Abstract: An apparatus for treating rock surrounding a wellbore comprises a hollow elongated housing having nozzles radially mounted therein, which are projectable externally thereof. Each nozzle has a through passage communicating with the internal space of the housing to pass the flow of a fluid with an abrasive filler therethrough. The external end faces of the nozzles are shaped so as to press against the wall of the casing. Each through passage of the nozzle has a larger-diameter portion at the end thereof, facing the casing wall. The body of each nozzle has auxiliary passages, each such passage having one end thereof communicating with the larger-diameter portion of the through passage and the other end thereof communicating with the space externally of the nozzle, the auxiliary passages affording the flow of the fluid with the abrasive filler from the space defined by the said larger-diameter portion of the through passage and the adjacent casing wall, as the nozzle is closely pressed against the casing.