Along-hole depth is the most fundamental measurement in our business of making and using subsurface measurements, tying together all downhole data services provided. Driller’s way-point depth (DwpD) correction is applied to calibrated drillstring length and, together with an associated uncertainty, provides true along-hole (TAH) depth. An earlier paper outlined DwpD theory. This paper reviews the methodology and describes the results of applying DwpD corrections in a field trial.
Logging-while-drilling (LWD) measurements are typically recorded using uncorrected driller’s depths while drilling down. When the drillstring is pulled out of hole (POOH) in a simple sliding state, DwpD correction of drillpipe depth can be applied in a way similar to the way-point correction used to correct wireline depth. The parameters necessary to calculate the correction include downhole temperature measured at the bottomhole assembly (BHA) and pipe axial tension measured as surface hookload (SHL). Both of these are measured as the drillstring is withdrawn from the well. These are the inputs to the DwpD thermal- and elastic-stretch corrections and these are applied to the calibrated length of the individual pipes that make up the drillstring.
DwpD corrections were applied in a field trial where two deep deviated off shore appraisal and development wells with along-hole depths of around 14,000 ft (Well 1) and 15,000 ft, (Well 2) that were drilled using composite 5- and 5.875-in. tapered drillstrings. Because the entire drillstring was under tension while being pulled out of hole, the corrections applied, amounting to around 100 ft at total depth (TD), are larger than those that might be expected using conventional methods. The field test results show that DwpD corrected depth is comparable to WLL logged depth.
The results show sensitivity of the corrected along-hole depth measurement to the tension profile, the temperature profile, the wellbore geometry and the drillstring architecture. The results highlight the differences between the originally logged LWD depth, the WLL logged depth and the DwpD corrected depth. The associated uncertainty of the DwpD corrected TAH depth then provides a context within which these differences can be resolved.