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 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 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), formation evaluation (especially for real time and high angle wells).

Description

Logging while drilling, along with measurement while drilling systems provide wellbore directional surveys, petrophysical well logs, and drilling information in real-time while drilling. MWD refers to measurements acquired down hole while drilling that specifically describe directional surveying and drilling-related measurements. LWD refers to petrophysical measurements, similar to open hole wireline logs, acquired while drilling. These systems are based on mud pulse telemetry, where variations in pressure exercised by the tool can be sensed on the surface via a computer or electromagnetic telemetry, and thus communication is established.

Measured parameters

A suite of tools record different parameters of the drilled rocks:

• Natural Gamma Ray (GR)
  • Average Gamma Ray
  • Azimuthal Gamma Ray
  • Gamma Ray Spectrometry
  • Indicates sandy or shaly formation layers
• Acoustic Properties (sonic)
  • Slowness (Δtc)
  • Shear Slowness (Δts)
  • Used to correlate seismic data to depth and to acquire formation porosity and pore pressure
• Resistivity of Formation (ohm-m)
  • Higher Resistivity is indicative of hydrocarbons (or pure rock)
  • Low Resistivity is indicative of salt water
  • Attenuation or Phase shift resistivities at different transmitter spacings
  • Resistivity at the Bit can indicate instantly when a new formation is reached
  • Deep directional resistivities make it possible to stay in a pay zone when drilling horizontal wells
• Estimated Porosity (phi)
  • More porisity means more pore space that may contain hydrocarbons
• Density/Lithology (rho and photoelectric factor)
  • Density Caliper can indicate under gauge hole or wash out
• Pressure/Temperature
• Seismic While Drilling (SWD)
  • Drillbit-SWD
  • VSP-WD (Vertical Seismic Profile While Drilling)
• Formation Pressure
  • Fluid type
  • Mobility (indicator of permeability)
• Magnetic Resonance
  • mineralogy-independent porosity
  • bound- and free-fluid volumes
  • permeability
  • pore size
  • ideal for locating low-contrast, low-resistivity pay

See also

• Geosteering
• measurement while drilling

References

• LWD definition on Schlumberger Oilfield Glossary
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