Tool for pulse-injecting fluid into the ground formation around a borehole or well, wherein: the tool includes an accumulator, containing pressurized fluid to be injected; the pressurized fluid is at accumulator-pressure, and fluid in the ground formation is at formation-pressure; the difference between the accumulator-pressure and the formation-pressure is termed the PDAF; the tool includes a pulse-valve, which, in operation of the tool, cyclically opens and closes a fluid path between the accumulator and the formation; the PDAF rises when the pulse-valve is closed and the accumulator is being recharged, and the PDAF falls when the pulse-valve is open and fluid is being injected into the formation, the tool includes a pulse-valve-member which is movable relative to a pulse-valve-housing in the direction to open and close the pulse-valve; the tool includes an operable pulse-valve-motor; the motor is effective, when operated, to move the pulse-valve-member in the direction to close the pulse-valve responsively to the PDAF reaching a low-threshold, and to open the pulse-valve responsively to the PDAF reaching a high-threshold; characterised in that: the tool includes a suckback-cylinder and a relatively movable sealed suckback-piston, which together define a suckback-chamber of variable volumetric capacity; the suckback-piston is mechanically free with respect to the pulse-valve-member, in that the suckback-piston is free to travel along the suckback-cylinder without being constrained by the pulse-valve-member; the tool is so configured that the distance the suckback-piston travels along the suckback-cylinder, during a cycle, is substantially greater than the distance the pulse-valve-member travels relative to the pulse-valve-housing, during the cycle; the suckback-cylinder includes an openable suckback-port; the suckback-port, when open, connects the suckback-chamber to the formation; the tool is so configured that, when the suckback-port is closed, the suckback-chamber is sealed off from the formation; the tool is so configured that, when the suckback-port is open, the PDAF exerts a PDAF-force on the suckback-piston in the direction to decrease the volume of the suckback-chamber; the tool includes a suckback-biasser, which exerts a biassing-force on the suckback-piston in the direction to increase the volume of the suckback-chamber; a suckback-equalization level of the PDAF is the level at which, the suckback-port being open, the PDAF-force on the suckback-piston is balanced by the biassing-force on the suckback-piston; the suckback-biasser provides a biassing-force of such magnitude that the suckback-equalization level of the PDAF is substantially above the said low-threshold of the PDAF. the tool includes an accumulator, containing pressurized fluid to be injected; the pressurized fluid is at accumulator-pressure, and fluid in the ground formation is at formation-pressure; the difference between the accumulator-pressure and the formation-pressure is termed the PDAF; the tool includes a pulse-valve, which, in operation of the tool, cyclically opens and closes a fluid path between the accumulator and the formation; the PDAF rises when the pulse-valve is closed and the accumulator is being recharged, and the PDAF falls when the pulse-valve is open and fluid is being injected into the formation, the tool includes a pulse-valve-member which is movable relative to a pulse-valve-housing in the direction to open and close the pulse-valve; the tool includes an operable pulse-valve-motor; the motor is effective, when operated, to move the pulse-valve-member in the direction to close the pulse-valve responsively to the PDAF reaching a low-threshold, and to open the pulse-valve responsively to the PDAF reaching a high-threshold; the tool includes a suckback-cylinder and a relatively movable sealed suckback-piston, which together define a suckback-chamber of variable volumetric capacity; the suckback-piston is mechanically free with respect to the pulse-valve-member, in that the suckback-piston is free to travel along the suckback-cylinder without being constrained by the pulse-valve-member; the tool is so configured that the distance the suckback-piston travels along the suckback-cylinder, during a cycle, is substantially greater than the distance the pulse-valve-member travels relative to the pulse-valve-housing, during the cycle; the suckback-cylinder includes an openable suckback-port; the suckback-port, when open, connects the suckback-chamber to the formation; the tool is so configured that, when the suckback-port is closed, the suckback-chamber is sealed off from the formation; the tool is so configured that, when the suckback-port is open, the PDAF exerts a PDAF-force on the suckback-piston in the direction to decrease the volume of the suckback-chamber; the tool includes a suckback-biasser, which exerts a biassing-force on the suckback-piston in the direction to increase the volume of the suckback-chamber; a suckback-equalization level of the PDAF is the level at which, the suckback-port being open, the PDAF-force on the suckback-piston is balanced by the biassing-force on the suckback-piston; the suckback-biasser provides a biassing-force of such magnitude that the suckback-equalization level of the PDAF is substantially above the said low-threshold of the PDAF.