual control line safety valve e g see pages 733 and 5328 of the composite catalog cited above wherein the high pressure side i e inlet 21 is open to annulus 22 while a balance line 31 extends to the surface as understood in the art the dual control feature provides balanced hydraulic pressures across valve 20 in deep wells to ensure that there will be minimum hydrostatic head to overcome during closing of valve 20 annulus 22 is filled with a noncompressible fluid e g salt water or diesel oil and is pressured up so that the annulus fluid acts through inlet 21 to move valve 20 to an open position the final annulus pressure at the valve will be substantially in excess e g 500.