Recently, an MWD acoustic technique for gas influx detection was evaluated under full-scale, controlled testing conditions. This technique continually monitors annular MWD pulse characteristics using a pressure transducer installed on the bell nipple riser pipe. Since the completion of the controlled testing program, the technique has been tested under actual drilling conditions.

Field testing of the acoustic gas influx detection technique has been conducted in both the North Sea and Gulf of Mexico. Analysis of annular MWD pulses has revealed the entry and presence of gas in a variety of hole sizes, mud densities, flow rates, drilling rates, depths, and lithologies. These results indicate that acoustic responses can provide an indication of penetration into a gas-bearing formation much earlier than traditional ditch gas methods.

The entry of gas into the annulus of the wellbore causes changes in the amplitudes and phase angles of the fundamental and harmonic frequencies of MWD pulses. Larger, drilled gas concentrations generate a "gas signature", which in its early stages is characterized by attenuation of pulse harmonics and high frequencies, and in its later stages is marked by amplification of pulse fundamentals and their harmonics. This "gas signature" is typically observed on the order of fifteen minutes or more before the gas reaches the surface. Influx of gas in a concentration that conceivably could result in a kick is distinguishable from large drilled gas concentrations by an acoustic signature possessing significant attenuation of both the pulse fundamentals and harmonics and a marked increase in their phase angles.

The results indicate how the technique may be used to:

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    detect the presence of significant gas in a reservoir before penetration by MWD sensors;

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    monitor the level of drilled gas in the annulus;

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    identify the presence of connection and trip gas before circulation out;

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    indicate the passage into a pore pressure transition zone;

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    detect the influx of gas in quantities that may produce a kick;

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    provide a much earlier warning of gas influx than traditional surface methods.

The use of this technique provides important information relevant to both the geological and drilling engineering programs, and may significantly enhance the safety and protection of the crew, rig, and well.

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