Patent EP-0621397-A1 (EP)
Inventor: CODAZZI DANIEL
IPC: E21B47/10
Filed: 1991-06-25
Abstract: Gas influx into a wellbore, which is commonly referred to as a "kick", is detected during active drilling of the borehole. One embodiment of the present invention is based upon the existence of standing wave patterns generated by pressure oscillations of the drilling rig mud pumps. The standing wave patterns form time sequences of maxima and minima as a gas influx or "slug" moves upwardly in the annulus. Time spacing between these peaks is equal to the time needed for gas cut mud to be displaced over a distance equal to a one-half wavelength of a standing wave at a frequency of the mud pump. The time between such peaks is measured and forms the basis for generating a first kick signal. A continuous increase in the phase between annulus and drill string standing waves forms the basis for another standing wave kick signal. Another preferred embodiment of the present invention is based upon the fact that the gas cut mud in the annulus affects acoustic transmission time differently than mud in the interior of the drill string. Accordingly, acoustic signals from a downhole source near the bottom of the borehole are transmitted at different speeds in the annulus mud and in the interior drill string mud. This downhole source may either be an MWD signal transmitter or drilling noise generated at the bit and resulting from the interaction between the bit and the rock. The annulus and drill string mud signals are detected at the surface and shifted in time and correlated with each other to remove phase ambiguity. A difference in arrival time between the signals is determined, and if large enough, causes a second kick signal to be generated. Another embodiment of the present invention may be used where at least two drilling pumps are used in the drilling system. This method determines the total travel time, from standpipe to drill string and up the annulus, of a beat frequency pressure wave caused by slightly different frequencies of the two pumps. A third alarm signal is generated if the total travel time is greater than a predetermined threshold. In a particularly preferred embodiment of the invention, both the first and second kick signals (and where possible, the third kick signal) are required to exist to establish a final kick alarm signal in order to substantially eliminate false signals. Another preferred embodiment determines mud pump noise round trip transit time T by evaluating a function        T=(n-Φ/2π)f          where Φ is the phase between standpipe and annulus signals and f is the frequency of a drill fluid oscillation caused by a mud pump and n is an integer which is incremented or decremented until the rate of change of T with respect to frequency is zero. An alarm signal is generated when the rate of change of T with respect to time exceeds a predetermined threshold. Another embodiment determines the Doppler frequency shift of a mud pump signal expressed as a ratio signal, Delta f/f, where Delta f is the change in frequency measured at the annulus from the frequency f of the mud pump signal measured at the standpipe. The ratio is compared with a threshold signal to determine whether or not gas influx has entered the annulus.