A straddle packer tool for setting against a constraining wall includes: a drag assembly with a locking mechanism for locking a position of the drag assembly relative to the constraining wall; a mandrel installed in and axially moveable through an inner bore of the drag assembly; and a packing element housing including a first annular packing element and a second annular packing element spaced from the first annular packing element, the packing element housing positioned between a stop shoulder on the mandrel and the drag assembly, the packing element being settable to expand the first annular packing element and the second annular packing element by compression between the drag assembly and the stop shoulder. A valve sub including a pressure actuated piston is also described and may be operated to open using the straddle packer tool.