The Design of BHA and the Placement of Magnetometer Sensors Influence How Magnetic Azimuth Is Distorted by the Magnetic Properties of Drilling Fluids
- Tor Inge Waag (Teknova A/S) | Torgeir Torkildsen (Wellpos A/S) | Per Amund Amundsen (University of Stavanger) | Erik Nyrnes (Statoil ASA) | Arild Saasen (University of Stavanger)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2012
- Document Type
- Journal Paper
- 393 - 406
- 2012. Society of Petroleum Engineers
- 1.6.1 Drilling Operation Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.11 Drilling Fluids and Materials, 1.12.1 Measurement While Drilling, 1.6 Drilling Operations
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The magnetic property of drilling fluid is one of the substantial error sources for the determination of magnetic azimuth for wellbores. Weight material, cuttings, clay, and other formation material in addition to metal filings from tubular wear may distort the magnetometer readings. This effect is obviously linked to the amount and kind of magnetic material that is in the drilling fluid, and the development of corrective means has therefore highlighted the drilling fluid. The problem has been studied by laboratory experiments and analyses of downhole-survey data. However, there are several inconsistencies in the results, and the phenomenon is not understood fully. We focus on the geometric properties of the bottomhole assembly (BHA) and the magnetic directional tool in this study.
The influence on directional measurement while drilling (MWD) from drilling fluids has been studied using finite-element modeling techniques. The simulations have been performed for several cases with realistic representations of MWD-tool geometries and varying location of BHA vs. the wellbore. One important result is that for situations with perfect axial symmetry, the magnetometer readings are attenuated proportionally to the square of the magnetic susceptibility. Because the magnetic susceptibility is a small number, the effect on magnetometer readings is generally negligible. However, if the symmetry is broken, the distortion on the magnetometer readings can be increased significantly. This means that segregation of cuttings, metal filings, or weight material can have a strong influence on the strength of the measured magnetic fields.
With a collar-based tool that is asymmetric by nature, the model shows a complex distortion picture. The influence varies with the tool-face angle and shows both attenuation and amplification. The distortion maxima and minima for the two cross-sectional magnetometers do not coincide. These findings underline how difficult it will be to find a straightforward corrective action.
The results from the simulations give increased knowledge about the influence from the magnetic properties of the drilling fluid. This is essential for accuracy estimation and multistation analysis of magnetic-survey data. Furthermore, the results demonstrate the importance of considering the geometry of the BHA and the tool and of the sensor placement when performing the mechanical design.
|File Size||9 MB||Number of Pages||14|
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