Well Depth Measurement Quality Improvement: Quantifying Uncertainty for Along-Hole Depth
- Harald Bolt (Depth Solutions, DwpD Ltd)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- SPWLA 60th Annual Logging Symposium, 15-19 June, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2019. held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors
- 6 in the last 30 days
- 97 since 2007
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Depth is the most fundamental subsurface measurement made in our business. Logging while drilling depths are based on driller’s depths. Driller’s depths have been plagued with accuracy issues, with numerous articles highlighting this.
Driller’s depth measurement is based on drillstring length, typically as identified in the “tally book”. Together with the dimensions of the bottom hole assembly (BHA), the pipe length represents the drillstring length.
Way-point wireline depth correction methodology (Bolt, 2018a) is applied to drill pipe derived driller’s depth. The methodology, Driller’s Way-point Depth (DwpD, Bolt 2018b), results in arriving at corrected drillstring depth based on the calibrated drill pipe length measurement. Using this, an associated uncertainty is quantified. The objective is to arrive at True Along-Hole (TAH) depth, providing users of depth data not only a depth value but also an evaluation of the accuracy of the depth data.
A crucial difference to conventional driller’s depth is that the way-point method is applied uniquely when pulling out of hole using simple sliding motion. Most of the parameters that cause complications in driller’s depth correction are mitigated using this rig-state. The correction contributions are limited to thermal elongation and elastic stretch but then based on measured, not assumed, parameters.
This allows a corrected depth to be defined for the bit and the associated measurement while drilling (MWD) and logging while drilling (LWD) sensors. A major improvement is that the measurement is not only repeatable but has also an associated calculated uncertainty. The correction profiles and the uncertainty profiles are unique to each well.
The way-point method described provides corrections in wells with complex and long reach trajectories. This includes deep offshore, horizontal and crooked wells and complex architecture drillstrings. TAH depth data from drill pipe can be independently evaluated against other sources of depth data, such as wireline. The typical “tie-in” is then no longer necessary, as depth data measurements can then be used to independently verify measurements instead of simply complying to an arbitrary maxim.
An example is discussed where DwpD is used to improve drill pipe derived well depth and quantify the depth measurement uncertainty.
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