A packoff nipple for sealing an annular space in a well pipe has a tubular body, a sealing assembly positioned around the tubular body capable of axial movement along the tubular body and a stop to restrain the sealing assembly from extruding into the annular space. The sealing assembly includes one or more elastomeric sealing rings having a circumferential groove formed in a downhole face. An axially moveable actuator, such as a packer cup, engages the sealing ring assembly to cause at least a portion of the sealing rings to expand radially outwardly to seal the annular space and cause the sealing ring assembly to contact the uphole stop. The sealing rings are axially compressed between the uphole stop and the actuator to further expand the sealing rings. The uphole stop may be concave to urge the sealing rings away from the well pipe to avoid extrusion or permanent deformation.