Top-mounted mud-telemetry pulser assembly for downhole communications, and downhole valve

A mud pulser for generating pressure pulses in a fluid flow in a wellbore, adapted for mounting downhole in a wellbore immediately uphole of a servo-valve and to be controlled by said servo-valve, the mud pulser comprising:

• a valve housing, having an uphole opening therein for receiving at least a portion of the fluid flow passing through said mud pulser;
• a downhole pilot port adapted to receive and be opened and closed by said downhole servo-valve;
• a piston chamber on a downhole side of said uphole opening and uphole of said downhole pilot port, wherein said downhole pilot port is in fluid communication with said piston chamber when said servo-pilot valve opens said downhole pilot port;
• a moveable piston valve moveable within said piston chamber and having a poppet tip on an uphole end thereof for opening and closing said uphole opening, wherein a cross-sectional area of said piston valve in said piston chamber is greater than a cross-sectional area of said poppet tip;
• one or more fluid egress ports in fluid communication with said piston chamber;
• said poppet tip on said piston valve forming with said uphole opening a main valve for regulating said at least a portion of said fluid flow and moveable between a main-valve open position uncovering said uphole opening and a main-valve closed position covering or closing said uphole opening, said movement of said piston valve and poppet tip causing the pressure pulses in said fluid flow;
• and
• a first channel within said valve housing, in fluid communication with a first region of said mud pulser uphole of said uphole opening and longitudinally extending downhole in a sidewall of said valve housing,
• (i) fluidly connecting, when said downhole pilot port is opened by said downhole pilot valve, said piston chamber with said first region;
• or
• (ii) fluidly connecting said first region with said piston chamber.