a wellhead configured to be installed at an entrance to a wellbore formed through a subterranean zone, the wellhead comprising an uphole wellhead end configured to extend upward from the entrance and a downhole wellhead end configured to extend downhole into the wellbore, the wellhead defining an opening extending from the uphole wellhead end to the downhole wellhead end,

a spacer tool attached to the downhole wellhead end, the spacer tool comprising an uphole spacer tool end attached to the downhole wellhead end of the wellhead and a downhole spacer tool end extending downhole into the wellbore, the spacer tool defining a tapered opening tapering from the uphole spacer tool end toward the downhole spacer tool end, and

a master valve attached to the downhole spacer tool end of the spacer tool, the master valve defining a flow pathway fluidically coupled to the opening defined by the wellhead and the tapered opening defined by the spacer tool; and