D5 Drilling and Well Logging

Topic

Well drilling and logging constitute the only source of direct information regarding the subsurface at depths ranging from tens to thousands of meters. Rock cores recovered during drilling allow for the direct measurement—on the extracted rock itself—of petrophysical, mineralogical, structural, and geochemical properties exactly as they existed in their original setting. Well logs, in contrast, are continuous measurements taken along the entire wellbore without the need to extract cores; they are performed using probes lowered into the well on a cable.

The sonic log measures the travel time of ultrasonic waves—both compressional and shear—between a transmitter and a receiver separated by varying distances within the well; these measurements provide a continuous profile of compressional and shear wave velocities along the entire wellbore.

The density log employs a gamma-ray source to directly measure the density of the rock formation penetrated by the well, whereas the natural gamma-ray log measures the natural radioactivity emitted by uranium, thorium, and potassium present in that same formation. This natural gamma-ray log is the most common method for distinguishing clay—which emits high levels of natural radioactivity—from sandstone or limestone, which emit significantly less.

The resistivity log, for its part, distinguishes between a water-saturated formation—which conducts electricity well and thus exhibits low resistivity—and a formation containing hydrocarbons or dry rock, which conducts electricity poorly and therefore exhibits high resistivity. The temperature within the borehole and the heat flow passing through the rock are fundamental to understanding the subsurface: the temperature profile measured along the depth of the well allows for the calculation of the local geothermal gradient. This gradient—combined with the thermal conductivity of the rocks penetrated by the well (measured subsequently in the laboratory on recovered core samples)—enables the calculation of the heat flow rising from the Earth's interior at that location; the higher the rock's thermal conductivity and the steeper the temperature gradient with depth, the greater that heat flow.

The deepest scientific boreholes ever drilled—such as one reaching a depth of nine kilometers in Germany, another drilled directly into the San Andreas Fault, and international drilling programs operating on the ocean floor—have revealed unexpected properties of the deep subsurface: pore pressures far higher than anticipated, fluids present at much greater depths than predicted, and temperatures far higher than models had forecast. These findings confirm the unique value of drilling and well logging as a source of direct information—the very concept with which this description began.

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