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. 

LWD types

• Electromagnetic logging
• Logging while drilling induction tools
• Acoustic logging
• Nuclear magnetic resonance (NMR) logging
• Nuclear logging

Depth measurement

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.

Along-hole (AHD) depth is the measured pipe length plus environmental and measurment correction. This is done to reduce uncertainty of the depth reference value used against which the bit position is established and the LWD sensor responses are logged. The combination of the AHD with uncertainty is named True Along-hole (TAH) depth. In many cases there is no correction made and the LWD depth measurment value is limited to the driller's pipe tally and measurment of the intermediate movement of the travelling block. Corrected depth is relatively simple to implement, but this is strongly dependant on the rig state. During downward drilling the drill pipe string is subject to a very complex regime of tension, compression, friction, toque, pressure differential, etc., and these individual influences can also be interlinked. This makes the quantification of the correction parameters extremely complex, to the point that the uncertainty of the correction figure arrived at may well be higher than the correction itself. As a funtions of this, while drilling there is often little point in attempting to include corrections. However, when the rig state is pulling out of hole (POOH) with simple sliding motion (SSM) almost all the complexities are either negated or dimish to zero, leaving temperature and drill pipe tension as the main contributors to correction. What is more, these paramters, using the same drill string in the same well are repeatable and hence an AHD measurment (including correction) can be repeated and verified. This allows the LWD sensors, such as navigation, GR, and resistivity, sampliing, etc. to be logged to calibrated and corrected AHD. This provides a definatve reference log for well construction, geological, pertrophysical and production operations.

A new methodology has been introduced to capture the correction information using interval segements where the correction effects are added to arrive at a correction profile for the well. Applying simple accuracy parameters to the contributing measurment allows uncertainty to be determined, and the TAH depth can be provided.

The TAH depth can be then compared to similar runs into the well, also with wireline, to verify the depth refernece arrived at.

This negates the need to run a cased-hole gamma ray during completion operations, a practice adopted by many operators as a check against LWD depth errors and lost-data zones

References

Bolt, H., 2017. Depth Data Quality Improvement: Way-point Methodology. SPWLA, Petrophysics, Vol 58(6), December

Bolt. H., 2017. A Method for Determining Well. Depth, Driller's Way-point Deptrh Ltd., International Patent Application No. PCT/GB2018/00030, February 2017

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

See also

Acoustic logging while drilling

Downhole tool communication

PEH:Drilling-Data_Acquisition