X-mas trees are a stack of valves that sit on the top of the wellhead. The name “vertical” or “horizontal” is derived from the arrangement of the master valves. In vertical tress said valves are arranged vertically and in horizontal trees said valves are arranged horizontally. The choice between vertical or horizontal is typically a decision made in subsea “wet” applications.
In horizontal trees the valves are mounted on the lateral sides of the tree and the tubing hanger is situated in the tree body. This allows easy access to the completion string for well intervention and tubing recovery as the tree does not need to be removed. Using horizontal trees the tree must be run prior to the completion. The tubing hanger in vertical trees sits within the wellhead and would require the tree to be removed prior to recovering tubing. Vertical trees will be run after the completion.
With purchase and installation costs similar (spec and market dependent) the choice between horizontal tree and vertical tree is largely driven by well interventions expected in the future, tree reliability, and the project schedule/field characteristics (layout/size/facilities).