A gravel pack zonal isolation apparatus, comprising: a sand control device having: an elongated base pipe extending from a first end to a second end, at least one alternate flow channel along the base pipe extending from the first to the second end, and a filter medium radially surrounding the base pipe along a substantial portion of the base pipe so as to form a sand screen; and at least one packer assembly, each of the at least one packer assembly comprising: an upper mechanically-set packer (212, 600) having a sealing element, and a lower mechanically-set packer (214, 600) having a sealing element, characterized in that: the upper packer (212, 600) and the lower packer (214, 600) each comprises at least one alternate flow channel in fluid communication with the at least one alternate flow channel in the sand control device to divert gravel pack slurry past the upper mechanically set packer (212, 600) and the lower mechanically set packer (214, 600) during a gravel-packing operation; and in that each of the upper packer (212, 600) and lower packer (214, 600) comprises: an inner mandrel, a movable piston housing retained around the inner mandrel, one or more flow ports providing fluid communication between the alternate flow channels and a pressure-bearing surface of the piston housing, a release sleeve along an inner surface of the inner mandrel, the release sleeve being configured to move in response to movement of a setting tool within the inner mandrel and thereby expose the one or more flow ports to hydrostatic pressure during the gravel-packing operation. an elongated base pipe extending from a first end to a second end, at least one alternate flow channel along the base pipe extending from the first to the second end, and a filter medium radially surrounding the base pipe along a substantial portion of the base pipe so as to form a sand screen; and at least one packer assembly, each of the at least one packer assembly comprising: an upper mechanically-set packer (212, 600) having a sealing element, and a lower mechanically-set packer (214, 600) having a sealing element, characterized in that: the upper packer (212, 600) and the lower packer (214, 600) each comprises at least one alternate flow channel in fluid communication with the at least one alternate flow channel in the sand control device to divert gravel pack slurry past the upper mechanically set packer (212, 600) and the lower mechanically set packer (214, 600) during a gravel-packing operation; and in that each of the upper packer (212, 600) and lower packer (214, 600) comprises: an inner mandrel, a movable piston housing retained around the inner mandrel, one or more flow ports providing fluid communication between the alternate flow channels and a pressure-bearing surface of the piston housing, a release sleeve along an inner surface of the inner mandrel, the release sleeve being configured to move in response to movement of a setting tool within the inner mandrel and thereby expose the one or more flow ports to hydrostatic pressure during the gravel-packing operation. an upper mechanically-set packer (212, 600) having a sealing element, and a lower mechanically-set packer (214, 600) having a sealing element, characterized in that: the upper packer (212, 600) and the lower packer (214, 600) each comprises at least one alternate flow channel in fluid communication with the at least one alternate flow channel in the sand control device to divert gravel pack slurry past the upper mechanically set packer (212, 600) and the lower mechanically set packer (214, 600) during a gravel-packing operation; and in that each of the upper packer (212, 600) and lower packer (214, 600) comprises: an inner mandrel, a movable piston housing retained around the inner mandrel, one or more flow ports providing fluid communication between the alternate flow channels and a pressure-bearing surface of the piston housing, a release sleeve along an inner surface of the inner mandrel, the release sleeve being configured to move in response to movement of a setting tool within the inner mandrel and thereby expose the one or more flow ports to hydrostatic pressure during the gravel-packing operation. an inner mandrel, a movable piston housing retained around the inner mandrel, one or more flow ports providing fluid communication between the alternate flow channels and a pressure-bearing surface of the piston housing, a release sleeve along an inner surface of the inner mandrel, the release sleeve being configured to move in response to movement of a setting tool within the inner mandrel and thereby expose the one or more flow ports to hydrostatic pressure during the gravel-packing operation.