Abstract: A frac pack system and method for operating a frac pack system wherein an isolation string assembly has a lower circulation valve with lateral flow ports that are operated to an open position without the use of shifters on a wash pipe. The isolation string assembly is incorporated into a frac pack assembly within a wellbore and includes an upper sleeve tool that provides selective production of fluids through the frac pack assembly. In addition, the isolation string includes a lower circulation valve having a sliding sleeve that is shiftable from an open position to a closed position upon receipt of a suitable pressure differential. This configuration is particularly valuable for permitting monitoring of pressure or other conditions in the annulus of the wellbore portion being packed during frac pack operations. Further, the lower circulation valve tool can be used to selectively allow fluid returns during the frac pack operation.