Logging while drilling

Logging while drilling (LWD) is a technique of conveying well logging tools into the well borehole downhole as part of the bottom hole assembly (BHA).

LWD tools work with its measurement while drilling (MWD) system to transmit partial or complete measurement results to the surface via typically a drilling mud pulser or other improved techniques, while LWD tools are still in the borehole, which is called "real-time data". Complete measurement results can be downloaded from LWD tools after they are pulled out of hole, which is called "memory data".

LWD, while sometimes risky and expensive, has the advantage of measuring properties of a formation before drilling fluids invade deeply. Further, many wellbores prove to be difficult or even impossible to measure with conventional wireline tools, especially highly deviated wells. In these situations, the LWD measurement ensures that some measurement of the subsurface is captured in the event that wireline operations are not possible. Timely LWD data can also be used to guide well placement so that the wellbore remains within the zone of interest or in the most productive portion of a reservoir, such as in highly variable shale reservoirs.^[1]

LWD technology was developed originally as an enhancement to the earlier MWD technology to completely or partially replace wireline logging operation. With the improvement of the technology in the past decades, LWD is now widely used for drilling (including geosteering), and formation evaluation (especially for real time and high angle wells).

Available LWD measurements

LWD technology was originally developed to partially or completely replace wireline logging. Over the years, more of the measurements have been made available in LWD. Certain new measurements are also development in LWD only. The following is an incomplete list of available measurement in LWD technology.

• Natural gamma ray (GR)
  • Total gamma ray
  • Spectral gamma ray
  • Azimuthal gamma ray
  • Gamma ray close to drill bit.
• Density and photoelectric index
• Neutron porosity
• Borehole caliper
  • Ultra sonic azimuthal caliper.
  • Density caliper
• Resistivity (ohm-m)
  • Attenuation and phase-shift resistivities at different transmitter spacings and frequencies.
  • Resistivity at the drill bit.
  • Deep directional resistivities.
• Sonic
  • Compressional slowness (Δtc)
  • Shear slowness (Δts)
• Borehole images
  • Density borehole image
  • Resistivity borehole image
• Formation tester and sampler
  • Formation pressure
  • Formation fluid sample
• Nuclear magnetic resonance (NMR)
• Seismic while drilling (SWD)
  • Drillbit-SWD

See also

• Geosteering
• Measurement while drilling
• Halliburton Logging-While-Drilling Services
• LWD definition on Schlumberger Oilfield Glossary

References