Method and apparatus for borehole fluid influx detection

A method of monitoring a well drilling operation for the presence of fluid influx into the bore hole, the drill operation comprising the use of a tubular drill pipe having a diameter which is less than the diameter of the borehole being formed wherein a generally annular space is defined between the drill pipe and the borehole, said monitoring being performed during the drilling of the borehole, and in which drilling fluid is pumped down the interior of the drill pipe, the drilling fluid existing at about the base of the drill pipe and returning to the surface via the generally annular space between the drill pipe and borehole wall, and in which data bearing primary signals in the form of primary pressure pulses are transmitted to the surface in the drilling fluid by the operation of pressure generating means in the drill string, the method comprising the steps of:

• sensing in said annular space reflected pressure pulses of the primary pulses in the drilling fluid in the drill pipe, said reflected pressure pulses defining secondary pressure pulses;
• measuring the difference between one selected parameter of the pressure pulses in the interior of the drill pipe with the same selected parameter of the pressure pulses in the annular space wherein the entire drill string is a reference with which to make a differential comparison to the entire annulus;
• and
• determining changes in said measured difference between the selected parameters of said primary and secondary pulses wherein fluid influx into the annulus is determined.