Logging While Drilling
Logging while drilling (LWD) is an oilfield service that provides a tool within the drill string that transmits real-time formation information. The LWD tools are located near the end of the drillstring. The measurements recorded provide drilling engineers with critical well information so they may make time sensitive decisions about future well operations.
LWD provides important well information on porosity, resistivity, acoustic waveform, hold direction and weight on bit. These measurements can be used to calculate ROP (rate of penetration) which is important in determining the speed at which the well is being drilled. Data is transmitted to the surface by pulses through the mud column.
Using the EWR (Electromagnetic Wave Resistivity) tool, one can measure resistivity of the formation and infiltration of drilling fluids into the formation. EWR uses formation exposure time (FET), the time difference between conditions at the end of the drillbit and the corresponding sensors on the drillstring. It is essential to measure the infiltration of drilling fluids into the formation because a large-scale invasion might cause a hydrocarbon-bearing reservoir to be obscured on the logs. The quantity of measurements taken is directly related to the Rate of Penetration (ROP). Logs will typically display tick marks corresponding to ROP to show the density of measurements taken over a given depth interval.
EWR can be used to image the hole while drilling. A small-button electrode is placed on the OD (outside diameter) of the stabilizer to measure the electrical current flowing through that electrode. The measured current can resolve up to 2 to 3 inches of vertical resolution within the well. Imaging can provide a picture of nonconformities, formation structure, and large fractures. Because of the resolution it provides, imaging can also be used in conjunction with geosteering.
Logging While Drilling Induction Tools
Schlumberger introduced the compensated dual resistivity (CDR) tool which allowed log data to be transmitted up the wellbore by mud pulses. Storage devices at the bottom of the drillstring allow the driller to retrieve raw data when the bottom hole assembly (BHA) is pulled. The CDR tool uses a 2-MHz electromagnetic wave to measure the difference between phase shift and amplitudes measured downhole.
Propagation measurements are made by subtracting the phase shift and the attenuation (amplitudes) of the voltages captured at the two receivers. Because both attenuation and phase shift are proportional to formation conductivity, these propagation measurements can be used to generate resistivity logs. The measurements are then averaged to account for the roughness of the borehole. This averaging is referred to as borehole compensation.
Phase shift and attenuation are recorded as two separate measurements, RPS (phase shift, shallow) and RAD (attenuation, deep). Service companies often perform there own dialectric correction on the data due to the 2 MHz dialectric effect at high resistivity levels.
Multiarray propagation tools
Schlumberger produced the ARC5 (array resistivity compensated tool) in the 1990s. The ARC5 is capable of making 5 independent phase shift and attenuation measurements. The ARC5 configuration consists of five transmitters and two receivers. This configuration is able to record 5 phase shifts and 5 raw attenuations.
Teleco produced the 2-MHz Dual Propagation Resistivity (DPR) tool in 1989. Borehole compensation was not used with this tool. After Teleco was bought by Baker Hughes in 1989, the tool was replaced with the multiple propagation resistivity (MPR) tool. Borehole compensation was performed with this tool by averaging measurements from long and short transmitter pairs. The MPR tool could then produce eight separate logs and could provide borehole corrected data. Borehole correction accounts for hole size and mud-resistivity.
Halliburton produced the 2-MHz compensated wave-resistivity tool (CWR) in 1993. The CWR is able to produce shallow and deep phase-shift and attenuation measurements. CWR data is also completed with borehole compensation.
Acoustic logging, ultrasonic caliper measurements, are used for improving neutron density measurements. Caliper transducers operate by generating a high-frequency acoustic signal. The signal is then reflected by the borehole wall. Ultrasonic caliper readings provide higher resolution than wireline mechanical calipers and are therefore the preferred method of acoustic logging in industry. Acoustic logging is used mainly to measure shear-wave velocity which can then be used to determine rock mechanical properties. The information acoustic logging provides can be especially useful when correlating lithologies with seismic data. The data gathered from acoustic logging is also very helpful in determining porosity. Downhole processing is often required because the massiveness and rigidness of the drill color affects the acoustic signal.
Seismic logging tools are mostly functional in exploration wells where previous seismic data is unreliable. Downhole seismic data can either be acquired while drilling or tripping out of the hole. In-situ seismic testing can be useful when determining the stability of the formation. Formation stability information can then help company men and geologists make decisions on mud weight and ROP.
Nuclear Magnetic Resonance (NMR) LWD
NMR is beneficial because it is a nonradioactive alternative for porosity measurement. NMR also offers high resolution imaging of laminated reservoirs and thin beds. NMR is particularly useful because it can provide an accurate measurement of permeability. NMR measurements are frequently used in shaly sand formations. Nuclear magnetic resonance is also able to achieve oil viscosity measurements as well as water, gas, and oil saturation of the target formation. One of the critical advantages of NMR is the ability to measure in situ formation characteristics.
NMR logging was first introduced by NUMAR in 1991. NUMAR essential inverted the equipment used for medical MRI logging. The tool is positioned in the wellbore where the center of the formation can be analyzed and measured. At the center of the tool, a large magnetic field is produced which magnetizes materials in the surrounding formation. An antenna around the magnet then pulses timed radio-frequency energy and listens for echos that bounce off of the target. Because NMR tools are sensitive to specific frequencies, they can be used to image thin slices of a rock formation.
Nuclear Logging While Drilling
Nuclear logging, or gamma ray measurements, is the most commonly used type of LWD. The essential requirements of nuclear logging are a depth-tracking system and surface computer hardware.
Rapid sampling is a nuclear logging method where statistical techniques are used in conjunction with incoming gamma data. Rapid sampling is able to determine where there is a significant difference between mud weight and formation density. A critical sharp decrease in mud weight can indicate an invasion of formation fluid into the well. An influx of formation fluid could be an indicator of the instability of the well or a kick. A kick is a critical well control condition where formation fluid travels up the wellbore. If a kick is uncontrolled and the well cannot be shut in, then conditions can exacerbate until the well is in a blowout. Aside from providing imaging of the hole, LWD tools are essential for observing and maintaining well stability.
Nuclear logging can also require the use of ultrasonic calipers. Ultrasonic caliper measurements are used to compensate for mud effects. Certain synthetic muds might attenuate sonic measurements so these errors must be corrected.