Magnetic contamination of the drilling fluid may contribute significantly to errors in directional surveying of wellbores. Such contaminations shield the field measured by the magnetic sensors in measurement-while-drilling (MWD) directional tools. The resulting error often exceeds typical sensor uncertainties, and so, the accuracy of the inferred wellbore position is severely degraded. Although the magnetic interference can be reduced by careful attention to procedures for transport, pumping, and handling of drilling fluids and their associated additives, the problem has not yet been eliminated.To understand the magnetic shielding effect in a well-defined setting, we have initiated a series of laboratory measurements where magnetic material of known properties are added to a well-defined nonmagnetic drilling fluid, and the resulting magnetic shielding is measured with a fluxgate magnetometer immersed in the fluid.In our experiments, powdered magnetite was added to a mixture of xanthan gum in water to obtain a mixture of nominal magnetic susceptibility typical of drilling fluids containing metal from pipe wear and erosion. Immediately after stirring, the vertical component of the Earth's magnetic field inside the liquid was measured and found to be significantly damped. Without touching the system, the field was subsequently logged. It was found to continue to decay for the next hour or so, reaching a fairly sharp minimum. The magnetic field then started to increase slowly and irregularly again over the next few days.This observed time dependency of the magnetic shielding is so far unknown to the surveying industry. The effect, combined with analysis of directional data, can explain errors encountered in directional surveying. Recommendations for how to minimize such errors are presented.
Magnetic materials in the drilling fluid used for drilling a petroleum well can significantly shield the Earth’s magnetic field as measured by magnetic sensors inside the drilling pipe. This has been shown to sometimes cause significant errors in the accuracy of borehole positioning using magnetic surveying. In this paper we present a physical approach for correcting the measured magnetic fields for such shielding. An explicit solution of the shielding problem is derived for the simplest case of a magnetic sensor on the axis of the borehole.
This paper describes a new method for estimation of well bore position accuracies, when using gyroscopic tools. The developed method represents a solution to the industry's need for a general and flexible error model which is applicable for all gyroscopic surveying tools and services. The general gyro error model consists of a new set of error terms and a mathematical description of how the different error sources contribute to position uncertainties dependent on sensor configurations and operational modes. The model is suitable for appropriate modelling of most gyro surveying services. The error propagation mechanisms are chosen to be identical to those in the ISCWSA's MWD error model (SPE 67616), which has become an industry standard during the last five years. Thus future standardisation and software implementation are simplified. The description of the model and the attached numerical examples should be sufficient to implement the model. The paper is a product of a collaborative work in ISCWSA (Industry Steering Committee on Wellbore Surveying Accuracy). Introduction Work in recent years by a group of industry experts, members of the ISCWSA, culminated in the publication of an error model for magnetic Measurement While Drilling (MWD) survey tools1 which has become widely accepted and used within the oil industry. The work described here was born out of a desire to extend that model to encompass the full range of surveying techniques available to the industry, and, specifically, to include gyro survey tools. The formative work, which has led to this paper, took place within the meetings of the ISCWSA, with subsequent detailed development being undertaken by a Gyro Working Group within that committee. Gyro tools are widely used for completion surveys and to control the drilling of well bores in regions of high magnetic interference, where the magnetic tools become less reliable. Recent advances in gyro technology have led to the application of gyro survey tools during drilling operations; the MWD gyro. This paper contains a description of a gyro survey tool error model, generated to provide:estimation of well bore position accuraciesa standardised and generalised model for the oil industrya model that is easy to implement in well planning and survey management software The model described here has been generated in response to a demand for a single model which is adaptable to the broad range of gyro based systems and services available to the oil industry, both now and in the foreseeable future. The formulation, as described, has sufficient flexibility to model the growth of survey errors in such systems, taking into account the types of sensor used, the sensor configuration and any operational procedures which will influence the performance of the system. The model also attempts to provide a fair representation of the physical processes which influence the propagation of errors, whilst avoiding unnecessary mathematical complexity. A balance between these two objectives has been sought in the selection of an acceptable format for the model. There follows a description of the model for the derivation of inclination and azimuth errors in both stationary and continuous modes of survey operation. This description includes the definition of terms, a statement of the assumptions made in the preparation of the model and details of the gyro and accelerometer error terms that have been included. A subsequent section illustrates the application of the model to some example survey tools. A simplified derivation of the error model coefficients and software implementation details are given in the Appendices.
fax 01-972-952-9435. AbstractThe drilling fluid may degrade the accuracy of magnetic azimuth and borehole position significantly so that it complicates the drilling operation and reduces the possibility to hit the planned targets. As an example: Azimuth is distorted up to 5° because of the magnetic properties (susceptibility) of the drilling fluid; and the wellbore is displaced 50 meter in one of the analyzed survey sections.Survey data from 30 well sections in the North Sea and the Norwegian Ocean are analyzed in order to see whether the weight material in the drilling fluid may affect the magnetic characteristics for the drilling fluid. The cross-axial magnetic field intensity is attenuated more when applying ilmenite instead of barite as weight material. Another significant finding is that the attenuation is stronger in 12 ¼" sections than in 17 ½" and 8 ½" sections.Laboratory experiments also indicate that drilling fluids with barite are less susceptible than those with ilmenite, and that fresh fluids are more favourable than used fluids. Attenuation factors for cross-axial magnetic components, which are derived from laboratory measurements and a simplified model of the downhole conditions, become much lower than those derived directly from the survey data. This contradiction is explained by some physical and operational conditions.Guidelines have been established for how to control, reduce and eliminate the negative effect on the wellbore positions. The precautions and actions, which are presented, are dependent on the composition of the drilling fluid, the drilling operation, the wellbore directions, the magnetic interference sources, the accuracy of the geomagnetic reference field and the requirements for wellbore position accuracy and reliability.The ultimate method for getting around this problem is to measure azimuth with a gyroscopic tool. However, multistation analysis and corrections of magnetic survey data is also a satisfactory solution when certain operational and geometrical requirements are met.
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