The paper illustrates the improvements in logging while drilling (LWD)
images and subsequent formation evaluation by using a new methodology for depth
and survey measurements corrections.
LWD depth measurements are often considered inaccurate and, therefore, not
as reliable for well-to-well correlations, correlations with data acquired with
wireline measurements and formation layer thickness determinations. The reasons
for these inaccuracies generally originate from the traditional practice that
LWD depth is purposely made equal to the driller’s depth, which is a static
pipe length measurement made by tape at the surface.
There is almost always a difference between the actual measured depth (MD)
of the LWD sensor downhole and this static pipe measurement, because downhole
the drillpipe is subject to an environment that is not representative of the
derrick (e.g., varying drilling mechanical conditions and temperature changes).
Here, we demonstrate the applications of the method, which allows dynamic
driller’s depth correction for the effects of drillstring weight, downhole
friction, weight on bit, thermal expansion, residual rig heave, and tide.
Another significant inaccuracy source is a standard practice of calculating
borehole position from stationary survey points typically taken every 90 feet
(ft) using the minimum curvature method. Neglecting the complex borehole shape
between survey stations can lead to a systematic error in determining the
borehole position. We consider using continuous inclination and azimuth
measurements along with stationary surveys to correct these errors.
We provide comparisons of LWD images before and after the depth and survey
corrections to illustrate how the measurement errors affect formation dips
interpreted from the images. We demonstrate how improved accuracy allows
filtering out the artifacts and provides more decisive and accurate
identification of geologic features. We show how using the corrected 3D
position improves accuracy of the formation thicknesses calculations and
therefore improves the reservoir summation results.
As a result, we propose a borehole 3D position measurement that is accurate,
consistent between wells (regardless of rig type or bottomhole assembly [BHA]
configuration), and independent of the drilling mode. Using this new
measurement significantly improves the quality of the formation evaluation.
© 2009. Society of Petroleum Engineers
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- Original manuscript received:
1 August 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
14 May 2008
- Manuscript approved:
17 July 2008
- Published online:
2 March 2009
- Version of record:
26 February 2009