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In this study, a model based fault diagnosis method for drillstring washout is proposed, which uses iterated unscented Kalman filter (IUKF) to detect the emergence of drillstring washout and to estimate washout depth and washout rate. To quantify the changes in pressure-loss and annulus outlet flow rate resulting from drillstring washout, pressure-loss factors and flow rate factor are introduced into the governing equations of circulating drilling fluid under normal drilling condition. Pressure-loss factors and flow rate factor are estimated by IUKF with updated measurements, and the emergence of drillstring washout is automatically detected by confirming the changes in pressure-loss factors and flow rate factor using generalized likelihood ratio test (GLRT). After drillstring washout is detected, continuously updated pressure and flow rate measurements are sent to the identification model of drillstring washout to estimate washout depth and washout rate by IUKF. In addition, the performances of fault diagnosis method for drillstring washout using IUKF and unscented Kalman filter (UKF) are contrasted. Numerical simulation indicates that IUKF and UKF have equivalent performance in drillstring washout detection, while IUKF shows a better performance in washout depth estimation. When using IUKF, the average relative errors of estimated washout depth and washout rate are 2.1% and 1.5% respectively. The errors of estimated washout depth and washout rate are 2.8% and 1.49% when using UKF. Robustness analysis indicates that IUKF is more robust than UKF to measurement noise. IUKF and UKF both have the ability to estimate washout depth and washout rate within the noise scope of 0%–0.8%, while IUKF is more accurate than UKF for washout depth estimation. • A diagnosis method for drillstring washout detection and identification is proposed. • Key parameters are estimated by iterated unscented Kalman filter firstly. • Temperature effect is considered in diagnosis state space representations. • The IUKF shows a more excellent performance than the UKF for drillstring washout. • The IUKF is more robust than the UKF to measurement noise. [ABSTRACT FROM AUTHOR]

Abstract A gas kick may occur when drilling with narrow pressure margin, which can lead to significant non-productive time. This paper proposes an early gas kick detection method applicable to water-based mud (WBM), which integrates a transient pressure and temperature coupling model into an unscented Kalman filter (UKF) algorithm. Three pressure factors and a flow rate factor are estimated with the UKF algorithm using updated measurement data in a detection estimator, and a generalized likelihood ratio test (GLRT) is employed to automatically detect changes in the pressure factors and flow rate factor for prediction of the gas kick. Simulated results show that the estimated pressures and outlet flow rate trace synthetic measurements very well with continuous inversion of the pressure factors and flow rate factor. All of the estimated pressure factors and flow rate factor fluctuate to a little extent around a base value under normal drilling condition, while the pressure factor in annulus and flow rate factor both deviate from the base value when the gas kick occurs. Performances of the proposed method, pit gain monitoring method and delta flow monitoring method are contrasted. The kick detection times are gradually reduced for all the methods with the increase of gas kick rate. Moreover, the proposed method herein shows a much better performance than the pit gain and delta flow monitoring methods, especially at small gas kick rate. Highlights • A new early gas kick detection method is proposed. • Pressure factors and flow rate factor are estimated by unscented Kalman filter. • Temperature effect is considered in the detection method. • Superior performance of the gas kick detection method has been proved. [ABSTRACT FROM AUTHOR]

Abstract The downhole weight on bit (WOB) measurement sensor can produce significant inaccuracies in its readings owing to the effects of temperature variation and well pressure differential. To improve the accuracy of downhole WOB measurement, the impacts of temperature variation and pressure differential on WOB measurement were studied via theoretical derivations, laboratory experiments, and field experiments. Based on the constitutive equations of thermoelastic materials and the strain theory of strain gauge, the equation of error induced by temperature variation was deduced. Through laboratory experiments, the conventional temperature compensatory method, and the new compensatory temperature method proposed herein were compared. The experimental results show that the proposed new temperature compensatory method performed better in eliminating the effects of temperature variation. The shortcoming of the conventional WOB correction method for eliminating the pressure differential impact is that it does not consider the pressure drop in the drill motor. A new WOB correction method is proposed based on the momentum equation of flowing fluid and floating body theory. The field experimental results indicate that the measurement error of the conventional correction method could reach up to 80 kN. The work presented herein can serve as guidance for the downhole WOB measurement. Highlights • A new method for removing strain errors due to temperature variation was proposed. • A new correction method for eliminating pressure differential's impact on WOB measurement was proposed. • Laboratory and field experimental results indicated that the new methods performed better in WOB measurement. [ABSTRACT FROM AUTHOR]

Non-uniform temperature distribution of DWOB measuring devices has a great effect on the accuracies of weight on bit (WOB) readings. To study the influence, two types of non-uniform temperature distributions of the WOB measuring device were created. The relationship between the two temperature distributions was studied with the further transform of heat conduction formulas. Then the strain of the measuring device was acquired under the two temperature distributions. And the relations of the strain in the two conditions was obtained with the further transform of compatibility equations and constitutive equations. To quantify the influence of non-uniform temperature distribution, a contrast simulation without non-uniform temperature distribution was carried out. The simulated results showed that the WOB errors were 9.33 kN and 46.25 kN. This paper proposed a novel compensating method based on the deduced relationships above, with which the WOB error was eliminated to 0.4 kN. To further validate the new method, the laboratory heating and cooling experiments were carried out. The experiment showed that the biggest WOB error was eliminated to 1.7 kN while the original error was up to 60 kN. Then a field experiment was conducted. Eight connection processes are chosen to further elaborate the temperature difference's impact and the related compensatory method. The economic loss caused by the WOB measuring inaccuracy was calculated, which was up to 1.17% of the drilling cost per meter. The work presented herein can serve as guidance for the DWOB measurement. • Temperature difference inside the WOB measuring tool results in great inaccuracies. • Temperature difference varies with the formation and drilling working conditions. • The WOB measuring error was up to 60 kN in heating and cooling experiments. • The WOB measuring error was eliminated to 1.7 kN using the proposed method. [ABSTRACT FROM AUTHOR]