Logging while drilling (LWD) is a well logging technique that measures formation properties like type of rock penetrated, water saturation and formation porosity while drilling progresses.
To achieve this, the logging while drilling tool is normally integrated with the bottomhole assembly located just above the drilling bit so that shortly after the bit drills a hole through a formation, the formation characteristics are almost immediately obtained.
It is possible to confuse logging while drilling (LWD) with measurement while drilling (MWD) but in reality, these terms are not the same. The information retrieved from a logging while drilling tool helps in characterizing the drilled formation whereas the information obtained through a measurement while drilling tool helps in navigating and controlling the well path (useful in directional drilling).
How Logging While Drilling Works
The data acquired downhole through logging while drilling can be transmitted to the surface through a method known as mud pulse telemetry.
Since the drilling mud is circulated from the surface, down to the bit and up to the surface again; pressure pulses are generated in the drilling mud downhole and these pressure pulses are transmitted to the surface through the drilling mud.
Logging while drilling data is encoded in these pressure pulses as drilling progresses and sent to the surface where it is detected by pressure transducers located at the surface. That way, logging while drilling gives real-time data about formation properties penetrated by the bit.
The logging-while-drilling tool also has a provision to store the data in a memory which can be downloaded when the drillstring is pulled out of the hole.
Logging while drilling data is transmitted at evenly spaced time interval and the logging while drilling tool is normally powered by batteries and/or a mud turbine. In addition to formation properties, a logging while drilling tool can also provide information about drilling parameters such as weight-on-bit (WOB), torque and downhole temperature and pressure.
Advantages of Logging-while-drilling
Logging while drilling has unique advantages over conventional well logging since data can be obtained quickly before sources of data contamination become severe.
For example when drilling with water based mud, drilling fluid could invade the formation; penetrating deeper with time. When using resistivity logs to obtain the water saturation of the formation, it is possible to mistake the water from the drilling fluid as part of the original water present in the formation.
This gives erroneous water saturation results. Logging while drilling minimizes this error since data is obtained before drilling fluids can penetrate deeply into the formation.
Running a gamma ray log with the bottomhole-assembly as drilling progresses helps in differentiating different rock types (like shale, carbonates and sandstone). That way, rocks penetrated can be logged in real time.
Again, compared to wireline, logging while drilling has no issues with deviated or high angle wells. It may be difficult to run wireline through deviated and high angle wells.
One risk however, is that since the logging-while-drilling tool is part of the drillstring and positioned close to the drilling bit, drilling vibrations generated from the bit and during drilling operation can damage the tool.