Crain's Petrophysical Handbook - ACQUISITION OF LOGS WHILE DRILLING (LWD)

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ACQUISITION OF LOGS WHILE DRILLING (LWD)

	This Page è	Logging While Drilling Basics
	See Also è	Coiled Tubing	Measurement While Drilling	Courses	Site Map

LOGGING WHILE DRILLING BASICS
Logging while drilling (LWD) is a technique of conveying well logging tools downhole as part of the bottom hole assembly (BHA) while the well is being drilled. LWD tools transmit partial or complete measurement results to the surface via a drilling mud-pulser or other improved techniques (Real Time Data). Complete measurement results can be downloaded from LWD tools after they are pulled out of the hole (Memory Data Logging). Sometimes all or portions of the wellbore are logged or re-logged while pulling out of the hole. This is referred to as logging while tripping.

The first patent for an LWD tool appeared in 1932, for a tool that was a mimic of an ES log. Developed by J. C. Karcher , President opf Geophysical Services Inc (GSI) at the time, it used an insulated rod inside each drill pipe secion to conduct power and transmit the resistivity measurement to the surface. The electrodes were on an insulated mandrill above the but. It was not very reliable and not widely used.

<== One of Karcher's LWD resistivity logs, circa 1930

Many other attempts during the 1930's through the 1960's demonstrated potential, but were not commercially successful. Measurement While Deilling (Mud Logging), discussed elsewhere in this Handbook, became possible in the 1950's and gained widespread use in the 1960's onward.

Real commercial tools arrived in the 1970's, mostly for geosteering applications. In the 1980's, more variety of measurements, including GR, resistivity, and neutron became available. By the 1990's nearly every measurement that could bne made by wireline in open hple, could now be made while drilling.

Aside from the real time measurements, LWD offers another significant advantage - there is very little drilling fluid invasion at the time of logging, so logs are less affected by this problem. Borehole conditions may also be better than is the case for wireline logging.

The LWD concept

<==LWD density image log (not available as a wireline log), black is low density (shale or porous),
white is high density (tight).

Measurement While Drilling (MWD) Is a term used to describe drilling related measurements made at the surface or made downhole and transmitted to the surface while drilling a well. The terms MWD and LWD are sometimes used interchangeably, but we like to think of LWD as the process of obtaining information about the rocks (porosity, resistivity, etc) and MWD as obtaining information about the progress of the drilling operation (rate of penetration, weight on bit, wellbore trajectory, etc). MWD today often refers to geosteering measurements made to help decide on changes to the wellbore path.

The measured results are stored in LWD and MWD tools and some of the results can be transmitted digitally to surface using mud-pulse telemetry. Certain MWD systems have the capability of receiving encoded control commands which are sent by turning on and off mud pumps and/or changing the rotation speed of the drill pipe. These messages allow the drill bit to be steered in a desired direction

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 high angle wells. In frontier areas, deep water offshore, and critical wells anywhere, the real time log data as the well is drilled may be critical to the success and economics of the well.

LOGGING WHILE DRILLING TOOL STRING (BOTTOM HOLE ASSEMVLY - BHA)

Schematic drawing of a typical LWD tool string with multiple sensors.

Over the years, a majority of the conventional wireline measurements have been made available in LWD. Certain new measurements are available only in LWD, for example density imaging logs. The following is a list of available measurement in LWD:

Natural 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
Attenuation and phase shift resistivities at different transmitter spacings
and frequencies
Resistivity at the drill bit
Deep directional resistivities
Compressional slowness
Shear slowness
Density borehole images
Resistivity borehole images
Formation tester and sampler
Formation pressure
Nuclear magnetic resonance
Seismic while drilling
Vertical seismic profile

LOGGING WHILE DRILLING EXAMPLES

Resistivity, density, neutron, gamma ray, and capture cross section (sigma) recorded while drilling. The sigma curve (scaled in GR units in Track 1) shows the lower part of the sand to be clean, even though the GR is somewhat radioactive (confirmed by constant density neutron crossover). Gas crossover is more obvious on LWD logs as there is little flushing of moveable hydrocarbons due to limited time for drilling fluid invasion. If the LWD is run while tripping or on a wiper trip, invasion can be deeper, and may vary from one run to another.

LWD log in horizontal well. GR and true vertical depth (TVDE) in Track 1, resistivity in Track 2, porosity and PE curves in Track 3. The low resistivity below the heavy black line on the log indicates that the well dropped below the oil water contact (TVD > 2418 +/-). Much of the horizontal leg beyond this point is in or near the water.

This LWD example shows GR, TVD, and caliper in Track 1, with drilling characteristics such as rate of penetration, torque at the bit, and bit RPM. Track 2 gives shallow, medium, and deep resistivity - separation between the curves and spikes to the left indicate fractures in an oil reservoir. PE, density and neutron curves are in Track 3. These logs were run in short "wiper trip" segments and spliced in the computer - each curve is spliced at a different depth because of their position in the tool string. Some curves may not splice perfectly due to changes in the invasion profile over time.