Characteristic Analysis of Downhole Pressure Signal while Drilling under Drillstring Washout Conditions

Abstract

The identification of drillstring washout conditions during drilling is mostly based on standpipe pressure, drilling fluid flow rate or turbine RPM. These methods are inaccurate or not applicable to other drilling methods. It is difficult to find the drillstring washout conditions in time. While the downhole measurement technology can provide important informations for real-time discrimination of downhole conditions, there are few studies on the fluctuation characteristics of pressure signals while drilling under drillstring washout conditions. Using finite element software to simulate wellbore pressure distribution under drillstring washout conditions, Based on the self-developed near-bit multi-parameter measurement tool, the downhole drilling pressure signal is collected, and the signal density analysis method such as probability density, wavelet, spectrum and time domain is used to discuss the relationship between the fluctuation characteristics of the pressure signal while drilling and drillstring washout. The results show that, (1) Before and after washout occur, the pressure distribution inside and outside the drill string changed significantly. After washout occur, there is fluid outflow at the puncture, and the fluid flow velocity in the drill string and the annulus at the bottomhole decrease,the pressure in the drillstring and the annulus smaller than before the puncture. (2) The time domain and frequency domain characteristics of pressure while drilling can judge the occurrence of the drillstring washout. When washout occurs, the annulus pressure near the drill bit drops sharply, and the pressure signal spectrum produces a low frequency band. (3) The wavelet analysis and probability density characteristics of the pressure while drilling can be used as an auxiliary basis for the drillstring washout. The wavelet detail signal of the pressure signal can judge the pressure fluctuation, and the probability density characteristic can judge the change of the pressure amplitude. The fluctuation characteristics of the drilling pressure signal under drillstring washout obtained in this paper can provide reference and guidance for the identification of complex conditions in the drilling process.

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