Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. More information
Logging while drilling (LWD)
Logging while drilling (LWD) refers to the addition of wireline-quality formation measurements to the directional data of a Measurement While Drilling (MWD) service. Although attempts to deliver LWD serices date back to the 1920's, the first viable tools were by J.J. Arps in the 1960's, but these did not become a commercial service. The growth of MWD in the late 1970's and early 1980's delivered the first commercial LWD services by the major service providers. The initial tools were natural gamma and resistivity, and these made geosteering possible, as horizontal drilling grew. Information is returned to the surface using the same methods as MWD telemetry options.
- Electromagnetic logging
- Logging while drilling induction tools
- Acoustic logging
- Nuclear magnetic resonance (NMR) logging
- Nuclear logging
Good, consistent knowledge of the absolute depth of critical bed boundaries is important for geological models. Knowledge of the relative depth from the top of a reservoir to the oil/water contact is vital for reserves estimates. Nevertheless, of all the measurements made by wireline and LWD, depth is the one most taken for granted (despite being one of the most critical). Depth discrepancies between LWD and wireline have plagued the industry.
LWD depth measurements have evolved from mud-logging methods. Depth readings are tied, on a daily basis, to the driller’s depth. Driller’s depths are based on measurements of the length of drillpipe going in the hole, and are referenced to a device for measuring the height of the kelly or top drive with respect to a fixed point. These instantaneous measurements of depth are stored with respect to time for later merging with LWD downhole-memory data. The final log is constructed from this depth merge. On fixed installations, such as land rigs or jackup rigs, a number of well-documented sources exist that describe environmental error being introduced in the driller’s depth method. One study suggested that the following environmental errors would be introduced in a 3000-m well :
- Drillpipe stretch: 5- to 6-m increase.
- Thermal expansion: 3- to 4-m increase.
- Pressure effects: 1- to 2-m increase.
Floating rigs can introduce additional errors with depth measurements for wireline and LWD from heave and tide. In LWD, these effects are sufficiently overcome by the placement of compensation transducers in locations fixed with respect to the seabed.
Wireline measurements are also significantly affected by depth errors, as shown by the amount of depth shifting required between logging runs, which are often performed only hours apart, since wireline depth is stretch corrected, but readily computable stretch and thermal expansion effects are not applied to drillipipe measurement. Given the errors inherent to depth measurement, if wireline and LWD ever tagged a marker bed at the same depth, it would be sheer coincidence.
Environmentally corrected depth would be a relatively simple measure to implement in LWD. Although this measure would certainly reduce gross depth errors, it probably would not eliminate them, due to the complexity of stretch models under dynamic conditions in a high angle well.Gross thermal effects would be simpler to correct for. The “cost” of corrected depth is an additional depth measurement that must be monitored. Driven by the increasing availability of wireline-quality measurements while drilling, the industry is beginning to realize the need to adopt a new process for measuring depth accurately. Running a cased-hole gamma ray during completion operations is a practice adopted by many operators as a check against LWD depth errors and lost-data zones
- Kirkman, M. and Seim, P. 1989. Depth Measurement with Wireline and MWD Logs. In Measurement While Drilling, ed. Rollins et al., Vol. 40, 27-33. Richardson, Texas: Reprint Series, SPE.
- Brooks, A., Wilson, H., Jamieson, A., et al. 2005. Quantification of Depth Accuracy. SPE Annual Technical Conference and Exhibition, 9-12 October 2005, Dallas, Texas. 95611-MS. http://dx.doi.org/10.2118/95611-MS
Noteworthy papers in OnePetro
Hansen, R.R. and White, J. 1991. Features of Logging-While-Drilling (LWD) in Horizontal Wells, SPE/IADC Drilling Conference, 11-14 March. 21989-MS. http://dx.doi.org/10.2118/21989-MS.
Jackson, Charles E., Fredericks, Paul D., 1996. Proactive Use of Logging-While-Drilling (LWD) Measurements Improve Horizontal Well Drilling and Subsequent Evaluation, SPE/IADC Asia Pacific Drilling Technology, 9-11 September. 37157-MS. http://dx.doi.org/10.2118/37157-MS