Summary
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.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
13 December 2011
- Meeting paper published:
7 March 2012
- Revised manuscript received:
30 June 2012
- Manuscript approved:
10 July 2012
- Published online:
16 August 2012
- Version of record:
18 September 2012
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