Rock Alteration
Most formations are altered when the borehole is drilled. More competent formations show either an imperceptible or massive change, while softer formations often suffer significant, obvious alterations.

Shales are altered by exposure to mud filtrate, caving, eroding, absorbing water and swelling. The degree of weathering is a function of shale type and mud property, such as water loss, filtrate salinity and weight. Exposure time and other mechanical factors involved in drilling a hole are also of importance.

Sandstones are altered by relaxation, which is a function of mud weight, as well as erosion - a result of mud weight, water loss characteristics, and bit hydraulics.

Carbonates under stress, as found in overthrust zones, often implode into the borehole, leaving a large, irregular hole which is difficult to log accurately. An example is given below.


Density Log in Rough Hole

Evaporites often dissolve and leave a large hole which cannot be logged.

All the above conditions can be minimized by proper attention to drilling procedures and drilling mud control.

Reconstructing a log, calculating from basic principles, using less affected data from the same hole may be attempted. Such results are often used for seismic modeling. In addition, reconstruction is also used to create missing logs or portions of logs to assist petrophysical analysis or geological correlation. There are numerous methods for reconstruction described in further Sections of this Chapter.

A dramatic example of rock alteration is the thawing of permafrost, by circulation of warm drilling mud, in frozen porous rocks in Northern climates. A sample is shown below.


This log clearly illustrates a severe case of rock alteration due to permafrost. In shallow or recent
sediments, the difference between long and short spaced sonic may be quite marked. The longer
 spacing is usually the better log.

In altered shales, a longer spaced sonic log, will often provide better data for geophysical purposes and reservoir calculations. Other sonic logs in the area can be edited using comparative data between the long and short spaced sonic logs run in selected holes. The log below is an example of rock alteration, showing a long and short spaced sonic log in a badly weathered borehole. Here, the density log has been reconstructed by calculating the log response from the long spaced sonic data and other known properties of the rock sequence.


An example of sonic and density log reconstruction, showing before and after curves.

 

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