Driller’s Depth Quality Improvement: Way-Point Methodology
- Harald Bolt (ICT Europe S.A.)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- December 2017
- Document Type
- Journal Paper
- 564 - 575
- 2017. Society of Petrophysicists & Well Log Analysts
- 0 in the last 30 days
- 58 since 2007
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In our business, accurate subsurface measurements are crucial. Depth is the most fundamental measurement made, tying together all the various along-hole measurements made and services provided. Logging-while-drilling (LWD) depths are based on driller’s depths. Driller’s depths have been plagued with accuracy issues, with numerous papers highlighting this.
This paper combines a wireline-depth-determination methodology with driller’s depth and shows how this can be used to arrive at a calibrated and corrected drillstring depth. The objective is to arrive at “true along-hole (TAH)” depth. Driller’s depth measurement is based on drillstring length, typically as identified in the “tally book”. Together with the dimensions of the bottomhole assembly (BHA), the measured pipe lengths represent the calibrated drillstring length.
Similar to wireline correction, a way-point method is described for correcting the drillstring length for thermal elongation and elastic stretch. This allows a corrected depth to be defined for the bit and the associated LWD sensors. A crucial difference to conventional driller’s depth is that the way-point method is applied during pull out of the hole (POOH). Most of the parameters that cause complications in driller’s depth correction are mitigated when pulling out of hole. The correction elements of thermal elongation and elastic stretch are the only ones then applicable. The waypoint method described can provide corrections in wells with complex and extended-reach trajectories.
The paper discusses how to arrive at an uncertainty so that measured depths are the TAH depth and that these TAH depths can be verified against repeat runs and wireline measured depth.
Along-hole depth is the basis of all well construction, reservoir and field modeling and economic evaluations. Depth impacts all aspects of the oil and gas upstream subsurface activity. Driller’s depth is derived from a composite record of the drilling activity as the drillstring moves into the well during the drilling process. The economic need for absolute and relative depth accuracy is not discussed in this paper, but it is clear that there are efficiency gains and asset value improvements with increased certainty and congruence of depth data.
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