BOREHOLE LOGGING
GENERALITY
Geophysical borehole logging can provide accurate data of the physical properties of geologic units and groundwater within the borehole environment.
Borehole logging is a time and money-saving approach for gaining detailed information which is otherwise obtainable only from performing and analyzing numerous cores. Borehole logging data is typically used to characterize geology, fracture patterns, fluid flow, and geologic structural properties.
Common borehole logging techniques such as video, resistivity, natural gamma, electromagnetic induction, caliper, spontaneous potential, borehole deviation, and temperature can be deployed and interpreted quickly and low charges.
These applications are commonly used to maximize the informations obtained from geotechnical borings. While a variety of other tools such as heat pulse flow meter, optical and acoustic televiewing, downhole seismics and ground penetrating radar (GPR) are available, these are usually reserved for more detailed subsurface analyses.
BOREHOLE
TELEVIEWER
(BHTV)
The Borehole Televiewer (BHTV) tool is an ultrasonic imaging tool. Borehole sidewall images are created by measuring variations in travel times and relative amplitudes of reflected acoustic pulses as the BHTV device moves up the borehole. A water column or a thinned slurry is required to act as medium to transit and receive acoustic signals from the borehole wall. The borehole images are presented as unwrapped 2-D false color plots that are oriented to the magnetic north. Planar features such as fractures and bedding are shown as sinusoidal traces acros each plot.
GAMMA RAY
Gamma Ray logs measure radioactivity to determine what types of rocks are in the well. Because shales contain radioactive elements, they emit lots of gamma rays. On the other hand, clean sandstones emit very few gamma rays.
SELF/
SPONTANEOUS
POTENTIAL
The Spontaneous Potential (SP) log measures the natural or spontaneous potential difference between the borehole and the surface, without any applied current.
The most useful component of this potential difference is the electrochemical potential because it can cause a significant deflection in the SP response opposite permeable beds. The magnitude of this deflection depends mainly on the salinity contrast between the drilling mud and the formation water, and the clay content of the permeable bed. Therefore, the SP log is commonly used to detect permeable beds and to estimate the clay content and the formation of water salinity. The SP log can be used to distinguish between impermeable shale and permeable shale and porous sands.
RESISTIVITY
LOG
Resistivity logging measures the subsurface electrical resistivity, which is the ability to impede the flow of electric current. This helps to differentiate between formations filled with salty waters (good conductors of electricity) and those filled with hydrocarbons (poor conductors of electricity). Resistivity and porosity measurements are used to calculate water saturation. Resistivity is expressed in ohms or ohms/meter, and is frequently charted on a logarithm scale versus depth because of the large range of resistivity. The distance from the borehole penetrated by the current varies with the tool, from a few centimeters to one meter.
CALIPER
A tool that measures the diameter of the borehole, using either 2 or 4 arms. It can be used to detect regions where the borehole walls are compromised and the well logs may be less reliable.
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