Gas-influx Detection With MWD Technology
- T.M. Bryant (Teleco Oilfield Services) | D.S. Grosso (Teleco Oilfield Services) | S.N. Wallace (Teleco Oilfield Services)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- December 1991
- Document Type
- Journal Paper
- 273 - 278
- 1991. Society of Petroleum Engineers
- 5.4.2 Gas Injection Methods, 4.3.4 Scale, 4.1.4 Gas Processing, 1.11 Drilling Fluids and Materials, 4.1.2 Separation and Treating, 1.10 Drilling Equipment, 1.12.1 Measurement While Drilling, 1.6 Drilling Operations, 4.1.5 Processing Equipment, 1.12.6 Drilling Data Management and Standards, 4.6 Natural Gas, 1.6.1 Drilling Operation Management, 1.7.5 Well Control, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties)
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This paper describes a new gas-influx detection technique that monitors the acoustic response of annular measurement-while-drilling (MWD) pulses to provide a rapid, early warning of the development of potential gas-kick situations. The technique has been evaluated in both water and oil-based muds during about 40 gas-kick simulations at two full-scale testing facilities. Free gas is identified by amplitude attenuation and phase delay of MWD fundamentals and their harmonic frequencies. Detection is independent of influx location because the entire length of the annulus between the bit nozzles and a surface-pressure transducer is sampled. Detection of potential gas-kick situations generally occurred within minutes of influx initiation, before any significant gas expansion. Some tests also evaluated a downhole MWD mud-resistivity sensor. Results indicated that both these techniques, and particularly the pulse acoustics can provide unequivocal confirmation of gas and an earlier warning of gas-kick situations than conventional kick-detection techniques.
The oil industry's trend to frontier deepwater exploration has increased the overall risks inherent in encountering a gas kick. In addition to a greater probability of a gas blowout offshore, technological advances in such detection have fallen short of other advances, increasing the risk of serious incidents resulting from gas -kicks. Conventional gas-kick-detection indicators suffer from reliability, accuracy, and credibility problems, and their reliance on gas expansion often results in a significant time lapse between gas entering the borehole and surface detection.
Safety in offshore drilling would be enhanced greatly by a reliable gas-influx indicator with a detection capability that was virtually independent of gas expansion. Two indicators that meet these criteria are the monitoring of the signal in the annulus and the measurement of downhole mud resistivity. Testing of these measurements offers convincing evidence of their sensitivity and reliability in detecting a gas influx. Both measurements can indicate a gas influx within minutes of kick initiation; are relatively unaffected by environmental variables; and, especially for annular acoustics, are relatively inexpensive to implement.
Annular Pulse Acoustics
A gas-kick transient is an uncontrolled gas influx into the wellbore annulus. This gas is introduced for a wide range of injection rates and volumes. Gas concentrations may vary from as low as that characteristic of drilled, connection, and trip gas to as high as that typically associated with blowouts. The evolution of a gas 16 is a complex, dynamic phenomenon involving fluid mixing, diffusion, dispersion, and expansion. These processes interact to produce a variety of temporal and spatial changes in the annular column. This behavior creates changes in the inertia, resistance, and capacitance of the wellbore annulus, which influence the natural frequency and damping of the fluid system. Hence, the amplitude and phase relationships of acoustic signals traveling through this system can be expected to change during the evolution of a gas-kick transient.
The Teleco MWD telemetry system transmits information through multifrequency, positive-pressure, binary codes. The pulser mechanism generates square waves, so in addition to the two fundamental frequencies, the resultant signal is rich in harmonics. The usual transmission channel of MWD signals is the drilling fluid contained within the drillstring bore. During the creation of a positive-p pulse, a considerably smaller negative pressure pulse is positive-p pulse, a considerably smaller negative pressure pulse is generated below the pulser mechanism. This pulse propagates to the surface within the annulus in the same manner as the pulse within the drill-string bore. Unlike the drillstring channel, however, the annular channel is frequently contaminated by gas.
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