An apparatus for modifying the wall surface of a borehole drilled through an underground formation. The apparatus comprises a tool body that can be introduced into the borehole using a conveyance system so as to be moveable along the borehole, and one or more members arranged so as to be able to be urged against, and moved across at least part of the borehole wall in order to work the surface thereof and modify its properties. A method of modifying the wall surface comprises positioning the tool body in the borehole at a location of interest using the conveyance system, urging the members against the borehole wall with sufficient force to modify the properties of the surface thereof, and moving the members across the surface of the borehole wall.