measurements taken to produce a caliper log (a graph of changes in the diameter of a borehole with depth). The measurement of borehole diameter was first proposed in 1935 by the Soviet scientist S. Ia. Litvinov.
The actual diameter of a borehole (the diameter of a circle equal in area to the cross section of the hole in a plane perpendicular to its axis) differs from the diameter of the boring bit with which it was drilled. An increase in diameter (formation of cavities) is observed in certain clayey rocks because of their crumbling as a result of swelling of particles; in rock salt and potassium salt, which dissolve in drilling fluid; in porous sands, which are washed away during drilling by the jet of drilling fluid; in cavernous and fissured rocks; and when a borehole is intersected by karst caverns. A reduction in the diameter of a borehole occurs in permeable rocks as a result of the formation of a clay crust because of seepage of the drilling fluid into the bed and swelling of some rock under the action of water.
Caliper logs are used in conjunction with data obtained by other geophysical methods for the precise determination of the geologic cross section of a borehole and, in particular, for the location of reservoir beds. In addition, caliper logs make possible monitoring of the condition of the borehole during drilling and the discovery of intervals that are favorable for the installation of sealing (packing) devices and determination of the amount of cement required for sealing the annular space when the casing string is run into the borehole.
REFERENCESpravochnik geofizika, vol. 2. Moscow, 1961.
V. T. CHUKIN