Grout delivery

A grout delivery system for delivery of a grout as a flowable substance comprising a mixture of two grout component materials at a downhole location. The grout delivery system comprises an elongate body comprises two reservoirs configured as cartridges for receiving respective charges of the two grout component materials. The two cartridges each comprise a piston. A selectively openable closure configured as a valve is provided adjacent the bottom end of each cylinder. Each piston is operably to progressively advance towards the valve, thereby expelling grout component material from the respective cartridge through the valve. When the grout delivery system is at the desired location downhole and a fluid seal established downhole, fluid such as water is pumped into the drill string and pressurized. Initially, fluid pressure is exerted on the two pistons indirectly via an actuator to initiate movement of the two pistons in concert. At a later stage, fluid under pressure is exerted directly on the pistons to continue their movement in concert. Retrieval of the grout delivery system may necessitate relief of the hydrostatic pressure differential across the fluid seal established downhole so that the grout delivery system can be lifted relatively easily from the downhole drilling assembly. A selectively operable pressure relief system is provided for this purpose.