Tool for pulse-injecting fluid into a ground formation, characterised in that: the tool includes a pulse-valve, having a pulse-valve-member that is movable between a valve-closed position (Fig.3) and a valve-open position (Fig.2); the tool includes an accumulator, for storing pressurized fluid that is to be pulse-injected into the ground formation; the tool includes a piston, which is connected to the pulse-valve-member; the piston has an accumulator-surface and an opposed formation-surface, and the tool is so structured that, in operation, the accumulator-surface is exposed to accumulator-pressure, and the formation-surface is exposed to formation-pressure; the pressure differential between the accumulator-pressure and the formation-pressure is termed the PDAF; the tool includes an area-divider, relative to which the piston is movable between a contact-position and a clear-position; the tool includes a biassing-means, which exerts a biassing-force on the piston in the direction to urge the piston towards its contact-position; the tool is so structured that, the piston being in its contact-position (Fig.2): (a) the accumulator-surface of the piston now makes sealing contact with the area-divider; (b) the area-divider sealingly divides the area of the accumulator-surface of the piston into two sub-areas, being area-A and area-B; (c) the area-divider keeps area-A sealingly separated from area-B, to the extent that fluid pressure in area-A of the accumulator-surface of the piston can be substantially different from fluid pressure in area-B; (d) only area-A of the accumulator surface is exposed to the accumulator-pressure, area-B being exposed to a lower pressure; (e) when the PDAF exceeds an upper trigger level, forces on the piston due to the PDAF acting over the area-A now exceed forces on the piston due to the biassing-means, whereby the piston now moves clear of the area-divider, towards its clear-position; the tool is so structured that, the piston having moved to its clear-position (Fig.2): (a) area-A and area-B are not now sealingly separated by the area-divider, but are connected; (b) whereby the accumulator-pressure now suddenly acts over the sum of area-A and area-B together; (c) whereupon the piston now is subjected to a sudden large force that acts to move the piston and to move the pulse-valve-member to its open-position. the tool includes a pulse-valve, having a pulse-valve-member that is movable between a valve-closed position (Fig.3) and a valve-open position (Fig.2); the tool includes an accumulator, for storing pressurized fluid that is to be pulse-injected into the ground formation; the tool includes a piston, which is connected to the pulse-valve-member; the piston has an accumulator-surface and an opposed formation-surface, and the tool is so structured that, in operation, the accumulator-surface is exposed to accumulator-pressure, and the formation-surface is exposed to formation-pressure; the pressure differential between the accumulator-pressure and the formation-pressure is termed the PDAF; the tool includes an area-divider, relative to which the piston is movable between a contact-position and a clear-position; the tool includes a biassing-means, which exerts a biassing-force on the piston in the direction to urge the piston towards its contact-position; the tool is so structured that, the piston being in its contact-position (Fig.2): (a) the accumulator-surface of the piston now makes sealing contact with the area-divider; (b) the area-divider sealingly divides the area of the accumulator-surface of the piston into two sub-areas, being area-A and area-B; (c) the area-divider keeps area-A sealingly separated from area-B, to the extent that fluid pressure in area-A of the accumulator-surface of the piston can be substantially different from fluid pressure in area-B; (d) only area-A of the accumulator surface is exposed to the accumulator-pressure, area-B being exposed to a lower pressure; (e) when the PDAF exceeds an upper trigger level, forces on the piston due to the PDAF acting over the area-A now exceed forces on the piston due to the biassing-means, whereby the piston now moves clear of the area-divider, towards its clear-position; the tool is so structured that, the piston having moved to its clear-position (Fig.2): (a) area-A and area-B are not now sealingly separated by the area-divider, but are connected; (b) whereby the accumulator-pressure now suddenly acts over the sum of area-A and area-B together; (c) whereupon the piston now is subjected to a sudden large force that acts to move the piston and to move the pulse-valve-member to its open-position.