to simplify the discussion of how the gravel packing will proceed it should be noted that while two producing zones a and c are shown separated by a non producing zone b the pattern can repeat and the distance between producing zones can vary and can be many meters down to a very small gap the basic idea is to limit cross flow between zones a and c when only one is desired to be produced without using barriers in the annulus 12 below the production packer 14 the annulus 12 is filled with gravel 16 if only one of zones a and c are aligned to produce such as for example if zone a is aligned to produce as indicated by arrows 18 and 20 any flow from zone c indicated by arrows 22 and 24 will only be able to reach zone a through the annulus 12 as indicated by arrows 26 zone c will not flow into producing screen 28 because the production string 30 can have a plug 32 at its lower end 34 when the string 30 has its lower end shown in fig2 inserted into a sealing relationship with the production packer 14 shown in fig1 the string 30 has a sliding sleeve 38 to selectively cover port.