A method for completing a wellbore in a subsurface formation, the method comprising: providing a packer comprising: an inner mandrel, alternate flow channels along the inner mandrel, and a sealing element external to the inner mandrel; connecting the packer to a tubular body; running the packer and connected tubular body into the wellbore; actuating the sealing element into engagement with the surrounding subsurface formation; injecting a gravel slurry into an annular region formed between the tubular body and the surrounding formation; and injecting the gravel slurry through the alternate flow channels to allow the gravel slurry to at least partially bypass the sealing element so that the wellbore is gravel-packed within the annular region below the packer; wherein: the wellbore has a lower end defining an open-hole portion; the packer and tubular body are run into the wellbore along the open-hole portion; the packer is set within the open-hole portion of the wellbore; the tubular body is (i) a sand screen comprising a base pipe, alternate flow channels, and a surrounding filter medium, or (ii) a blank pipe having alternate flow channels; and the base pipe or the blank pipe is made up of a plurality of joints, characterised in that the packer additionally comprises: a movable piston housing retained around the inner mandrel; and one or more flow ports providing fluid communication between the alternate flow channels and a pressure-bearing surface of the piston housing, and in that the method additionally comprises: running a setting tool into the inner mandrel of the packer; manipulating the setting tool to mechanically release the movable piston housing from its retained position; and setting the packer by communicating hydrostatic pressure to the piston housing through the one or more flow ports, thereby moving the released piston housing. providing a packer comprising: an inner mandrel, alternate flow channels along the inner mandrel, and a sealing element external to the inner mandrel; connecting the packer to a tubular body; running the packer and connected tubular body into the wellbore; actuating the sealing element into engagement with the surrounding subsurface formation; injecting a gravel slurry into an annular region formed between the tubular body and the surrounding formation; and injecting the gravel slurry through the alternate flow channels to allow the gravel slurry to at least partially bypass the sealing element so that the wellbore is gravel-packed within the annular region below the packer; the wellbore has a lower end defining an open-hole portion; the packer and tubular body are run into the wellbore along the open-hole portion; the packer is set within the open-hole portion of the wellbore; the tubular body is (i) a sand screen comprising a base pipe, alternate flow channels, and a surrounding filter medium, or (ii) a blank pipe having alternate flow channels; and the base pipe or the blank pipe is made up of a plurality of joints, a movable piston housing retained around the inner mandrel; and one or more flow ports providing fluid communication between the alternate flow channels and a pressure-bearing surface of the piston housing, and in that the method additionally comprises: running a setting tool into the inner mandrel of the packer; manipulating the setting tool to mechanically release the movable piston housing from its retained position; and setting the packer by communicating hydrostatic pressure to the piston housing through the one or more flow ports, thereby moving the released piston housing. running a setting tool into the inner mandrel of the packer; manipulating the setting tool to mechanically release the movable piston housing from its retained position; and setting the packer by communicating hydrostatic pressure to the piston housing through the one or more flow ports, thereby moving the released piston housing.