Method and apparatus for mud pulse telemetry in underbalanced drilling systems

A telemetry system conveyed on a drilling tubular for use in an underbalanced Measurement-while-Drilling system for use in a borehole having a fluid therein, the telemetry system comprising:

• (b) a pulser for generating positive pressure pulses in the borehole fluid corresponding to a selected pattern;
• (c) a fluid supply line supplying fluid under pressure to the wellbore;
• (d) a flow rate sensor measuring fluid flow rate through said supply line corresponding to said pressure pulses and generating signals representative of the flow rate;
• and
• (f) a processor at the surface operatively coupled to said flow sensor, said processor determining from said flow sensor signals the selected pattern of pulses generated by said pulser.

A method of drilling a borehole in a subsurface formation comprising:

• (a) conveying a bottom hole assembly on a drilling tubular into the borehole,
• (b) connecting a fluid line to communicate with a borehole fluid through the drilling tubular;
• (c) providing a drilling fluid to the fluid line for maintaining an underbalanced condition wherein a pressure of the borehole fluid is less than a formation fluid pressure;
• (d) operating a downhole pulser in the bottom hole assembly to generate positive pressure pulses in the borehole fluid corresponding to a selected pattern;
• (e) measuring a rate of flow of fluid in the fluid line corresponding to said pressure pulses using a flow rate sensor and generating signals indicative of said rate of flow;
• (f) processing said signals using a processor to determine the selected pattern;
• and
• (g) using a drill bit at an end of the bottom hole assembly to drill the borehole.